Sound waves are pressure waves, but pressure is not sound.

To accurately measure the pressure of any oscillating wave one needs sophisticated high speed transducers. These are very expensive and are generally computer controlled.

It should be noted that waves in a medium (pipe for example) are oscillating. With an oscillating waveform, the amplitude of the wave will be constantly varying. There is usually a negative and a positive component (value). The shape of the wave can be uniform or very non-uniform (like I suspect an exhaust pressure wave would mimic)

When measuring any wave with a mechanical device (such as a pressure gauge) this gauge is very over-tasked for this sort of a measurement. The result would be a RMS (root mean squared) value, AT BEST, and would not accurately represent the actual value.

Let’s assume you have a steady frequency sinusoidal wave in the pipe (which we do not but we will assume for the moment) Taking a measurement with a gauge will give you the RMS value of this wave. The RMS value will be an “average” at best.

Now assume a pressure wave that is constantly varying it frequency, and amplitude (like a true pipe pulse) and now, you get a pressure that can not be measured correctly with a simple gauge-type device.

As we rise in elevation, atmospheric pressure decreases. This is a given. So, if you do NOTHING but rise in elevation, you drop pressure. So, ANY pressure measurement in a non sealed medium will be LOWER just from the rise in elevation.

Now the pipe pulses are treated as sound waves and the speed of sound is used in these calculations. The speed of sound (in a non-sealed medium) is temperature dependant. As we rise in elevation, temperature usually decreases and therefore; the speed in which the pipe waves travel are slower and this why we need to have increased pipe heat as elevation increases. You can increase the pipe heat via several methods. 1) Minimize radiation losses via wrapping the pipe with a TRUE heat barrier. 2) Increasing the internal gas heat via added power (engine enhancements etc.) these are a few. OR, you can alter the tuned length of the pipe to compensate for the slower travel.

So, one would surely benefit from a pipe that is designed around the lower atmospheric pressures that are present at the higher elevations.

BUT, let’s say you increase your engine’s internal heat and pressure via some true power increasing modifications. Now the internal pipe pressure may still be lower than if you were at sea–level BUT, it would be higher than if your engine did not have the power enhancements and you would have more rapid travel in the pipe.

So, the higher in elevation we go, the less power the engine makes unless something changes to process more air through the engine (Engine enhancements etc.). The naturally aspirated engine loses power because it can't process any more air. The engine can't magically grow displacement or increase rpm

So, what is the point to all this?? ONE point would be that the increase in elevation will produce less internal engine pressure and, as a result, less pipe pressure. Increasing the pipe’s internal pressure via stinger choking or restrictive silencers CAN allow for more pressure to be placed back in the engine via the “plugging pulse” of the pipe. BUT, this can come at a cost. The pulse’s strength and length are determinant mainly by the cone sections of the exhaust, NOT the stinger. The diameter of the stinger is a critical component. WHY? Because it has some control over the pipe’s operating temperature (internal heat). Too large of stinger and you can lower the pipe heat to a point of power-loss. Too small and you get an exhaust that can not bleed off pressure effectively and the engine will be the recipient of this “non-exhausted” heat and can cause engine failure, especially on long WOT pulls).

Get the stinger diameter just right and the engine acts like a tad shorter pipe and you can gain some power. NOTE: I said “STINGER” not “SILENCER” The silencer is located AFTER the stinger. Yes, the silencer does effect the rate of exhaust bleeding but having too large of a silencer does not effect the stinger’s operation. It does effect overall pipe temp.. having too small of silencer diameter can effect overall pipe operation due to raising the internal heat and can cause engine failure with prolonged running.

OK, so what do we know?

1) Increasing elevation will decrease pressure EVERYWHERE.

2) Pipe pressures are largely determinant on the power of the engine.

3) Internal pipe heat affects the speed of the waves in the pipe.

4) The proper way to increase pipe heat and pressure is to design a pipe that accounts for the higher elevation.

5) The Stinger section of the pipe has a strong effect on the RETURNED internal pipe heat to the engine and how fast the pipe cools or heats.

6) Increasing the returned pipe pressure via stinger or silencer choking can cause severe engine failure but can also aid in some cases.

7) Internal pipe pressure can not be accurately measured via ANY mechanical gauge.

8) More power creates more internal pressure/heat and can allow a pipe designed for low elevation to work at high elevation.

