I have a stock 1975 L-82 with 47K Original Miles on it.

For my upcoming winter project:
I’ve already purchased an Edelbrock Performer RPM Manifold and 600 CFM Carb. Got a great deal on them used. $200.00 for both.

I plan to stick with the stock L-82 Cam since it’s not too bad:
222/222 @ .050, .450, .460. 114 LSA.

ISSUE/QUESTIONS
I’m looking at aluminum heads. The Factory Heads are the real performance robber issue on this motor due to the Intake Runner and Combustion Chamber (Chamber drops the Compression Ratio to 9:1). Plus, I have come to understand that they are also prone to cracking. In the early 70s, the same engines ran with 11:1 compression ratios due primarily to different heads.

I’m shopping for performance heads and have narrowed the search to the list below:
HEAD Intake Exhaust Intake Runner/Exhaust Runner Chamber
L-82 2.02 1.6 161cc 62cc 76cc
Edelbr 2.02 1.6 185cc 60cc 70cc
5073
Summit 2.02 1.6 185cc 65cc 64cc
60899

Using the compression ratio calculators you can find on line, I estimate that with this engines stroke and flat top pistons, a 70cc Combustion Chamber will put me at about 10:1 and 64cc would probably be 11:1 or so. I’ve also read that the difference between 10:1 and 11:1 is about 8% HP. I’d like to take that advantage if I can. For 11:1, I think 93 Octane will be just fine but I’m concerned about detonation issues with the L-82 Cam at 11:1.

I don’t want to change the CAM. Will this be an issue that introduces the risk of detonation and limits me to a 70cc Chamber with 10:1 Compression?

 

Keep in mind you can play around with different head gaskets and easily change the comp ratio too..

For example, the difference between a steel shim that compresses to .018 and a standard Fel-Pro gasket is about a 1/2 point in compression.. Allot of people dont realize when they rebuild their motor and replace the steel shim gasket with a stanard Fel-Pro (which most do), they are actaully giving up power..

 

Max CR on pump gas with full ignition advance is right around 10.5 or so with an .040 quench. You can certainly push that, and someone will no doubt pop up and start talking about dynamic compression ratio, aluminum heads allowing more CR, etc....but you'd do well to think more around 10.2 or so. You want to be very careful adjusting your head gasket thickness as you're also adjusting quench...which has a huge impact on the detonation sensitivity (and power) of the engine.

As for Vettes running 11:1 in the "early 70's" - the decline of the LT-1 is a perfect case with 11:1 in 1970, 9:1 in 1971. As came emissions and went premium fuel...so went CR.

The impact of an increase in CR is directly proportional to the HP/CID the engine will make - so on your relatively mild 350 the difference between 10:1 and 11:1 is a lot more like 2-3% max. See some of David Vizard's books regarding this.

Your stock heads (881/882) have a 76cc chamber and 9.0:1 CR with a fairly thick head gasket. The rule-of-thumb is about 1 point of CR for each 10cc chamber size change on an SBC...so dropping from a 76cc chamber to a 64cc chamber is right around 10.2...ideal.

But we do need to factor in quench. I believe (someone needs to check me here) is that the stock head gasket is .041 thick. If we move down to a .028 gasket, we're closer to 10.6...and IMHO that's too much. An .039 gasket gets around 10.2...but with .064 quench, which sucks.

So; no easy choices, which there never are in doing a top. A 76cc chamber head with an .015 gasket will deliver around 9.5:1 and is the way I would go - it's a good match overall for the cam and the quench is right on at .040 assuming an .025 deck.

Otherwise, you can play with the heads that are out there...factoring in quench in your CR calcs and looking for around 10.2.

 

Agree with all said above.

Only thing I would disagree with is the little 600 cfm carb. The stock Q-Jet is 750 cfm and a well tuned 750 cfm will not be too much carb for a 350.

 

Keep in mind that a Q-jet is a spread-bore with very small primaries and very large secondaries...and was designed to support the entire engine line. Most 350s use only a small part of the CFM available in the secondaries, but the unique design of the Q-Jet makes this a non-issue...much more so than "standard" vacuum-secondaries.

