Jump to content

YZ125/144 Compression Ratio

Recommended Posts

I was reading this webpage http://www.hpbikes.com/news.php

and it was talking about compression ratios. Its says he is looking at running a static compression ratio of around 16:1 compared to the stock 125 of 12.9:1.

Is it really possible to run a 144 at 16:1, and still benefit from the compression? I would have thought that all extra power would mainly be lost as heat. Also would pump gas handle this?

I am about to modify a stock YZ125 head to fit a Athena 144 barrel. The athena head is too lightweight, gets hot, distorts and dumps the gas into the radiators (thats my theory). so ill machine some o-ring grooves in the barrel and see how that goes with the stock head.

Also, I was thinking of making a stroker crankpin, anybody done this before? i was either gonna make a whole new pin or build one up with weld and machine it..

Share this post


Link to post
Share on other sites

16:1 is where Diesels start. I doubt he's running that. And if he is, he isn't using pump gas, but either JP7 or kerosene. You would need 150 octane or something in the Cetane range for that compression and spark ignition. Anything much past 12-13:1 is asking for it. With the sonic wave action caused by a two-stroke expansion chamber, at the perfect tune point you get a supercharger effect. That does the equal of upping the compression ratio far beyond the static point, which is why most people end up running race gas, as pump gas has enough problems with lower compression ratios.

So I remain skeptical about a static CR of 16:1. Of course, if he never starts the engine, he can go to 35:1 with no problems. Sounds like those that used to say that had a 3/4 race cam (whatever that is) installed in their two stroke. Sure they did........taped to the head.

Share this post


Link to post
Share on other sites

Separate issue, separate answer.

Stroker: how are you going to get the port timing to line up with a stroker crankshaft, no matter where you get it? Now the piston goes higher each TDC, and lower each BDC.

Next: your own crankpin? Unless you are well versed in metallurgy, and understand that the crankpin has to have a fairly ductile core with a hard surface, in the Rc 45-60 range to prevent excessive wear and brinelling, probably ought to buy that one.

Tinkering is great, and you learn a lot, but don't tempt fate and engine demolition with home built critical parts. If that crankpin lets go at full load, you won't have an engine to rebuild. We built our own expansion chamber years ago out of sheet metal. they looked like deranged elves built them, but worked well. Vibration cracked almost immediately, as the metal we used wasn't designed for such things (non-galanized sheet metal for ductwork) but we sure learned a lot. We saved our money and bought Bassani pipes and no more problems.

What part of Oz are you in? I used to live in WA in the 70s.

Share this post


Link to post
Share on other sites

you can do your pin out of 4140, flash treat it and draw it back, then grind/polish to size.

I will have to look in my secret recipie book, but I seem to have notes for a mini that was running compression ratios in the "you can't do that" range and it worked well. you may be stuck in the c20 range, which around here is getting about $ 20 per gallon.

Share this post


Link to post
Share on other sites

I live in Perth WA.

the stroker idea is just something i might play around with when i get some time off. im with you greg, was thinking case harden then surface grind to size.

Share this post


Link to post
Share on other sites

We run almost all out high performance 2 strokes at our above 16:1 static compression ratio, the 250 two stroke right around there, 85's and 65's around 18:1 and the 125 in between.

Considering that fact that the exhaust port is not closed until 82-83 degrees before top dead center (as compared to 140-150 degrees on a four stroke) the effective compression ratio is much lower

Yes the supercharging effect from the pipe makes the dynamic running compression HIGHER than the "corrected" compression ratio, but it's no where near the geometric compression ratio.

That's on race fuels only, such as a 110 leaded, we like VP c12.

And yes, they run awesome...

With pump fuel - anything over 13:1 on a 125 is pushing it - combustion chamber shape, ignition timing, squish, and purity of charge (as little ex gas mixing as possible), and latent heat will determine exactly what compression ratio you can tolerate - and of course these days, the consistancy of the fuel used.