Exposing the Myths of High Octane Fuel and the Definition of Detonation

It is often stated that " The higher the octane, the slower the burn" AND there is PLENTY of discussion on the internet that would support this statement... BUT... It is my opinion that it is really not that "cut and dry"

First, a little tech info..

Octane is the fuel's ability to resist detonation. The higher the octane, the higher its resistance to detonation.. This is understood.. It is not this article's intent to dive into how these octane numbers are derived. Suffice to state that different companies/countries derive these ratings quite differently. We can leave it at that for the time being.

Detonation occurs AFTER IGNITION... This is very important point in the discussion. MANY confuse/equate detonation to pre-ignition. Pre-ignition is just that.. PRE IGNITION and occurs BEFORE IGNITION.. So, detonation and pre-ignition are very different. Now, pre-ignition will generally lead to full blown detonation.. but it is its own entity and should be treated as such.

During the compression process, the fuel/air mix is being "squeezed" into a small area. Shortly after this squeezing process begins, ignition occurs. Ignition will start a flame kernal / flame front that will propagate and expand from the ignition source (generally a spark plug in a gasoline engine) outward towards the oncoming piston.  Meanwhile, the fuel/air charge is still being compressed and forced into the combustion chamber area. This compression creates a pressure and this pressure creates heat. The flame kernal is also creating heat and aiding in the rapid rise is pressure.

Side note: The speed at which this flame kernal propagates is ,greatly, influenced by the head geometry, the amount of pressure surrounding it, the residual "mix" present and its chemical makeup.

OK.. back to the topic , at hand... So, we have a rapid expanding flame kernal, a fast approaching piston, and a fuel/air mix all occupying the same space and this space is getting smaller, VERY QUICKLY! This, my friends, is compression in an internal combusted engine.  When the temp gets to a point that this fuel/air mix FULLY ignites, then we have combustion.

The key to proper combustion is to have it occur at the best time to extract the most energy from the fuel/air mix , at the best rod angle to impose the max amount of leverage on the rod, and to perform work on the the crank train via force.

IF the combustion process before or after the above criteria are met, then you still have combustion but it is not optimum combustion. For this tech article, we will only concern ourselves with combustion that occurs too soon with respect to the rod angle. This is what we will term uncontrollable combustion or DETONATION.

Detonation from Wikipedia:

Detonation is a process of combustion in which a supersonic shock wave is propagated through a fluid due to an energy release in a reaction zone. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration, which propagates at a subsonic rate (i.e., slower than the sound speed in the material itself). Because detonations generate high pressures, they are usually much more destructive than deflagrations.

Detonations can be produced by explosives, reactive gaseous mixtures, certain dusts and aerosols.

In more simplistic terms (via RK Tek): Detonation is when the charge is prematurely combusted due to very high heat/pressure. This is more of an explosion than a combustion and the forces produced from this are VERY large and are super-sonic in nature.

So, why is detonation so bad? After all, you have extreme pressure occurring at very high speeds. This should produce some serious force on the rod and spin that crank with much more authority. Yes, it produces forces that we would love to have acting on our crank, unfortunately, these great forces occur at the wrong time in the stroke.  They occur at a rod angle that causes great STRESS on the crank, piston, bearings, and rod. So, this force becomes destructive vs. productive and since the force is so powerful, it WILL cause engine damage if not addressed.

So, how do we address and prevent the onset of detonation?

1) Proper combustion chamber design (i.e. geometry and volume) See RK Tek for this variable.

2) Proper ignition timing (adjust timing until optimum)

3) Proper Fuel amount and Octane (resistance to detonation) fuel.

OK, FINALLY, we get to fuel octane...

The octane rating of the fuel is directly related to its resistance to detonation. The higher the octane the higher pressures/heat the fuel can withstand before ignition. The rate at which this fuel burns has VERY little to do with the octane rating. The chemical soup of the fuel will determine the fuel's burn rate under IDEAL conditions. Since we KNOW that under combustion (active radicals, residual un-burnt mixes , squish action etc.) is NOT IDEAL, then this burn rate can not be properly established nor calculated. We do not need to know this burn rate because we will adjust our ignition and combustion chamber design to accommodate ANY burn rate.  So, it is really irrelevant for us. 

Does the higher octane fuel burn at a slower rate than lower octane fuel? IMO.. MAYBE, SOMETIMES, YES, and NO..  See above.. the rate of burn is dependant on many variables but to state that just because the octane is higher, then the burn is slower, MAY NOT always be a correct assumption.

