How much is too much?

[Subareubs]

Hey all,

Its been a long time since I last posted - had to sell the RSB but have now bought a 1998 Lancaster with the EJ254. I bought it with the blown head gasket fairly cheaply with 124,000km on the clock and have just got the engine and heads back from the engineers.

They say they took 10 thou (0.254mm) off the block surface (might have been total from upper and lower, not sure) and having breezed through the manual it says the grinding limit for the block is 0.1mm. When the pistons are at the top of their stroke there is about 0.3mm between the to of the piston and the block surface.

Now here is the tricky question:

Assuming that they did take 0.254mm off the block surfaces, is this going to cause problems?

I know that its going to bump the compression up nicely (do I need to worry about anything here other than bumping up the octane I feed it?) but I was concerned about whether there will be any interference with valves etc...

Am I worrying too much about nothing?

Your thoughts would be greatly appreciated!

Cheers

[boon]

I would say run a thicker head gasket to bring things back to nominal values. You can't just make an arbitrary change to the CR and hope that running higher octane will save it... for all you know it's going to knock like buggery. If you had a tuneable ECU you could potentially adjust the timing to compensate but I wouldn't just leave it to the factory knock detection to deal with.

[Rosssub]

Using my own engine specs:

92.25mm bore

75mm stroke

48.8cc combustion chamber (version 6 STI heads/machined)

17cc piston dish

-0.5mm clearance to deck/CTD (piston head above block 0.5mm at TDC)

93.3mm HG bore diameter

1.37mm HG thickness crushed

I have CR - 7.979:1, with a 0.87mm quench.

Or using Summit Racings compression ratio calculator I get 8.05:1.

http://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator

If I take 0.254mm off the block decks, taking the CTD to -0.754 (pistons now 0.754 above deck at TDC). It raises my CR to 8.22:1 and lowers my quench gap from 0.87mm to 0.616mm.

So as Boon said, changing to a thicker HG. Going from 1.37mm HG up to a thicker 1.5mm HG, the CR goes back down 8.05:1 and quench gap back up to 0.746.