BigBlockThunder

Hi everyone, I'm having a valve job done on my 1967 L-79. I have two important questions, and one has no bearing on the other. It's been bored 30 over and runs really, really well. The first is, should I bother to do the unleaded conversion on the heads or not. I run the car on 30-40% 110 leaded (Cam 2 C-12) and the balance 93 unleaded. Compression ratio is a solid 11:1, and the car runs great. Is thare any disadvantage to doing the conversion? The second question is regarding the replacement head gasket. I know that if I go with a Fel-Pro, I will loose compression due to the thickness, relative to the steel gasket. The last think I want to do is detune this engine. Is there any good reason to go with the Fel-Pro over the steel head gasket? This car sees at most 2000 miles a year on it, and is in mint condition. Thanks in advance

Joel 67

You'll probably get a few different opinions on this but here are mine. Take them for what they are worth.

1. I would not spring for the hardended valve seats. Unless you are doing a lot of hard driving I do not think you will have a problem with premature valve seat errosion over the normal course of an engine's life. I would spend the money saved on a nice valve job and pocket porting.
2. I would go for modern head gaskets. The sealing capability is so much better than a shim gasket that it is extra insurance. I do not think you will have a noticable performance drop between the two, especially if you do the above mentioned head work
3. You did not ask about this but I will mention it to you anway--I do not think you need to mix any racing gas into your car. As stated, you don't need the lead, and unless you are hearing detonation you do not need the extra octane. Of course I do not know anything about your timing, etc so it is hard to state for sure, but a stock L79 should be ok on 93 octane pump gas.

Good luck

Scott Marzahl

I would not opt for the hardened seats and because you are mixing race gas you are helping the seats and preventing seat recession. From what I understand, you only need to mix about 25% leaded race fuel in a tankful of gas every few months to eliminate seat recession. The stock "151" cam reduces your Dynamic Compression enough to run pump premium or mid grade depending upon your timing curve.

How do you know you are at 11:1, did you remove the stock pistons and install new pistons or deck the block since stock compression was around 10.25:1 on a L-79.

As far as head gaskets, you have more options than just a .040" Fel Pro. You can use a NAPA/Victor Reinz P/N 5746 (0.026") thin composition head gasket or a coated steel shim Fel Pro 1094 which is .015". The Victor Reinz would be a good compromise as it is the same if not very close to a stock .022" - .026" steel shim gasket.

wmf62

per an earlier thread. leaded 'racing and aviation fuel' is on it's way out..... soon there will be no more leaded fuel of any kind for any purpose. unless you want to rebuild again for unleaded sometime in the future, i'd plan for it now.....
Bill

Allcoupedup

You don't need the hardened seats unless you are hot lapping or doing something else that sees continuous heavy load on the engine. Or... if your valve seats show signs of recession from multiple valve jobs.

AS far as the head gasket goes, the compressed thickness on steel shim vs. composition may be as much as .020 which will drop your compression ratio by up to a half a point. If your deck is straight and your head is straight, steel shim will be just fine. If your heads are getting milled your CR will increase but you can offset it by using a composition head gasket.

Whatever you do, make sure you take measurements to actually compute your new CR. I don't know if you've measured or not but many people assume their CR is as stated in the manual which is most likely a false assumption.

brian

GCD1962

NASCAR is going fully unleaded next year. I would imagine Sunoco will drop the CAM2 leaded very shortly thereafter

63 340HP

The lack of leaded fuel should not be a concern with the low miles and usage that you practice. The loads needed to accelerate seat recession are heavy towing (not likely to occur) or racing (where there is more to worry about before the next rebuild than seat recession).

The gasket thickness does more than seal; the gasket thickness determines the distance between the piston at TDC and the flat sections of the head chamber surface. This distance or clearance between piston and head is termed the "quench height" (or simply "quench").

The quench height is important because if you let it grow past ~0.060" the gas trapped in the resulting space has the potential to promote detonation. The distance or clearance where the quench height becomes excessive and promotes detonation is different for every chamber (because the chamber geometry is different for every head & piston combination).

Guidance given to me from experienced 64cc SBC engine builders is to keep the quench height under 0.050" to minimize detonation (most of the books advise tighter is better down to ~0.035"). This advice is offered with the warning that if you let the quench height grow into the 0.080" range you will promote detonation even if the static CR is under 9:1 (the problems experienced in the 1980's when the poor octane solution was double gaskets and thick gaskets with no consideration to quench height). Select the gasket that will locate the head somewhere close to 0.038” to 0.045” away from the piston at TDC (you will need to know the piston deck-height and gasket thickness, and add the two to determine the quench height).

Current textbook advice on detonation prevention is to keep quench height tight and alter the static compression ratio (CR) with piston dome/dish, and alter the detonation potential with the dynamic CR by cam selection & cam timing. The dynamic compression ratio is altered by using a cam with wider lobe center or a longer duration intake profile, or retard the stock cam, to delay the intake closing event and effectively shorten the dynamic compression stroke. The DCR usually needs to be around or under 8:1 for 92 octane fuel.

Someone with knock-engine testing experience using the old style 461/462 chamber (or simple experience with multiple SBC engine configurations) might be able to offer better guidance on when the quench height is large enough to become a problem (what gasket and deck height combination to select)?

Advice for how to time the 141' cam for the best compromise between performance and detonation resistance on modern USA 92-octane is also valuable (anyone want to offer experience on where to time the cam for a good DCR matching 92 octane fuel)?

I hope this information helps you to prevent a potential problem with detonation.

SWCDuke

How do you know the CR is a "solid 11:1? Did you measure the deck clearance of all cylinders, and use the specified piston compression height and dome volume with various available gasket thicknesses and run it through a CR calculator to see what CR range you can achieve. Don't believe ANYTHING you read or hear about CR. It has to be MEASURED on each individual engine.

As OE machined few L-79s left the plant at more than 10.5, and 93 PON is just fine, and there is no need for valve seat inserts.

Duke

Kensmith

Duke,

Is that true of the 10.5 compression ratio as everything I have read about the L-79 327/350 states 11.0. I know that is probably an advertised number from Chevrolet but how do you figure 10.5? Thanks,
Ken

SWCDuke

The 10.5 is based on actual MEASURMENTS of all the parmeters of actual engines required to compute CR and then running them through a CR calculator.

Chevrolet's advertised CRs are about as accurate as their advertised gross horsepower.

In the month that's passed you've had plenty of time to measure deck clearance and run the numbers through a CR calculator to see what CRs you get for various head gasket thicknesses, and the MOST you want is 10.5 with a minimum of 10, and 10.25 is a good target for the L-79 cam.

Duke

Donny Brass

my stock L79 measure 9.4:1 in the rear cylinders and 10.2:1 in the front.........

now it runs a true 11:1

knight37128

If you like to throw money around for no good reason; throw it this way. Just Pm me and I will give you my address.

No; you do not need the hardened seats. It is the most wasteful money you can spend.

ghostrider20

Victor makes a .028 compressed thickness gasket. Used on my 65 L76 with +.125/5.3CC stock domed pistons and .025" deck height, produced 10.53:1 compression. (Heads CC'd out at 64.5CC.)

With this combo you will need some good gas, I run 5 gal. of 100LL avgas and 15 gal. pump 93. This gives me detonation free use with
14*-16* BTDC. 8*-10* on pump 93 only, with 8* producing no detonation at any range, and 10 having some high load, low RPM transient.

Mark