Don't believe me?? Why should you? there are countless articles contradicting what I just said.. Well.. see an interview from the Sunoco Engineer below (I will highlight the best parts LOL)

Popular Hot Rodding Magazine, January 1998
By Scott Parkhurst

Octane is a measurement of a fuel's resistance to ignition. Ideally, the air/fuel mixture will ignite at the proper time and burn smoothly through the power stroke. The idea is that one powerful combustion of the air/fuel mixture is better than several randomly-ignited small flame fronts. When you can precisely control the point at which the fuel will ignite, maximum performance of the engine can be achieved, and power-robbing knock and ping will be eliminated. Knock and ping are a result of abnormal ignition, or multiple flame fronts colliding within the combustion chamber during the compression stroke.

All reputable fuel manufacturers determine the octane rating of their gasoline in the research lab using a special, dedicated single cylinder engine. Comparing the gasoline to a series of standard reference fuels in the test engine results in either a research octane number (RON) or a motor octane number (MON) depending on a set of operating conditions. The RON is determined with the test engine operating at 600 rpm, at standard barometric pressure, and the intake air temperature set at 125 degrees Fahrenheit. RON is primarily used to address part-throttle knock and ping problems. The MON addresses wide open throttle operation and is determined with the test engine spinning at 900 rpm, also at standard barometric pressure, and the intake air temperature pumped up to 300 degrees.

The best predictor of a fuel's performance in a street/strip machine is the Anti-Knock index (AKI). This is simply the average of the RON and MON numbers, or (RON + MON) / 2. Most all octane ratings posted at the pumps are determined by this AKI formula, and are the minimum values you could expect to see. The minimum octane requirement
of your engine is determined by several variables besides the compression ratio. The engine and cylinder head configuration, air/fuel mixture, timing, coolant temperature, atmospheric pressure, relative humidity, and ambient air temperature will also affect the octane required to make your mill produce maximum power.

The burn rate of a fuel is a measurement of the time required for complete combustion of the air/fuel mixture. The notion that octane ratings affect the burn rate of fuel is about 180-degrees from reality; burn rate is a function of several variables, and the two are completely independent, although there is generally a correlation between octane ratings and burn rates.

To give you a good example of this, we contacted Jim Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal, which is our fastest burning fuel, and coincidentally one of Sunoco's highest octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel has to burn pretty quickly to achieve complete combustion".

Octane boosters offer little help in the quest for higher octane. Most popular street-legal octane boosters claim increases in octane ratings up to five points, and those boosters intended for off-road use only claim up to seven points. That's a lot of octane to hope for simply by pouring an additive in a tank. Sunoco told us that before they launched their GT-100 Unleaded retail pilot program, they wanted to be sure that a 100 (R+M) / 2 octane street legal fuel would be of value, and that enthusiasts would not be able to get the same (or better) results using an octane booster. Nine of the most popular retail octane boosters were put through a series of tests to determine where the consumer could get the most bang for the buck. The test results were verified by an independent testing facility, using several brands of regular unleaded and premium gasolines, just to make sure everything was legit.

According to Mark Borosky, Vehicle Test Engineer for Sunoco,"Of the nine octane boosters tested, none showed a significant increase, and one actually lowered the octane number of the test gasolines."Testing repeatedly showed a maximum increase in octane of 3.5 points by only two of the six street legal octane boosters when the recommended treatment rate was blended with lower base 87 octane gasoline. The best the remaining four products could muster was less than a one point increase. "While clearly no one would actually use an octane booster in a low base octane fuel, we wanted to give the manufacturers the benefit of the doubt relative to their claims of five-to-seven point increases," explained Borosky.

When tests were performed using 93 and 94 octane fuel, even the two best products from the previous tests produced a disappointing 1.5 to 2 point maximum increase. The remaining four street-legal octane boosters showed less than a .5 point increase. Those products designated for offroad use only didn't fare any better than the street-legal products. Subsequent tests where the dosage of octane booster was doubled, tripled, and even quadrupled produced only minimal improvements in octane, regardless of the base octane number of the test gas. In fact, quadrupling the treatment rate of the most powerful additive produced only a 3.5 point increase in octane when added to 93 premium, resulting in a cost of $3.25 a gallon.

