Compression vs octane
Thread Starter
Burning Brakes
Joined: Mar 2001
Posts: 1,203
Likes: 147
From: dalton ma
Compression vs octane
Will the High comp C1 & C2 engines, 340hp and up survive on todays premium fuel? Just about ready to assembly my 62 340 with the original pistons and am having second thoughts on 11.5 to 1 comp. Don't want to become a chemist to drive 2000 miles a year. I'm sure there are a lot of original engines still out there, how are you coping? Thanks, Bob
Race Director
Joined: Jun 2006
Posts: 17,901
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From: Inverness FL
St. Jude Donor '07
i also have a 70 LT1 with 11:1 cr; i have had to turn the timing back to 6 - 8 degrees to minimize the pre-ignition 'ping'. if you listen closely when you 'put -your-foot-into-it' and back off the timing when you start to hear spark knock until the 'ping' goes away, then you'll be OK.
performance may suffer a bit, but it's cheaper than buying new heads and/or pistons.
Bill
performance may suffer a bit, but it's cheaper than buying new heads and/or pistons.
Bill
Drifting
Joined: Oct 1999
Posts: 1,675
Likes: 493
From: Corrales NM
I am having my L79 327/350 rebuilt as I write this. I am lowering the compression ration to 10:1. There is no way I want to be chasing octane boosters, racing gas, avgas or any of the other things needed to stop detonation. The octane of gas is only going down from here. In New England I can get 93 octane all day long but many parts of the country can only get 91.
I did compensate for the lower compression ratio by putting in a roller cam and rockers, gasket matching the intakes, and some mild clean up of the intake ports on the heads. This will more than compensate for the slightly lower compression.
My engine builder strongly discouraged keeping the 11:1 compression. He builds a many high performance motors. I listened to him.
NEVERL8
I did compensate for the lower compression ratio by putting in a roller cam and rockers, gasket matching the intakes, and some mild clean up of the intake ports on the heads. This will more than compensate for the slightly lower compression.
My engine builder strongly discouraged keeping the 11:1 compression. He builds a many high performance motors. I listened to him.
NEVERL8
Race Director
Joined: Nov 2002
Posts: 12,142
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From: Taylor Michigan
I have no issues with my crate engines of a 70 LT1 and a 66 L72 with max lead and pump premium so far. This E85 crap being jammed down out throats just might upset the cart however. If that happens aviation is a good substitute. :D
Team Owner
Joined: Mar 2003
Posts: 26,118
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From: Greenville, Indiana
My 327/360 engine is built using stock 360 hp replacement components including the .120 domed pistons and heads. The block deck had a light cut taken on it and the cylinders went .040 over.
I changed the distributor vacuum source from the factory venturi port to manifold full time vacuum and the vacuum advance pot to the quicker pull in VC 1810. In other words the vacuum advance comes in and stays in quicker and longer than a factory 340 or 360 hp set up. The mechanical advance curve, I left alone. Initial timing is set at 12*. The engine runs fine on mid grade 89 octane fuel.
I can make it knock a little with a low rpm, high load throttle tip in. As quick as the throttle is open, the little bit of knock is gone. Or, I can ovoid low speed lugging and never hear it.
I see no reason why a stock rebuild on a 340 hp engine wouldn't do the same. I have also have a '65 327/250 hp engine that requires a higher grade fuel to prevent spark knock even though initial timing is 2* (that's by the book), with 10-1 rated compression.
I changed the distributor vacuum source from the factory venturi port to manifold full time vacuum and the vacuum advance pot to the quicker pull in VC 1810. In other words the vacuum advance comes in and stays in quicker and longer than a factory 340 or 360 hp set up. The mechanical advance curve, I left alone. Initial timing is set at 12*. The engine runs fine on mid grade 89 octane fuel.
I can make it knock a little with a low rpm, high load throttle tip in. As quick as the throttle is open, the little bit of knock is gone. Or, I can ovoid low speed lugging and never hear it.
