Turbo M20 2.8L Stroker w/ 135mm rods

[SLEEPYDUB]

Hey guys, my engine is at the machine shop, and after a few set backs Im on track to have the engine finished today. I need to verify some things first though. They said my pistons are sticking out of the block by 0.035. I will be using a cooper cutring 0.080 headgasket. Does this sound right?

[digger]

IMO the only time a dome is going to hurt you any meaningful way is a ultra high CR small capacity NA engine. then the dome might inhibit flame travel because you need so much dome to achieve the CR. this is one reason why old school hemi is not very good for NA high CR.

[varg]

There is no inherent advantage to a dished piston, the only reason the dish is there is to reduce compression ratio. Why would he recommend it? Because it's what he's used to or is conventional knowledge. Ideally you'd want a piston that has been designed the way manufacturers design their OE pistons, CFD and testing to optimize quench and flame front travel, typically one doesn't design custom pistons, so they go with a forged dish. The best route for a reasonable engine is essentially a forged replica of a stock B25 piston, but you're getting into hypothetical territory talking about 6-700hp, people just don't do that often and when they do they just go with what's conventionally used for high power turbo builds - a forged piston with a big dish.

For what it's worth, turbocharging doesn't change the basic characteristics of an engine's operation. People will parrot things like 'squish doesn't matter with boost' or 'port design is less important than flow with boost' because they think an extra atmosphere or two worth of pressure makes some drastic difference in how the engine makes power or the combustion process behaves. It doesn't. Not on the levels we're talking about at least, when you get to top fuel style insanity things change. In the combustion chamber and ports, what's most efficient without boost is what's most efficient with it, provided you don't exceed the capabilities of something because you're just making more power than it was ever designed to make. You won't see any stereotypical turbo engine build stuff in a modern turbocharged engine, we've moved beyond that. Peek inside of 1,500hp Nissan GTR's engine and there aren't deep dish pistons, the compression ratio isn't 8:1.

If you're really trying to make 600-700whp it's probably best to move on to the M50 family in my opinion. The M50 family heads flow better and will support the power more easily with less boost pressure, and the directly actuated valve lifters are an advantage over M20 rockers for high revs. Also, see Nisse Jarnet's insane cars. If I wanted to make that kind of power it's what I'd do.

(edit) Actually, no, if I wanted that kind of power I'd put a supercharged V8 in it, but if I wanted to make that power with a BMW I6 I'd go 24 Valve.

[SLEEPYDUB]

The local using the same turbo as you had flat top forged pistons and 885 head in his first engine, so I have seen it first hand back to back. The factory style pistons made more power across the board with the 885 head - just like they do in N/A form. Irregardless of quench numbers, if an engine makes more power N/A, then it will naturally be more powerful when adding pressure, so it only makes sense to start with the most powerful combination.

Would you happen to have any dyno results to compare the oe crown piston vs the dished? I am not doubting your findings, but I would like to see where and how great the power loss is.

Also, I need this question answered: At what point does the OE crown piston become a problem? I see that it held ~520whp on your friends build, but what about 600? 700? Does it ever become an issue when really trying to push the limits? Does a dished piston have ANY ADVANTAGE what so ever? Why would Steve at TEP recommend these over OE crown? What does he know that we dont?

Lets say I want 600whp. Would the difference in making that power come down to something like this, where the OE crown makes it at 30psi but the dished style requires 35psi? Are the differences that great? Or are we talking about a small percentage of power difference?

I think Im going to stick with conventional rings, so this is my last debate before placing my order. Thanks again guys and hopefully this will help some others in the future.

[ForcedFirebird]

Wasn't an intentional back to back, his first engine blew, 2 broken ring lands. He went back to the same engine builder and used the OEM crown the second time. We have purposely tested N/A m20 combinations using mismatched OEM parts, though.

[digger]

The local using the same turbo as you had flat top forged pistons and 885 head in his first engine, so I have seen it first hand back to back. The factory style pistons made more power across the board with the 885 head - just like they do in N/A form. Irregardless of quench numbers, if an engine makes more power N/A, then it will naturally be more powerful when adding pressure, so it only makes sense to start with the most powerful combination.

