Piston Comparison
Dec 18, 2008, 08:17 PM
#16
If everything else stayed the same you always make more power with more boost. More boost is more airflow, more airflow is more power. However when you reach the end of the airflow rope the next best thing is more compression...like I mentioned with the 4% rule, if you are at 500whp at 10:1, theoretically the next full point at 11:1 would net 520.
Dec 18, 2008, 08:55 PM
#17
So what is the ideal static compression ratio?
There is no correct answer to that question, simply because there are too many variables to consider. Alcohols like Methanol and Ethanol will tolerate static cylinder pressures that make gasoline unstable. But one question is, just how much can a mixture of 85% EtOH and gasoline withstand?
Keep in mind that factors like valve timing, head gasket thickness, intake air temps, turbo/turbine housing size, etc., all factor into just how much static cylinder pressure the engine will tolerate. That is because all of these things factor into the only compression ratio the engine really sees, which is dynamic compression. A car with smaller cams and turbo will have a tougher time with high compression than long duration cams and a turbo with a big hotside.
As always, octane is the primary limiting factor. For a car that will never see anything but pump premium, the factory SCR (static compression ratio) is probably the best bet given the ample data we've seen of what is possible with that. With E85 used as a secondary fuel, I'd still stick to the factory-range SCR if strong performance on pump premium is desired.
For a dedicated E85-E98 car with larger turbo, I'd guess 10:1 should improve torque without drawbacks. Does higher SCR create faster spool? It creates a bit more torque, but it increases effciency, which reduces heat (the primary spool influencing factor). It also improves fuel economy and emissions somewhat. FWIW, WRC cars make stupid torque around 3500 rpm with a tiny turbo, 40psi boost, and 10:1 SCR using 98 RON. That doesn't make it practical for a street car however.
For an E98 car, 11:1 might be viable, but who knows? At some point, even with straight alcohol, going to a higher SCR is too much of a good thing, and the result will bring the detonation threshold down to a point whereby no more power can be realized because the ignition advance is set too far from BMT.
Just my thoughts. Now ... where's my martini?
There is no correct answer to that question, simply because there are too many variables to consider. Alcohols like Methanol and Ethanol will tolerate static cylinder pressures that make gasoline unstable. But one question is, just how much can a mixture of 85% EtOH and gasoline withstand?
Keep in mind that factors like valve timing, head gasket thickness, intake air temps, turbo/turbine housing size, etc., all factor into just how much static cylinder pressure the engine will tolerate. That is because all of these things factor into the only compression ratio the engine really sees, which is dynamic compression. A car with smaller cams and turbo will have a tougher time with high compression than long duration cams and a turbo with a big hotside.
As always, octane is the primary limiting factor. For a car that will never see anything but pump premium, the factory SCR (static compression ratio) is probably the best bet given the ample data we've seen of what is possible with that. With E85 used as a secondary fuel, I'd still stick to the factory-range SCR if strong performance on pump premium is desired.
For a dedicated E85-E98 car with larger turbo, I'd guess 10:1 should improve torque without drawbacks. Does higher SCR create faster spool? It creates a bit more torque, but it increases effciency, which reduces heat (the primary spool influencing factor). It also improves fuel economy and emissions somewhat. FWIW, WRC cars make stupid torque around 3500 rpm with a tiny turbo, 40psi boost, and 10:1 SCR using 98 RON. That doesn't make it practical for a street car however.
For an E98 car, 11:1 might be viable, but who knows? At some point, even with straight alcohol, going to a higher SCR is too much of a good thing, and the result will bring the detonation threshold down to a point whereby no more power can be realized because the ignition advance is set too far from BMT.
Just my thoughts. Now ... where's my martini?
Dec 18, 2008, 09:06 PM
#18
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Good post Aaron. There is a company that is testing there 63 engine for next season currently. They are running 14:1 compression and keeping everything together. They mentioned that its all in the tolerances. Everything has to be perfect and if you can do that then there are huge gains to be made. Guess we will see how it does.
Chris
Chris
Dec 18, 2008, 09:09 PM
#19
Next time I am in Baton Rouge visiting my brother I'll drop one off 
Thanks for the post. That is one secret that seldom seems to get mentioned is that the Cams can make or break high compression. Being able to bleed off some of the compression through overlap can overcome some of the other issues you encounter...as I recall.
I know that on some fuels (methanol) compression really seems to be irrelevant to the amount of boost you run. There is at least one non-mitsu motor I know of running obscene compression and running levels of boost that make some diesels look weak.
