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Old 06-21-2004, 02:55   #1 (permalink)
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Question 9.5 compression ok for blown LPG engine?

as the title reads, is 9.5 compression ratio ok for running 8 pounds boost on straight lpg? or am I going to have to lower it to get better power from my supercharger?

stats; eaton m112 supercharger, 351c setup for gas, GRA dual carbie & converters all in an 84 4x4 Ford Bronco
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Old 06-21-2004, 03:10   #2 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

Sounds a bit high, can you get it under 9:1 with some head relief work?
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Old 06-21-2004, 04:32   #3 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

I was thinking i might just go with some thicker head gaskets if i need to drop the compression ratio.
alot of people run high compression ratios with lpg from what i hear... so i was wondering if i needed to drop it in my case.
the lower the comp ratio, the more air/fuel mixture the supercharger can push in, the more powerful the engine, right?
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Old 06-21-2004, 04:41   #4 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

im doing some thing similar, a 400ci, b&m 162ci blower, 302 heads. with a rv cam, close overlap, it blew the boost out the exhaust, making about 2lb's, and pinging with twin convertors, and twin 425's. ive rebuilt the motor, blower cam, and opened up the 302 heads, almost as big as open chambers. i can still run 2 head gaskets, if needed, as i dont want to go coppers. i dont think you can get thicker composite gaskets, but hopefully im wrong. i dont know what the comp was, but 4" bore, 4" stroke, dished pistons and closed chambers, would be pretty high.
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Old 06-21-2004, 04:49   #5 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

Quote:
Originally Posted by broncosean
I was thinking i might just go with some thicker head gaskets if i need to drop the compression ratio.
alot of people run high compression ratios with lpg from what i hear... so i was wondering if i needed to drop it in my case.
the lower the comp ratio, the more air/fuel mixture the supercharger can push in, the more powerful the engine, right?
8.5-9:1 compression for a blown gas engine is still high. You need to reach the right compromise otherwise you won't be able to get enough boost to make effective use of your blower and reach a good volumetric efficiency.
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Old 06-21-2004, 07:28   #6 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

ok, so if i'm running 9.5 C:R at the moment, how thick will i have to make the head gaskets before i reach say, 8.5? and where can i get copper head gaskets for my 351c ?
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Old 06-21-2004, 15:25   #7 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

I'd go for the thicker copper gaskets but get the motor O ringed at the same time.
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Old 06-21-2004, 17:58   #8 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

when you say get the motor o-ringed... what do you mean? the 351c was completely rebuilt 1000 k's ago to LPG specs and 350hp. I'm still interested as to what size head gasket i'm going to need to drop the compression ratio down to 8.5 if i'm currently at 9.5??
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Old 06-21-2004, 23:18   #9 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

http://www.popularhotrodding.com/eng...07pon_ringing/

Has a short piece on o ringing a pontiac
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Old 06-22-2004, 04:34   #10 (permalink)
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Re: 9.5 compression ok for blown LPG engine?

ok, thanks wartide. now i know what o-ringing an engine is.
ok, so that process really strengthens the head gaskets... now how thick of a head gasket will i need to drop my current 9.5 C:R down to 8.5? can anyone help me with that one?

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