Turbo M20 2.8L Stroker w/ 135mm rods

[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?

[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.

[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

[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.

[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.

[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.

[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.

[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?

[ForcedFirebird]

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.

I started a dyno thread. You can see there that a stock tired 2.5 will make 160whp and the 885 head on an eta bottom end made 130. While it doesn't sound like much in HP, when you look at percentage, the flat pistons made 80% the power the stock dome made.

I have had the chance to test it on a turbo car. The same client I mentioned earlier started with flat pistons, but his second setup had a different turbo and cam. But, I can say that I have tuned 2 of the same turbos on two different m20 cars (one having flat, other domed pistons). The flat pistons made 250hp/280tq and domed pistons made 290/304, both at 12psi. Hard to say if there was any other factors since it was 2 different cars after all.

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?

That will be fine. .080" gasket minus the .035" piston protrusion leaves you with .055" quench which is more than enough - shooting for ~.040" is best, so if you can get a .070" gasket, you will be even more gold.


EDIT: What shape pistons did you end with?

[SLEEPYDUB]

That will be fine. .080" gasket minus the .035" piston protrusion leaves you with .055" quench which is more than enough - shooting for ~.040" is best, so if you can get a .070" gasket, you will be even more gold.


EDIT: What shape pistons did you end with?

Wouldn't that actually net me a 0.045" Quench? Or is my math off?

I went with the dished 8:8:1 JEs with Top Seal gapless rings, anti detonation grooves, and HD pins. I debated the OE crown with Steve over and over, and he has recommended against the use of it for my application. I understand the benefits of the OE crown over the Dished, and I am still nervous that I may have made the wrong decision. Although, I do see the benefits of the dished as well especially at extremely high boost levels.

I think the Gapless rings will help to restore some of the loss of power from the dished pistons being that it will dramatically decrease leakdown. Ive seen some pretty amazing dyno results back to back with conventional rings vs gapless, and I must say that the amount of power that was picked up all across the curve was pretty phenomenal. Couple that with boost and I think it may make up for the power loss from the lack of oe crown.

[digger]

I must admit I've not seen evidence of large gains from gapless top posted anywhere and would be interested in seeing such data.
Sometimes people change from thick conventional to thinner gapless so is the gain the thinner ring or a smidge less end gap. I've heard people say that power gains are small when you compared rings if same thickness despite often showing much better leakdown numbers

There are mixed reviews on them mainly from an oil consumption point of view but this seems sketchy that the top ring has such a large effect on oil control

[ForcedFirebird]

My math was wrong, leave the .045" quench (in reality it will be more with the angled band on the head). The flat pistons work just fine, that's been proven over. Would be interesting to do a comparison some day.

[ForcedFirebird]

As far as gapless, I haven't seen any test data, but have seen a ring gap test Hot Rodding magazine did years ago. They kept opening ring gaps and dyno testing the engine taking note of power and oil consumption. By the end of the article they ended up making the gaps ridiculously large. The power loss was minimal, but oil consumption did increase.

[SLEEPYDUB]

My math was wrong, leave the .045" quench (in reality it will be more with the angled band on the head). The flat pistons work just fine, that's been proven over. Would be interesting to do a comparison some day.

Well thats reassuring that the pistons Im using will be fine. The 0.080 gasket should compress some and make the quench even closer, so I think it should work. As for the gapless ring, i saw hotrod mag do a comparison and they saw an increase across the entire power band when using gapless. This was in an NA engine, however I think the same would hold true to a FI engine as well, and may provide even better results

[McGyver]

Fantastic thread! SLEEPYDUB , thanks for asking all the questions. ForcedFirebird and digger , thanks for providing such clear answers and theory.

Any updates or changes in thinking since you guys put this together? I'm embarking on my own turbo 2.7i build and this info has been invaluable.