steven74

I am rebuilding my 1986 turbo engine as a hybrid and have received it back from the machine shop. After offset grinding the rod bearings and sleeving and decking the block, I ended up with the pistons at 0.070” below the deck. When I ran the numbers in a compression calculator, it resulted in a calculated compression of only 7.32:1 . This is much lower than anticipated and to get to about 8:1 as expected , I would have to deck the block 0.050” more to get 7.97:1. I think this amount of decking may be too much and would require an offset cam key and possible tensioner issues with the timing belt and would like to avoid that.
Would 7.32:1 be too low for a street engine? I was hoping to run boost around 16-17psi based on my turbo efficiency. Please advise. About every 0.010” decking raises about .13 point of compression in this setup.

gruhsy

What gasket thickness were you calculating with?

Not sure what Cometic's thinnest gasket is for out engines?

steven74

Cometic mls 103mm bore 0.040”

V2Rocket

it might feel a little "soft" off boost but not undrivably so.
advancing the camshaft and playing with ignition timing will help a lot.

the original porsche 911 turbo (930) had like 6.5:1 compression, lol.

on the plus side you could probably run 87 octane, LOL

steven74

I was thinking the same. I may ask about re-decking the block about .020 to .030 to raise some compression. I was actually hoping to have more power off boost since my turbo doesn't get into the powerband until 3000 rpm or more.

V2Rocket

Were it mine, I'd be maybe a little disappointed because it came out lower than my target, but I'd run it.

However, how did your compression end up lower than Shawn's, with less deck clearance? Different pistons/stroke ?

steven74

I did have less deck clearance than Shawn's build in the end. He had 0.012 below deck and I have 0.070 below deck so 0.058 less than him. I figured out 0.050 more decking would net me a 7.97:1 static compression .

steven74

I spoke to the machine shop and he said it was only decked 0. 010" after the sleeves were installed. I asked him and he said he would suggest more decking to take up the quench area for better combustion.
Can a 944 block be decked 0.060" overall?

I may need an offset cam key and have a manual tensioner on my 1986 block for belt tension.

Tom M'Guinn

I'd run it as is myself. 7.3 is not "that" low for an old-school turbo car like this and will give you and edge against pinging, etc. Once you build up a little carbon you'll be close to 8 anyway My motor came out with higher than expected, close to 9, and I'd take 7.3 over that any day. You can also probably skim the head a little extra to reduce the combustion chamber size, which will sneak the CR up a pinch...

V2Rocket

Can you please post all your engine data to double check the CR calculation?
Bore, stroke, piston cc, gasket thickness, deck clearance, approx. head CC (8v 944 should be 56-60cc depending)

steven74

Sometimes it's good to be a moron. Maybe if I properly torque the girdle before measuring next time, I would get the correct measurement
Using torque crank and rods with starrett straight edge and feeler gauges, I got 0.032 negative deck clearance. Per Summit racing compression calculator, this is 7.8:1 now.