How much compression is too much?

I have a YZ144 done by Eric Gorr and ported for low/mid powerband and pump gas. I was a little surprised to see that the compression is about 215 psi. This seems high to me.

I was wondering, how do you know if a 2 stroke engine is running too high a compression ratio? I can't hear any pinging, and I took the head off after about 1 hour of riding and I don't see any pitting suggesting detonation. It seems to me this engine is very happy as is, so my question is why am I able to run such a high compression on regular gas? Is it due to the porting perhaps? Just curious.

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That is up there for sure for a 144, but if you have no detonation then run it. I have built race engines for two stroke jet skis and know that the higher the comp the more wear on the crank. I set my boats up with 230 to 240 and the crank wear was greatly accelerated. But if you have no pinging, joy to the world! I had my cranks welded and trued but the bearings take a beating.

I know 250 2 strokes normally have about this much compression. Does the yz250 have heavier crank bearings than the yz125?

Yes, they are larger.

Try running it on VP C12 race gas

...if there's a noticable power increase,

you'll know your close to the limit

using pump gas.

What head volume / cc's is it now ?

Try running it on VP C12 race gas

...if there's a noticable power increase,

you'll know your close to the limit

using pump gas.

What head volume / cc's is it now ?

Good idea. My understanding is that higher octane fuel should make no difference at all unless there was preignition with the pump gas, correct?

How do I measure the head volume? Fill it with water? Also need to consider piston extension above deck right?

For anyone wanting to do an actual check of their head volume, I've done it this way a couple of times and it worked pretty well. You'll need these things; A piece of Lexan (not as brittle as Plexiglas), a graduated beaker and an eyedropper. Cut a piece of Lexan to cover the head area and drill a small hole in the Lexan that will end up in a corner of the combustion chamber. Put a layer of heavy grease or Vaseline on the head to seal the head-to-Lexan joint. I use mineral oil for the liquid because it's not too thin in case the seal isn't perfect around the Lexan. Tip the head slightly and fill up the combustion chamber right to the hole with the eyedropper, noting on the beaker how much is needed to completely fill it up. If it's a 4 stroke head, I've used the Vaseline to seal the valves to but that may not be necessary Keep that Lexan for next time! The only part that may be difficult is finding a beaker or measuring cup that's graduated small enough to work. A c.c. graduated beaker works perfect.

Interesting idea. How about you plug the spark plug hole, turn the head upside down, and lay the it perfectly flat and fill with water to the max, then suck the water out with a dropper into a graduated cylinder/beaker? Maybe the surface tension of the water is too high and it builds up a big miniscus this way. Could use alcohol I guess?

Wait, fill it with spackel, let harden, then pop it out and measure the displacement of the casting in a glass of water? No, wait, wait, fill it with water and then freeze it, pop out the ice cube casting and let it melt in a beaker? This is fun :rolleyes:

With water and no Lexan., obviously you have to have it perfectly level and if it's not and it spills out, & assuming you were measuring it as you go, there goes your measuring out the window. No advantage with alcohol . Less surface tension may even make it harder to handle a leak or not level. OBVIOUSLY, YOU HAVE TO HAVE THE SPARK PLUG IN.

On second thought, I think you SHOULD try the spackle method and report back to us.

Is combustion chamber volume supposed to take into account the volume inside the spark plug (i.e., around the electrode)?

Is combustion chamber volume supposed to take into account the volume inside the spark plug (i.e., around the electrode)?

Yes, you would measure the volume with your normal spark plug in.

Ok. Thanks. WIth these little combustion chambers (125 and below) it would seem the spark plug makes up a considerably amount of the cc volume. It would seem one could adjust the compression with plug choice (if such plugs were available).

But what do we do about the piston crown extension? The CC must be only the volume remaining after the piston has reached TDC, and in many cases this means it will be protruding beyond the cylinder top and into the head. How do we compensate for this in order to measure the true CC volume?

Only thing that comes to mind is positioning the piston at TDC, removing the plug, and slowly measuring in a volume of heavy oil until the chamber is filled. I don't need this measurement for any reason, just curious how it can be done.

Edited by rjpjnk

it's true that to actually calculate the CR, you would have to know the piston crown volume, the deck height if the piston goes above or below the top of the cylinder at TDC and head gasket thickness compressed if it's that type of gasket. You know the swept volume. It's common to be able to find the dome volume of a lot of pistons - but not ALL of them so that may be a problem. As you can see, there's a lot of monkey business involved in actually calculating CR. I really don't think your compression is too high anyway. Some healthy YZ250's are 240 lb. and even higher. As someone mentioned, if it isn't pinging, it's OK. If it is, I would check timing, then jetting, then squish band dimensions are super-important in being a ping resistant motor. You could still do a 25% or 50% mix of race fuel but I really don't think you need it and it's a pain in the butt to do it anyway.

it's true that to actually calculate the CR, you would have to know the piston crown volume, the deck height if the piston goes above or below the top of the cylinder at TDC and head gasket thickness compressed if it's that type of gasket. You know the swept volume. It's common to be able to find the dome volume of a lot of pistons - but not ALL of them so that may be a problem. As you can see, there's a lot of monkey business involved in actually calculating CR. I really don't think your compression is too high anyway. Some healthy YZ250's are 240 lb. and even higher. As someone mentioned, if it isn't pinging, it's OK. If it is, I would check timing, then jetting, then squish band dimensions are super-important in being a ping resistant motor. You could still do a 25% or 50% mix of race fuel but I really don't think you need it and it's a pain in the butt to do it anyway.

