Common mis-conceptions about compression/head design and fuel requirements as it relates to a 2 stroke internal combustion engine

First off: This is NOT to be considered a “Covering EVERY Scenario” or “Lesson in Science” type article. It will, most likely, have some “holes” in it that could be seen as incorrect under certain scenarios.

This article is being written to touch on certain misconceptions regarding this area. It will be “Generalized” and NOT all encompassing.

This will, most likely, be an ongoing discussion as more data and topics surface. It is difficult to recall everything on the spot.

It will be based on Theory, Reality, Direct Experience, Automated Testing, and Common Sense. All topics and references will have been “realized” using one of the above at some time. NONE will have been “fabricated”.

If you do not agree with any of it, please ask yourself if you have DIRECT EXPERIENCE, by “Direct”, it is meant-> 100% direct as the example you disagree with, NOT 99% and not lumping , components (like cylinder head design or other) into one general “bag”.

NOTE: ALL references to Compression Ratio will be full stroke, uncorrected

OK.. let’s get started….

2 Stroke Cylinder Head Misconceptions:

1) Cranking compression via compression gauge will tell you what octane your engine requires in order to run safely…   100% FALSE!

The cranking compression will not tell you much of anything about the required octane. Cranking compression is a good tool used to compare a previous, known, value to a current value in order to tell if anything has changed. This comparison is good for a “health check” of an engine. Again, not real useful in terms of determining the “required” octane for any engine.

2) ALL 10:1 UCCR compression ratio heads (or any other compression ratio) will function the equally on the same engine. 100% FALSE! 

This could be one of the largest mis-conceptions out there. Let’s break it down: 10:1 is a ratio of the volume trapped at TDC as it relates to the volume of the cylinder and head together.  For example, if you have a 100cc engine, a 10:1 compression ratio head would be 11.117cc-> 100cc+11.117cc (full stroke cylinder volume) / 11.117cc (head volume at TDC).

As you can see 10:1 will always be the SAME volume given the same engine.  What will NOT be the same as how that volume is geometrically accomplished.

There are infinite geometric designs that can arrive at 11.117cc volume.

This is where the fun begins! The same VOLUME head will yield the same compression ratio but can also yield VERY different results in terms of how it performs when installed on the engine.

The geometric design of the head is critical as to how it performs. A simple .001” change in a certain area can have a huge influence on its “effectiveness”.

Back to the misconception… Brand “A” 10:1 head can, and usually will, perform VERY different from Brand “B” head. 

Think of fast food hamburgers. If you have a double cheeseburger from one fast food chain, it will/can taste totally different than a double cheeseburger from a different fast food chain even though they have basically, the same ingredients. SAME with Head design.

3) If you raise the compression on an engine it will ALWAYS require more fuel to run safely… 100% FALSE!

This is a loaded subject.  It also relates, heavily, to #2 above. It is also going to be difficult to explain. Let’s give it a go anyway.

2 Stroke head design is a huge player (as is exhaust, ignition timing, piston, reeds, and porting etc.) in the tuning and performance of any 2 stroke engine. They all work together.

Changing ANY of these variables can have a pronounced effect on performance (HP, Torque, Over-Rev, Timing, Heat etc.) and fuel requirements (octane and volume etc).

Suffice to say, they are all “Shaking Hands” or “Punching Each Other in the Face” depending on the change made.

This article will be limited to Head Design but wanted to mention that there are other “players”, directly, involved.

Fast combustion is what is always desired. The faster we can complete the combustion process, the better. The head design has large influence on how fast this process completes and what ignition timing is optimum. Advanced ignition timing is not desired. The later we can start the ignition process and still have a good rod angle ATDC, the better. Again, the head design is has a large influence in this.  This area can become very detailed discussion. We will stop here for now.

Head Efficiency or Combustion Efficiency is the measure of the head’s ability to convert the supplied Air/Fuel into useful energy.  Obviously, we want the highest efficiency we can. Of course, there are exceptions to this as well where we do NOT want a highly efficient head ie. A tuned exhaust system that requires more energy to function better… but that is discussion for another time.

For the purpose of this article, we want the most efficient head design possible.

This, directly, relates to how much fuel/air (F/A) is “needed” to be supplied to the head. 

When the head is more efficient, it is converting more F/A mix to useable energy. Does this require MORE Fuel to accomplish? Not always. Anything that is more efficient usually requires less to get the same or better result. This is the definition of efficiency. But, with head design it is not always that simple.

Whether or not your engine requires more or less fuel when adding a head with a higher combustion efficiency directly relates to how good or poor the previous head design was and how the fueling was set up.

For example..  2014 and newer KTM 250 and 300 Enduro or SX OEM Head. 

This head design is so inefficient that it is plagued with a high frequency of misfires. These misfires manifest themselves as a rich running condition when, in fact, the F/A supplied is close to correct with stock jetting.

The misfires are so prominent that most will lean the jetting to lessen this rich condition (caused by the head’s inability to combust). It stands to reason, if you have a head that is struggling to combust the given F/A mix, you will have residual (non-combusted) F/A mix “lingering” in the engine and exhaust that will be combined with the new F/A charge and push it to a F/A mix that is too rich. Of course, this does NOT occur every stroke, but is frequent.  