Unless we're getting past 1.4 HP/CID or so, or doing "special purpose" driving such as drag racing, the rule of thumb CFM = ((CI x RPM)/3456) x VE works fairly well. Smaller carbs often deliver better off-idle and mid-range throttle response and only give up a few ponies up top. He'd have to turn a 350 making around 1HP/CID to 7K to run out of flow in a 600 CFM carb

 

When you push that close in compression ratio, keep one thing in mind., if you are out traveling and get hold of some fuel that isn't up to your expectations, you make kill and engine before you get that fuel run out and replaced with some good stuff.
IMHO the risk isn't worth that suspected 8%. I leave about a half point wiggle room so I don't loose an engine over a bad fuel issue.
Just because it says 93 on that pump I wouldn't risk an engine with that bet. I hear folks say they just add octane booster to every tank, but that can get old also.
You can dump some of that compression with a little valve over lap also, if you're set on high compression.

 

FishnDude, billa, Steveg75, Indiancreek,

Thanks for the scoop! All great info and I really appreciate it. I need to finalize the research and get ready for Carlisle.
I got this info from a friend and fellow vette owner:

”A Chevy 350 is a 4.00 bore by 3.48 stroke = 43.73 cu in. This converts to 716.66 cc per cylinder.
The total starting volume is the swept volume of the cylinder plus the head volume or 716.668 + 64cc = 780.668ccCompression ratio is them 780.668 / 64 = 12.19 to 1. Too high! “

Using a 72 cc head, it is 788.668 / 72 = 10.95 to 1
Using a 75 cc head, it is 791.668 / 75 = 10.55 to 1.

If there is a convention that says the numbers are done simply using swept volume divided by head volume, then the numbers will come down.

716.668 / 64 = 11.19
716.668 / 72 = 9.95
716.668 / 75 = 9.55

Using assumption #1 above, including chamber volume with swept volume, I get 10.42:

716.668 + 76 / 76 = 792.668 / 76 = 10.4 for the original Heads

Using assumption #2, Swept Volume/Chamber Volume:

716.668 / 76 = 9.4. This one's a bit closer to the original claim and probably a safe method to choose as I suspect that GM overstated the Compression Ratio to begin with.

I plan to go with 70 (10.23:1) or 72cc's based on formula #2. The benefit/HP will come from the intake runner. Valves stay the same. Any small % increase in HP due to CR increase will be a bonus but shouldn't compromise me with detonation potential.

 

Try this calculator.

I get 9.8 with a 350 using a piston with a 5cc valve relief (?) and 64 cc heads and .020 deck plus .040 head gasket.

10.28 with a .020 gasket.

http://www.angelfire.com/fl/procrastination/motor.html


Need to verify what the valve relief is. If you get real crazy you can calculate the space between top ring and top of piston with the .001-.002 wall clearance. That's for people who can't sleep at night!


JIM

 

When I rebuilt my 355, I used flat top/4 valve relief pistons, zero deck and .040 quench. Compression calculated to 10.05:1 with 64cc Vortec heads. This is in a computer controlled truck, so I wasn't too worried about compression. To keep yours down to a pump gas level, you'll need to use 70cc heads, I think 76's would drop it too far.
Go to the Ross Pistons website and find their compression ratio calcluator. It's the most accurate of the online ones I've found.

Oh, and don't believe that you're going to get 8% horsepower increase going from 10:1 to 11:1. It's about that from 8:1 to 9:1, then it drops as each point is added. You're probably looking at around 3% or so from 10 to 11 to 1.

 

If you like spending money, get all your figures stroke, bore, deck height, gasket thickness compressed, and chamber size.
Call JE or Wiesco or one of the big hitter piston companies and tell them what ratio you want to end up with and have them machine a custom set of pills for your engine.
They'll come to you looking to nice to hide in an engine.
If my wife wouldn't detonate, I'd hang them on the walls instead of some of those goofy pictures.

 

Thanks Jim,

I just tried your calcualtor and my compression ratio went up to 11:1

 

Not sure how you're doing that.

Is the block truly *zero* decked? Even .005" makes a difference. Stock is usually around .020" down in the hole.

What head gasket thickness? You need to add deck height and gasket together for that program.

Your pistons surely have some valve pockets of some sort. You need to know how many cc's they hold. That is input a a negative number.

JIM

 

Sorry Jim, I misled you.

I tried your calculator out on my engine, not the OP's

 

No where on here did I see a piston change or a decked block. So until you know the neg cc of the piston or how far down in the holes the pistons are you can't really buy a head cc size.

The lobe size on a L-82 cam makes it wimpier than the numbers would seem to indicate.