+1 on all this. You can get really in depth to measure compression ratio. Then, do you want swept volume, or just volume after the ports close? Power valve open or closed? Include the volume above the ring, or assume near zero crown-to-wall clearance due to heat expansion, and the fact that gasses won't burn well there anyway due to quenching from thr cool surroundings.

In the end theres no solid answer on what "too much compression" is anyway. Timing, squish, jetting, pipe, intended usage, rider style, etc all factor in.

Fyi

My 250 does 245-255 psi and runs pump gas. Many a top end thru it, never any signs of detonation.

Edited by adam728

That compression ratio you find by using the port timing is called "corrected compression ratio"...I think. I've never used it for any purpose and hadn't even seen it mentioned until the last few years. You could even apply that system of measurement for 4t's and use valve timing as a factor, couldn't you? I'm not familiar with the wildest profiles of todays 4t mx bikes and if both valves are absolutely closed when the compression stroke starts.

BTW Adam - with your bike so good at not being prone to detonation, did you have the squish band massaged or was it that good as delivered?

Edited by motoxvet

That compression ratio you find by using the port timing is called "corrected compression ratio"...I think. I've never used it for any purpose and hadn't even seen it mentioned until the last few years. You could even apply that system of measurement for 4t's and use valve timing as a factor, couldn't you? I'm not familiar with the wildest profiles of todays 4t mx bikes and if both valves are absolutely closed when the compression stroke starts.

BTW Adam - with your bike so good at not being prone to detonation, did you have the squish band massaged or was it that good as delivered?

You will notice most manufacturer's use "corrected" compression ratio in their specs, that's why there's a high & low speed called out, accounting for powervalve position. That's why Yamaha lists the YZ250 as being 8.9~10.6:1 compression.

My bike has a lot of work done. Long rod, KTM piston, head has around 60% squish area, 0.045" clearance, porting, etc etc. Was told I had to run at least a pump/race blend when I got it. After years of riding, jetting, fiddling, I've found pump works just fine for me on it. I attribute most of it to the squish setup, and I am sure a fast MX rider or going to the dunes would have it pinging. Trails, harescrambles, enduro, etc, just doesn't get the cylinder temps high enough to push into that problem.

You will notice most manufacturer's use "corrected" compression ratio in their specs, that's why there's a high & low speed called out, accounting for powervalve position. That's why Yamaha lists the YZ250 as being 8.9~10.6:1 compression.

Yes, I have noticed they always specify two numbers and I reasoned it was due to powervalve position changing. One thing I wonder about, though, is how the powervalves seal enough to contribute to compression. Clearly they don't touch the rings, and air can flow all around them. I'm guessing that at normal engine speeds the air can't escape past them? Or maybe th oil helps seal them?

My 250 does 245-255 psi and runs pump gas. Many a top end thru it, never any signs of detonation.

This is the real bottom line I suppose.

What are the signs of detonation that we can see?

Yes, I have noticed they always specify two numbers and I reasoned it was due to powervalve position changing. One thing I wonder about, though, is how the powervalves seal enough to contribute to compression. Clearly they don't touch the rings, and air can flow all around them. I'm guessing that at normal engine speeds the air can't escape past them? Or maybe th oil helps seal them?

I've never done a compression check with the valve open vs closed. Obviously they have an effect on the air flow, or else there'd be no power difference from open to closed. Since they vary the port height, flow, pulsing, etc, I am sure it would effect the dynamic compression as well. There's a lot going on in a running engine.

This is the real bottom line I suppose.

What are the signs of detonation that we can see?

On a tear down detonation can be seen as:

Aa sandblasted look to the piston crown, typically near the outer edges (in the squish band).

Errosion of the top edge of the cylinder (which can lead to blowing out the o-ring to the water jacket).

Errosion of the piston edge

Gray spark plug, being aluminum from the piston depositing itself there after melting off the crown (the sandblasted or pitted look).

On a running engine you can sometimes hear the "knocking" or "pinging" of detonation, although sometimes it's difficult. Overheating is another tell tale, as piston, head, plug, and cylinder temps climb rapidly when detonation sets in, which then just makes it occur more, raising temps again, etc. On a monitored engine you'd see this as cylinder head temps suddenly climbing, likely with EGT's falling off a bit. Power suffers as well if ignition timing is at best torque, as peak cylinder pressure now occurs too early.

On a tear down detonation can be seen as:

Aa sandblasted look to the piston crown, typically near the outer edges (in the squish band).

Errosion of the top edge of the cylinder (which can lead to blowing out the o-ring to the water jacket).

Errosion of the piston edge

Gray spark plug, being aluminum from the piston depositing itself there after melting off the crown (the sandblasted or pitted look).

Thanks. I'll be looking for any signs of this at the next teardown.

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