While I never publish the majority of our testing efforts, with youtube and my goals to add some truth in media and some real data vs whatever we wish to call the lack of data that is currently NOT published in "testing" reviews - I recently did a 4 head shootout on a Husky TC85 specifically for youtube (in reality we've done 20 different heads when developing), and this test will apply to all KTM 85 and Gas Gas 85 bikes as well. But the over arching goal here is not to make you buy a specific head but rather make all of us smarter through more education and testing.
Everyone is "convinced" that higher compression is always better.
Everyone is "convinced" that x pipe or y pipe is better.
Everyone is "convinced" some massive secrets in R&D exists that allow 1 engine builder to be head and shoulders better than the rest.
But if we take a step back - and think logically for a minute - we should ask some simple questions. If all ktm had to do was raise their compression to sell a more powerful bike - why wouldn't they? If some shop has a magical head design - and sells it - why doesn't ktm just buy it, and copy it? If porting is as simple as raise a few ports, make them bigger - and go faster - then surely KTM can hire a guy to do that, right? The reality is that engine power gains are hard to come by, hard to find, and extremely nuanced. They require lots of testing, in multiple manners to get an idea of what you really did. Two strokes in particular are very special engines. They work as a system that needs to be in harmony - not as one item you can improve at a time.
I've attached our youtube video for you to check out, of note - the lowest compression, smallest narrowest squish banded head made the most power, and the tightest squish, highest compression head made the least power.
Everyone is "convinced" that higher compression is always better.
Everyone is "convinced" that x pipe or y pipe is better.
Everyone is "convinced" some massive secrets in R&D exists that allow 1 engine builder to be head and shoulders better than the rest.
But if we take a step back - and think logically for a minute - we should ask some simple questions. If all ktm had to do was raise their compression to sell a more powerful bike - why wouldn't they? If some shop has a magical head design - and sells it - why doesn't ktm just buy it, and copy it? If porting is as simple as raise a few ports, make them bigger - and go faster - then surely KTM can hire a guy to do that, right? The reality is that engine power gains are hard to come by, hard to find, and extremely nuanced. They require lots of testing, in multiple manners to get an idea of what you really did. Two strokes in particular are very special engines. They work as a system that needs to be in harmony - not as one item you can improve at a time.
I've attached our youtube video for you to check out, of note - the lowest compression, smallest narrowest squish banded head made the most power, and the tightest squish, highest compression head made the least power.
So let's talk two stroke head science for a second. Two stroke heads do a list of things. Firstly - via their volume they set the compression ratio. If you do some basic reading on the modern otto cycle, you can learn a lot about the theoretical differences in compression ratio. In doing so, you might realize that past a certain compression ratio the power gains become very very small even in a perfect world where fuel capability doesn't matter.
Two stroke heads also influence combustion speed. They do so by using a squish band that forces all the compressed gaseous mixture towards the spark plug and into a small tiny space, the chamber. This is a huge advantage a two stroke carries over modern four strokes - as four stroke heads have most of their area taken up by valves and cant have large squish band/area (referred to as quench in a 4 stroke - yeah both worlds didn't work together often). As an example to how big of a difference this can make, a 2 stroke typically requires 8-12 degrees peak ignition advance at peak torque (moment of highest cyl pressure). A four stroke in the mx world (state of the art for NA 4t tech) typically requires 30-33 degrees ignition advance at peak torque. Because the four stroke has less mixture turbulence and speed, it has a harder time lighting it off and getting it to combust as rapidly and as such it requires more spark advance. It's not that the 4 strokes WANTS to have to use 30+ degrees of spark advance, but it has to due to the chamber shape. The Nascar world with 2 valves, and a much larger quench (squish) area than the 4 valve mx head world uses much less timing, in the mid 20's currently. While the 2 less valves ultimately can't produce the power per CC the 4 valve heads can, they do produce as much torque per CC - potentially with better efficiency due to the better spark and quench.
Two stroke heads also use design considerations to help cool the piston dome, via the squish action.
Two stroke head designs influence the scavenging of the engine. Scavenging is what 2 stroke builders call the interaction of the outgoing exhaust flow with the incoming intake charge. This flow uses a loop type motion that comes out of the transfers, up the cylinder back side and into the head and around, then down the exhaust side in a loop type shape. The head shape - has a large effect on this loop. Imagine a totally flat head spaced up from the piston for the volume we want, vs a perfectly semi circular shape, or hemi shape. The hemi shape SHOULD help that loop flow shape more than a flat head. Now add in the dynamic of a squish band, size, shape, angle, and then combustion chamber shape, and things get complicated! Remember we want to get ALL of the spent charge out, and sharp tight corners or dead space in the loop that doesn't flow - can leave old spend charge still in the engine - not desirable.
Two stroke heads, with careful placement of the spark plug, can influence burn speed even more. I wont detail more of this - but imagine a combustion chamber that is really deep, plug far away from piston, then a chamber where the plug is closer to the piston - but we equalize the volumes... Which one would burn faster?
Clever designs of heads have used the coolant water to do helpful things for power. I wont detail this either, but you can find as much as 1 hp via some modern water routing techniques on engines that are pushing the detonation limits.
