Fuel for 13:1 compression?
Thread Starter
On The Tree
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Oh no, no way in hell am I trying to run that much with pump gas. I am mainly just trying to find anyone who IS running this much compression, to see if a fuel mixture of pump and race fuel will work.
Originally Posted by KGSloan
there are people with higher compression. they run race gas like advised. i would not try that much compression with pump gas. i think you're just looking for trouble and it won't be worth the extra power you get.
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You should investigate the differences between static and dynamic compression. On a 348 CI motor running a 234/238 .595 .605 110lsa a friend of mine runs 12.2 on pump gas. I run 11.45 with a 114lsa....we don't have knock retard as seen on A-TAP. If the cam you run has tons of overlap such as a 242/250 .625 .630 108lsa you can run 12.9 CR on pump gas. The same motor with a 224/224 114 lsa cam would have problems at >11.5 CR. The octane needed is cam-overlap dependent on every motor not a fixed value by what the displacement is. Do a search on static vs. dynamic compression ratio and you will get a better answer.
why would u want to chance running pump gas... 10k+ engine build w/ 13:1 CR and you want to chance things or baby it with pump gas... do it right the first time. either build it for pump gas or race gas...
when someone wants to race you don't say well i need to pour this 5 gal jug of race gas i have in the trunk first because i cant do more than 1/4 throttle with this pump stuff...
and not only that but unless u have ls1edit for yourself with 2 tunes... you are gonna leave some hp sitting around ... gonna need low Timing for your pump junk, and a higher for the race gas
when someone wants to race you don't say well i need to pour this 5 gal jug of race gas i have in the trunk first because i cant do more than 1/4 throttle with this pump stuff...
and not only that but unless u have ls1edit for yourself with 2 tunes... you are gonna leave some hp sitting around ... gonna need low Timing for your pump junk, and a higher for the race gas
Due to the same problem a few years ago, I sold my GTP heads and got ARE heads for my then ARE 422ci stroker. It was:
4.060 bore iron block
4.075 offset ground Lunati crank
In the end I was still around 12:1. I ran 25 degrees of timing at WOT and went 11.doh on 93 pump gas.
I would not run 13:1 or more on 93 ever. I would at least run straight 104 unleaded.
4.060 bore iron block
4.075 offset ground Lunati crank
In the end I was still around 12:1. I ran 25 degrees of timing at WOT and went 11.doh on 93 pump gas.
I would not run 13:1 or more on 93 ever. I would at least run straight 104 unleaded.
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There are 2 reasons to run the compression:
First the more octane you have the slower the burn rate is....you lose power. Running 104 is going to cost about 7-8 rwhp just from the loss in burn rate. Second every point in compression you go up is about 14rwhp gained so going from 11:1 to 13:1 gets you 28 rwhp. In addition the motor doesn't really see 13:1 with a big cam. If you would take the time to investigate the difference between static and dynamic compression you would see that the CR is bleeding off due to the effect from cam overlap. He isn't 'taking chances with a 10k motor', he is running a fair amount of compression due to the specific set-up. 13:1 CR is not 13:1 on all motors. The PCM would take timing away if there was any knock and Auto-tap/tuning is where he will find his answer. The fact is almost every set-up I have seen runs far too little compression. The stock LS1 cam runs fine at 11:1. If you run any bigger a cam with more overlap you can run more compression. In fact if you don't raise the compression the cam / intake valve and power stroke are effectively lowering it for you. Here I will make it easy for you: CLICK:
http://www.e30m3performance.com/myth...comp_ratio.htm
Please understand that I am not trying to be a know it all. I just hate it when old myths like this one persist no matter what research is available. I do this stuff for a living.
First the more octane you have the slower the burn rate is....you lose power. Running 104 is going to cost about 7-8 rwhp just from the loss in burn rate. Second every point in compression you go up is about 14rwhp gained so going from 11:1 to 13:1 gets you 28 rwhp. In addition the motor doesn't really see 13:1 with a big cam. If you would take the time to investigate the difference between static and dynamic compression you would see that the CR is bleeding off due to the effect from cam overlap. He isn't 'taking chances with a 10k motor', he is running a fair amount of compression due to the specific set-up. 13:1 CR is not 13:1 on all motors. The PCM would take timing away if there was any knock and Auto-tap/tuning is where he will find his answer. The fact is almost every set-up I have seen runs far too little compression. The stock LS1 cam runs fine at 11:1. If you run any bigger a cam with more overlap you can run more compression. In fact if you don't raise the compression the cam / intake valve and power stroke are effectively lowering it for you. Here I will make it easy for you: CLICK:
http://www.e30m3performance.com/myth...comp_ratio.htm
Please understand that I am not trying to be a know it all. I just hate it when old myths like this one persist no matter what research is available. I do this stuff for a living.
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TECH Enthusiast
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13:1 with the right cam --93 IS the correct fuel. Too much octane is BAD for your engine. You only run the amount of octane you need to prevent detonation unless of course carbon deposits are your goal. Oh yeah: why does Phil99vette run 93 with 12.2:1 and get 10.77 N/A with his lil' ol' 5.7 liter???? Note the word MYTHS in the web address:
Here I will make it easy for you again: CLICK:
http://www.e30m3performance.com/myth...comp_ratio.htm
Here I will make it easy for you again: CLICK:
http://www.e30m3performance.com/myth...comp_ratio.htm
Thread Starter
On The Tree
Joined: Oct 2003
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Everyone thanks for the help! Spinmonster, thanks for the link! I am not afraid to run small chamber heads on this engine now, I just know I need to have an engine builder help me pick out the right cam. I've got all kinds of friends running popup pistons in their SBCs, but it seems I just don't see a lot of the LS1 folk running higher compression. Maybe I can trade up for some Patriot Performance LS6 style heads with sub-60 cc chambers and see if I can get my static compression past 14:1. 