I know what MBT is and assume by BMT you mean the same...or do I need to go back to the books? lol
Thanks for the post. That is one secret that seldom seems to get mentioned is that the Cams can make or break high compression. Being able to bleed off some of the compression through overlap can overcome some of the other issues you encounter...as I recall.
I know that on some fuels (methanol) compression really seems to be irrelevant to the amount of boost you run. There is at least one non-mitsu motor I know of running obscene compression and running levels of boost that make some diesels look weak.
I know what MBT is and assume by BMT you mean the same...or do I need to go back to the books? lol
Last edited by JohnBradley; Dec 18, 2008 at 09:11 PM.
Dec 18, 2008, 09:29 PM
#20
Bleeding off some of the pressure with long valve overlap is a secret to getting high SCR to work where octane is limited (e.g. WRC engines). At higher rpm, where the effect of overlap is reduced, there is usually sufficient turbulence to mitigate the effects of the high SCR, so can make for a win-win situation, usually at the expense of low speed performance however.
Straight methanol is very forgiving. There are souls who know far more than I do about where the real limits lie with it. One thing however is that very high compression pistons can impede the flow of gases around the valves. This is where the details make the difference.
Yes, I meant "MBT". My mind was on gin and olives . . .
Straight methanol is very forgiving. There are souls who know far more than I do about where the real limits lie with it. One thing however is that very high compression pistons can impede the flow of gases around the valves. This is where the details make the difference.
Yes, I meant "MBT". My mind was on gin and olives . . .
Dec 18, 2008, 10:53 PM
#21
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There is one thing that can bite you in the heinricher--the peak cylinder firing pressure. It grows exponentially with SCR even in the absence of detonation.
When mixing high SCR with high octane and plenty of boost, you are compounding this situation. So you either build the bottom end, or accept that it is living on borrowed time.
When mixing high SCR with high octane and plenty of boost, you are compounding this situation. So you either build the bottom end, or accept that it is living on borrowed time.
Dec 19, 2008, 05:17 AM
#23
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And thats where my mind was when I built my motor. Mine is a "Slightly more streetable" version of what was discussed here. Mine is a 9.7:1 compression with a touch over 11lbs removed from the rotating assembly. It is run strictly on E85, and with everything ported and a set of GSC S2's on a IX turbo, it is almost impossible to not get into boost because of how fast spool-up is. I wanted my motor to do a few things:
1. Spool up extremely fast.
2. Have more power off boost.
3. Abuse the E85.
4. See how far I can get with my IX turbo.
She seems to be sitting around 440/450hp now, but I am not finished with my tuning yet. (I am working on all the "non-WOT" areas of my map to make things like startup & cold idle/warmup work better.)
What I can say is after building a high(er) compression turbo motor running on E85, I will never go back. It truly is a KILLER set-up, and a rediculous amount of fun to drive on the street. Plus, 4-wheel burnouts in 1st and 2nd are fun because of how fast the power comes on....
Dec 19, 2008, 05:27 AM
#24
I have been considering going to 9.5:1 compression pistons for a while now. Unfortunately in my area the only fuel available on the pump is 93 w/ 10% ethanol. Have you done any testing with that compression ratio? I have seen a lot of stock, 9:1 and 10:1 but it seems the majority have ignored everything in between. Any reason for that?
Great write up btw
. I always love pictures too
Great write up btw
. I always love pictures too
Dec 19, 2008, 08:35 AM
#25
There is one thing that can bite you in the heinricher--the peak cylinder firing pressure. It grows exponentially with SCR even in the absence of detonation.
When mixing high SCR with high octane and plenty of boost, you are compounding this situation. So you either build the bottom end, or accept that it is living on borrowed time.
When mixing high SCR with high octane and plenty of boost, you are compounding this situation. So you either build the bottom end, or accept that it is living on borrowed time.
I was talking about this thread with Sean our tuner and mentioned to me that he rarely sees Evo's running Wisecos but then again we don't build many extreme motors to go to or near 1000hp so i understand the use of diff manufacturers designs and such.
Aren't all pistons essentially made of the same materials when you discuss the top brand names?
Just wondering as we have very few failurs of the Wisecos in our dsms/evos over the years and we are the type to not change stuff if its working out for us.......we don't have the exstensive time nor funds to do the testing of other pistons/rods other company's can
Dec 19, 2008, 08:49 AM
#26
We have done one 9.5:1 motor for Turbojunkie. He is running 92 octane right now for winter (we are currently in the middle of a blizzard) since the extreme temps dont make for the best starting.
