Calculating Head Squish Measurements - (REP)

[E-man811]

Originally Posted by Faded

If you measure total squish first (first step in my post) prior to pulling off the head then you have a number to work from. After pulling off the head you can measure the squish height in the head and the head gasket's compressed thickness. By adding these two numbers together and subtracting them from your total squish you can get a fairly good idea of deck clearance. Not the most precise way to do it, but close enough without having to buy costly equipment. I hope this clarifies things a bit.

I'm sure everyone will get that now. Including me...

Thanks again for all of your input faded.

Ian

[ossagp]

Two things that I would ad,,,kind of like two cents. If you are going to use the solder method of determining your squish area and clearance, apply the solder to the exhaust side of the piston and THE INTAKE side. If you dont the piston will rock in the bore and the measurement on a 60-70 mm piston can easily change by .010. That is still the easiest way.

On TDC or finding it: Critical to accurate volume. For this I use a bridge that bolts to the top of the cylinder. I have a stop which is just a screw set in the center so that when the piston comes up it stops below top dead center,,,(doesnt matter where) then you rotate the crank back the other way until it hits the stops again(gently hits it I should stress). Now put a degree wheel (you have one right?) on the crank. Take the total of the degrees that you can rotate the crank back and forth. Using a wire indicator determine where the midpoint is on the degree wheel. Do it several times and you can find where TDC really is. I am assuming your lower end bearing is pretty fresh,,this also necessary for accuracy. Just about impossible to find it by eyesight. If finding it by eyesight is close enough for the work you are doing, or using the factory supplied mark (occasionally in the time I have been using this method, I have found the factory mark to be perfect according to my method) is close enough,,then don't bother with the other stuff thoughtfully mentioned by Faded, and just go from the factory drawings.

Now lots of "real" engine builders have better means I am sure, but for the garage tuner, this way works.

Again, it is critical to find exact TDC and to use the stop right on the center of the piston so you eleminate as much rocking of the piston as possible.

On the actual figuring of the compression ratio: I am still trying to figure out a real value to figuring the closed valve compression ratio. The sealing varies so much from design to design and so much leaking occurs. What is useful to me is the timing change.

[Faded]

Originally Posted by E-man811

I'm sure everyone will get that now. Including me...

Ian, thank you for helping to clarify. It doesn't help anyone if it isn't easily understood. I appreciate your help.

[Faded]

Originally Posted by ossagp

On TDC or finding it: Critical to accurate volume. For this I use a bridge that bolts to the top of the cylinder. I have a stop which is just a screw set in the center so that when the piston comes up it stops below top dead center,,,(doesnt matter where) then you rotate the crank back the other way until it hits the stops again(gently hits it I should stress). Now put a degree wheel (you have one right?) on the crank. Take the total of the degrees that you can rotate the crank back and forth. Using a wire indicator determine where the midpoint is on the degree wheel. Do it several times and you can find where TDC really is. I am assuming your lower end bearing is pretty fresh,,this also necessary for accuracy. Just about impossible to find it by eyesight...

Now lots of "real" engine builders have better means I am sure, but for the garage tuner, this way works.

Andy, thank you for that excellent post on finding true TDC and helpful hints on how to make your own piston stop. Definately something else for the garage tuner to consider.

Originally Posted by ossagp

On the actual figuring of the compression ratio: I am still trying to figure out a real value to figuring the closed valve compression ratio. The sealing varies so much from design to design and so much leaking occurs...

You make another good point; some designs are far better (or worse!) than others.

[Thumprding]

Okay I understand about how to measure the combustion chamber volume and to deterime the compression ratio. I also understand that once I have these numbers I can compare it to minium recommeded fuel octane rating.

Now how does the octane rating requirement at this point comapre to a 10% decrease in combustion chamber volume. Do I need to increase the fuel octane rating by 1 point or 2 or?

I am also curious as this relates to how the octane fuel number was attained. Are we talking about a (RON+MON)/2 rating, a straight RON or a straigt MON rating.

On My KTM I installed a modified head. The head was cut not to increase compression but alter the combustion chamber shape to get rid of an off idle flat spot. The off idle flat spot went away but the changes resulted in the motor pinging. This was common in the 01 and 02 300 EXCs. KTM later fixed this issue by decreasing the compression in 03 and again in 05. The fuels in europe are more resitant to pre-ignition. 93 octane fuel was not enough. I could get the ping to go away at the cost of fouling plugs. Once I started running close to 100 octane rated fuel the ping went away.

