piston compression ratio []=o

[gerknme]

ok, im not sure how advanced this is but here gose. how do you find out the compression ratio of a piston? what equation would you use?

[Nick 92 TSi AWD]

You tell the person who is selling you the pistons what compression ratio you want.

[gerknme]

oooohhhhhh thats how it works!!! really thought, that cant be the ONLY way you find out.

[99gst]

You would compare the volume of the cylinder when the piston is at bottom dead center (BTC) to the volume when the piston is at top dead center (TDC). 2G compression ratio is 8.5 to 1, so the volume at BTC is 8.5 times the volume at TDC.

How could you figure this out? I have no idea. Car manufacturers publish the compression ratio of their cars so you could find yours out that way. If your ordering pistons its like Nick said, whatever you order.

[Rdy2race]

to deep to answer on the net with people working and typing :D

In short you need to know
bore
stroke
CCof the head chamber
height of the head gasket

this will allow you to figure out what it is but I do not have the math book in fromt of me that shows how to figure this out. Your best bet is to get a book on things like this. The two books to the left of your screen are VERY good and I have both ( had them for domw time now.) they are well writen and understandable.

Good luck
BM-

[99gst]

Originally posted by Rdy2race
to deep to answer on the net with people working and typing :D

In short you need to know
bore
stroke
CCof the head chamber
height of the head gasket

this will allow you to figure out what it is but I do not have the math book in fromt of me that shows how to figure this out. Your best bet is to get a book on things like this. The two books to the left of your screen are VERY good and I have both ( had them for domw time now.) they are well writen and understandable.

Good luck
BM-

Hey, I was at work when I replied to this. There hasn't been much to do there lately. However, your answer is a good start but it does not take into account whether the piston is dished or domed.

[AWDDOHC]

If you are buying pistons, I am assuming you are having machine work done. Save yourself some risks, and order all of your parts through the machine shop.

They figure that kind of thing all the time and will be able to figure it out perfectly.

If your head is milled, then you have to compensate for that on the cc's of the combustion chamber.

If your block is resurfaced, you have to compensate for that.

If your bloock is bored out, another thing to be compensated for.

If you relocate the ring pack, thats another.

The heads are commonly measured by dripping water 1cc at a time until the chamber is full- the rest of it I'm not sure.

Basically, you could save yourself a lot of trouble, and room for error by letting a proffesional order the parts for you. I know its against the DIY charter, but if you're shooting for a specific CR, its best to have them do it.

Make sure you go to a performance oriented shop, though. The monkeys that do straight rebuilds usually don't have the tools (or brains) to do this kind of stuff.

[gerknme]

k, my problem is that i ordered pistons from my freind. he has a hook up at je so i gave him the ok to get them. when igot them he said they were 9.5/1, but when i opend the box the paper work said it was shelf stock. i heard somewere that je's shelf stock was 9/1. can anyone confurm that info? if i call je and give them the specs on the piston, wouldnt they be able to tell me the compression ratio? the ratio is not on the paper.

[AWDDOHC]

ok, it does not matter what the paper says, because all of the variable I listed, they could not have known.

I have no idea how to calculate the CR with taht info, a good performance shop could do it.

BTW- tell your buddy to hurry up with my piston! I need my dimesnsion so I can order my rods and get the rest of them made!