Got a spare 16vt head lying around, so thinking of skimming the head a bit to increase the compression ratio from the lazy 8.0 : 1 to a slightly higher ratio, just wondering what is the maximum tolerance of the head available for skimming?

Everything else is fairly standard, keeping the boost at 1.0~1.1 bar max via mechanical PRV, the car is chipped with fairly moderate setting, nothing aggressive like the Evo chip. Will be using 98 octane fuel all the time.

In answer to your question it comes down to how much clearance there will be between the valves and pistons at TDC. Guy croft will know a figure for general assembly but to purposly increase the CR that way perhaps a little risky. Any with std cam pullies you will send the timing out also if you remove any significant amount from the head. I wouldn't bother if i were you.

hadn't actually realise that tbh.
Rich, how much is the "significant amount" though???
0.3mm or more? (sorry not good with these thousands of an inch...)

V.

I was only thinking about skimming by 1.0~1.5mm, however I don't know how to calculate how much of compression would it increase. I would have thought this should be well within the design tolerance of the pulley and tensioner.

Vs = Volume Swept
Vc = Volume Clearance
CR = Compression Ratio

CR = (Vs + VC) / VC

For example 20VT
400 + 53.3 = 453.3 / 53.3 = 8.5 CR

By using the formula backwards you can set yourself a target CR then work out what the VC needs to be thus working out exactly how much is needed to be skimmed.

Vc = Vs / (V1-V2)

For example I want 9:1 on 20VT so

V1 = 9
V2 = 1

Vc = 400 / 8 = 50

So to achieve 9:1 I need a Vc of 50cm3 so i would need to remove 3.3cm3 from each chamber. Don't forget to consider headgasket thickness and dome and vlave pockets on pistons and bore size if re-boring. Make sure you properly CC the head after skimming to be sure you are at the correct Vc.

Hope this helps and good luck.

ok, you're not understanding the relation between timing based on pulley to pulley distance, this has nothing to do with tension, yes the tensioner will take the slack out of the belt. But the relation has changed thus the timing will also, trust me I didn't write that because I was bored.

1-1.5 mm is huge 40-60 thou, you will definitely run into problems.

Oh hell! Going to skim around 0.7mm instead, do have a spare head, in fact do have a complete spare engine available just in case if things go horribly wrong!

On some heads there are also problems with skimming associated with widening of oil or waterway apertures into the gasket area as well.
On the 20VT block there is a problem decking it too much as there are a couple of oilways that then will sit under the gasket and cause it to fail prematurely. Really should do any change in CR with change in piston design IMHO..

Cheers, thanks everyone for the info here. Changing the piston is indeed the best way to change the C/R without distributing the overall timing, however as I have a friendly machine shop that I can do CNC stuff for minimal cost, while this spare head + block has been lying around idle for a while, I would go ahead & take my chances.

Seriously mate. From experience, messing around with the head to change CR is the perfect route to destroying your engine through detonation. New pistons or don't do it!!

can you please elaborate how a high comp piston (say 8.5:1) is so much better than a thinner head gasket and a bit of skimming?

Detonation is potentially the same sort of threat you'd have on any badly tuned engine.
Am I missing something?

V.

Because you can design the pistons to clear the valves radially and longitudinally. But of course detonation being what it is could occur and be more likely with no other changes to the engine calibration.

The combustion chamber in an engine is designed to have an approx. 1cm thick, washer-shaped volume around the rim. This is an extremely important design feature to prevent detonation. Anything that changes this "squish volume" is a very bad idea. As one of my tuning books says, "consider the squish volume sacred and do not tamper!"

Ever looked at an aftermarket high/low comp piston? Manufactures could just make it thicker or thinner to change the comp. But they are always flat around the rim and either dished or raised in the middle. This is to keep the squish volume the same.

'Fraid the only truly safe way do to it is to buy new pistons!