Some cams like the Crane PowerMax are made to trap pressure Lumpy sounding cams bleed off pressure. The l-82 is so mild that it doesn't do a good job of filling the cylinders. So it can actually run higher static compression VS something like the 272 Crane

 

To the OP all of the early engines with 64cc heads to get to a factory
rated 11.0 compression ratio 302 (290hp) 327(350,365,375 hp) 1970, 350 (LT1 370 hp) all of them had a dome piston. obviously the dome on a 302 because of the stroke was bigger then a 327, dome on the 327 bigger then the dome on a LT1 350. All of these engines had .018 compressed steel shim gaskets. Pistons were down in the hole. 1971 heads all went to 76cc. Domed pistons were all gone even on the 71 LT1. L/48s used a dished piston. 71/72 LT1s flat top for there 9.0 rating. L82s showed up in 73 same forged flat top piston. They still used the 0.18 compressed steel shim gaskets. Anyway if for sure your engine has never been decked you really should check the depth your pistons are down. But most likely you can use the .015 compressed fel pro gasket thats made for aluminum head use. With flat top pistons of your L/82 don't even consider a head with more then 64cc chambers. If you just slap on the standard .041 compressed gasket that come in most full gasket sets with your pistons down in the hole even with 64cc chambers you won't have didly for compression.

 

There is a lot to setting the right compression for your use and combination. A comparison of two engines I have is a good case in point. I built the one in my Corvette that is in my signature, the one in my 64 SS I did not build and would not have built it like it is.

The engine in my 64 Impala SS is a 69 Corvette 350 block. The pistons are .030" over generic hypers with 4 valve pockets. They are down in the hole .025" with a .041" gasket making a .066" squish (quench). It has iron 462 Camel Back heads @ 64cc making for a 9.57 to 1 compression ratio. The cam is a generic cam that companies like PAW and Summit sell with 218/224 duration, .445/.465 lift, 114 LSA. The intake is a Performer with a 600 CFM Carter Competition AFB. 1 5/8" long tube headers.

For what it is the 64's 355 runs strong, but not as strong as it could have been. It is a little fussy on gas when it's hot outside, but is fine most of the rest of the time on 92 octane.

Had it been built with a tighter squish and a more suitable cam it would run stronger and have less sensitivity to detonation. My 406 in the Vette is running 11.8 to 1 with a .034" squish with the rest of the specs in my signature. It has less trouble with detonation than the 355. Combination and tuning are everything.

On the squish, I do not understand why Chevy guys never seem to tighten the squish up beyond .040" I have built a lot of small blocks with squish set between .034" and .036". I never set them looser and I have never had any issues, quite the contrary actually. They run crisper, make better power and have less detonation sensitivity.

 

FWIW if your doing that much to your engine, I think you should change the cam as well. You can pick up some extra power with a little more lift and a fast lobe, even if you keep the same 222/222 sized duration. Most SBC's can use a little help on the exhaust side too. 222/228 etc, I'm sure several cam guru's can drive far better advice than I can.

My 383 is supposed to be 10 to 1 w/alum heads...it's probably actually 10.5 to 1 looking at the parts used etc. It has dieseled on hot days on 93. Wasn't tuned perfect but even after tuning the tuner said during the hot summer a few gallons of 100 octane gas might be needed to completely avoid that issue.

If you go 11 to 1 compression, might keep that in mind 100 octane unleaded is fairly easy to find if pricey, if you have any pinging etc and don't want to back off the timing etc.

 

I though I was going to be the fist to say it.... but looks as if I were beet!! lol...


Change the cam..... the L82 cam is not that great... you seem to be going to a lot of work changing other high-priced components not to replace one of the most important components... you talking 3-8% hp gains... want HP? change the cam also, its that simple, and make sure its a roller cam.

and make sure your chose components match, if they dont you will not be happy... a 600cfm carb, an RPM manifold and talking 10.5+:1.... stock cam, aftermarket high-performance heads.... to me this is not matching........

of the heads you chose, I would go the Ededbrocks.... but if I were to choose again on mine, I would go AFR or TrickFlow...

 

If the OP sticks with his Edelbrock Performer RPM Manifold and 600 CFM Carb and changes his mind about about switching out the cam, what cam/rollers, heads and gasket thickness would you recommend for him (and others)? I guess this is based on an original stock motor.

 

Tops are always a crapshoot to some extent. Ideally, the heads are pulled and deck height and piston volume are established before any decisions are made. Failing that, you make some smart estimates, play it conservative and roll the dice. Not ideal, but it works if done carefully.