An alternative path to octane euphoria is to blend gasolines of different octane levels yourself. It's easier than you may think, safe, and the results are predictable. The formula for mixing gasolines of the same type is pretty straightforward. When you mix a 50/50 blend of two unleaded fuels, simply average the two octane ratings to determine what's in the tank If you mix 94 and 100, you get 97. The same generally holds true for leaded gasolines, assuming the lead content is
nearly equal.

Blending a leaded fuel with unleaded, however, pushes the octane up a bit more than the math would suggest, due to the effect of the lead. Just a gram or two of lead blended into the unleaded fuel will raise the octane number significantly. Commercial leaded racing fuels contain anywhere from a trace to six grams of lead per gallon. If you were to mix 50 percent 110 octane leaded fuel with 100 octane unleaded, you would actually end up with an octane number around 106 to 107. Keep in mind that even the smallest amount of lead or leaded gas line with unleaded, could spell the end of your catalytic converter or oxygen sensor. The same holds true for using octane boosters intended for off-road use only. A word to the wise, check for any lead content in all the additives you might mix with your unleaded gasoline. And check with your state emissions regulations for street use.

We asked Sunoco's Wurth about using aviation fuel in an automobile engine. He was emphatic when he said, "Don't do it. Even though Sunoco is a major producer of aviation fuel, this fuel is specifically blended for aircraft engines. Aircraft operate under very different conditions than automobiles, and the fuel requirements are quite different as well. Aircraft engines generally.... run within a very narrow rpm range. There's no need for transient throttle response in an airplane because after the pilot does the initial engine run-up, the throttle is set in one position and the rpm doesn't normally change until landing. Also, airplanes fly where the air is cold and thin, and the atmospheric pressure is low. These are not even close to the conditions your street machine will see on the ground. Also, since most piston-driven aircraft cruise at 3,000 rpm or so, the burn rate of aviation gas is much too slow for any high performance automotive applications."

What is it that makes race gas so different? What's it made of? Sunoco tells us their GT PLUS 104 octane unleaded race gas is only 15-20 percent traditional gasoline, and about 85 percent additives! Actually there are about 120 different chemicals in GT PLUS. One reason it isn't street legal is the high oxygen content. The EPA requires that the oxygen content of a street legal fuel cannot exceed 2.9 percent. GT PLUS is about 3.5 percent oxygen. This fuel is light in weight at only 6.14 lbs-per-gallon. The high oxygen content improves the octane, and when the induction system is properly calibrated, this fuel will help make additional horsepower. The high oxygen content has a supercharging effect, since 3.5 percent oxygen is the equivalent to about 17 percent more air. Different fuels can actually alter horsepower 5-to-10 percent or more.

We wanted to to know more about the different types of race gas Sunoco had, and didn't realize there were five different types of racing fuel alone.

GT-100 Unleaded, is a clear fuel with a pump octane of 100, and will handle compression ratios of up to 12:1, and is street legal in all 50 states.

GT PLUS, is also unleaded, and is rated at 104 octane. It is suitable for compression ratios up to 14:1 and is colored light blue. It will not harm oxygen sensors or knock sensors in computer controlled engines. It is not street legal.

STANDARD, is a leaded fuel rated at 110 octane, is colored purple, and is intended for drag racing, road racing, and race boats.

SUPREME, is also a leaded fuel and is dark blue. It was developed to help resist vapor lock and meet the demands of sportsman, modifieds, offshore powerboats, and endurance racing where engines regularly run in excess of 7000 rpm.

MAXIMAL, we mentioned earlier, is colored red, has 116 octane, and is leaded. It is intended for exceptionally high performance applications like Pro Stock where extremely high cylinder pressures are common. Its extremely fast burn rate is satisfactory where rpm exceeds 10,000.

Now that you're an expert on gasolines, you probably would like to know where to buy and store the stuff. If you are fortunate enough to live in the mid-Atlantic states, you can take advantage of Sunoco's GT-100 Unleaded retail pilot program and get 100 octane race fuel at pumps located at select Sunoco stations. The rest of us have to purchase from local speed shops, at race tracks, or directly from Sunoco distributors.

When you plan on buying fuel in quantity, say a 55-gallon drum, you'll be happy to know that racing fuel has a shelf life of about a year, if you store it properly. The container must conform to all safety standards, and should be made from metal or polymer. Make sure the container is opaque and solid in color. The white plastic jugs we see at the track should be used for short-term storage only. They let in sunlight, which will affect the fuel. The lead in leaded fuel and other chemicals in unleaded fuel are photosensitive, and will dissipate if they am exposed to the sun. Keep any container tightly sealed to prevent evaporation.

