2.9l stroker compression question

[itsmedant]

Hey everyone,

Just signed up for this forum and i have to say, i'm impressed. I have done all the searching i can on here and i have gotten alot of information that is going to help me in my project for this summer.

Let me give everyone alil backgroung first, I have had no experince with anything car wise except the sound sytems, but i am a very fast learner.

Basically i have become addicted to my friend's e30 and i have decided that i have to get one. I think the best way to learn about it is to build my own engine and do all of my own work. I have decided to go with a 2.9l stroker m20.

Right now i am thinking about going with custom pistons with a 86mm diameter that will have the compression at 10.0:1.

If later on i decide to go with a turbo, what is the lowest i can drop my compression to without having to get new pistons.

Hopefully this isnt something that can be found already by using search (dont want to be one of those annoying n00bs)

Thanks ahead of time for all of your help!

-Eric

[nando]

if you want to turbo later, don't use an 86mm bore. even N/A that's pushing it with an M20 - metric mechanic gets away with it because of their specially designed pistons.

[itsmedant]

i was actually going to get metric mechanics pistons for my car.

but even getting those, do you still think it would be pushing it?

[nando]

N/A it would be fine, but the cylinder walls will be awfully thin for a turbo.

[itsmedant]

so how far can i bore it out and get good compression but still leave it safe for a possible turbo in the future?

[mspiegle]

at 10.0, your car will be on the edge. I personally think you should pick a direction and stick with it.

You mentioned lowering the compression without changing pistons... I assume you mean thicker headgasket or enlarging the combustion area in the head?

[itsmedant]

at 10.0, your car will be on the edge. I personally think you should pick a direction and stick with it.

You mentioned lowering the compression without changing pistons... I assume you mean thicker headgasket or enlarging the combustion area in the head?

okay basically what i was trying to say was that i am going to do this stroker and aim for something around 10:1 compression. some day down the road if i want more power and decide to turbocharge my m20, how much will a thicker head gasket, ect. drop my compression. will it be able to drop it enough to turbo the engine. or will i have to buy new pistons.

[SA E30]

Dunno where this myth comes from, that the cylinder walls r 2 thin for a Turbo... I know that there is increased heat and friction with a turbo motor.

With the correct cooling, there is not problems, and it has been proven time and time again, over here in South Africa, where our summer temps r comparable 2 Arizona.

U don't even need 2 use a custom piston... a cutdown piston is still strong enough, so long as u not gonna be boosting more than about 1.5bar... or so.

[Sean]

Dunno where this myth comes from, that the cylinder walls r 2 thin for a Turbo... I know that there is increased heat and friction with a turbo motor.

With the correct cooling, there is not problems, and it has been proven time and time again, over here in South Africa, where our summer temps r comparable 2 Arizona.

U don't even need 2 use a custom piston... a cutdown piston is still strong enough, so long as u not gonna be boosting more than about 1.5bar... or so.

So, if you lower compression enough using 86mm bore pistons, you can run up to 21 lbs of boost?

If I ever did, I'd want to lower compression to around 9.2 and hopefully run 7-8?

I have the same question - what kind if compression would a thick headgasket yield? How THICK is the thicker headgasket? If it's only like .010" it will only lower the 10.1 to like 9.6 or so, I would think.

[nando]

Dunno where this myth comes from, that the cylinder walls r 2 thin for a Turbo... I know that there is increased heat and friction with a turbo motor.

With the correct cooling, there is not problems, and it has been proven time and time again, over here in South Africa, where our summer temps r comparable 2 Arizona.

U don't even need 2 use a custom piston... a cutdown piston is still strong enough, so long as u not gonna be boosting more than about 1.5bar... or so.

I guess it comes from the same place as the myth than an 86mm bore is too much for an M20 - out of somebody's arse. :(

yeah, looking at your engine shot (assuming that's an 86mm bore), looks like there's plenty of meat between the cylinders. I still don't think I'd want to turbo a 10:1 compression engine though! :p

btw what crank/rods are you using for your 2.9l?

[itsmedant]

I guess it comes from the same place as the myth than an 86mm bore is too much for an M20 - out of somebody's arse. :(

yeah, looking at your engine shot (assuming that's an 86mm bore), looks like there's plenty of meat between the cylinders. I still don't think I'd want to turbo a 10:1 compression engine though! :p

btw what crank/rods are you using for your 2.9l?

i'm really not sure yet....this is still on the drawing board waiting for funds and cooperation from the parental units (i think i got them now though!)

I've read that i could use the 325e crank or the 525 td crank (isnt that one forged?) or do you think since i am going to be pushing this sucker i should go with an aftermarket crank? See i'm really new to cars and engines so i am trying to get every bit of info possible before i start, just got my bentley manual in a couple days ago!