Well... not really.
Well... not really.
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you can run 11.5 cr on a 422 and run 11.xx's but I would run 12.2 cr on a 348 and turn 10.7's.
J/K
Andy at A&A corvettes has build many huge motors and has seen 12.5:1 many times in Cali.....where they only have 91 octane.
J/K
Andy at A&A corvettes has build many huge motors and has seen 12.5:1 many times in Cali.....where they only have 91 octane.
Last edited by Spinmonster; Feb 25, 2004 at 11:30 PM.
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From: Deer Park
I would like to add something to Spinmonsters info-
When they test fuel for the RM/2 method, they take the given fuel and do a lab test. Then they take the same fuel and run a motor. This motor literally has an adjustable cylinder head head that can be cranked down by hand to increase CR. This fuel is run through the motor adn the head is cranked down increasing CR until knock appears. At this point the actual CR is calculated for the motor method, then added to the research number- then this is divided by 2 to give you the RM/2 Octane rating.
When plants make this fuel, they add Alkylate, Reformate, and other additives to raise the octane to the desired standard.
Remember, all three grades are shipped via the same pipelines (in most cases). The kick a huge pump on and run it for so long with 87, switch a few valves, then run 93 through the same pipeline.
You may be getting 94 octane fuel, you may be getting 91 octane fuel. It's a toss up- especially where you go for your fuel and how they treat their storage tanks.
Also, different manufacturers use different additives. Some cars respond better to these additives than others. Personally, I prefer Valero/ Diamond Shamrock gasoline, because of the methanol content. I think my car runs much better with this mixture. SOme people prefer Exxon, Chevron, etc.... and for good reason- their cars may respond positively to a certain additive for whatever reason. You should experiment and keep track of your findings with each fuel.
When they test fuel for the RM/2 method, they take the given fuel and do a lab test. Then they take the same fuel and run a motor. This motor literally has an adjustable cylinder head head that can be cranked down by hand to increase CR. This fuel is run through the motor adn the head is cranked down increasing CR until knock appears. At this point the actual CR is calculated for the motor method, then added to the research number- then this is divided by 2 to give you the RM/2 Octane rating.
When plants make this fuel, they add Alkylate, Reformate, and other additives to raise the octane to the desired standard.
Remember, all three grades are shipped via the same pipelines (in most cases). The kick a huge pump on and run it for so long with 87, switch a few valves, then run 93 through the same pipeline.
You may be getting 94 octane fuel, you may be getting 91 octane fuel. It's a toss up- especially where you go for your fuel and how they treat their storage tanks.
Also, different manufacturers use different additives. Some cars respond better to these additives than others. Personally, I prefer Valero/ Diamond Shamrock gasoline, because of the methanol content. I think my car runs much better with this mixture. SOme people prefer Exxon, Chevron, etc.... and for good reason- their cars may respond positively to a certain additive for whatever reason. You should experiment and keep track of your findings with each fuel.
I think that Spinmonster is partially right.
12:1 can be done on 93 pump gas, I did it and other have done but it works because...
-I ran less timing 25, rather than the 28 that I ran when I was stock compression
-Alumnimum heads don't retain heat as much as iron heads
While there is a difference between static and dynamic compression, the reality is that the overlap with a 112 lsa cam might only be like 2-4 degrees at .050 so I don't think that much compression is bled off by the cam.
And to run 13:1 AF safely I would run straight 104 but you can always do some testing on the dyno and share you results.
I am running 746rwhp on C12 with a blower and we thought I'd have to run C16 to do that.
12:1 can be done on 93 pump gas, I did it and other have done but it works because...
-I ran less timing 25, rather than the 28 that I ran when I was stock compression
-Alumnimum heads don't retain heat as much as iron heads
While there is a difference between static and dynamic compression, the reality is that the overlap with a 112 lsa cam might only be like 2-4 degrees at .050 so I don't think that much compression is bled off by the cam.
And to run 13:1 AF safely I would run straight 104 but you can always do some testing on the dyno and share you results.
I am running 746rwhp on C12 with a blower and we thought I'd have to run C16 to do that.
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While lsa is a big factor in overlap, duration is a bigger factor. The longer the intake valve is left open, the less volume the cylider has due to the volume decrease in where the valve closes during the power stroke. More intake duration=less volume left in the cylinder went the intake valve closes regardless of LSA. 224/224 112 has far less overlap than 234/234 112. More overlap = less dynamic compression. These engines have knock retard as a protection against damage. You run enough octane to have zero knock retard. I run 93 octane, 114lsa on a 7200rpm revving motor, 36 degrees timing in some places, and 11.45 CR with zero KR. When I shoot N2O the timing is pulled back 3 degrees from a timing tuner with no KR.
Last edited by Spinmonster; Feb 29, 2004 at 02:11 PM.