Share this post


Link to post
Share on other sites

When he stated 16:1 compression, I was under the (wrong) assumption that was real, actual compression, not a no-port cylinder vs the combustion chamber formula.

Sort of like saying you have a 100 watt amplifier when that is power consumption from the wall, not output. Correct, but not correct.

Sorry...we always figured for the ports and adjusted accordingly.

If it REALLY was a true 16:1, my earlier statement applies: JP7, diesel, kerosene, maybe 150 octane. maybe.

Share this post


Link to post
Share on other sites
I live in Perth WA.

the stroker idea is just something i might play around with when i get some time off. im with you greg, was thinking case harden then surface grind to size.

If you have the stuff, grind first, then harden, then polish.

Lived up by the Northwest cape.

Share this post


Link to post
Share on other sites

I want to do the 150 stroker to my 144 also.

What are the additional requirements

Grind/machine the crank cases

Spacer plate

Offset crank pin or offset big end holes through the crank halves (which is better/easier/safer)

What about porting? Will the current porting suffice or will the port timing need to be adjusted?

What else it there to do?

I have a spare cylinder and head here

I lived in Perth, Esperence, Karratha, Mount Newman, Brisbane, Shark Bay and a few other places.

Share this post


Link to post
Share on other sites

You get around about as much as I do. We were in Exmouth from 77 to 80.

You can do this, but you would be be better off looking for a proper crankshaft/ crankpin. You can machine your own crank, would work well, just be sue not to get the case walls too thin.

Neat project.

Share this post


Link to post
Share on other sites
You get around about as much as I do. We were in Exmouth from 77 to 80.

You can do this, but you would be be better off looking for a proper crankshaft/ crankpin. You can machine your own crank, would work well, just be sue not to get the case walls too thin.

Neat project.

The fishing back then would have been shit hot.

Share this post


Link to post
Share on other sites
The fishing back then would have been shit hot.

It was: wife and I went with friends out on their boat. As fast as I could bait the hook and she could toss it in, she would pull in a red snapper. This went on for a few minutes until I decided it was too easy, and if she wanted more snappers, she could bait her own hook. Astounding levels of fish, great diving and all sorts of outdoor stuff.

We used to run our enduros over what we called the "trans-cape highway" which started up the back side of the airfield at Learmouth. A very nasty single track with sections of deep sand mixed with head sized rocks, then back up the ocean side, around the point and home. Whoever managed to be first would dust the guys behind on the trip up to the lighthouse on the point. You didn't want to ride drag on that. We had to strap aux fuel tanks on the backs of our bikes so we could go the distance. Great riding in all of the old water well and oil exploration roads that crisscrossed the entire peninsula. I became a master at relacing wheels from all of the dented rims.

Fun times. Poor bikes had a full and tough life. I had a DT3 Yamaha in those days, which was on par to what everyone else had. Great, tough engines with scary handling frames.

Share this post


Link to post
Share on other sites

Last tme I was in Exmouth I rode down from Karratha on my 600 Tenera, to run the engine in.1988 it was

Share this post


Link to post
Share on other sites

Our Skidoo Summit's 800HO's come from the factory with 13.25:1 CR, the flatlander MXZ model comes with 12.5:1 CR. At our elevation (5K') we can run aftermarket heads at 14:1 CR on pump 91 without problems. Its getting tough to find pump fuel w/o ethanol, but it will run it.

Share this post


Link to post
Share on other sites
Our Skidoo Summit's 800HO's come from the factory with 13.25:1 CR, the flatlander MXZ model comes with 12.5:1 CR. At our elevation (5K') we can run aftermarket heads at 14:1 CR on pump 91 without problems. Its getting tough to find pump fuel w/o ethanol, but it will run it.

Ahh: back to the original subject.

Up front: I'm not up on sleds. Don't know if what you're referring to are two or four strokes.

Not to sound arguementative, but I doubt you're getting that CR if they are two strokes; we're back to the portless cylinder again. Sometimes I really wish the professional liars would stick to politics, and not marketing. They went through this back in the late seventies, it went away, now it seems to be back.

