Compression ratio

andy

Can someone help me with what the difference is between a high compression ratio and lower and how to get there
plan on doing a complete truck rebuild and motor is first
thanks

.just as it says.. higher compression is more pressure in the combustion chamber... more for a NA engine...   lower compression is lower pressure in the compression chamber.... generally for boosted motors..... the higher number for the cc in the heads, the lower the compression, the lower the number, the higher the compression.......

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Originally Posted by fastdurango00the higher number for the cc in the heads, the lower the compression, the lower the number, the higher the compression.......While the combustion chamber size of the cylinder heads does play a part in the compression ratio there are other things that need to be considered as well. For example pistons, are they flat top, domed, do they have valve reliefs cut in them are they deck height. Another crossly overlooked item is head gasket thickness, just changing head gasket thickness wil increase or decrease compression ratio. Keep in mind that this is just for static compression, dynamic compresison in a whole different story.

I could build 2 different engines using the exact same cc cylinder heads on both engines and have one at 8.5:1 and the other at 10:1

.Compression ratio is just that...  A ratio of the air volume between the top of the stroke and the bottom of the stroke.

top of the stroke:  total chamber volume displacement (chamber size +/- piston dome/dish), head gasket thickness, cc's of the piston valve relief, cc's between the top ring and piston top.

bottom of the stroke:  The cubic inches of the motor/# of cylinders

Compression ratio does not change once the motor is built unless one of the above factors is altered.

Cylinder pressure is the amount of air pressure in the cylinder generated by the cam timing of the valves.

.You can find a lot of sites that will allow you to input detail information to determine your overall ratio, and sites that will compare an N/A ratio vs. boost etc. Google is your friend

.i was reading some article in motorcraft magazine not long ago that argued about the lower compression for boosted motors. generally, ppl say you want about 8.5-9 compression for boost, but they built a motor (chevy, of course) that had 10.5-11 compression, ran like 15psi on it and put out over 800 ponies. they said that if the compression were lower it would only have put out about 600 something. pretty interesting.

.The reason to lower compression ratio when running boost is to be able to run lower octane fuel.  

That motor with 11:1 compression and 15lb of boost certainly had to run around 115 octane whereas around 8:1 compression it may be able to run on 100 octane.