I see no reason why a stock rebuild on a 340 hp engine wouldn't do the same. I have also have a '65 327/250 hp engine that requires a higher grade fuel to prevent spark knock even though initial timing is 2* (that's by the book), with 10-1 rated compression.
Le Mans Master
Joined: Apr 2005
Posts: 6,350
Likes: 423
From: Seattle Area WA
Static compression alone doesn't mean much, you need to know what your dynamic compression is, and that is also determined by your camshaft timing events. These GM cams have typical late closing inlet valves that bleed off compression so running 11:1 is typically not a problem. Also confirm what your actual compression is by measuring everything like head volume, deck height, etc before assembly. You can adjust your compression somewhat with the various head gasket thicknesses available.
Advanced
Joined: Jun 2007
Posts: 75
Likes: 0
Compression?
First of all, if it's pure stock you're not running a true 11.25-1 c.r. as rated by the factory (it never was rated 11.5-1). They also used stamped steel head gaskets, which are much thinner, and gave a higher reading. Assembly line motors were generally about one point lower and I know this because I personally cc'd my stock 365 h.p. heads and a set of 396/425 h.p. & heads came up with less than the advertised values. There are after market "sandwich" gaskets available in different thicknesses that you can pick from to come up with a 10-1 c.r. which I've found to be optimal for street use with MY cars. I can also tell you that my '66 396 Chevelle runs a true 10.8 compression and seems to handle street use pretty well with unleaded gas although it does like an occasional gallon or two of leaded gas mixed in. No two cars will be alike so my advice is to check with a qualified mechanic that is hip to camshafts and ignition timing.
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Burning Brakes
Joined: May 2004
Posts: 782
Likes: 27
From: Bend OR
Neverl8,
Don't mean to rain on your parade but don't be surprised if your engine is noticeably weak on the bottom. It's a common mistake to think that
"bigger" cams always add power. In fact, the bigger the cam, the more compression is needed for good low end performance. The "racier" the grind the lower the dynamic pressure due to overlap and timing figures biased for high rpm power. In other words, by lowering the compression AND putting in the roller(which I'm guessing is "higher performance") you've done two things that add together to reduce torque. If you are disappointed with the performance, put the stock cam back in and keep the lower compression.
I just built my L-79 to 10.6 with the stock cam and I do not expect any detonation problems. Mr. Marzahl may have a comment on this......
Don't mean to rain on your parade but don't be surprised if your engine is noticeably weak on the bottom. It's a common mistake to think that
"bigger" cams always add power. In fact, the bigger the cam, the more compression is needed for good low end performance. The "racier" the grind the lower the dynamic pressure due to overlap and timing figures biased for high rpm power. In other words, by lowering the compression AND putting in the roller(which I'm guessing is "higher performance") you've done two things that add together to reduce torque. If you are disappointed with the performance, put the stock cam back in and keep the lower compression.
I just built my L-79 to 10.6 with the stock cam and I do not expect any detonation problems. Mr. Marzahl may have a comment on this......
Thread Starter
Burning Brakes
Joined: Mar 2001
Posts: 1,203
Likes: 147
From: dalton ma
I'll try it
Since no one has said " Oh my God, don't do it" I shall press on blindly and see what happens. I have two gasket sets available a Victor Reinz and Felpro both which have the same # KS2600 ident. Would either have significantly thicker head gskts? I know there was a thread not too long ago listing the various gaskets and compressed thickness but didn't keep it. Thanks to all for a lot of "good" answers. Bob
Drifting
Joined: Apr 2004
Posts: 1,302
Likes: 36
From: Allen Park Mi
runing 11 to 1 or even 11.5 to 1 on a 327 with the 30-30 cam is no issue on 92 octane.. unless you floor it in 4th gear at 15 mph... LOL
i do it in 2 different motors and have no issues whatsoever.
Truth be told, you need that extra compression for that big ole cam.
A
i do it in 2 different motors and have no issues whatsoever.
Truth be told, you need that extra compression for that big ole cam.
