View Poll Results: what compression ratio
10.5 or under
38
18.10%
11.0
69
32.86%
11.5
49
23.33%
11.5 or higer
54
25.71%
Voters: 210. You may not vote on this poll
what compression ratio r u running??
02-27-2008, 08:31 AM
#1
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what compression ratio r u running?? Going to be getting heads for my 402 soon. The Guy i bought it from said the cam likes 11.5 or better, just curious what other people are running on pump gas.
02-27-2008, 09:20 AM
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Static compression numbers alone are meaningless what really dictates compression is the cam in the car. A 13:1 car may only actually have a compression ratio of 7.8:1 with a particualar cam while a 10:1 car might have an actual compression ratio of 8.2:1 due to the cam differences.
That said Im at 11.9 SCR and 8.7 DCR.
That said Im at 11.9 SCR and 8.7 DCR.
02-27-2008, 01:43 PM
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the cam that is going in the car is a comp (did not get the card with the cam) 238/240 @.050 605/609 lift 112 lsa. That is what i got from comp when i called them and gave them the #s off the cam. The guy i got it from said there is 2 degrees of advance ground in and he ran it another 2 on the crank gear. That is all the info i have.
02-27-2008, 06:53 PM
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You can go here and find all the info you need for the cam, look at page 18 (it appears to be an XER grind):
http://www.compcams.com/Technical/Ca...obeCatalog.pdf
Next you need to know specific info about your internals such as bore diameter, stroke, piston height in relation to the deck, combustion chamber size and head gasket thickness. With all of this info you can calculate your DCR in addition to your SCR, or have someone do it for you. I'm sure someone would be willing if you had all the necessary info. Like stated above, the DCR is what you need to pay close attention to. How high you can go depends on several things. My combo is running 11.3 SCR and 8.7 DCR, which I think is about max for 93 octane gas.
http://www.compcams.com/Technical/Ca...obeCatalog.pdf
Next you need to know specific info about your internals such as bore diameter, stroke, piston height in relation to the deck, combustion chamber size and head gasket thickness. With all of this info you can calculate your DCR in addition to your SCR, or have someone do it for you. I'm sure someone would be willing if you had all the necessary info. Like stated above, the DCR is what you need to pay close attention to. How high you can go depends on several things. My combo is running 11.3 SCR and 8.7 DCR, which I think is about max for 93 octane gas.
04-27-2008, 12:18 PM
#7
Here is some good info:
I would just rather a camshaft add VE than subtract it. Anotherwords why put a cam in to reduce VE since you have too much compression for pump gas?
On the other hand if you have poor VE like with a really big cam or too big a heads or overly short runners and too much cross section and no runner length you CAN run more compressin for sure and it will also usually help you.
I would rather choose stuff that promotes good VE across the range you want to make your power in and then add the most compression you can reliably get by with on pump gas and so far that's been around the 11-11.5 to one range for us.
On the other hand if you have poor VE like with a really big cam or too big a heads or overly short runners and too much cross section and no runner length you CAN run more compressin for sure and it will also usually help you.
I would rather choose stuff that promotes good VE across the range you want to make your power in and then add the most compression you can reliably get by with on pump gas and so far that's been around the 11-11.5 to one range for us.
I have not seen a good running engine be able to run that much compression on even slightly crappy pump gas and be able to tune them right.
To put it in perspective, in the Engine Masters contest, that is run by Popular Hot Rodding and has had over at times 100+K prize money and many NHRA and IHRA builders including Jon Kaase, no one has successfully run that much compression and made it work very well when they were really trying to make power on pump gas. In fact over the half the 12+ to one engines blew up in only a few dyno pulls since the gas in NY was a little worse than in other areas of the country.
Most winners were in the 11 to one range and even some of them had detonation damage after the tests which were only a few dyno pulls. Now the rules are in the 10 to one range on compression and people are still making 700hp in 400 inch small block at under 6500 rpm but almost no one blows up anymore from detonation.
Keep in mind that these engines blew up with perfect cooling and on an engine dyno and not in a real car on a hot summer day with the AC on and with out half the stuff going wrong that happens on 90 percent of the street engines I see. Several of these engines had NHRA and IHRA Pro-Stock tuners working with them and several melted pistons in three pulls or less! MANY were pissed that running a half point too much compression on pump gas had cost them a hundred grand after the pistons fragged.
Granted that the average engine on LS1tech is not getting the VE numbers that these EMC engines are making but they are close so I would not run that much compression on anything I built for regular pump gas myself. Also the FAST 90 type deals are VERY good manifold for the rpm range these engines are working in and are actually banned for EMC competition and it's not because they suck!
To put it in perspective, in the Engine Masters contest, that is run by Popular Hot Rodding and has had over at times 100+K prize money and many NHRA and IHRA builders including Jon Kaase, no one has successfully run that much compression and made it work very well when they were really trying to make power on pump gas. In fact over the half the 12+ to one engines blew up in only a few dyno pulls since the gas in NY was a little worse than in other areas of the country.
Most winners were in the 11 to one range and even some of them had detonation damage after the tests which were only a few dyno pulls. Now the rules are in the 10 to one range on compression and people are still making 700hp in 400 inch small block at under 6500 rpm but almost no one blows up anymore from detonation.
Keep in mind that these engines blew up with perfect cooling and on an engine dyno and not in a real car on a hot summer day with the AC on and with out half the stuff going wrong that happens on 90 percent of the street engines I see. Several of these engines had NHRA and IHRA Pro-Stock tuners working with them and several melted pistons in three pulls or less! MANY were pissed that running a half point too much compression on pump gas had cost them a hundred grand after the pistons fragged.
Granted that the average engine on LS1tech is not getting the VE numbers that these EMC engines are making but they are close so I would not run that much compression on anything I built for regular pump gas myself. Also the FAST 90 type deals are VERY good manifold for the rpm range these engines are working in and are actually banned for EMC competition and it's not because they suck!
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Reread those posts! 
05-30-2008, 09:24 AM
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thread back from the dead, anyway my engine is done and i have around 11.5 compression, have not drove it yet still dont have the body work done.
06-01-2008, 04:45 PM
#19
Since this was brought back up.... I agree 100% with who this came from. A lot of people get caught up in trying to work out the highest compression ratio they can run. The thing is you are typically going to have less than ideal valve events to bleed off the compression to bring your DCR down. The base idea is that 1 point of SCR is about a 2% power increase. For a 500hp engine that's only 10hp. You can gain more than that with a cam that keeps your volumetric efficiency up and has good valve events.
06-01-2008, 07:03 PM
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11.82 scr 8.69 dcr with a tight quench thanks to the .040's and my afr's i have no det problems.
I typ run 110 leaded when i spray 150 on it but still no problem's in 20k mile's
I typ run 110 leaded when i spray 150 on it but still no problem's in 20k mile's