Sorry if this is a stupid question....im still learning about this stuff....but after browsing the posts on here....I keep seeing people talking about compression ratios...and knock or ping.....and pump gas......I have no idea what this means...all I know is that a stock LO3 which is what I have has a 9.3:1 compression ratio......now when you raise or lower the compression (by getting bigger heads or whatever) what does this have an effect on as far as performance, emmissions, and the type of gas you have to run? The reason I ask is because Im in the market for a new 350....and different engines have different compression ratios...and I do need to pass inspection.....and I just want to know so I can learn more! Thanks for the help.

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http://users.ticnet.com/mult68
1992 Teal Camaro RS
T-Tops
305 TBI V8
Open air element
3" Flowmaster Exhaust System w/Quad Tips
Aiwa CD/Mp3 player
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Dynomax High Flow Cat Coming
More to come.

 

Ok i just read that higher compression engines need higher octane gas....otherwise the engine will start knocking....what other effects does it have? I think I remember reading that you get better gas mileage with higher compression engines....what about emissions.......now i am completely guessing now...that lower compression engines will give you crappy mileage....and maybe high emission numbers? I dunno

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http://users.ticnet.com/mult68
1992 Teal Camaro RS
T-Tops
305 TBI V8
Open air element
3" Flowmaster Exhaust System w/Quad Tips
Aiwa CD/Mp3 player
10 Disc CD Changer
Rockford Fosgate Speakers
SLP Headers Coming
Dynomax High Flow Cat Coming
More to come.

 

What is being referenced is "static compression ratio", which is the volume of the cylinder at the bottom of the piston stroke divided by the volume of the cylinder at the top of the piston stroke. A higher compression ratio raises the pressure of the fuel/air mixture at the time of ignition, and if you stayed awake in physics class, you'll remember that raising the pressure also raises the temperature.

Since the temperature is higher, ignition occurs more easily, and therefore can occur too soon. Higher octane fuel burns more slowly, so the rapidly rising pressure from combustion doesn't try to force the piston down while it is still moving up on the compression stroke (why you ignite the fuel/air mixture before the piston gets to the top is another topic). "Pump gas" is a general term for what is available at the corner gas station, and when used in reference to street performance engines, typically means the highest octane available with the gas taxes paid. Race gas is also available, but typically does not include the road taxes (and is much more expensive), so its use is not assumed when building an engine for street use. This limits the compression ratio you'd want to build into a street car engine.

Generally speaking, higher compression will produce more power for the same amount of fuel, therefore economy is also improved. Emissions, however, can be adversely affected (more complicated stuff), but your 9.3 to 1 compression is a good street compromise. 13:1 isn't uncommon is race engines using only race gasoline.

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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam and intake, World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet, GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).

 

So I should stay around 9.3:1......I am looking at starting with the ZZ4 short block which says on Scoggin-Dickeys site that it has 9.6:1 w/ 64cc heads....this is a good start correct? Oh and the short block itself has nothing to do with the EGR or anything right? Just the intake and heads? Like I said...i need to pass visual inspection as well as the sniffer.

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http://users.ticnet.com/mult68
1992 Teal Camaro RS
T-Tops
305 TBI V8
Open air element
3" Flowmaster Exhaust System w/Quad Tips
Aiwa CD/Mp3 player
10 Disc CD Changer
Rockford Fosgate Speakers
SLP Headers Coming
Dynomax High Flow Cat Coming
More to come.