Compression Ratio: How Much Is Too Much

[Joe Ciaravino]

I am building a 1965 327/365 motor and would like to put it back to its original configuration.Now,that is well intended, but I am not sure whether those 11:1 domed buckets will tolerate today's 93-94 RON octane gas.I am reasonably sure that I will have to use an octane booster, but if so, how much.

I have been told by some that "the engine won't run" on today,s fuel. What does that mean? I would rather rebuild using the 11:1 ratio rather than flattop pistons. I also had hardened exhaust seats installed in the heads for unleaded.

Anyone with any thoughts/experience here? If I must use the flattops,will this kill the performance? Will the hi-lift cam be happy with the lowered compression? As I said before,I don't mind using the octane booster, but within reason(1 bottle at about $3-$4/tankful is livable).Can anyone recommend a good product?

Before disassembly, the car ran fine on hi-test without any additives, I could not hear any detonation (those sidepipes are loud!!), and no run-on at shutdown. When I opened the engine for rebuild, I found 11:1 forged pistons,was running stock 12 degree timing, and a hydraulic cam which is now going to be replaced with the original 30-30 unit.

The original 1965 owner's manual recommends running 101-102 octane gas(1965 rating) with the 365 HP engine.

Joe

[g.r.rogers]

Re: Compression Ratio: How Much Is Too Much

Contact Jack Podell 219-232-6430,they can get your octane up to where you won't ping using "tetraethyl" ask about the product Max Lead 2000. I have used a product in the early 90s called Lead Supreme 130. Same stuff made by Kemco fuel 1-801-768-4408. I used it on 1970 LT1, basically the same engine and ran a lot more advance than the book called for w/out any problems. The lead supreme 130 will work.GRR #33570

[Joe L.]

Re: Compression Ratio: How Much Is Too Much

Joe-----

If your engine prior to tear-down ran well on the pump gas which you were using, I don't see any reason to be concerned now. Empirical evidence is always the best evidence, and that's what you have: your engine with 11:1 compression ratio ran well on today's pump gas.

I would not expect the switch to the original configuration solid lifter cam to alter the detonation picture here, either. The fact is that the hydraulic lifter cam that you were previously running probably had less overlap, with consequent higher cylinder pressures, than the original mechanical lifter cam that your L-76 came with. So, if anything, I would expect installation of an original configuration cam to reduce the tendency of your engine to detonate.

As I've mentioned befoe, if your engine does not detonate on a certain octane gasoline, YOU DON'T NEED MORE OCTANE. Under such circumstances, more octane is NOT, in any way, "better" for your engine.

[dale pearman]

Re: Compression Ratio: How Much Is Too Much

Please let me know when you take this Corvette out on the road so I can stay home! With the 30-30 cam you'll be leaving the traffic light at 4000 RPM minimum because your engine won't have the torque to get out of it's own way running any slower. Heaven help anyone in the way from 4000 to 6500 RPM because trhat's where this puppy comes on! I wish you luck in achiving an idle. Never mind the roughness. This cam belongs on a race track.

I wish you could see my number eight piston which I melted during an altercation with another enthusiast. I was running 11.0 to 1 with real/lead, (which fouls spark plugs) and what I thought and paid for, 92 octane from my friendly gas station. The gas turned out to be 87 octane and at wide open throttle the detonation cracked the rings which embedded in the cylinder head (aluminum) and the piston! So this "compression education tuition in the college of hard knocks,(pun intended), cost me about $5000.00 by the time it was over!

First of all, you don't want a NASCAR type wide open-all-the-time torque at the top end engine for the street. It's a pain in the DONKEY! Build yourself a bottom end torque motor that pulls stumps out of the ground! The torque should start as soon as you turn the key on and remain "flat" throughout the RPM power band. These engines idle well, provide high vacuum and I can win a race every time over short distances aginst a 30-30 cam. I'm a BIG fan of Rhodes lifters and a reall high lift-short duration hydraulic bump stick that closes the intake valve soon in the cycle for maximum cylinder pressure.

If you are building a show car or one that is driven sparsley, then go with 30-30 cam, or the GM 140 1st racing design, which is close. Use racing fuel, aviation fuel, or add some REAL tetraeythyl concentrate.

Good luck

Dale Pearman

[Duke W.]

Re: Compression Ratio: How Much Is Too Much

Agree with everything Joe L. said, but I've got a question. What was the head gasket configuration when you disassembled the engine? The special high performance and FI engines were equipped with two steel shim gaskets from the factory beginning sometime in '62, but it's not known when, or even if this practice ceased on the high compression 327s. It's noteworthy that the '64 and up versions were rated at 11:1 versus 11.25:1 for the '62 and '63 versions, eventhough it was the same piston and essentially the same head. Actual piston deck clearance and the type or number of head gaskets can very the compression at least a half a point.