So, the next time somebody tells you that higher octane fuel burns slower, just refer him to this article.

PISTON DECK HEIGHT: How to Measure it and it's importance

What is Piston Deck Height?? Also known as Piston Deck Clearance.

    How do we determine Piston Deck Height??

    Why is it important when designing performance items for an engine??

    Is there a benefit to having a smaller or larger Piston Deck Height??

    Can the Piston Deck Height be easily changed??
 
 
    Above are some of the questions I will try and answer in this technical article.

_____________________________________________________________________
 
What is Piston Deck Height?? or Piston to Deck Clearance?
 
    The Piston Deck Height is the distance the piston EDGE is in relation to the top of the cylinder deck. NOTE: This
distance can be negative (the piston edge is recessed in the cylinder deck) or positive (the piston edge is sitting above the cylinder deck). Please remember... we are talking about the relationship of the EDGE of the piston not the center. Of course, if the piston is of flat-top design then the edge and the center will be the same.
 
 
How do we determine Piston Deck Height??
 
    1.  Remove the cylinder head.
 
    2.  Grab your Dial Indicator and Magnetic Base (very accurate), Flat-Blade Depth Micrometer (accurate), Dial Caliper(less accurate), or straight edge and feeler gauges (much less accurate) OR your Step Micrometer (needed for pistons with a positive deck height   .
 
    3.  Place piston at TRUE Top Dead Center (TDC) NOTE: Since every crank has dwell at TDC, determining TRUE TDC can be a bit tricky. Using a custom piston stop is the most accurate method for finding true TDC. Since some of you may not have a custom piston stop, finding true TDC can be determined using a dial indicator and a mag base. WHAT!! you don't have a spare mag base and dial indicator laying around??
 
Hmmmm... OK, Here is a fast way to determine true TDC without using a piston stop or dial indicator. This method, if done with finesse and time, will yield an accurate, or fairly accurate, measurement. This will require a very thin, but rigid, piece of material with a very fine point (if the piston is crowned).

  a. Rotate the crank to what you believe is true TDC.
 
  b. Take your rigid piece if material and place the sharp end in the cylinder to where it touches the piston. Be sure to hold this material in such a manner where it can be held in place without the backing of the piston.
 
  c. While holding the material on the piston, very carefully rotate the crank (by hand) forward and backward while keeping your material STATIONARY. If at any point the piston raises the material, even just a little bit, then you were not at true TDC.

With this in mind continue this method until the piston no longer displaces the rigid material.
Once, this occurs, you will be very close to true TDC.
 
OK.. now, you have found true TDC.. you are half way there!
 
Since you know when you are at true TDC, all you have to do is measure the distance from the piston edge to the cylinder deck.
 
How this measurement is done will be based on the measuring equipment you have chosen to use.
 
Please keep in mind... when measuring the piston deck height on a crowned piston, it is nearly impossible to get to the actual edge of the piston. This is because your measuring device does have mass and will hit the piston crown before it hits the actual edge. The finer the measuring device.. the more accurate the measurement. So.. if you are using the butt end of a dial caliper on a crowned piston, please keep in mind that this butt end is very wide and flat and will surely hit the crown way before the edge. Since the dial caliper is the instrument most commonly available for this measurement, I have determined an approximate offset factor for measurements obtained with the dial caliper. Of course, the accuracy of this offset will be determined by the accuracy of the initial measurement given. So, PLEASE take your time in measuring.
 
For pistons with a positive piston deck height, a step micrometer seems to be the most accurate tool for this measurement. It is nearly impossible to accurately measure positive deck without a step micrometer.
 
WITH ALL THAT SAID...MATH, along with measuring tools, can be used effectively to help determine piston deck height.

If you know the piston's crown height, then this can be used to aid in determining deck height.

For example.. if you can determine how far above the deck the center of the crown is at true TDC, you can subtract the known crown drop from this number to determine your piston deck height.
 
Why is it important when designing performance items for an engine??
 
 
Piston Deck Height is a very important piece of information when determining engine parameters and designing combustion chambers.
 
Let's start with its role in determining engine parameters.

