Compression, Timing and Detonation - Chevelle Tech
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post #1 of 9 (permalink) Old Aug 1st, 20, 12:29 AM Thread Starter
Greg
 
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Compression, Timing and Detonation

So... my 1970 Elkie has what is supposed to be the original 300 HP (optimistic) 350 in it. These motors were rated at a 10.25:1 CR when new. The guy I bought it from, when asked, said he ran regular unleaded in it, with the carb set up rich and the timing not fully advanced to factory spec. It runs like it. I put premium in it to drive it home to Spokane (2,000' elevation) from his sea-level location. Given that running 10.25:1 compression on 87 octane unleaded is supposed to be impossible, I suspect these are either modified/different heads or pistons. I plan to tune it up soon, including leaning out the 650 Holley and setting the timing to spec. I will be checking the compression when I change the plugs.

Anybody know what compression readings I should expect, if it is somehow factory spec; and what should I expect if it is actually 8.5 or 9:1? Cam is stock. I would like to maximize the tune but not go to detonation land!

Thanks,

G
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post #2 of 9 (permalink) Old Aug 1st, 20, 1:50 PM
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Re: Compression, Timing and Detonation

Those compression ratios are nothing new, ZZ4 engines come with that C/r.

THE single most important part is to GET THE QUENCH RIGHT. Early Chevy engines ran a quench of between .039 and .045, they did it this way, pistons down in the bore between .020 and .025, and a .020 compressed thickness steel shim head gasket.

Along comes top tuners complaining about head gasket failures after 150K miles, and, too high a C/R, so, two things were done, both disasters.

Silv-O-Lite builks a "destroked" piston for all sorts of engines, stock piston with the piston pin moved up another .020 to lower the piston dome down into the bore, lowering the C/R, which wasn't the problem in the first place.

Gasket makers actually listen to moron engine bozos, and make thicker head gaskets, so they don't blow as easily as the steel shom stuff. Sizes, oh, heaven, compressed thickness, .040, .042, .045, .055, .060 and more.

DISASTER. Think about a deck with .025 stock, Destroked another .020 down in the bore (this gets us a strange build, the quench to be right, would need to NOT use a head gasket), and a .055 thick head gasket. On this one, quench will be a whopping disaster of .080, PING AND DETONATION CITY.

So, this is why most saavy engine people ZERO DECK blocks, and use a .040 to .045 thick pack type head gasket, the gasket creating the proper quench distance, as Chevrolet did it in the first place, only now, done differently.

And that is the evolution of worse and worse pinging "for no reason", and detonation. NOTHING ever happens "for no apparent reason", NOTHING.
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post #3 of 9 (permalink) Old Aug 1st, 20, 4:46 PM Thread Starter
Greg
 
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Re: Compression, Timing and Detonation

Well that was not much use. Let's simplify: How much cranking compression should a stock 1970 300 HP 350 show? I am looking on the web and no luck so far.
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post #4 of 9 (permalink) Old Aug 1st, 20, 7:15 PM
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Eric
 
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Re: Compression, Timing and Detonation

Quote:
Originally Posted by GTOlympia View Post
Well that was not much use. Let's simplify: How much cranking compression should a stock 1970 300 HP 350 show? I am looking on the web and no luck so far.
Any number you find will be a total crapshoot anyway. Too many variables.
Starter cranking speed, camshaft spec more specifically intake valve closing point, whether timing set is actually accurate of off 1-2 degrees retarded or advances, ring seal condition.

Generally, if it is under 190 psi you should be able to tune it to run on pump gas....maybe not 87, but certainly 91-93
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post #5 of 9 (permalink) Old Yesterday, 12:27 AM
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Re: Compression, Timing and Detonation

Greg, you may or may not already know this, but if you're concerned about pinging/detonating, then be sure to remove a few of the spark plugs after laying on the throttle during a drive on a hot day, and look closely to see if there are dark speckles on the white parts of the plugs. If there are, then those are tiny pieces of the pistons embeded in the plugs, and that's a sure sign of detonation whther you heard the pinging or not. In that case, you would need to change the plugs, back off on the ignition timing advance, and go for another hard drive, and look at those plugs again.