[Thumprding]

I also have a question of curiosity. A two stroke runs a lower compression ratio than a four stroke but the four stroke does not ping or detonate on 91 octane fuel. Am I missing something here?

I do know that combustion chamber design plays a lot into fuel requirements. Take like the origina Genesis engine. In 1986/7 Yamaha released the FZR750. The compression ratio of that motor was 11.8:1 and ran fine all day long on 89 octane fuel. Heck I had a car with 10.5:1 compression in a 327 and 93 octane rated fuel was on the edge of the fuel requirements for that motor. Is the combustion chamber design of a two stroke prone to detonation or pre-ignition as compared to a four stroke. I have always wondered why the compression ratios are so much lower. Or does it have to do with being case inducted?

[Faded]

Originally Posted by Thumprding

Now how does the octane rating requirement at this point comapre to a 10% decrease in combustion chamber volume. Do I need to increase the fuel octane rating by 1 point or 2 or?

Sadly enough this will be engine/design dependant and there really is no rule to go by. Too many design criteria should be looked at and making a generalized statement wouldn't be wise.

Originally Posted by Thumprding

I am also curious as this relates to how the octane fuel number was attained. Are we talking about a (RON+MON)/2 rating, a straight RON or a straigt MON rating.

Here's a quote I found, should help out:

Octane is tested in a single cylinder octane test engine. The MON is a measure of the gasoline's ability to resist knock under sever operating conditions. MON affects high speed, part throttle and performance (under load such as in passing). The RON on the other hand, is a measure of gasoline's ability to resist knock under less sever conditions. RON affects low to medium speed knock and engine run-on (dieseling). For a given AKI, RON is typically 8-10 points higher than the MON. As an example, 87 AKI (pump octane) fuel would have a MON of 82 and a RON of 92.

Again, something that will be engine/design dependant.

Originally Posted by Thumprding

On My KTM I installed a modified head. The head was cut not to increase compression but alter the combustion chamber shape to get rid of an off idle flat spot. The off idle flat spot went away but the changes resulted in the motor pinging. This was common in the 01 and 02 300 EXCs. KTM later fixed this issue by decreasing the compression in 03 and again in 05. The fuels in europe are more resitant to pre-ignition. 93 octane fuel was not enough. I could get the ping to go away at the cost of fouling plugs. Once I started running close to 100 octane rated fuel the ping went away.

How was the combustion chamber shape altered? IMHO it sounds like one of two ways; 1) Tighten the squish by milling the gasket surface and not relieving the bowl enough to get it back to stock-like compression ratio(s) OR 2) Milling a deeper squish recess and not touching anything else. Excessive squish clearance can cause more end gasses (compared to stock) to gather at the outer ends of the bore where the local temperature can increase due to accumulating pressure waves. Once the ignition temperature of the fuel is reached pre-ignition of the excess end gasses begins. This can happen at low(er) compression ratios as well. The solution is to tighten the squish clearance to force more of the intake charge to the the center of the combustion chamber and will also increase the flame front propogation through more turbulence of the intake mixture. The result is the potential to produce more power (more of the intake charge to combust) and a faster, more efficient combustion process.

[ossagp]

Your question about the 4 stroke having a higher compression ratio without pinging has several answers. One is that heat has more of a chance to disspate when you have 720 degrees between firing.

Now,,when you are figuring the compression ratio on your KTM of say 8.5 to one, you are figuring it in purer terms than you may be getting on your 327. If you haven't heard of "overlap" go ask your cam manufacturer about it. To figure the true compression ratio on your 327 you need to do it from the point that the valves are totally closed on the way up on the compression stroke. Things may look a little bit closer then (or not).

Your speculation on chamber design is a good one though. One 4 stroke to the next, there is a lot of difference in what is acceptable octanewise,,,if you want to compare the Honda XR line to the CRF line you have to go back to my comment on overlap to be able to compare apples to apples though.

[mtk]

I've also heard that premix oil actually lowers the effective octane rating of your fuel, as in makes it less detonation resistant. Which is why manufacturers recommend 93 octane for a 8.6:1 compression two-stroke.

Now, I can't remember if it was a in my Combustion Theory class in grad school or in an SAE seminar on oils or in a Paramins seminar about oil additives, but it was one of those three. It's all become a big jumble of mostly useless trivia now.