Digger, as far as that thread goes, it also used to be common practice for a mechanic come to your house, grind a bad rod pin, shim the bearings and send you on the way. Those same mobile mechanics would also have a deck-mounted boring machine, would bore one cylinder, back shim new rings - but neither of these are practice in modern days. Does it work? Yes. But, will it last and/or make optimum power compared to modern machining practices? When building a fresh engine like the OP, and shopping for appropriated parts, why skimp?

if you have tested back to back with absolutely no change other than squish vs no squish (same CR) then i don't doubt the results, i was just posting something i read not that long ago that i found interesting, its not what i would do. i would use OE style then if i wanted lower CR id go to the Paul burke FI style with bigger dish. its also hard to put some things in context reading on forums

[SLEEPYDUB]

The local using the same turbo as you had flat top forged pistons and 885 head in his first engine, so I have seen it first hand back to back. The factory style pistons made more power across the board with the 885 head - just like they do in N/A form. Irregardless of quench numbers, if an engine makes more power N/A, then it will naturally be more powerful when adding pressure, so it only makes sense to start with the most powerful combination.

This is my dilemma. I am being pulled in two different directions.
Steve says go dished style because at the high boost levels, the oem crown really doesnt play any role in power delivery, but FFF says that it is imperative. So with that said, im at a stand still till I know the best route to take. $1500 is a lot to spend on uncertainty

Gapless rings are being recommended because I run ethanol, but other then that I dont see much benefit, maybe a little bit better seal.

[ForcedFirebird]

Even with the high amount of boost that will be thrown at it?

The local using the same turbo as you had flat top forged pistons and 885 head in his first engine, so I have seen it first hand back to back. The factory style pistons made more power across the board with the 885 head - just like they do in N/A form. Irregardless of quench numbers, if an engine makes more power N/A, then it will naturally be more powerful when adding pressure, so it only makes sense to start with the most powerful combination.

Digger, as far as that thread goes, it also used to be common practice for a mechanic come to your house, grind a bad rod pin, shim the bearings and send you on the way. Those same mobile mechanics would also have a deck-mounted boring machine, would bore one cylinder, back shim new rings - but neither of these are practice in modern days. Does it work? Yes. But, will it last and/or make optimum power compared to modern machining practices? When building a fresh engine like the OP, and shopping for appropriated parts, why skimp?

[digger]

some of the guys in that thread work for a piston manufacturer

http://speedtalk.com/forum/viewtopic.php?f=1&t=50883

[SLEEPYDUB]

Digger has a good point, there is no perfect solution for what you are after. You are going to start chasing your own tail. I say this with all due respect, have seen it happen often.

One point to add, not sure if it's been mentioned. Rather than focus on the side skirt coatings, consider ceramic coating the top. If you are looking to maximize longevity on a FI engine, while still having to run 2618, then it's a very nice touch.

Steve mentioned the top coating not being a big deal because I will be running strictly E85, and the cooling benefits from that fuel negate the help from the coatings. But, I may as well just throw that on there too at this point.

I can understand that there is no perfect solution, but I just cant understand why one professional says to use the OEM crown, and one says to use the dished style. I dont know enough myself about the dynamics of it to make a decision, so this is really messing with me

[SLEEPYDUB]

Without a doubt match the piston shape to the head. If using 885 head and flat pistons, you eliminate the squish band. I have worked with flat pistons and 885 heads, but they always make less power and require more timing.

Even with the high amount of boost that will be thrown at it?

[SkiFree]

nobody can tell you in uncertain terms as it comes down to the execution of it all

At this point, my mind is not made up on what I should order.

Digger has a good point, there is no perfect solution for what you are after. You are going to start chasing your own tail. I say this with all due respect, have seen it happen often.

One point to add, not sure if it's been mentioned. Rather than focus on the side skirt coatings, consider ceramic coating the top. If you are looking to maximize longevity on a FI engine, while still having to run 2618, then it's a very nice touch.

[ForcedFirebird]

Without a doubt match the piston shape to the head. If using 885 head and flat pistons, you eliminate the squish band. I have worked with flat pistons and 885 heads, but they always make less power and require more timing.

[SLEEPYDUB]

Steve said that he uses gapless top rings on every turbo M20 he has ever built and has had good success with it. I mentioned to him that Ill be running 30+psi and he said to use the dished pistons vs oem crown due to the way it combust under high boost?? Said OEM crown is good for lower hp levels, but when getting into the big power territory he suggest to go with dished style. I dont think he mentioned anything about gas porting with the gapless rings.

At this point, my mind is not made up on what I should order.
Based off of our conversations in this thread, and my conversation with him, I cannot come to a conclusion as to what I truly need. So, would you guys mind giving me some questions to ask him so that we can figure this out? He responds very quickly and has been easy to work with, i'm sure he wouldnt mind clearing some things up for me.

Thanks again guys

[digger]

For an extra 230 hard to justify when there is no overall consenus.

What is turbo dish style ?