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Old 01-28-2009, 07:31 PM
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How LOW compression ratio can be?

I'm working a set of vette heads to install on my SUPERCHARGED 350 SBC

the blower is a 144 B&M that pumps 8 psi with the current 882 head, I guess the high boost comes partially from the restrictive heads, it also has the smallest driven pulley that B&M had made for the smaller 305 engine.

it is a STREET engine and will burn pump gas

I want to extract as much power out of it and with the current heads I have to limit timing a lot because it pings like crazy so actually does not make much power.

I've seen racing turbo engines with compression ratios as low as 5.5:1 but they are racing engines, what would happen if I go that low on a street engine? will it be a real dog when there is little or no boost?

I used to have a '62 Land Rover that had 7.1:1 compression ratio and was a very smooth and torquey engine.

My plans are carving the heads a lot unshrouding the valves, this will let them breathe very well and will reduce compression at the same time.

please give me your opinions, thanks.


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Old 01-28-2009, 07:53 PM
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Eight pounds of boost isn't excessive, combined with a cr of 8:1 you're fine with pump gas.
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Old 01-28-2009, 08:09 PM
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but pings like crazy and I have to run very low ignition timing to compensate, our gasoline is not that good, I should of say this first, I was thinking of installing water injection but I think I will be better with a lower compression that will allow me to pump more air into the cylinders and make more power, in other words make the engine think it's bigger.

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Old 01-28-2009, 09:15 PM
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Post the complete details of your engine build. Details matter.
And the blower drive and driven pulley diameters.
What fuel are you using?- carb?- jetting?- timing?(initial/total) - Sparkplugs?

Here, on your typical roots blown 350SBC with 92 pump octane fuel {average} (R+M/2) you want a 7.5:1 measured compression ratio to allow the most boost/timing
(least amount of power robbing retard) and power with a non intercooled roots style blower and the old GM smogger heads and proper .040" net quench clearance. Assuming the typical street/strip performance hyd camshaft.

8.5:1 will need less boost and or less total timing at full boost to avoid pinging on the same fuel.

If you make the heads flow more air, more air will get into and out of the motor at the same blower drive ratio and will lower the observed boost as well as improve power/lbs of boost.
The exhaust has to get out in order for the boost to get in and make power.
The 882 heads are very, very poor on the exhaust side. (the intake ports suck too) They need max/serious work to get the exhaust flow up. You also need to completely block off the center twined exhaust EGR riser ports on these heads.

Water/methanol injection works wonders. So does toluene, ethanol or 110-114 octane race gas mixed in with the pump gas.

Last edited by F-BIRD'88; 01-28-2009 at 10:08 PM.
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