compression ratio - Team Camaro Tech
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post #1 of 6 (permalink) Old Mar 26th, 03, 05:08 AM Thread Starter
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How does compression ratio affect overall horsepower? For example if I was to have an 8 to 1 compression ratio as opposed to 12 to 1 what would the difference in performance?

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post #2 of 6 (permalink) Old Mar 26th, 03, 07:04 AM
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A higher ratio will provide more HP and TQ. It also depends on how the rest of the engine components work with each other. If it is a street engine, 9.5 is about the max with cast iron heads. 10.5 is about the max for aluminium heads. Keep in mind, this is running pump gas.

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post #3 of 6 (permalink) Old Mar 26th, 03, 09:33 AM
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As Codi said, compression ratio needs to be matched to the rest of your setup. 9 to 1 compression is plenty with a mild cam that idles smooth. That is way too low for a big lumpy cam that has enough intake and exhaust overlap to bleed the compression down. Do a search for Dynamic Compression on this site. Dynamic Compression is what you really need to worry about. It takes in to account valve overlap and is much more accurate and real world. To answer your question, on an average 350 making about 350 hp each compression point is worth about 15 to 20 hp and about 15 to 20 ft. lbs. of torque.
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post #4 of 6 (permalink) Old Mar 26th, 03, 02:33 PM
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Just don't be fooled into thinking that Dynamic Compression will save you from detonation problems at high RPMs. Dynamic and static compression as well as timing and fuel octane must be in balance to maximize HP and minimize detonation.


68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #5 of 6 (permalink) Old Mar 26th, 03, 04:36 PM
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In a nut shell, the higher the compression the more effecient the engine and therefor more power and torque.

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post #6 of 6 (permalink) Old Mar 27th, 03, 04:02 AM
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As a thumbrule 12-14 HP per point of compression. So increasing compression from 8:1 to 12:1 you could potentially gain 50 HP on compression alone. There are many other variables involved too, so don't rely too heavily on this rule of thumb. Combustion chamber shape, camshaft profile, ignition timing, induction and exhaust flow all play an important part when optimizing a combo for a given compression ratio.

Also, when compression gets very high, like 15:1 for example, the added benefit from increasing compression decreases per point added. I don't know why this is, but practical octane limits may come into play here.

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68 Camaro, E85 powered 427" small block. 9.96 @ 133 MPH, 1.319 sixty foot on motor. 5.92 eighth @ 116 with a 1.42 sixty breaking beams with back tire on the bottle
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