Compression Ratios

pablo · Post by **pablo** » Sat Nov 22, 2008 6:55 am

How do you work out the compression ratio of an engine. Need to know what ours is hopefully before we remove it from the car

Area 52 Autosport · Post by **Area 52 Autosport** » Sat Nov 22, 2008 10:05 am

You need to know the stroke, bore, piston deck height, piston dish cc, piston volume above top ring, head gasket thickness, head gasket bore and head cc.

Then,

(Cylinder volume + head volume + piston volume + clearance volume + gasket volume) / (Head volume + piston volume + clearance volume + gasket volume) = Compression Ratio.

Jase

LR6GSR · Post by **LR6GSR** » Sat Nov 22, 2008 2:23 pm

A usefull tool on JMR web site a VW tuner but the calculator might help.
http://www.johnmaherracing.co.uk/toolbox.htm

Liam

Gerry H · Post by **Gerry H** » Sat Nov 22, 2008 3:00 pm

To measure the volume of the piston dish and head volumes, you'll need a graduated pipette or burette like this but it seems expensive for what it is:

http://cgi.ebay.co.uk/Burette-compressi ... 240%3A1318

Using a thin oil in the burette, start off with a known level of oil and allow it to run into the head chamber until it fills it. You can then read off the amount of oil it's taken to fill the space and there's your volume. Valves and plug in of course. If you need to do any work on the head you can use candle wax pr gease to seal the valves rather than fit springs, etc.

That may be crap but iirc it's how I did it many moons ago.

Gerry H · Post by **Gerry H** » Sat Nov 22, 2008 4:02 pm

I also remember using a flat piece of perspex sheet, big enough to cover the chamber, with a hole to feed the oil through. The perspex lets you see the oil has filled the chamber with no air bubbles and no surface tension to worry about.

Something like that anyway iirc.

pablo · Post by **pablo** » Sat Nov 22, 2008 7:21 pm

Wow

....Thats a bit more indepth than I thought it would be.

thanks for the info and help gents most useful