Many performance shops utilize software and / or math equations as an aid in determining engine parameters and upgrades. For example, it is important to know the position of the piston, in inches or millimeters, at different points along the stroke ie. exhaust opening, transfer opening etc.. This information can be calculated using software or by punching the numbers in the mathematical equation (software is much easier).

OK, you may be asking yourself, "What does the piston deck height have to do with the piston position?" Well, not much from an initial design point, but for the person porting your engine, it can be very important. For example... many people send their cylinders out to be ported. Once the shop gets the cylinders they have the pleasure of determining how to modify it in order to yield performance gains. Many performance shops determine what to change based on the port timing of the engine. Now, if they only have your cylinders, not the entire engine, how can they determine what your port timing actually is??? Well, you may tell yourself that they have seen that engine before and know what the porting arrangements are and your engine will be exactly the same. Well, if you are convinced this is true then, you have nothing to worry about. But if you are like me, and KNOW that this is not always true, then you should be concerned if the shop you are sending your cylinders to does not ask for a piston deck height measurement or a base gasket thickness. There are often variances in cylinder castings, piston heights, connecting rod lengths, and base gasket thicknesses. ALL these things effect port timing and piston deck height. Knowing these parameters will assure that the shop has the necessary engine measurements to do the best job they can in modifying your cylinders.
 
How about combustion chamber design? How does piston deck height effect the design of the combustion chamber?
 
 
Piston deck height is a VERY important measurement to consider when designing a combustion chamber.
 
Piston deck height is a major player in determining the squish clearance of an engine. While it is beyond the scope of this article to discuss squish band design (maybe later), let's just say the squish action within a combustion chamber is very important in the combustion process and power making process.

Squish clearance is the distance from the edge of the piston, at TDC, to the outer edge of the combustion chamber's squish band. So, one can easily see how the piston deck height effects the squish clearance measurement.
 
So, you may ask yourself..." How can one design a proper combustion chamber without knowing the piston deck height??" Well.... the answer is simple.. ONE CAN'T!! Sure, they can get close. All I am stating is that they can get a lot closer if they have a list of engine measurements, like the piston deck height.
 
It has already been determined that there are many factors that effect the piston deck height.. cylinder casting, piston casting (or forging), con rod length, and cylinder base gasket to name a few. So, with this in mind, how can a head designer properly modify your head, or design a new head, for your engine without knowing the piston deck height of your engine? THEY CAN'T!
 
You might be saying to yourself " So, what if I am off on my piston deck height measurement .008", how big of a deal can that be?"
 
Well... While I will not go into the difference .008" has on squish action within a combustion chamber, let's look at what .008" does to the compression ratio and volume of an engine.
 
Volume of a cylinder: PI * R^2 (radius = 1/2 bore) * H (Stroke)
 
So, let's take  the 800 Rotax twin engine with an 82mm bore and a 76mm stroke. We will convert the .008" to mm so the numbers in the equation coincide. .008" = .2mm
 
What effect will .008" have on the compression ratio?
 
Let's  do the math: 3.14 * (82mm/2)^2 * .2mm = 1.05cc of volume change. So, what does 1.05cc of volume increase or decrease on this particular engine? Well.. 1.05cc equates to a 0.4 change in un-trapped compression ratio. OK.. 0.4 change in un-trapped compression ratio will change a 12:1 engine to a 12.4:1 or a 11.6:1 engine. Well..may be that is not so bad. so, lets change the engine by .015" or .38mm
 
3.14 * (82mm/2)^2 * .38mm = 2.0cc  of volume change. So, what does that do to the un-trapped compression ratio? Well it changes the un-trapped compression ratio on a 12:1 engine to 12.79:1 or to 11.21:1.
 
So, you can see that the compression ratio is effected but what is also effected is the squish action within the head. Squish action is important in determining power characteristics of an engine. The squish band acts as a cooling layer to help cool the end gases as they are being rapidly compressed. By keeping these gases below their combustible temperature, one can prevent undesired combustion of these end gases in the squish band area. If these end gases are allowed to combust before the oncoming, spark initiated, flame front chooses to combust them, then you have the receipt for detonation and engine damage.
The squish action also creates turbulence within the combustion chamber. This turbulence has a direct effect on the flame front speed so ,in actuality, it effects ignition timing.
 
OK, another measurement for the squish action is the Maximum Squish Velocity (MSV). In short, this is the max velocity of the end gases as they are be compressed. It is actually a lot more complicated than that, but I will leave it at that for now. It is measured in meters/sec (m/s).
 
