Author Topic: Compression Adjustment Help (Read 2663 times)

flyingbrickperformance · « **on:** March 26, 2011, 08:14:00 PM »

So I've been spending some time this weekend looking at various part combinations for my engine and need some help with compression calculations.

My proposed engine uses the M44 crank, S52 pistons, and M42/44 rods. If my calculations are correct, shaving the pistons by 1.7mm puts the compression around the original M44/42 at 10:1. Since I'm supercharging this I'd like to lower it to about 9:1.

How much more would I need to take off the pistons to get compression closer to 9:1? Should I shoot for something lower, in which case I could drop down to the 135mm rods and probably land right where I need to be?

flyingbrickperformance · « **Reply #1 on:** March 27, 2011, 01:19:23 PM »

Okay, so I just measured my parts and it turns out I don't have S52 pistons, I have the S50 - which have a higher compression height and requires I shave them closer to 3.65mm - not sure I want to go that route. My only other option is to then go back to the M42 crank to get the shaving back down to around 1.15mm.

Still need the amount of shaving required to drop compression from 10:1 to 9:1 - will make a decision on parts once someone can help me with this.

DesktopDave · « **Reply #2 on:** March 27, 2011, 05:51:30 PM »

Wish I could help, maybe someone's done it before. Glad you're posting this info for future builds though.

pdxmotorhead · « **Reply #3 on:** March 27, 2011, 10:56:22 PM »

I would not shave piston tops in a boosted motor... At least not that much... Your upper ring will be getting awful close to the piston top.

There is a point where its cheaper to pay someone who does this kind of work to do the assembly. Not necessarily all of it but if you build a good relationship with a race engine builder in your local it might be useful.. I'd have Wiseco or JE make a set before I'd shave them down. It is also possible to have the crank Offset ground to destroke a MM or two to drop compression. Cam timing can also help.

Dave

a930rocket · « **Reply #4 on:** March 28, 2011, 10:50:46 AM »

I'm not familiar with turbo or supercharging a BMW, but with the 911 Turbo, going from 7.5:1 to 8:1 gave a good bump in compression for lower end grunt, but without the dangers of detonation and runnng 100 octane at all times. A BIG intercooler was mandatory.

YMMV.

flyingbrickperformance · « **Reply #5 on:** March 28, 2011, 09:30:58 PM »

The shaving I'm talking about is pretty minimal - less than 1/16th of an inch with even less from the center. I can't justify spending $700+ on pistons for a project car that in total is targeting no more than $2012. The engine work alone to bore the block is already annoying me - tempted to do that myself as well.

I'm leaning towards a more stock compression ratio and just running race fuel - it is after all a track car.