Compression ratio..shaved head m42

[projekt EPiK]

1995 Ti, m42 overheated and I rebuilt the top end. while the head was off I sent it to the machine shop and it was pretty twisty. the shop took off .020" to clean it up and give me a little extra bump in compression ratio.

my question is, how do I calculate the approximate new compression ratio? stock replacement headgasket, .020" off a virgin head.. I'm thinking it may have been a decent bump after all.

I was running 89 octane before the radiator blew out and overheated the car and the car ran well on that. Since the shave and new hg it has been not so good, I did use a can of Seafoam and it didn't smoke much at all tho.. It just seemed sluggish and ran out of breath it seemed.

So for the hell of it, I pumped in 93 tonight and it runs great now.. pulls smoothly and pretty hard all the way to redline. I'm guessing that the car has been running pretty lean on the 89 since the headswap.

well I guess its premium from now on, unless the sluggishness comes back again.

CLIFFS*** How to determine new CR on m42 head shaved .020"

[jca]

Welcome to the world of premium grade fuel. My m44 head is stock, but the engine runs noticeably stronger on high octane gas. Sorry that I can't help you with your question. John

[xxxJohnBoyxxx]

Higher compression requires higher octane fuel. If you run low grade fuel the knock sensors pick up a ping and retard the timing = less power. 93 octane allows the motor to run without ping so you get full advance on timing and a stronger better running motor. Also 93 octane should give you better gas mileage and make up for the extra cost to run the good gas

[projekt EPiK]

I know how octane affects fuel tuning, slower burn..etc.. what I dont know is the compression ratio on my engine now.

[Marv17]

Could you not just do a compression test on all the cylinders and use those numbers to calculate the ratio? Or is it not as simple as that?


Sent from my iPhone using Tapatalk

[projekt EPiK]

No a compression test won't give me the answer I'm looking for.

[xxxJohnBoyxxx]

Correct you won't get compression ratio with a compression test. 0.020 Milled off will not change much with compression numbers. Stock is 10:1 I believe and 0.020 is a minor mill job to remove minor warp or very minor damage around the water jacket transfer holes. If you were at 150psi per cylinder you might get an extra 5psi maybe 10psi at the most.

This is the gas I run in my motors. When I had my stock 318 M4x compression at 150psi 93 premium or never lower then 89 mid-grade. Race car 11:1 compression @180psi airport fuel 100LL 50/50 mix with 93 octane. Kawasaki Factory race motor 220psi compression VP110 Purple. Love the smell of the purple race fuel. M4x stroker FI motor with 8.5:1 compression @ 110psi. 93 ocrane boosting to 15psi. From 15psi to 28psi I shoot Meth with a progressive controller. Meth is like 138 octane

[projekt EPiK]

Just wondering because on my 944 na 8v a .020" mill will bump from 10.2 up to 10.6:1
I always used 89 prior to this in my ti..now just 93...a few cents difference is worth the safety and smooth running

[xxxJohnBoyxxx]

Originally Posted by projekt EPiK

Just wondering because on my 944 na 8v a .020" mill will bump from 10.2 up to 10.6:1
I always used 89 prior to this in my ti..now just 93...a few cents difference is worth the safety and smooth running

It really depends how big the combustion chamber is on the head. Small combustion chamber obviously will be more sensitive to milling and net a higher ratio. I know nothing about Porsche motors but I do consider the M42 head as having a large combustion chamber. If you want the ratio you will need to take the head and cc it to find out it's volume then piston size and stoke and you get your compression ratio.

I think compression numbers are more important when choosing what gas to run. Just my opinion.