Author Topic: compression ratio for turbo (Read 7819 times)

Romrog318is · « **on:** June 27, 2009, 07:11:14 PM »

what do i need to lower my compression ratio to run 12lbs of boost? ive been told a 9:1 CR will work pretty good for this boost im just wondering what i need to do to make it lower? how much do i need to take off the pistons or how much thicker of a head gasket will i need?

bearsbmw · « **Reply #1 on:** June 27, 2009, 07:28:51 PM »

you can make 12lbs of boost on stock m42 internals without problems.. At around 14-15 psi the head starts to lift and you will push the head gasket... ARP or Metric blue Head studs can solve this for around 15psi

Romrog318is · « **Reply #2 on:** June 27, 2009, 07:34:18 PM »

i know that but i want to make it lower for when i get tired of my 14psi for like 18 then 21 later on its all about tunning once u have lowered the compression its my daily driver so i want to make sure i will not hav aany BIG problems once its done... it also depends on the octane rating of the gas... but do you kno what i need to lower it to? and how much i need to take off the pistons or how much of a thicker head gaket

Boosted E30 · « **Reply #3 on:** June 28, 2009, 12:11:47 AM »

on my car i have 8.5:1cr and it works great and if you think 14psi will get boring ive been running my car on 7psi and its hella fun for around the city you dont need more....more is more fun but where are you gonna do most of your driving???track or street??

Romrog318is · « **Reply #4 on:** June 28, 2009, 10:38:50 AM »

well on my way to work is like 5 miles of curvy roads then 7 or 8 miles of highway. and plus i wanna be able to keep up with my friend in his s52 swap haha
but if ur running an 8.5:1 how did u get it that low?

e30guydownunder · « **Reply #5 on:** June 28, 2009, 04:54:14 PM »

the bottom end is forged, so once you get some forged pistons you are set. If your getting pistons you can get whatever comp ratio you want. I'm personally going to be running stock internals with a MLS headgasket, Head Studs and E85 by the end of the year.

Romrog318is · « **Reply #6 on:** June 28, 2009, 05:16:33 PM »

im going to keep all stock internals i just need to find out gasket thickness..... and these motors heads have great flow along with the intake manifold which is great for turbos! the rods are also very strong in these cars!!

Boosted E30 · « **Reply #7 on:** June 30, 2009, 09:38:36 PM »

Do not go the route of a super thick gasket to lower c.r its sucks and will not work ive tried many times its just better to get pistons mls gasket head studs and it will be worrie free.........sorta.

Deftones · « **Reply #8 on:** July 01, 2009, 12:42:57 PM »

i read from some forums that pistons and rods are very good, but the big problem is with the main bearings which are only 180�and can melt down... is this real? how did you solve this problem?

nuvolarossa · « **Reply #9 on:** July 01, 2009, 02:27:03 PM »

I read something about bearings only in really high powerful engines, over 300hp... some "drag" turbo M42 use diesel main bearings.

Romrog318is · « **Reply #10 on:** July 01, 2009, 05:03:57 PM »

Yeah the main bearings are generally good for around 220 whp its when you go for REALLY HIGH boost that you runs into those major problems... and thanx on the gasket idea
what is a good website for pistons?

ApocolypseAutoEngineering · « **Reply #11 on:** July 02, 2009, 04:27:32 AM »

If your Going to get pistons you might as well change the bearings for good measure.

Boosted E30 · « **Reply #12 on:** July 02, 2009, 07:35:31 PM »

i got mine from vacmotorsports.com i didnt change the bearings or rods just pistons, mls, and studs but then again im happy with 10psi its safe and with a good tune i made 232whp which in a small lite car is more then enough for around the city.

bearsbmw · « **Reply #13 on:** July 02, 2009, 08:42:56 PM »

Quote from: Boosted E30;74720

i got mine from vacmotorsports.com i didnt change the bearings or rods just pistons, mls, and studs but then again im happy with 10psi its safe and with a good tune i made 232whp which in a small lite car is more then enough for around the city.

that is good numbers for those changes ... what are specs for gasket and pistons?

Boosted E30 · « **Reply #14 on:** July 04, 2009, 01:00:40 PM »

gasket is stock thickness and pistons are 85mm 8.5:1cr