[Thumprding]

The head was modified by option number 2, and yes that is the exact reason I thought that it pinged so much. The flame front pressure and temperature was causing the thin areas of air fuel mixture to pre-ignite. A KTM exc is also a dual purpose bike in europe and as such is ridden a lot at light throttle settings for a continued amount of time. The combustion chamber has to be designed for it not to buck and kick, but it tends to produce a flat spot off idle that no amount of jetting or reed changes can remove.

In regaurds to overlap yes I understand it. I also understand how it effects power output at various engine RPM and compression ratios at various RPMs. This is if the exhaust is capable of passing the gases equal to the intake.

I mentioned the Genesis engine because I feel it has the most efficient combustion chamber design. When I think about it, that motor has been production for almost 20 years. That is amazing in the motorcycle world. Then think of what Yamaha has done with it. 400cc, 600,cc, 1000 cc four cylinder eninges. 750 and 850 twins, and 250, 400 upto 650 singles. All the same engine design. And if you really look at that motor it is hard to improve on it. Many can screw it up but few can improve on it. Most changes are made in the intake and exhaust to try and improve air movement and management in the engine. When I really think of it look at what Honda, Suzuki, Kawasaki, KTM and the rest had to do to make a high HP thumper for the off road market. They had to design and build a motor. Yamaha just had to make a single cylinder Genesis engine from a 15 year old design. But again look at the stack of patents on that engine. There is a reason no one else runs that combustion chamber design.

[ossagp]

I wouldnt get too warm and fuzzy about that original Genesis design. I am not sure that Yamaha would have had to patent the original one to keep it from being copied. There was a lot about the FZ750 to make it desireable, but from what I keep calling the garage tuner or builders standpoint, there was a good reason that other designs were more desireable. In a nutshell, any engine that required you to run advances as high as mid 40's (ignition advance in degrees before TDC) was not a particularly good design. Compressing gasses that are expanding is not considered desireable (a horrible waste of energy in my opinion). Yamaha changed that, kept the amount of valves and kept the name. But that original chamber design is probably safe for the ages.

With all that you know about overlap and how compression ratios are figured, I guess you were just testing us with that question about 2 strokes and 4 strokes weren't you?

[2kHondaCR500R]

Wow! Great info guys!

[FFRacing11x]

As far as the Genesis engine design, ever wonder why Rossi's MotoGP Yamaha only has FOUR valves per cylinder?? One way to make a 5 valve YZF achieve the same HP as a 4 valve CRF? Make the YZF a 4 valve head! LOL Tdub

[ossagp]

So did you get that 4 valve working on the blue rig? I noticed last year that Yam's new 4 valved 3 cylinder snowmachine engine is called,,,,(what else) a "Genisis". Interesting chamber on it and it works well.

[Kels]

I have some tech articles on my website that discuss these issues .

They are located at [Only registered and activated users can see links. ] if you wish to have a look

Kelsey

[E-man811]

Welcome to ATM.

You do have some great articles on your sight. Thanks for putting that out their for people. It would benefit most people to go read them.

If I may offer some feedback to Kelsey... I couldn't get past some of the terminology discrepancies. Small albeit, but significant enough to cause confusion in someone who is learning. i.e. One of your articlea is spot on about the importance of measuring and knowing the relationship between the top of the piston at TDC and the top of the cylinder. But your article refers to this as "Piston Deck Height" which as your article moves on, abbreviates to "Deck Height". This relationship of the Piston to the Deck is called "Piston Deck Clearance" or "Deck Clearance". But describing this incorrectly as "Deck Height" will certainly lead to confusion for a few people as they try to learn more about building engines.

"Deck Height" is the distance from the centerline of the crank to the top of the cylinder. "Deck clearance" is Deck Height minus Stroke + Rod length + piston compression height +/- variances = where the top edge of teh piston will stop at the top of the stroke.

To Kelsey... Don't mean to nitpick, it's just a peeve of mine. I welcome them back also.

To everyone else go read Kelsey's articles

Ride safe

Ian

[Kels]

I appreciate the feedback..

You are correct.. By me abbreviating "Piston Deck Height" to "Deck Height" .. I can see how it could take on a diffrent meaning to those 4 stroke engine builders..

I do believe that piston deck height is a clear and correct term and is also known as piston deck clearance.. While deck height by itself could relate to block deck height..

Thanks for pointing that out.. I will modify my article..

Kelsey