Compression and stroker info for a 2.9 or 3.1

[lukasbmw]

When you stuff an S50 3.0 crank in a 2.8 aluminum block, what is compression?

When you stuff a S52 3.2 crank in a 2. 8 aluminum block what is compression?

How difficult will it be to build up a stroker? I have a complete 2.8 block in my yard. Is swapping in either the 3.0 or 3.2 crank a simple procedure? Will I have to change/modify anything else?

I want to keep this as simple as posible which means using as much of the parts I have as possible.

[Stu]

Putting in a 3.0 or 3.2 crank will increase the stroke, and therefore increase the compression ratio if you don't modify the pistons. Figuratively, you'll have about 12:1 with a 3.0 crank and about 14:1 with a 3.2 crank.

You don't want to setup an M50 with 84mm pistons and a 3.2 crank (89.66mm stroke). That's way too much stroke for the bore-size and the motor will run out of steam at 5800-6000. Your best bet is to go with the 3.0 crank ($300-400 second-hand), have the pistons machined down (talk to my dad), and then you'll have a 2.9, with about 11:1 compression (or less, if you desire). This is the setup that's in my dads M3. It gives you an 85.8mm stroke to go with the 84mm bore pistons. It makes for a very civilized drive at lower RPM's, with a LOT of midrange, and it screams nicely up top. The ideal bore-size for a 3.0 crank is 86mm or larger, but 84mm will work fine. You can't take the 2.8 block past 85mm anyway (aluminum block can't be bored over 84.5).

You should go with:
3 liter crank
machined pistons
11:1

[lukasbmw]

Exactly what I needed to know. 11:1 is perfect as I can only get 91 oct here in AZ and CA. I wouldn't want anything higher.


What if instead of using machined pistons I used different rods?

My research shows the m52b28 has 135mm rods so couldn't I reach 11:1 compression with 130mm rods?

What is the highest comp you would go with for a car that gets 91oct and is a daily driver in a state where the summer time temp reaches 120o?

Obviously using US S50b30 rods is out of the question since they are 140mm correct?

[Stu]

Use the normal 135mm rods. If you can only get 91oct, I'd keep the compression around 10 7 or 10 8, to be safe. You'll still make plenty of power.

You don't want to run short rods, stick to the 135mm, and machine the pistons.

[M3 Ryan]

5mm difference in rod will make your motor like 6:1CR instead of 12:1. When you get into an engine like that, a little goes a LONG way. You cannot simply add a stroker crank and not use custom pistons unless its a small difference that can be machined out of the piston heads.
I am running 10.7:1 CR now and on 91 with a chip it pings unless its under 50 degrees outside. I need to install my schricks to lower the compression back down and then I can put the chip back in.

[SlammedE30]

I don't understand how putting cams in will lower compression, unless you are planning to do something else during the cam install :?:
-Ted

[Jonathan M3]

Not to get too off topic, but can an alum. m52 be resleeved for a bigger bore? Just curious since LS1 guys have this option from a company called Darton, or just going with a C5R block :twisted: .

[Stu]

Not to get too off topic, but can an alum. m52 be resleeved for a bigger bore? Just curious since LS1 guys have this option from a company called Darton, or just going with a C5R block :twisted: .

My dad didn't sleeve/bore his. He thought it was too fragile, especially since it has a longer crank though and is turning to 7400. The 84mm bore in the alum. block works fine, the pistons have a special coating on them to reduce friction, just gotta throw in the 3liter crank. :)

You probably could sleeve it, and see what happens. No real answer from me though, sorry.

[EVOIIIM3]

I don't understand how putting cams in will lower compression, unless you are planning to do something else during the cam install :?:
-Ted

Cams bleed off compression. The higher the duration cam and thus more overlap, the lower the combustion pressure and that makes a lower CR. This is how some one can use 11.5 CR pistons and not have detonation.

Erik, why get a stroker crank, and then use shorter rods? That just going to kill the smoothness of the engine, and make it more like a domestic engine.

Stock a 2.8 is perfectly square with 84x84 B/S That is why it has good low end torque and pulls up high. Try to keep the B/S as close to that as you can.

I agree with Stu, get a 3l crank and overbore to 85+ pistons. Don't forget the Cams.

[RedBaron86325]

Cams bleed off compression. The higher the duration cam and thus more overlap, the lower the combustion pressure and that makes a lower CR. This is how some one can use 11.5 CR pistons and not have detonation.

Mike, you read my mind on that one.

It isn't extremely difficult to figure out how much overlap is good/not good on an "old school" engine (motor math), but how much effect would VANOS have on overlap? I don't know much about variable timing. If I learned more about it, I may be more accepting to it. Having a cam not "absolute"ly locked in place puts a slight bit of fear in me.

[Kell]

Stu,

just wondering, how much HP/ torque is your dads M3 pushing out with this setup?

[Stu]

Stu,

just wondering, how much HP/ torque is your dads M3 pushing out with this setup?

plenty.

[e30Matt]

Stu,

just wondering, how much HP/ torque is your dads M3 pushing out with this setup?

plenty.

If he told you, he'd have to kill you.

[Kell]

Stu,

just wondering, how much HP/ torque is your dads M3 pushing out with this setup?

plenty.

sounds good to me :D

If he told you, he'd have to kill you.

im willing to face the consequences ;)

[Stu]

It probably makes around 280 horses, similar torque (maybe less).