Piston : Head Clearance

[Dale FZ1]

What is the typical piston : head clearance targeted by any of you RB30 builders using a head with the quench retained?

I've measured my 25DET bottom end, found the lower flat areas are proud of the block by .011" @ TDC. I'm intent on retaining a similar relationship between piston and quench in specifying how much to deck the RB30 block.

Discussing this point with a mate, he's suggested a minimum clearance of 1.00mm to allow for rod stretch, and carbon buildup. His view makes sense. If I specify .015" decking, this would yield 0.97mm clearance when using a 1.3mm HG.

Any ideas / experiences / warnings?

[4drfts]

my opinion is it doesnt really matter, quench is the enemy. in a turbo engine at higher boosts quench is the cause of pre-ignition. the best thing to o is remove the quench, otherwise take the sharp edges off. although it is common knowledge that quench helps N/A engines, there is ongoing debate as to the best solution on turbocharged engines, and it depends on boost levels, fuel type etc. the easiest solution is to reduce the effect of quech as much as possible. just my opinion.

[Dale FZ1]

It'd be nice to have that as a viable option and not spend a lot of $$ achieving it. The increase in combustion chamber volume would require a corresponding increase in piston dome volume to maintain static CR. I suppose forgies are available in that configuration.

Going the budget route of stock 30E type pistons, limited boost (1 bar) and rpm should see a healthy result but means retaining the quench to even keep CR ~ 8.3:1.

Any comments from those who have done the build and know their desired clearances? At this stage the minimum of 1.00mm is still winning out.

[Adriano]

You can go less than 1mm, i usually work at a minimum of 30 thou(.75mm). 2 engine builders i have spoken to have run 25 thou, but insist thats the absolute minimum. IMO the smaller the squish clearance the better, as it improves combustion turbulance, flame propogation, emmisions torque and engine efficiency, remember these are street cars.

[4drfts]

your right, adjusting the shape of combustions chambers can be a costly exercise. but if the only way you are adjusting quench is by decking the head/block or using a gasket, then you are directly going to alter the CR anyway. i think really you need to work out what CR you want, and whatever quech that gives you, you want be able to alter. I would agree with adriano though, in your case you would want minimum squish, as you are not running high boost and it is a street car. also, at you boost levels you could safely run 9:1 compression ratio so i wouldnt worry about CR going too high as a result of lowering squish.

Edited October 2, 2008 by 4drfts