Duke

[Duke W.]

Re: Compression Ratio: How Much Is Too Much

Agree with Dale's comments about the 30-30 cam. It was a torque killer. I'm currently researching the design of all the small block mechanical lifter cams and hope to write an article for The Restorer. I know Chevrolet also learned a lot about valve train dynamics during the fifties and sixties, and the ultimate result was the LT-1 cam. According to computer simulations I've run it makes more low end torque and more top end power than the Duntov, so this is the cam I would recommend for an original solid lifter 327. Also, use the old "585 edge orifice lifters" from the Duntov cam rather than the "piddle valve lifter" that were OEM on the 30-30. I do disagree with Dale on his characterization of the "140" cam. This was a pure racing design intended for headers with open exhaust and had considerably more overlap than the 30-30, and it would make a very poor street cam through any kind of reasonably legal muffler system.

Duke

[dale pearman]

Re: Compression Ratio: How Much Is Too Much

The 140 cam stinks on the street! Thats why I called it the 1st design racing cam It was designed for racing. The 30-30 is not far behind. Actually, all those 40 year old cam designs are passe, the 097 Duntov included. I've had GREAT luck with a Melling C80-P in a 350 cu in with 10.0 to 1. Mechanical lifter that pulls really well on the bottom and will rev to 6500. The 151 hydraulic with rhodes lifters and 10.0 to 1 is also hard to beat but it too is old fashioned. A Comp Cams 280 Magnum is torquey but I don't trust Comp Cams any more. I installed one of their cams and before proceeding with engine assembly, I measureed each lobe for duration, lift, base circle run-out, and slope, (drop-off). On three lobes, the base circle run-out was more than 0.004 inches! This cam would have chewed up the lifters in 15,000 miles! I pulled it and drove it back to Memphis where I got a hard time until THEY measured it. Then I was offered another cam or a refund. I took the refund and went to Ultradyne in Mississippi. Best hand made cams in the world come from Ultradyne!

We could go on forever with cams & CR's. I gonna hit the sack because tomorrow I'm going to Brent Ferguson's funeral in Kentucky. I gotta leave at 4 a.m.! gulp!

[motorman]

Re: Compression Ratio: How Much Is Too Much

edge orifice lifters are still avaliable from speed pro under part# AT 840 R. also if you are using stock stamped rockers make sure that you lube them good with moly coat(moly di sulfied) because these lifters are slow to oil at new engine startup. if you use moly coat for camshaft breakin be sure that you change the oil filter after the first start up because the moly plugs the filter and on the next startup up will have no oil pressure. this will only be seen if you run a blocked oil filter bypass as the bypass will still show oil pressure.

http://clemz9@hotmail.com

[Robert C.]

Re: Compression Ratio: 365hp rebuild

Joe, I've got the same year and engine combo. I was afraid too. The engine was bored .30 over and original type domed pistons used. The hardened seats was the only difference. The car runs perfect on 103oc. gas. I've driven over 3500 highway miles and not a miss. Used 102 oc. gas when i drove to Oklahoma and the car had a slight ping. Good luck, the 365 engine is almost as fast as my old LT-1!! Bob


Texas Chapter NCRS

[Larry Maust]

Re: Compression Ratio: How Much Is Too Much

I have a 1969 roadster with a 350/350 hp 11:1 compression, I burn 93 octane. If I were to do a rebuild with todays gas I would consider going down in compression. I have set the timing back to run with out much pinging, and it helps alot but not completely. I like to drive the car alot and put about 6000 miles on it each summer. Getting good gas is hard. I haven't found any lead additive or octane booster that helps much. Good luck

[Jim T.]

Re: Compression Ratio: How Much Is Too Much

One of my drivers is a original 68 L79 and has 11:1 ratio compression. It has a lot of age and agout 93K miles, but runs real just fine on 93 octane available in TN. I just don't lug the engine and like the owner manual says don't drive less than 40 MPH in third. In reading an article in I believe Chevy High Performance, they did a cam test with the L79's 151 cam and one from Competition Cams. If I remember the test correctly, the 151 faired okay until about 5500 RPM's, then the other cam really made a difference. I hardly ever run my L79 beyond 4500-5000, so I don't miss that extra horsepower. I am not a great engine builder, but I don't think your Corvette will have that sound you like if you lower your compression. Rebuilt a 327 lower compression engine for my brother and used the 151 cam. The sound at idle just wasn't there compared to my 68 L79.