High Compression Engines/Pump gas
Thread Starter
Melting Slicks
Joined: Apr 2002
Posts: 2,337
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From: Norfolk VA
High Compression Engines/Pump gas
I have been reading on this forum that when building an engine with cast iron heads with a compression ratio higher than 10.0:1 cannot run on pump gas. The c6 engine has a compression ratio of 11.0:1 and runs on pump gas. I wonder if that is because both the engine block and heads are made of similiar metals. I am aware that the high compression engines (speaking of SBC engines only) ran on leaded gas.
I would like to build an engine with a 11.0:1 CR using cast iron heads that can run on pump gas. The '70 LT-1 engine for example has an 11.0:1 CR and I'm sure some owners run pump gas without octane boost. I'm also certain that today's gasoline is of better quality than that of 1970.
To make a long story short, why can't an engine with a 11.0:1 CR cannot run on pump gas?
I would like to build an engine with a 11.0:1 CR using cast iron heads that can run on pump gas. The '70 LT-1 engine for example has an 11.0:1 CR and I'm sure some owners run pump gas without octane boost. I'm also certain that today's gasoline is of better quality than that of 1970.
To make a long story short, why can't an engine with a 11.0:1 CR cannot run on pump gas?
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
The ability of newer cars to run high compression ratios has more to do with combustion chamber shape and fuel mixture consistency than what material the head is made out of. Modern engines have much more science in them.
BigBlockk
Later.....
BigBlockk
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Instructor
Joined: Jun 2002
Posts: 176
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From: New Albany IN
I have a 1970, 350 with a 11 to 1 CR. It will ping with pump gas if it is under any type of load. I use aviation fuel I get at the airport. It's not much more than pump gas and it is 130 octane.
Team Owner
Joined: Jul 2001
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From: Lakeland, FL
Cruise-In IV-V-VI-VII-VIII Veteran
St. Jude Donor '03, '06, '17
depending on your cam, heads and timing - you can run up to 12:1 CR engines with pump gas...
1) the longer duration the cam has - the more CR you can run
2) if you retard the timing you can run more CR
3) if you run aluminum heads you can run more CR
My new engine will have anywhere from 11 - 11.25:1 CR and I'm planning on running on pump gas with about 4 - 5 degrees retarded timing.. On the track - I'm going to advance the timing back and run 100 octane... My heads are aluminum and my cam has a 270 degrees .050 duration which will bleed off some of the compression..
1) the longer duration the cam has - the more CR you can run
2) if you retard the timing you can run more CR
3) if you run aluminum heads you can run more CR
My new engine will have anywhere from 11 - 11.25:1 CR and I'm planning on running on pump gas with about 4 - 5 degrees retarded timing.. On the track - I'm going to advance the timing back and run 100 octane... My heads are aluminum and my cam has a 270 degrees .050 duration which will bleed off some of the compression..
Safety Car
Joined: Oct 1999
Posts: 4,101
Likes: 10
From: NorCal
Another big reason is camshaft technology. I'm able to run 92 octane 76 gas in my BOSS with no problems using a Doug Herbert cam that rocks!
Any of the cheaper high ethanol content blends do not work as well.
Keep in mind that the modern engines also benefit from electronics (knock sensors, EFI, etc...) to keep them at optimum settings.
Hans
Any of the cheaper high ethanol content blends do not work as well.
Keep in mind that the modern engines also benefit from electronics (knock sensors, EFI, etc...) to keep them at optimum settings.
Hans
Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
A lot also depends on the cam. A short duration cam will ping on a high compression engine. That same engine with a longer duration cam might be fine. Also with the newer cars you have a computer adjusting the timing in milliseconds. If it detects any noise it will retard the timing. There has been a lot of technology in 30+ years tha tallow you to run high compression with lower octane.
Doh! Wrencher beat me to it.
Doh! Wrencher beat me to it.
Melting Slicks
Joined: Dec 2000
Posts: 2,041
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From: Wichita, KS
As mentioned above, the camshaft has a lot to do with the ability to run pump gas in a high compression engine. You can read more about it and download a program to help you determine if you can use pump gas with your high compression engine at http://cochise.uia.net/pkelley2/DynamicCR.html
Thanks to this program and the help of forum members here I run 92 octane pump gas in my 11.5 to 1 406.
