C/R on a 1.8L bored .040 over?

Post by **Rennwagen1** » Dec Fri 15, 2006 9:06 am

Hi!

I have found a super cheap .040 oversized pistons/rings/pins set for my 84 GTI, and I was just wondering if anyone knew how much the C/R would be affected. I am looking to raise it a bit(not much maybe .5-1.0) and I was wondering if I would need to deck the block, or what? I do not want to lose any compression, for sure, just keep it the same, or maybe increase it a bit. ANyone have any experience with this? I have a few people I can call, but I figured that I would ask you guys first! Thanks!

Post by **ValveFloat** » Dec Fri 15, 2006 9:55 am

I'll take a shot at this.
I am making a bunch of assumptions, but here we go:
stock bore: 81mm
Stock stroke: 86.4mm
Cylinder volume: 445219mm^3 or .445 liter.

Bored bore: 82mm (.04" = 1mm)
Stroke: 86.4mm
bored cylinder volume: 456280mm^3 or .456 liter

Stock CR is 8.5:1?

.445L/Combustion Chamber Volume = 8.5
CCV = .445/8.5 = .052L

Bored CR = .456/.052 = 8.7

Post by **Rennwagen1** » Dec Fri 15, 2006 11:38 am

okay, well you are far too smart for your own good. SO buy that calc, I would actually be raising the C/R fromthe base line by about .2? So then if I were to deah the blocka abit, that would only help it further? Hmmmm I am beginning to think I might get my snorty little 8V going faster than I thought.....

Post by **benny_mech** » Dec Fri 15, 2006 4:31 pm

Hey Aeron,
I fired off an e-mail to you about this, but ValveFloat's math looks good to me. There's a lot of other considerations when decking the block or head to bump compression. Any meaningful bump in CR means you'll be out of spec for machining, and you'll have to do some valve to piston clearance checking.

Post by **Rennwagen1** » Dec Fri 15, 2006 5:58 pm

Thanks for the info and hello from the Big Sky!

Post by **Custom VW** » Dec Wed 20, 2006 12:56 pm

I love it when people read....... correct formula's make a big difference in the world. Valve clearance isn't a big issue on that block, so your GTG there, just don't ever go past 11:1.

My little 2.0L 8v, is currently at 11:1, and down from 11.5:1. at 11.5:1 it was hard to keep it streetable on pump fuel. It's still hard, but seems to be at the limit and stable.

Post by **Rennwagen1** » Dec Wed 20, 2006 5:36 pm

so are you saying that the formula above is correct, or not? ANd how is your 8V on power, I bet it is fun! Running premium fuel in it?

Post by **Custom VW** » Dec Wed 20, 2006 8:29 pm

CR = SV+CCV+HGV+DHV+EDV
CCV+HGV+DHV+EDV

Where CR = compression ratio
CCV = combustion chamber volume
HGV = head gasket volume
DHV = deck height volume
EDV = effective dome volume
SV = swept volume

Post by **Custom VW** » Dec Wed 20, 2006 8:32 pm

http://www.imperialclub.com/Repair/Engi ... ession.htm

*Big Sky Euro

C/R on a 1.8L bored .040 over?

C/R on a 1.8L bored .040 over?

Git on it!