I would like to increase my compression to 8.5 from 7.8 . I have a 6 bolt turbo block with stock pistons, stock head gasket, and my head has been machined about .002".
How much do I deck the block? Ten thou??

I am aware of the risks that are involved.

Thanks

hmm I really can't give you an honest answer but why not just spend the money and get 8.5:1 pistons if it's all coming apart and the block is being machined anyways? I'd be afraid if you deck it too much you'll have the valves slap the pistons even with the timing dead on?

I would recommend just swapping in a set of 2g pistons. I dont know that you could reach 8.5:1 just by decking the head down. I do know for a fact that you can deck the head all the way until it touches the valves in the closed position and still run it with no problems. I have done it.

Decking the block costs far less than machining the connecting rods to fit the 2g pistons, and purchasing the pistons themselves. That is something I would like to avoid, but I probably won't be able to. Or maybe I should just run high boost.

High boost > high compression.

8.5:1 is nice on part-throttle operation though. Let me do some calculations and we'll see if we can't get some numbers.

EDIT:
From what I've been able to find 4G63s have no deck clearance at all. That said, milling the deck makes the pistons rise above the deck, pushing into the gasket thickness area!

Can anyone with more engine experience (Kyle? Crankbender?) confirm this?

Personally, I think it'd be much less hassle to go the 2G piston route. Otherwise, while you're in there, you could do a complete engine rebuild with 8.0:1 pistons with SBR's rebuild kit...

RoasT BeeF said:

I have done it.

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Did you machine the head, or the block?

Sure you can get to that compression ratio if you only deck the block .005” and assemble the head without any gasket.
Think pistons if you want higher compression period.

idk i had to have 14tho shaved off my head due to the fact it was warped and i've put 14k on the car w/ no problems. how much do u think i upped mycompression??? (sorry for the jacking but i put in soem useful info)
Ryan

Your not jacking the post. That's totally related info.
I think I have the formula for calculation compression in one of may textbooks.
I can calculated for you. Do you have a Turbo or n/t 4g63?

You said 0.014" right??

Let's run some numbers to figure out how compression changes with decking.

First, the formula for compression ratio is:

(Displacement + TDC Volume)/(TDC Volume)

Displacement is (bore/2)^2*pi times stroke, which in a stock 4G63 is 85mm and 88mm. The resultant displacement is 1.9974 E 6 mm^3, or 1997 cubic centimeters. That is 499356 mm^3 per cylinder.

When you deck a head, or a block, the displacement remains the same, but the volume at TDC decreases. First, let's find the stock volume at TDC: If the stock compression ratio is 7.8:1, then the stock TDC volume is 73435 mm^3.

How much of this do we take off if we deck the head .010? Well, you take a circle with the same radius as the bore, and multiply it by the thickness of the deck taken off.

Pi*(85/2)^2*.254 = 1441 mm^3 taken out of the TDC volume.

So, our new compression ratio is (499356+71993)/(71993), or 7.94:1. Not a huge change, but definately something.

If you wanted the compression go up to 8.5:1, you would need to lower the TDC volume to 66581 mm^3, which means removing about 1.21 mm of material. That's 50 thou.

The moral of the story: Don't EVER use deck height to raise your compression.

In addition, deck height changes WILL throw your cam timing off as well.

kpt4321 said:

In addition, deck height changes WILL throw your cam timing off as well.

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This is very interesting .. both my block and head have been decked. ( head twice ) .. I am running the metal mls headgasket which I figured would still lower my compression a tad. Anyways, my spool up is horrible, so maybe I need some adjustable cam gears because my timing is way off to fix that?