shaving 20V heads?

[Sam_Q]

I would like to increase the compression of my silvertop 20V. I already have a 0.8mm TRD gasket which if my calculations are correct change my compression ratio from 10.5:1 to 10.8:1. I would like to know if it's ok to shave 20V heads? I understand that it will mess with my squish areas but doesnt any other way of increasing the compression also do that? I worked out that 0.5mm would increase it to 11.6:1, is this a good idea? and is it a good compression ratio for this engine in terms of power and fuel efficiency?

[RWDboy]

Really with any N/A car i usually come to the conclusion to either get high compression pistons, or leave the compression ratio standard.

Shaving heads/thinner gaskets will alter your camshaft timing (usually it retards the timing! as the path from the crank pulley to the tensioner to the cam sprocket needs to be lengthened to make up for the shorter 'other side') this can cause poor performance - especially given that raising compression ratio is a case of 'diminishing returns' (a rise of 9.0:1 to 10.0:1 gives something like 7 or 8% increase approx).

I can't remember the bore of yer motor, but 0.5mm will give you around about 3.3cc less combustion chamber. So for a 400cc engine (1 piston of your motor) if you had 10.8:1 compression you will go up to 11.8ish

[Sam_Q]

so basicly with a thinner head gasket its worth it but anything after that will not be worth it because the retarded cam timing will offset the power and economy gained from the higher CR?

[RWDboy]

Well you wouldn't know until you put it on a dyno, but yeah I would assume as much

If you stuck some vernier adjustable cam gears on you'd have no worries though.

Problem is that it's usually a reasonably small amount of retardation, and cam timing can have a huge effect on power. You could be 5-10degrees retarded, but then even if you slipped the belt over a single tooth to try and compensate, you'd probably find yourself 5-10degrees too advanced again!

I have never touched a 4A-GE though - so my word isn't gospel.

[Sam_Q]

well I did some simple maths and I found that shaving the head to rais it to this compresion would retard the cams by less than a single degree, now I couldnt imagine that affecting anything much. I will very likely make my own Vernia cam wheel for the exhuast later on anyway.