A
Team Owner
Joined: Mar 2002
Posts: 44,653
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From: Huntsville AL
I will be building up my L79 this weekend with L2166F.040 pistons, stock 2.02 heads, LT1 Cam, stock intake and 2818 Carb. I have researched this setup, run the numbers many times over and feel very good about the end product. I had help/input from Duke, Ironcross, Aron, 427Rat and Scott just to name a few.
When I statred all this most engine builders said you have to run flat top pistons. I just don't buy into the lower compression bigger cam thing with these 327 engines, I'm sure I could be proved wrong but for me it was "if it's not broke don't fix it".
The one thing I did do was move the screw in rocker studs from the LT1 462 heads over to my stock 461 heads.
When I statred all this most engine builders said you have to run flat top pistons. I just don't buy into the lower compression bigger cam thing with these 327 engines, I'm sure I could be proved wrong but for me it was "if it's not broke don't fix it".
The one thing I did do was move the screw in rocker studs from the LT1 462 heads over to my stock 461 heads.
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My cars all run fine on pump gas without retarding the timing. I agree with the earlier poster that the advertised CR's were overrated.
65 L79 11:1, stock specs runs fine on 93 Octane. 68 427/390 10:25:1 stock specs. Runs fine on 93. 61 is 10.25:1 and runs fine on 93 (not stock. 383).
65 L79 11:1, stock specs runs fine on 93 Octane. 68 427/390 10:25:1 stock specs. Runs fine on 93. 61 is 10.25:1 and runs fine on 93 (not stock. 383).
Instructor
Joined: Aug 2006
Posts: 170
Likes: 2
From: East Fremantle Western Australia
Don't you guys have 98 octane readilly available?? Gary, (Perth, Australia).
Melting Slicks
Joined: Sep 2004
Posts: 2,917
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From: NE Illinois IL
Same with my L71. We pay a fair bit more for gas over here in Aussie, but have a ready supply of "premium unleaded petrol - PULP" at most outlets (BP, Mobil, Shell, Caltex etc) rated at 98 octane - the L71 loves it, with a dash of lead additive Don't you guys have 98 octane readilly available?? Gary, (Perth, Australia).
Don't you guys have 98 octane readilly available?? Gary, (Perth, Australia).I believe the US and AUS use different octane ratings. In the US we use the average of RON and MON whereas in other parts of the world they use RON only (which is typically higher than MON).
Cheers!
Joel
Melting Slicks
Joined: Nov 2002
Posts: 2,460
Likes: 5
From: Wheaton IL
I'd guess that the majority of old corvette owners on this forum have no idea what there TRUE static compression ratio is. Over the years blocks get decked, heads cut, flat tops installed, big fat gaskets , etc... make sure when you ask people about their cars the responses include the MEASURED static compression ratio.
Here are a few things to minimize in your analysis - compression ratio advertised by Chevy and compression ratio advertised by piston manufacturers.
That being said I run a measured 10:1 on a .030 327 with an LT1 and I can run full timing on 91 octane. I wish I would have gone up to 10.5 as others have run true 10.5:1 with 91-93 octane without having to retard the timing but the heads I bought had more combustion chamber volume than expected and the piston/deck height was larger than specs.
My heads were CCd at 66-67cc (larger than stock) and my deck height was .031 on one side and .041 on the other. Stock is supposed to be about .025 I think. Moral of the story?? Take measurements and then build your motor (assuming you are building).
Brian
Here are a few things to minimize in your analysis - compression ratio advertised by Chevy and compression ratio advertised by piston manufacturers.
That being said I run a measured 10:1 on a .030 327 with an LT1 and I can run full timing on 91 octane. I wish I would have gone up to 10.5 as others have run true 10.5:1 with 91-93 octane without having to retard the timing but the heads I bought had more combustion chamber volume than expected and the piston/deck height was larger than specs.
My heads were CCd at 66-67cc (larger than stock) and my deck height was .031 on one side and .041 on the other. Stock is supposed to be about .025 I think. Moral of the story?? Take measurements and then build your motor (assuming you are building).
Brian