Is there a way to determine the compression ratio of a motor when you don't know any of the mechanical specifications?

My answer is "Nope", because you don't know the actual cylinder bore, the actual piston size/configuration, actual piston rod length, the actual valve chamber displacement, etc., etc., etc. However, there is a remote possibility. If you have the block number AND the assembly code you can determine what the motor was built to be - including the ORIGINAL compression ratio. If you know for sure that the engine has NEVER been touched, then the original ratio will be your current ratio.

If you don't have any of that data, then incense and a OUIJA board are probably your best bet ...now that the "Psychic Friends Network" is out of business.

John

P.S. ...and if they really were psychic, why didn't they know they were going bankrupt?

|UPDATED|4/29/2004 10:03:16 PM (AZT)|/UPDATED|

Well you can make some assumptions if you think the engine has never been touched. The casting number on the head should be able to be crossed over to a chamber size.

You will need to look at the head gasket closely to detemine if its a standard gasket or the older copper shim. You can usually use a compressed gasket thickness of .039 with a 4.120 bore to get your gasket area.

for the piston you can fiture two valve reliefs of about 2cc area each.

Next you need to figure the swept volume That is dependant on the stroke and overall bore size.

One other figure you will need is the piston compression height in relation to the deck height to determine if the piston is setting in the hole. If the engine has never been touched then you can figure the piston has the stock comp height it came with from the factory.

There is of course one other way. There is equipment out there that will allow you to measure the combustion area to give you its cc volume. Then its just a matter of knowing the stroke to get the swept volume.

Well, this is not an original motor. It is supposed to be a 300hp crate motor, origin unknown. All I really know about it is that it has a mild cam. I was really just trying to determine if the ratio was higher than 8:1.

If the heads are 64 cc and it does not have dished pistons, it probably has at least 9 to 1. Pull the valve covers and get the number off the heads to determine if they are 76 or 64 cc. Of course, someone could have milled them down as well. There are just too many possibilities. You can pull a head and check the piston travel, bore size, pistons, and measure the head's combustion chamber. There are articles available to walk you through the process.