Author Topic: Compression ratio (Read 4709 times)

hesgone2fast · « **on:** April 16, 2008, 10:25:11 AM »

Hey guys, I�ve just received my M47 crank from germany. And I�m about to receive a set of 6 US S52 pistons + rods from the states.
As you can guess I�m planning a big stroker (around 2070cc) and very possible in a near future complete the project with a DASC on top of it.
Now, with these pistons and the stock M42 rods I would get a CR of about 12:1, which I�m not comfortable with...I would like to go down to about 9,5:1
I would like to find out what lenght are the S52 rods (US version) I�ve heard 135mm, up here in europe they are 140mm.
Using the S52 rods(assuming they were 135mm...) it would be around 9,5:1
Another posibility and main question of this thread is the M40 rods(318i 8 valve engine)they share the same block and crank, the only difference in the block are the pistons and rods, which are 135mm.
What do you guys think about using these rods with the S52 pistons and M47 crank?I�ve already mounted them with the M42 piston and leaves the piston sitting 5mm down in the bore at TDC.
Any guesses on power output with this setup(2070cc + DASC) is very welcome.

Thanks!!

hesgone2fast · « **Reply #1 on:** April 17, 2008, 03:16:49 PM »

anyone?

colin86325 · « **Reply #2 on:** April 17, 2008, 08:50:11 PM »

I was looking at a set of US S50/52 rods today. They are the same length as the M42 rods but had a wider cross-section and weighed less than the M42 rods.

colin86325 · « **Reply #3 on:** April 17, 2008, 11:25:42 PM »

Quote from: colin86325;47366

I was looking at a set of US S50/52 rods today. They are the same length as the M42 rods but had a wider cross-section and weighed less than the M42 rods.

***correction--the M42s are 140mm. I was confused.

See pics:
http://www.e30tech.com/forum/showthread.php?t=57666

kowalski · « **Reply #4 on:** April 17, 2008, 11:40:34 PM »

sounds like quite the project. you could try milling the tops of the pistons down as well.

mkodama · « **Reply #5 on:** April 18, 2008, 06:31:10 AM »

I'm not sure you can use shorter connecting rods (or at least much shorter than stock). At the bottom of the stroke, the pistons are already getting pulled down 3mm more than they are originally designed to, and with shorter connecting rods, the pistons would be pulled down even further. At that point you might start running into problems that would create accelerated wear or other problems. I'm not expert, but this is just a guess from other information I've gathered.

Other things that will help you are shaving the top of the pistons down. This has been done before and safely lowers the compression ratio. Also you can use a thicker head gasket. A person who has done the crank swap, along with euro S50B30 pistons decked 3.8mm, originally had a compression ratio of 12.1 but replaced the head gasket with a thicker one and brought compression ratio down to 11.6.

But honestly, I don't get why you don't just bore the block to fit some decked M3 pistons and reuse the stock crank. With this setup, you would have a low compression ratio good for forced induction, and the stock crank and connecting rods in the M42 are forged and have plenty of extra strength.

hesgone2fast · « **Reply #6 on:** April 19, 2008, 05:37:51 AM »

Hi there guys, thanks for your replies.
Well, the truth is I didn�t think of the so low position of the piston with a shorter conecting rod, but hey, Metric mechanic is using a 134,5mm rod with the 88mm crank, intended for forced induction.
Also keep in mind that the S50/S52 piston is a little shorter(in height) than the M42(see gizmo�s pics on BimmerFMs, so it wouldn�t sit so low in BDC. the difference would be 8,5mm lower in the bore.
Once I get all the parts, I will play around with the pistons, rods, cranks, etc... measure everything, and see what�s the best solution.
Cheers!

mkodama · « **Reply #7 on:** April 24, 2008, 12:31:38 AM »

Quote from: hesgone2fast;47519

Hi there guys, thanks for your replies.
Well, the truth is I didn�t think of the so low position of the piston with a shorter conecting rod, but hey, Metric mechanic is using a 134,5mm rod with the 88mm crank, intended for forced induction.
Also keep in mind that the S50/S52 piston is a little shorter(in height) than the M42(see gizmo�s pics on BimmerFMs, so it wouldn�t sit so low in BDC. the difference would be 8,5mm lower in the bore.
Once I get all the parts, I will play around with the pistons, rods, cranks, etc... measure everything, and see what�s the best solution.
Cheers!

Good job with the research and best of luck with the project!

hesgone2fast · « **Reply #8 on:** April 25, 2008, 04:33:05 PM »

yeah, it seems a nice project, and real, not as some people say that will do... but never becomes real.
I will do it in two steps, first the stroker, with a 11,5:1 CR to get verything settled in the engine, and then I will add the supercharger with lower CR, just changing rods or pistons.
My job is taking engines apart all day on a workshop, so I don�t mind doing it at home over and over again on my personal car.
I already have the pistons and crank in possesion, I will take them to machine the aditional keyway into the crank, and shave the pistons in a few weeks.
I have a spare M42 intake manifold, so I might try to make the same as MM, to see what happens.
I�ll keep you posted on a new thread.

pungky29 · « **Reply #9 on:** December 02, 2008, 11:11:50 PM »

Quote from: hesgone2fast;47187

Hey guys, I�ve just received my M47 crank from germany. And I�m about to receive a set of 6 US S52 pistons + rods from the states.
As you can guess I�m planning a big stroker (around 2070cc) and very possible in a near future complete the project with a DASC on top of it.
Now, with these pistons and the stock M42 rods I would get a CR of about 12:1, which I�m not comfortable with...I would like to go down to about 9,5:1
I would like to find out what lenght are the S52 rods (US version) I�ve heard 135mm, up here in europe they are 140mm.
Using the S52 rods(assuming they were 135mm...) it would be around 9,5:1
Another posibility and main question of this thread is the M40 rods(318i 8 valve engine)they share the same block and crank, the only difference in the block are the pistons and rods, which are 135mm.
What do you guys think about using these rods with the S52 pistons and M47 crank?I�ve already mounted them with the M42 piston and leaves the piston sitting 5mm down in the bore at TDC.
Any guesses on power output with this setup(2070cc + DASC) is very welcome.

Thanks!!

I know this is an 8 months old thread but still interesting for M42 stroker.
Here I quote the comment from Pete Mc Henry of Precision Performance in Greensboro, North Carolina when I asked through Mike Miller the editor of Bimmer TQA magazine about M42 stroking possibility:

Pete says:

"Using a 135mm rod from the 3.2 US M3 and a 86mm piston from a '95 US M3 (86mm), I would have a 2045cc engine with a 10.2/10.4 compression. Compression distance is 32.8. with the 135 rod you have: 212.15 - 135 = 77.15, minus 44 = 33.15 vs a CD of 32.8 for the 86mm piston leaves you 0.35mm (12 thou) short of the deck. With the 86 bore you use the M-44 head gasket. The early M-42 blocks are heavy; I've got one downstairs that's bored to 86."