Thanks to this program and the help of forum members here I run 92 octane pump gas in my 11.5 to 1 406.
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Melting Slicks
Joined: Aug 2004
Posts: 2,165
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From: Keizer Oregon
I run 10:3 to 1 CR 383 on super unleaded with a very advanced timing curve and have never heard any knock or ping!! If I went to the dragstrip i would still run racing gas "just in case". 
Melting Slicks
Joined: Apr 2002
Posts: 2,103
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From: Tucson AZ
I am running on 91 octane (best available here) . No pinging. The engine was timed accordingly. Some power is lost, but for a driver with still plenty of get up and go, I have no complaints.
Burning Brakes
Joined: Sep 2004
Posts: 917
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From: Vernon British Columbia
St. Jude Donor '05
First C3 General Poster!
I do believe carb jetting has something to do with it as well, maintaining a proper A/F mixture is key, Id say a bit on the rich side of things, 12:1 would help. Dialing back timing helps with a blower on high boost, should do the same naturally, as does cam retard of 2 degrees or so.....basically everything that was said here.
Main story is that you wont get much in the way of high compression on pump gass with small CID, small cam, small carbs, and iron.
Main story is that you wont get much in the way of high compression on pump gass with small CID, small cam, small carbs, and iron.
Le Mans Master
Joined: Mar 2004
Posts: 7,465
Likes: 1,485
From: Little Rock AR
Originally Posted by 1979toy
As mentioned above, the camshaft has a lot to do with the ability to run pump gas in a high compression engine. You can read more about it and download a program to help you determine if you can use pump gas with your high compression engine at http://cochise.uia.net/pkelley2/DynamicCR.html
Thanks to this program and the help of forum members here I run 92 octane pump gas in my 11.5 to 1 406.
Thanks to this program and the help of forum members here I run 92 octane pump gas in my 11.5 to 1 406.
I run a stock spec 11:1 engine with a GM "30-30" cam and it will run fine on pump gas. Tight quench helps along with the long duration cam to make the engine less sensitive to octane requirements.
I also have a 10.4 C/R, flat top piston, cast iron aftermarket head, tight quench, L-46/L-82 cam engine that also runs great on pump gas.
Watch the DCR, keep the quench tight, and use detonation resistant heads and you should be fine on pump gas.
-Mark.
Race Director
Joined: May 2004
Posts: 11,920
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Originally Posted by GrandSportC3
depending on your cam, heads and timing - you can run up to 12:1 CR engines with pump gas...
1) the longer duration the cam has - the more CR you can run
2) if you retard the timing you can run more CR
3) if you run aluminum heads you can run more CR
My new engine will have anywhere from 11 - 11.25:1 CR and I'm planning on running on pump gas with about 4 - 5 degrees retarded timing.. On the track - I'm going to advance the timing back and run 100 octane... My heads are aluminum and my cam has a 270 degrees .050 duration which will bleed off some of the compression..
1) the longer duration the cam has - the more CR you can run
2) if you retard the timing you can run more CR
3) if you run aluminum heads you can run more CR
My new engine will have anywhere from 11 - 11.25:1 CR and I'm planning on running on pump gas with about 4 - 5 degrees retarded timing.. On the track - I'm going to advance the timing back and run 100 octane... My heads are aluminum and my cam has a 270 degrees .050 duration which will bleed off some of the compression..
One of the big engine builders (like Lingenfelter) suggested not to use more than 10,5:1 on aluminum heads and 9,5:1 on cast iron.
You have to take a lot of variables into consideration as a lot of people have mentioned in their posts to be able to accomodate the higher compression without doing damage.
Go and read a book or two of one of the big well known builders and take everything else you hear with a pinch of salt! File it in the back of your mind for cross referencing.
Melting Slicks
Joined: Apr 2000
Posts: 3,286
Likes: 8
From: San Jose California
I agree with all the suggestions given that are considerations for high compression motors. One very significant item left out is incoming air temperature. The colder the air, the more compression you can run. Underhood temps can be extremely high. True fresh air intakes also make 1% more power for every 7.5 degree drop in air temp.
