figuring out compression

[dubie]

How do you figure out the compression ratio of an engine? I am running a 350 with stock pistons, moly rings, some 60 CC chamber heads from a 305, stock intake and 750 cfm Qjet. I was told by a couple guys that I have to run 92 octane because of the heads I chose. Can anyone fill me in on how you calculate the compression

[Bowtie67]

General rule Dubie is that 350's had 8.5cr with 76cc heads if you were to take 76 - 60 = 1.6 + 8.5 = 10.1cr , this is only a general rule there are some other factors as well like how far down the cylinder the piston is at TDC and how thick the head gasket is as well. 305 heads have small valves and small chambers and usually aren't that great about putting them on a performance oriented 350 because of the huge compression gain with smaller valves basically defeating itself. If you are running the stock cam this will be a killer because it isnt releasing some compression while running causing you to run higher octane in some cases even more than 92 octane hence you need a bigger cam to drop your running compression. Another problem you have is if these heads are cast iron with the stock cam there will be an awful lot of heat and detonation and could further damage an engine. Recomended Compression with Cast Iron is about 9.6, Aluminum - 10.5, Higher can be run but it but conditions have to be met.

[JimKshortstep4x4]

Assuming flat pistons and zero piston to head clearance, I came up with a calculated compression ratio of 12.92 to 1. The most accurate way is to cc the cylinder and head.

The way I calculated the ratio is as follows: CR = beginning volume/ ending volume

Total volume is cylinder volume + combustion chamber volume

The cylinder volume is 43.73 cubic inches and the combustion chamber volume is 3.67 cubic inches, (converting cc to cubic inches) for a total of 47.4 cubic inches With the piston at the top, (based on my assumptions) the compression ratio is total volume 47.4/ending volume 3.67 or 12.92 to 1

If you factor in piston to head clearance and a dished piston, the CR will be less but probably above 11 to 1. Whatever the number you will have spark knock unless you use a high enough octane fuel. Generally a 10 to 1 ratio will spark knock with the available gas station premiums. To be safe with iron heads a 9.5 to 1 ratio is good.

Jim

[dubie]

As far as I know it's the stock cam. I never replaced it because it was in nice shape and I wasn't going for a huge performance type engine. This truck is just a daily summer cruiser. But now ya got me thinking if I did the right thing. I had 2 sets of 350 heads and they were both cracked whcih made me use these 305 heads. A couple board members have run them on their 350's without any troubles, besides starving the engine after 5000 rpms. They are just supposed to give you some lower end torque. Man, now I need some re-assurance on my choice of heads

[meathead95]

try this web site http://www.dunegoon.org/compression.html