CCed a 3100 piston, now im lost on compression

[SappySE107]

Stock heads are 28.2cc, stock 3100 piston is 28.6cc. Plugged these into a calcluator with a 1.5mm gasket thickness and it spit out 7.x:1 compression. So...um...wtf?

Anyone else measured a piston or head? Id like to compare but something really isn't adding up for me unless the piston sits higher than the block, which I can't check as I dont have an assembled block to look at.

[SappySE107]

RE: CCed a 3100 piston, now im lost on compression

Ok, i used a different site http://www.csgnetwork.com/compcalc.html and it spit out 9.598628100327078:1 comression. Finally, something that works!

I used these values. 2, 89, 84, 97.4, .5334, 28.2, -28.6, 0.

Are we good to go on these now?

[Rhedalert]

RE: CCed a 3100 piston, now im lost on compression

[Doubt_Incarnate]

i beive the piston sits out of the block something like .014 or .016". i tried that link and dd2k with a .016" and they gave me 9:1. looks like GM likes to lie about stuff. i've always read the 3100 piston had like 25cc dish or so, but if you cc'd one, i'll take your number over what gm says.

[Guest]

Yea I always heard like 26 cc for the piston. I think the aftermarket ones state they are 26. I always thgouht the heads were 26 too. Can manufacturing tollerances vary 2 cc's from head to head so that someone can get 28 and someone else can get 26?

[SappySE107]

Well, i had over 1cc difference between chambers. Ive got plenty of stock heads sitting around so ill be getting the numbers on all of them eventually. I have a set of 3.1 heads as well, which I think I read are supposed to be 26.4cc.

I really need to know how far above or below the deck these pistons come though. I measured a 3.4 DOHC gasket for the bore diameter as well.

[v6h.o.]

Gen 3 pistons are .020 above deck height yes.

[XLR8ING]

According to an ancient scroll I dug up from some ruins, the great philosopher "GPXSS" once said
"The stock GM 3100 SFI pistons are -0.02125 deck height.....dumbass!"

[SappySE107]

hahaha!

From what I remember, mick was saying there is .030 quench distance, which would put the piston about .020 above the deck. But Chris measured it below the deck. I need a bridge for my dial indicator and a 3100 block:P

[XLR8ING]

Actually, -0.02125 means it comes out of the bore that far. You use the negative number to calculate the compression. Below the bore would be a positive number on the calculations.
Sorry, I should have clarified.

[SappySE107]

ok cool, well then, that settles that part Now we need a 3100 gasket bore diameter.

[bszopi]

I might be able to help with that, since I have 2 3100s in the basement.

[torq455]

I have a set of 3400 heads that Ben p&p'd cc'd. they were at 29cc's.

[RacerX11]

I measured a 2.8, 3.1, and 3100 gaskets (some OEM, some aftermarket). They all measured ~3.69". The 3.4/3400 gaskets measure ~3.75".

Marty

[SappySE107]

Awesome Guess we have all our measurements for compression now.