But I'm not selling this concept, so no one has to buy it.

Share this post


Link to post
Share on other sites
Last tme I was in Exmouth I rode down from Karratha on my 600 Tenera, to run the engine in.1988 it was

Some guy named Graham was taking a lap of Oz while we were there, broke down on his Rickman framed KZ1000 just outside of town. We put him up in the spare room until the replacement coil arrived. I'd like to see how he's doing these days. I think he was from Victoria.

Share this post


Link to post
Share on other sites
Ahh: back to the original subject.

Up front: I'm not up on sleds. Don't know if what you're referring to are two or four strokes.

Not to sound arguementative, but I doubt you're getting that CR if they are two strokes; we're back to the portless cylinder again. Sometimes I really wish the professional liars would stick to politics, and not marketing. They went through this back in the late seventies, it went away, now it seems to be back.

But I'm not selling this concept, so no one has to buy it.

I think you are missing something basic here...just my two cents.

Share this post


Link to post
Share on other sites

I probably am, wouldn't be the first time. I suppose that level of CR is probably correct. I remain skeptical about the diesel level CRs though. I'm no design engineer, but I have done plenty of 2 stroke mods for years, though none recently since I went over to the dark side (four strokes). When we calculated CRs after machining heads, we were careful to do it from Exhaust port closing to TDC, and not go too far above a factory performance engine level. We had access to TDs and other engines to use as comparison. because of the wave action of the intake and the pipe, we believed the same general rule applied as for adding a supercharger or turbo: what you calculated as the CR was in fact much higher due to the "pumping" effect. It didn't take long to see you had overdone it, and we didn't like just throwing parts away.

So yes, many probably are running very high CRs, but my take on it is as you approach diesel level, your engine becomes less and less reliable, until you end up with top fuel dragster engine life: one full throttle run down the strip, toss everything inside and fit new. There is a tipping point where the law of diminishing returns takes over. Bad place to be. Or at least a very expensive place to be.

I'll shut up now.

Share this post


Link to post
Share on other sites
I probably am, wouldn't be the first time. I suppose that level of CR is probably correct. I remain skeptical about the diesel level CRs though. I'm no design engineer, but I have done plenty of 2 stroke mods for years, though none recently since I went over to the dark side (four strokes). When we calculated CRs after machining heads, we were careful to do it from Exhaust port closing to TDC, and not go too far above a factory performance engine level. We had access to TDs and other engines to use as comparison. because of the wave action of the intake and the pipe, we believed the same general rule applied as for adding a supercharger or turbo: what you calculated as the CR was in fact much higher due to the "pumping" effect. It didn't take long to see you had overdone it, and we didn't like just throwing parts away.

So yes, many probably are running very high CRs, but my take on it is as you approach diesel level, your engine becomes less and less reliable, until you end up with top fuel dragster engine life: one full throttle run down the strip, toss everything inside and fit new. There is a tipping point where the law of diminishing returns takes over. Bad place to be. Or at least a very expensive place to be.

I'll shut up now.

I think everyone is talking uncorrected - or pure static compression ratio.

18:1 corrected (to ex port closing) on most mod 125's is 10:1.

Share this post


Link to post
Share on other sites
Ahh: back to the original subject.

Up front: I'm not up on sleds. Don't know if what you're referring to are two or four strokes.

Not to sound arguementative, but I doubt you're getting that CR if they are two strokes; we're back to the portless cylinder again. Sometimes I really wish the professional liars would stick to politics, and not marketing. They went through this back in the late seventies, it went away, now it seems to be back.

But I'm not selling this concept, so no one has to buy it.

I guess I could have been more descriptive. These are only 2 stroke models. 4 strokes don't do so well in the mountains unless they are turbo/supercharger, because of the weight penalty. Even then they only go up in a straight line well...

These figures are static compression, NOT overhyped marketing gimmicks.

I run 12.5:1 CR in my turbocharger 800 2stroke making 10lbs boost on 100 octane safely.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×