Squish velocity has a very large effect on the heat release and rate of burning in a two stroke engine. Hence power output and engine reliability.
 
Software exists to give a close approximation of this velocity but.

Let's take our above examples and see how squish velocity is changed by a small variance in squish clearance.
 
The first example showed a change in squish clearance of .2mm. Using 2 stroke software this .2mm change in total squish clearance will increase the squish velocity in a 13.5:1 head by 7.4m/s if this .2mm is removed from the total squish clearance. If this .2mm is added to the total squish clearance, then the squish velocity is decreased by 5m/s.
 
The 2nd example, showed a squish clearance difference of .38mm
This .38m change in total squish clearance will change the squish velocity by 18m/s when this .38mm  is removed from the total squish clearance. If this .38mm is added to the total squish clearance, then the squish velocity is decreased by 8.4m/s.
 
NOTE: The above MSV calculations were taken from a specific head design. Since MSV has many determining factors, the changes in MSV could be much less or much greater than the ones listed above.

The overall head design and cylinder port timing determines the magnitude of the MSV changes.
 
One can see that the relationship between adding and subtracting squish clearance is not linear and does have pronounced effects on squish action.
 
So, you can see how one needs to be careful when purchasing a new aftermarket head or modifying a stock head. Next time you are talking to a head maker or a shop that may be modifying your head, ask them what the piston deck height is for that engine. If they do not know, how can they design a head for that engine that will have acceptable squish velocity, squish clearance, and compression ratio? OR when speaking with these people, tell then that you have added an extra base gasket to your engine. Ask them how that will effect the "on the shelf" head they want to sell you. I have already shown you what a .008" and a .015" difference in deck height will do to compression ratio and maximum squish velocity.
 
So... the old adage of shaving off .015" of a stock head can get you into excessive squish velocities in a very big hurry. Excessive squish velocities can lead to piston breakage and severe engine damage.
 
 
Is there a benefit to having a smaller or larger Piston Deck Height??
 
There maybe some benefits to having a large or small piston deck height.
 
The one that comes to mind first is in the cooling effects of the engine. For example.. if the piston deck height is large in the cylinder, then there may be an argument for the end gases retaining more heat due to them being trapped in the cylinder vs. the head. One may argue that end gases trapped in the head portion of the squish band would be subject to the better cooling properties of the head. This would be a hard theory to prove, but it does have merit.
 
Can the Piston Deck Height be easily changed??
 
Yes, it can be easily changed.
 
Below are several methods of altering piston deck height, which, as I have shown, also alters MANY other operating factors.
 
1. Changing base gasket thickness
2. Decking cylinder base
3. Decking cylinder top
4. Changing piston
5. Changing crank
6. Changing rod length
7. Changing stroke
8. Altering piston crown

DOES YOUR FUEL INJECTED SLED REALLY NEED A FUEL CONTROLLER? ARE THE PRE-PROGRAMMED CONTROLLERS OFFERED REALLY RIGHT FOR YOUR ENGINE?

You always hear how you can get a pre-programmed fuel control box for your Cat or Polaris and it will be set up to work perfectly with YOUR engine!

Is this really the case?

Are ALL engines the same and, therefore; require the same fueling requirements?

Is this pre-programmed (1 size fits all type) controller really allowing to you extract the full power potential from the expensive "kit" you just spent your hard earned money purchasing?

OK, let's look at these questions in a little more detail..

First, it would be wise to get a few facts straight about fuel flow in these throttle body injected power-plants we love so much..

1) Injectors operate on what is called a "Duty Cycle". What does this mean? A Duty Cycle is the fraction of time that a device (in this case your fuel injector) is in an "ACTIVE" state.  The injector is fed a square wave pulse (see below for a sample of a square wave pulse)

When the amplitude is at a "1" ,the injector is active and will open and flow the fuel that is present in the fuel rail and pumped from your fuel tank via the electric fuel pump. This fuel is INJECTED via the injector to your engine. This is why they call it "Fuel Injected".

When the amplitude is at a "0", the injector will close its orifice and fuel flow will stop traveling from your tank through the injector to your engine.

This square wave signal is not necessarily "even" in its "on" and "off" times. Meaning, the square wave will not give equal "on" time and "off" time to the injector.  The amount of "on" time is  directly controlled by your engine's electronic brain box.