FI makes for a much more accurate mixture delivery allowing higher compression. Even if 6 of 8 cylinders are happy at 11:1 compression; one or two cylinders knocking because things are a bit lean, hot, etc. can ruin your whole day. Much like a chain being only as strong as its weekest link.
Mappable water injection is something I am very interested in.
Chuck
FI makes for a much more accurate mixture delivery allowing higher compression. Even if 6 of 8 cylinders are happy at 11:1 compression; one or two cylinders knocking because things are a bit lean, hot, etc. can ruin your whole day. Much like a chain being only as strong as its weekest link.
Mappable water injection is something I am very interested in.
Chuck
Safety Car
Joined: Mar 2002
Posts: 4,366
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From: Las Vegas NV
Originally Posted by Raydon 3000
I have a 1970, 350 with a 11 to 1 CR. It will ping with pump gas if it is under any type of load. I use aviation fuel I get at the airport. It's not much more than pump gas and it is 130 octane.
Race Director
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From: Corsicana, Tx
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You also have to distinguish between running a "real measured" 11.0 and a "nominal" 11.0. The old LT-1's were a lot closer to 10.2-10.5 at best in real life. I have a buddy with a .060 over 327 using 11.0 pistons, but it actually has around 10.5 or so. Runs fine on pump gas with iron heads and a pretty stout little solid lifter "roundy pounder"cam.
My 427 was an honest 11.2 and ran fine on 93 octane at the track and street with a 244* street roller and iron heads. But the chambers were smoothed out and it ahd tight quench. Just for reference it had approx 195 psi cranking compression.
My 540 is a measured 11.12 and does fine too on 93 octane, but it has a pretty stout cam in it and aluminum heads.
I personally think you're better off to use slightly less compression and crank in the timing than to go the other way. The timing will give you more power/response/driveability than the extra .5 point of compression.
But I like running on pump gas at all times. Hate the idea of chasing down good gas. I want to be able to rock anytime I want to regardless of what's in the tank.
JIM
My 427 was an honest 11.2 and ran fine on 93 octane at the track and street with a 244* street roller and iron heads. But the chambers were smoothed out and it ahd tight quench. Just for reference it had approx 195 psi cranking compression.
My 540 is a measured 11.12 and does fine too on 93 octane, but it has a pretty stout cam in it and aluminum heads.
I personally think you're better off to use slightly less compression and crank in the timing than to go the other way. The timing will give you more power/response/driveability than the extra .5 point of compression.
But I like running on pump gas at all times. Hate the idea of chasing down good gas. I want to be able to rock anytime I want to regardless of what's in the tank.
JIM
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
Originally Posted by Summerfun
One of the big engine builders (like Lingenfelter) suggested not to use more than 10,5:1 on aluminum heads and 9,5:1 on cast iron.
You have to take a lot of variables into consideration as a lot of people have mentioned in their posts to be able to accomodate the higher compression without doing damage.
Go and read a book or two of one of the big well known builders and take everything else you hear with a pinch of salt! File it in the back of your mind for cross referencing.
I do suggest reading books, more than the Lingenfelter book, and use that information as a guide line if they can back up what they suggest with data and theory. If it's their own opinion without adequate data, then take it with a pinch of salt. I ran 11:1 back in the late 70's on cast iron heads without detonation on premium pump fuel. There is no hard set number you can go by in regards to static CR. If you start to crunch numbers, you'll start to see the dynamic CR start to drop dramatically when a cam has a longer duration. For example, almost a full point of compression is lost when comparing a cam of 260 degrees to one with 288 degrees of duration.
Last edited by mandm1200; Oct 17, 2004 at 11:10 AM.
Instructor
Joined: Feb 2004
Posts: 111
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From: Bloomington MN
My 69 has the 350-350HP with 11:1 and I roughly mix premium with racing gas 50/50, costs more to drive but I did`nt by a Vette to putz to the grocery store. It is to much fun having run like it`s suppose to!!
Le Mans Master
Joined: Mar 2001
Posts: 6,237
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From: Watkinsville, GA and Glen Cove, NY
On a related note here. Lets say I am running 11 to 1 with a big cam on premiom pump gas. Because the cam is so big I have no detonation but because this cam is so big it will rev very high. Since it is filling the cylinders so well would there not be a point where I could get detonation at the higher rpms. I have always wondered this.