What we found on Lucas for example is the 10:1 would tolerate 21-22psi on west coast 92. The main reason is piston availability for the 9.5:1 motors. There are some tricks to getting that compression ratio easily and off the shelf, but there are always compromises somewhere.
What we found on Lucas for example is the 10:1 would tolerate 21-22psi on west coast 92. The main reason is piston availability for the 9.5:1 motors. There are some tricks to getting that compression ratio easily and off the shelf, but there are always compromises somewhere.
Dec 19, 2008, 08:54 AM
#27
Patrick,
The Wisecos seem to do ok, but we havent pushed them over 500whp or so. We have used Ross, Arias, and JE. The problems with Wisecos always seemed to be the piston to wall and then they'd wanna black death. For awhile they were getting called SeizeCo because of it.
I think its all about clearance when building and overall metallurgy because some pistons have more silicon on in them or the alloy is a little different so there is some variation on expansion. Strengthwise they might be really close.
The Wisecos seem to do ok, but we havent pushed them over 500whp or so. We have used Ross, Arias, and JE. The problems with Wisecos always seemed to be the piston to wall and then they'd wanna black death. For awhile they were getting called SeizeCo because of it.
I think its all about clearance when building and overall metallurgy because some pistons have more silicon on in them or the alloy is a little different so there is some variation on expansion. Strengthwise they might be really close.
Dec 19, 2008, 09:40 AM
#28
Patrick,
The Wisecos seem to do ok, but we havent pushed them over 500whp or so. We have used Ross, Arias, and JE. The problems with Wisecos always seemed to be the piston to wall and then they'd wanna black death. For awhile they were getting called SeizeCo because of it.
I think its all about clearance when building and overall metallurgy because some pistons have more silicon on in them or the alloy is a little different so there is some variation on expansion. Strengthwise they might be really close.
The Wisecos seem to do ok, but we havent pushed them over 500whp or so. We have used Ross, Arias, and JE. The problems with Wisecos always seemed to be the piston to wall and then they'd wanna black death. For awhile they were getting called SeizeCo because of it.
I think its all about clearance when building and overall metallurgy because some pistons have more silicon on in them or the alloy is a little different so there is some variation on expansion. Strengthwise they might be really close.
Also, I have seen people claiming smoking from the rings with JE pistons. I have no idea if that's true. I have seen others say that the 1.0mm top ring on the CP isn't enough, and that the JE 1.3mm top ring is better for boosted applications. I have no idea if that's true either. I have seen others claim Wiseco's don't "slap" or aren't "noisy" because of the alloy...even though they seem to use 2618 everyone else does.
What I do know is that Aby seems to like the Wiseco rings better than some other companies. Don't know why, but then again, I've never built an engine before.
Inquiring minds want to know
Dec 19, 2008, 09:49 AM
#29
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From all the piston manufacturers I've seen with the Evo, they use the 2618 alloy. It would be interesting to me if one piston exhibits different properties than another. If true, could that be the forging process and not the alloy? Could it be same alloys of different quality?
Also, I have seen people claiming smoking from the rings with JE pistons. I have no idea if that's true. I have seen others say that the 1.0mm top ring on the CP isn't enough, and that the JE 1.3mm top ring is better for boosted applications. I have no idea if that's true either. I have seen others claim Wiseco's don't "slap" or aren't "noisy" because of the alloy...even though they seem to use 2618 everyone else does.
What I do know is that Aby seems to like the Wiseco rings better than some other companies. Don't know why, but then again, I've never built an engine before.
Inquiring minds want to know
Also, I have seen people claiming smoking from the rings with JE pistons. I have no idea if that's true. I have seen others say that the 1.0mm top ring on the CP isn't enough, and that the JE 1.3mm top ring is better for boosted applications. I have no idea if that's true either. I have seen others claim Wiseco's don't "slap" or aren't "noisy" because of the alloy...even though they seem to use 2618 everyone else does.
What I do know is that Aby seems to like the Wiseco rings better than some other companies. Don't know why, but then again, I've never built an engine before.
Inquiring minds want to know
I am around 1,000 miles on my build now with CP's, and I have only heard piston slap once, and it was rediculously cold outside. (For the area). I can tell you if the CP's top ring isn't enough, well, I will be finding out eventually as I beat my Evo like it owes me money now that I have a built motor. (And a spare)