2) The longer the amplitude stays at "1" the longer and more fuel that will be injected to your engine. The reverse is true as well.

Aftermarket fuel controllers, alter this square wave. The injector pulse (Square Wave) is fed to the controller and the controller will adjust its duty cycle based on the controller's programming. This is how you add or subtract fuel to your engine. NOTE:  there are only 2 controllers that can removed fuel from your engine. ALL can add more fuel but only 2 can subtract it.

3) The pressure present in the fuel lines will DIRECTLY effect the amount of fuel that will be injected when the injector is "on". The higher the fuel pressure, the more fuel that will be injected when the injector is turned on. Conversely, the lower the fuel pump pressure, the LESS amount of fuel will be injected when the injector is turned on. So, you see, that the amount of fuel that is delivered, via injector, to your engine, is NOT just related to your injector pulse duty cycle, but is also directly related to the line pressure.

There is a very good calculator and description of this located at: http://www.rceng.com/technical.aspx  You can play with numbers and see directly how the fuel flow is altered. VERY good article, play with the fuel flow calculator. Please check it out!

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OK, now, you have a good understanding of how the fuel gets from your tank to the engine on a throttle body injected system.

Now ask yourself: "Are ALL fuel pumps outputting the SAME pressure?"

Are ALL fuel rail regulators outputting the SAME?

Answer: "ABSOLUTELY NOT!"

We KNOW that the fuel pump and rail pressure directly effects the amount of fuel flow through your injectors and we KNOW that all fuel pressures WILL vary between sleds.

Please ask yourself: "How can there be ONE Fuel Control Box setting for, say ALL M8 sleds?  All M10's? All Polaris 800's? etc. etc.

Answer: "There Can't!"

WHY? Because there is NO WAY to KNOW the fuel flow of each and every sled. WHY? Because the fuel pressures vary!   It is really that simple!

There are MANY performance enhancement kits available that require a fuel controller to allow for better fuel flow.  These kits are supplied with a fuel controller that has pre-programmed fuel maps that optimize the kit. These controllers are non-adjustable (one size fits all of the same models) and are said to be adjusted , just right , to maximize the performance enhancement kit that they are selling.

Hmmm.. how can this be? Is the fuel flow of your injectors known? Is it the same for all engines?

Is your fuel pump's output pressure known? Is it the same for all fuel pumps?

Are ALL regulators at the same level?

Well.. since there are variances in the pressures, it stands to reason, that there will need to be variances in the fuel controller's settings as well. 

So, what happens when you purchase a power enhancement kit with a non-adjustable fuel controller? Are you getting the FULL enhancement from the kit you just laid down your hard earned $$$ to purchase? OR are you only getting a percentage of the power that is really available due to the fuel controller's settings NOT be optimized for your engine? Please ask yourself this question.

What happens is that the "kit" suppliers will pre-program the fuel controller for the lowest fuel  pressure they have seen. In this manner it will be safe for all engines. What it will NOT be is optimum for all engines.

Since this programming is based around the "exception" and not the "norm" if your engine falls in the "norm" category (like most do) you will have a controller that is OVER-FUELING your engine. While this will be "safe" , it will not produce the best power. You could be seriously down on power (even lower than stock power outputs) because of this over-fueling scenario.

In conclusion... It is nearly impossible to have a pre-programmed fuel controller that is OPTIMUM for your engine. There are simply WAY too many variables (fuel pump pressure, rail pressures, exhaust, riding elevation, head, temperature etc. etc) that factor when programming ANY fuel controller.

Having said all that, fuel controllers are a great thing.... They allow you to TUNE your engine for optimum power output! The "key" word here is "tune" you must be able to tune your controller or you could be missing out on a lot of hidden power simply because you are over-fueling your engine. Fuel controllers are very easy to tune and one should NOT shy away from running them.

I am sure you have all witnessed or even owned a "Stock" sled that simply performs above other stock sleds (and even some mod sleds). This "Super- Stock" sled's superior performance can usually be traced to a better (more accurate) fuel supply. The proper fueling of ANY engine is critical in determining the power output.. Too much.. low power, too little.. low power and low life... just right.. great power and long life.

So, while you may feel more confident purchasing a "kit" that has a "hands off", "Set to go"  advertisement surrounding it. Keep in mind, it may be "hands off" but it will rarely be "optimum" and , after-all, you purchased the kit to extract more power and be optimum. Why not get it?