And last but MOST importantly - the head design influences the EGT. A 2 stroke relies on the exhaust system to provide the majority of air movement and thusly POWER. In fact, a 2 stroke is not truly NA - the pipe is actually like a super charger or turbo charger. Pipes vary their functional dimension based off their average gas temperature inside. It's really complicated - but a hotter pipe works at higher rpm better, and cooler exact same pipe will work at lower rpm better relative to one another. AKA hotter the pipe gets, the higher the rpm it works at.
When we design a head, if it happens to lower egt into a range the pipe isn't designed for - we will loose power. Same applies for a head that increases EGT.
Ultimately - heads are not as simple as just go for more compression or less! They have an influence on every aspect inside the engine, and finding the perfect match for the engine combo is what yields best results.
I routinely see claims from various head companies that I know to be false because we have tested them. Sure, tighten up that squish for "better faster combustion with less detonation". That's the blanket "theory" statement you learn about two stroke heads that was written 50 years ago. And, in theory, it should be correct. But then why is it that, when I tighten the squish on a stock yz250 - I usually have to add spark advance to get the power back to where it was? Shouldn't I need to retard the ignition?
What happens if you tighten the squish, open the bowl and equalize the compression ratio to before the changes, but in doing so lower the pipe temperature, or alter the scavenging flow such that now the engine produces less power? Or shifts the power in a direction you didn't want? These are the things that actually happen, rather than theoretically happen. My goal is to get you thinking. The more you think, the better decisions you can make when you buy a product.
Two stroke heads also influence combustion speed. They do so by using a squish band that forces all the compressed gaseous mixture towards the spark plug and into a small tiny space, the chamber. This is a huge advantage a two stroke carries over modern four strokes - as four stroke heads have most of their area taken up by valves and cant have large squish band/area (referred to as quench in a 4 stroke - yeah both worlds didn't work together often). As an example to how big of a difference this can make, a 2 stroke typically requires 8-12 degrees peak ignition advance at peak torque (moment of highest cyl pressure). A four stroke in the mx world (state of the art for NA 4t tech) typically requires 30-33 degrees ignition advance at peak torque. Because the four stroke has less mixture turbulence and speed, it has a harder time lighting it off and getting it to combust as rapidly and as such it requires more spark advance. It's not that the 4 strokes WANTS to have to use 30+ degrees of spark advance, but it has to due to the chamber shape. The Nascar world with 2 valves, and a much larger quench (squish) area than the 4 valve mx head world uses much less timing, in the mid 20's currently. While the 2 less valves ultimately can't produce the power per CC the 4 valve heads can, they do produce as much torque per CC - potentially with better efficiency due to the better spark and quench.
Two stroke heads also use design considerations to help cool the piston dome, via the squish action.
Two stroke head designs influence the scavenging of the engine. Scavenging is what 2 stroke builders call the interaction of the outgoing exhaust flow with the incoming intake charge. This flow uses a loop type motion that comes out of the transfers, up the cylinder back side and into the head and around, then down the exhaust side in a loop type shape. The head shape - has a large effect on this loop. Imagine a totally flat head spaced up from the piston for the volume we want, vs a perfectly semi circular shape, or hemi shape. The hemi shape SHOULD help that loop flow shape more than a flat head. Now add in the dynamic of a squish band, size, shape, angle, and then combustion chamber shape, and things get complicated! Remember we want to get ALL of the spent charge out, and sharp tight corners or dead space in the loop that doesn't flow - can leave old spend charge still in the engine - not desirable.
Two stroke heads, with careful placement of the spark plug, can influence burn speed even more. I wont detail more of this - but imagine a combustion chamber that is really deep, plug far away from piston, then a chamber where the plug is closer to the piston - but we equalize the volumes... Which one would burn faster?
Clever designs of heads have used the coolant water to do helpful things for power. I wont detail this either, but you can find as much as 1 hp via some modern water routing techniques on engines that are pushing the detonation limits.
And last but MOST importantly - the head design influences the EGT. A 2 stroke relies on the exhaust system to provide the majority of air movement and thusly POWER. In fact, a 2 stroke is not truly NA - the pipe is actually like a super charger or turbo charger. Pipes vary their functional dimension based off their average gas temperature inside. It's really complicated - but a hotter pipe works at higher rpm better, and cooler exact same pipe will work at lower rpm better relative to one another. AKA hotter the pipe gets, the higher the rpm it works at.
When we design a head, if it happens to lower egt into a range the pipe isn't designed for - we will loose power. Same applies for a head that increases EGT.
Ultimately - heads are not as simple as just go for more compression or less! They have an influence on every aspect inside the engine, and finding the perfect match for the engine combo is what yields best results.
I routinely see claims from various head companies that I know to be false because we have tested them. Sure, tighten up that squish for "better faster combustion with less detonation". That's the blanket "theory" statement you learn about two stroke heads that was written 50 years ago. And, in theory, it should be correct. But then why is it that, when I tighten the squish on a stock yz250 - I usually have to add spark advance to get the power back to where it was? Shouldn't I need to retard the ignition?
What happens if you tighten the squish, open the bowl and equalize the compression ratio to before the changes, but in doing so lower the pipe temperature, or alter the scavenging flow such that now the engine produces less power? Or shifts the power in a direction you didn't want? These are the things that actually happen, rather than theoretically happen. My goal is to get you thinking. The more you think, the better decisions you can make when you buy a product.
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