70 Chevelle SS clone (632 CI powered).
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post #6 of 9 (permalink) Old Yesterday, 11:45 AM
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Re: Compression, Timing and Detonation

160 to 170

I have run 87 with no problem 150 and below. 150-170 we have an 89 grade that works most of the time, anything over 170 requires 93. I really cannot get much over 180 to run successfully on pump gas unless you reduce timing and can get the engine to run really cool in the 170 range. Just what I have found on basically stock every day driving engines. Modified engines vary with the modifications.
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post #7 of 9 (permalink) Old Yesterday, 1:00 PM
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Re: Compression, Timing and Detonation

"Well that was not much use" Sorry to cause you pain, but, you asked about it, and that is a GIANT part of it. Care to overlook it, live with the consequences.

"Any number you find will be a total crapshoot anyway. Too many variables.
Starter cranking speed, camshaft spec more specifically intake valve closing point, whether timing set is actually accurate of off 1-2 degrees retarded or advances, ring seal condition."

Eric has it full-on correct, too many variables, and one he didn't allude to, not his over looking, but, carbon buildup in the chambers and piston tops. There are many more, like a very restrictive air filter, not holding the throttle WOT during the tests, but, you already know all that, don't you. So, why did we get involved if you didn't want to fix it in the first place? Didn't help us one bit.

Enjoy it, no need to fix it, is there.
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post #8 of 9 (permalink) Old Yesterday, 1:56 PM
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Post Re: Compression, Timing and Detonation

Quote:
Originally Posted by GTOlympia View Post
So... my 1970 Elkie has what is supposed to be the original 300 HP (optimistic) 350 in it. These motors were rated at a 10.25:1 CR when new. The guy I bought it from, when asked, said he ran regular unleaded in it, with the carb set up rich and the timing not fully advanced to factory spec. It runs like it. I put premium in it to drive it home to Spokane (2,000' elevation) from his sea-level location. Given that running 10.25:1 compression on 87 octane unleaded is supposed to be impossible, I suspect these are either modified/different heads or pistons. I plan to tune it up soon, including leaning out the 650 Holley and setting the timing to spec. I will be checking the compression when I change the plugs.

Anybody know what compression readings I should expect, if it is somehow factory spec; and what should I expect if it is actually 8.5 or 9:1? Cam is stock. I would like to maximize the tune but not go to detonation land!

Thanks,

G
Quote:
Originally Posted by GTOlympia View Post
Well that was not much use. Let's simplify: How much cranking compression should a stock 1970 300 HP 350 show? I am looking on the web and no luck so far.
With the engines age and mileage I think 140 to 160 would be the reading. You should be more concerned with 1 or 2 cylinders readings being way off from the other cylinders.

I had a 68 Catalina 4 door with a 2bbl. 400,the advertised compression was the exact same CR as yours is.I ran it on regular unleaded all the time except for when I had that big trunk and the back seats loaded with people and cargo then I would mix in half a tank or more super (93) in the tank.
That was just insurance though.
There's a good chance that 10.25-1 CR isn't really up there,it's probably around 9.5 and maybe even less.I never had any pinging with that Pontiac unless it was loaded down along with travelling to my Brothers place up in the mountains.
However there is a good chance the gas in NJ is way different to the gas where you live.Shop around and find a gas that runs good in your new (to you) car.You can also add 1/2 a tank 93 at a time to regular to increase the octane rating that way
Also you shouldn't lean out the idle mixture too far as the idle mixture affects the carb some of the way into its part throttle mixtures.Just do a high altitude tune-up and then see where it goes from there.Pop off one of the valve covers and post what your cylinder heads casting number and date is,that way you will know if they are still the OE L48 heads.

John
71 El Camino SS
406 SBC M21 3.31 12 bolt
"Quality is always remembered,long after the price is forgotten"
"I would rather have questions I can't answer than have answers I can't question" :R.Feynman

Last edited by 71350SS; Yesterday at 2:04 PM. Reason: had to add something
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post #9 of 9 (permalink) Old Yesterday, 2:28 PM Thread Starter
Greg
 
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Re: Compression, Timing and Detonation

Thanks for the guidance on the numbers and observations on how similar-period motors are working on today's fuel. The immediate future of this car is to become optimized for what it is for the time being, then probably a 400 HP 383 will replace the stock motor. Just wanted to know what I should expect when I check compression, and thoughts on how a stock L48 should work on today's fuel- not how to build a motor!

Thanks again.
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