Leaning the carb mix via jetting alterations will provide less Fuel and, in turn, lessen (not eliminate) the frequency of the head misfires. Of course, those times (frequent) when the head is NOT misfiring, allow for a lean condition. This lean condition will create added heat and will lessen power output. If left too long it can damage your engine.

Point being.. You are treating the “symptoms” of the problem vs. addressing the “source” (head design). You end up with a “compromise” with less power BUT ,possibly, a “cleaner” running engine. So, you feel you have fixed your issue.

If you address the source of the misfires (head design not rich jetting) you get rid of all the symptoms as well. This is the better solution. It is like having a cold. You can treat the runny nose (symptoms) and feel better OR you can get rid of the cold (source) and the runny nose goes away with it.

Now, how does this relate to installing a highly efficient head in terms of fueling?

In the case of the KTM 250 and 300, like mentioned, the head was the real culprit of the rich condition and the leaner jetting was a compromise to lessen the misfires. The engine required the larger jetting spec, but the poor head design limited the engine’s ability to utilize the required fuel.

When you run in a leaner state, you, generally, will make less power even if the engine is running cleaner. The engine requires sufficient amount of F/A in order to produce the maximum power.

They can run fine lean, they are just down on power and you would not realize this unless you compared to one that was not running lean. You don’t know what you don’t know!

When you install an efficient head, like the RK Tek Head, you do not have this frequent misfiring issue because the design of the head helps to prevent it.  With the higher efficient head design, the engine can benefit from the larger F/A volume and produce more Torque and HP.

How about fuel injection?   With fuel injection, you are only as good as your Fuel Mapping, Injector Firing, and Fuel Pump Pressure. If any of these are “out” you can have a poor running engine. It can be “lean poor” or “rich poor”.

What happens when you add a more efficient head to a fuel injected 2 stroke? Well… There are many scenarios. Suffice to state that the head designer better have some good knowledge of how to accommodate the inner-workings of 2 Stroke FI. With this knowledge, nice gains can be had with no fuel alterations or controller required.

HOW? you ask?--> It all reverts back to how the exhaust, head, ignition timing, and reeds “hand shake”.

Knowing how these interact with each allows the head designer to “compensate” in the design to allow for more power without upsetting the “balance”.  This is a very complicated topic.

It is often the case, that the design that works best for a carbureted induction engine will not be optimum for a FI engine.

Testing is required to find which design can keep all the other players “happy”.

4) Air Fuel Ratio Pre-Combustion can be measured, and is the same as AFR measured Post-Combustion.

There is a huge mis-conception that  Air/Fuel Ratio O2 sensor readings (in the pipe AFTER combustion) have a direct (one to one) relationship with the Air/Fuel Ratio reading Pre-Combustion (Carb to Head Path).

O2 or Lamba sensors measure the amount of Oxygen present in an exhaust sample and relate/compare this reading to a known reference oxygen level (Usually the ambient air).

While these readings can provide valuable information about POST Combustion efficiency and ratio, they do NOT tell you about the F/A ratio PRE-Combustion.

It relates back to #3 above. How efficient is the head? If you have a head design that is very efficient, you will have a more complete combustion process resulting is a “better” AFR shown on the exhaust sensor.

What this sensor reading does NOT tell you is what the head had to work with to create this output. It also does not tell you how “optimum” you are because you have not pushed the head to its limits (ie. Adding/Subtracting more fuel or air) to determine what it can efficiently combust. This is controversial. You can have a 13:1 AFR measured.. add some fuel or air (or both), and STILL have a 13:1 measured AFR. This will largely depend on the head’s design and how it works with the other primary engine components. It is like stressing a frayed rope., you really do not know when then rope is going to break until it does.

Point being, there are many variables in play here, including the limitations and design of the AFR measuring device, itself.  Suffice to say, that measured AFR is NOT the “End All” and can lead you astray if not careful.

With any carb system, unless the head is increasing air-flow (possible), fuel flow into the engine will remain near constant for a given RPM, heat, and load EVEN if the AFR reading changes. What, really, has changed the AFR reading is the head’s ability to combust a higher or lower percentage of what it was given.

If the head is very efficient, usually, LESS F/A is required for the same output. Again, this is all relative and not always the case.

For example, on the 2000-2012 Ski Doo 800 engine. We were able to increase the compression, significantly, and also reduce the main jet 5 sizes. This resulted in a large power increase and better fuel economy. This was due to the OEM head design being very inefficient and hot.  The OEM design needed extra fuel to keep combustion temperatures in a safe range. Without this added fuel the OEM head would run too hot and detonate. It was, essentially, cooling temps with fuel. The RK Tek Head design focused on increased combustion efficiency and minimizing hot spots within the chamber. This allowed for a much better and safer running engine.

5) Is Higher Compression required to extract more power? In short NO!

One can easily create more power using the SAME compression ratio as before but using a better design. How much more power depends on how much better the design.

Increasing compression (leaving all else the same), is always good for increased performance but it can come at a cost unless you optimize the design and “hand shaking” that is happening with the other primary engine components.

IF you optimize these relationships you can, successfully, increase the compression ratio without any compromises (gains across the board).

It is, usually, never as simple as a head “shave”.  However, while this can be effective, it, usually, is not and comes at a cost. Robbing Peter to pay Paul type scenario.

It ALL reverts to the HUGE complexity of the 2 stroke engine and how ALL primary components interact. 

‘Nuff for now.. Already way too long!!