Raise comprssion or not??

[browneskids]

Hi all, just wondering wether a standard 2.0 M10 running on 40DCOEs would benifit from raising the compression WITHOUT fitting a vernior type timing gear to re-correct the valve timing. Has anyone skimmed the head during a rebuild & noticed any difference??

Thanks, Kev.

[TobyB]

Yup, it does help performance,

but it's also worth retiming the cam...

Pistons are really a better way to raise compression, and 10:1's not outrageous.

t

[peteinjp]

If you will be running a higher overlap cam higher compression is a good thing.

[browneskids]

Thanks for the replies. Just weighing up the costs really, the valve stem seals need to be replaced anyway. The car's only done 84000 miles & I don't think the heads ever been off!! I thought I might as well flow the exhaust manifold a bit better & re-face the head. Just wondered how much to skim it by - in thery, 0.7mm skim should raise com. ratio to about 9.2:1. I would like a different cam but the bodywork's already over budget (surprize)!! Maybe re-grind original cam??

[Harv]

Yea, I would not skim the head to get the comp ratio you want. I would go with pistons. I can't remember how much you can skim it but it's not very much. Also, if you do get it resurfaced, DON"T FORGET THE CAM GEAR COVER.

If you have the head off, you could go with some useable used (from someone on the FAQ?) stock size higher compression pistons. Once the head is off, you can pull the pistons with the rods. put on the new pistons on your old rods, and fit iron rings (after you hone the bores with a cylinder hone) and let it rip.

Is this the right way? Not really, but it would work on a budget.

John

[73malaga]

If you need new pistions do it that way.

With 0.5 increase from tii pistons I had to

retard timing some even with super. Not sure

I gained much HP trading compression for timing?

[browneskids]

Cant afford new pistons as well!! This is going way over budget............... LOL

[TobyB]

Then I wouldn't cut the head just for compression. Uncut heads are

getting far too hard to find...

But DO measure the head thickness (129mm is the 'uncut' range)

because you might already be milled a bit.

I run 128mm heads on the racecar (because that's what it came with)

and I find I have to adjust valve timing about 3 degrees...

and that I have to change timing chains more frequently, because I run

out of adjustment range a lot quicker.

hth,

t

[browneskids]

Looking at your avitar, you probably give it a wee bit of a harder time than me!!! LOL