Running a thicker head gasket to drop compression, Good idea? or not?

[e92livin]

My car is currently at BMW getting rod bearings done, I have the opportunity to run a thicker head gasket. How thick can the gasket be before compression drops to much? I have been looking for some info on this and can't seem to find the answers I am looking for.

[kawasaki00]

If you ran a head gasket that was .080 instead of .040 on a 92mm bore it would drop it from 11.9 to about 10.5 (assuming the stock gasket is .040). In other words doubling the stock gasket is about 1.4 Ratio lower
Be carefull running too thick of a gasket as it changed the center line of the crank/cams though.

[e92livin]

Originally Posted by kawasaki00

If you ran a head gasket that was .080 instead of .040 on a 92mm bore it would drop it from 11.9 to about 10.5 (assuming the stock gasket is .040). In other words doubling the stock gasket is about 1.4 Ratio lower
Be carefull running too thick of a gasket as it changed the center line of the crank/cams though.

Thanks, I will keep that in mind.

[Patrón]

why would you want to change the cr, unless you are going fi?

[s85e90]

Originally Posted by Patrón

why would you want to change the cr, unless you are going fi?

did you read his sig?

OP no offense but if you have to ask then this may be over your head.. That being said, I would see no issues doing it properly as long as it isn't over kil (i.e. too thick) Lowering the CR to 10.5.1 would be mated nice to a blower @ 15 lbs

[MilehighM3]

Also note there no overlapping labor involved between rod bearings and head gaskets. Short of an oil change.

[e92livin]

Originally Posted by s85e90

did you read his sig?

OP no offense but if you have to ask then this may be over your head.. That being said, I would see no issues doing it properly as long as it isn't over kil (i.e. too thick) Lowering the CR to 10.5.1 would be mated nice to a blower @ 15 lbs

I asked to see if anyone has done this before on this motor. I know dropping from 12:1 to 10:5 with just a gasket isn't a good idea. You could mess up the clearances and it wouldn't run the way it should after. If that were the case and it was easy as you make it sound, than I'm pretty sure no body would
be doing builds and dropping compression with low comp. pistons and such.

[e92livin]

Originally Posted by MilehighM3

Also note there no overlapping labor involved between rod bearings and head gaskets. Short of an oil change.

When BMW does it, they pull the whole motor out of the car. Instead of dropping the oil pan, subframe and oil pump.

[pbonsalb]

Many of us with E36M3 have dropped CR with a thicker HG or HG and spacer. Cometic makes HG for the M50/M52/S50/S52 motors in varying thicknesses. Works great for FI. I have run 0.080, 0.098 and 0.140 on my 99M3, for example, with superchargers and turbos, though I now run a built motor with LC pistons. Not sure what is available for the S65, and having two heads with variable valve timing on each cam in each head would make the labor more involved. I eventually built a motor because I wanted stronger rods to hold the torque of a turbo, which has the potential to be much higher than a centrifugal.

What you do not depends on your longer term plans for the car. I have always found that I get used to boost and want more. This has always happened sooner with centrifugals because the power curve is so predictable you get used to it and it is no longer as exciting. If no one has used a thicker HG on the S65, that makes you a pioneer, and pioneering is often expensive and troublesome. I prefer to follow the beaten path. That way I spend more time driving and less time on the project. Project creep can be deadly. I have read about people who have their car off the road for a year while their project meanders all over the place. It is even worse to me if the person is paying someone else to work on the car while it sits there month after month waiting for this part or for that problem to be sorted out. I do almost all my own work (its the only way I can afford to mod the car).

[Patrón]

Originally Posted by s85e90

did you read his sig?

OP no offense but if you have to ask then this may be over your head.. That being said, I would see no issues doing it properly as long as it isn't over kil (i.e. too thick) Lowering the CR to 10.5.1 would be mated nice to a blower @ 15 lbs

No.

[MilehighM3]

Originally Posted by e92livin

When BMW does it, they pull the whole motor out of the car. Instead of dropping the oil pan, subframe and oil pump.

Do you have supporting evidence for this claim?