Team Chevelle banner
  • Hey everyone! Enter your ride HERE to be a part JULY's Ride of the Month Challenge!
1 - 8 of 8 Posts

·
Premium Member
Joined
·
2,173 Posts
Discussion Starter · #1 ·
As I get closer to assembling my 1969 L78 396 + .060 I began searching old postings for head gasket recommendations. Most who expressed opinions preferred steel shim over composite head gaskets. I have built Big Block engines using both. Given most of the discussions I reviewed were a decade (or more) old, I thought I would ask the question again.

The block was finish honed (with torque plates) using composite head gaskets, but considering the block was not decked I am now concerned about quench, given the top of the pistons (Speed Pro L2242) will probably sit .015-.020 below deck height at TDC. Cam is a Howards enhanced L78 (.539 lift - 242 @.050 duration - 114 degrees lobe separation). Heads are 840 castings with one light surfacing.

1. Should I be using steel shim head gaskets in order to achieve (approx) .035-.045 quench?

2. Am I OK switching to steel shim gaskets after torque plate honing with composite gaskets?

3. Which steel shim head gasket shall I use? (manufacturer and P/N)

Thanks!
 

·
Premium Member
Joined
·
1,528 Posts
I would suggest doing the measurements and determine compression ratio based on various gasket thickness. .o39 is average for composite gaskets with other thicknesses available. Steel should be .015 (?) not sure of that). Easy time to fix a high cr to offset detonation problem later
 

·
Registered
Joined
·
2,445 Posts
As I get closer to assembling my 1969 L78 396 + .060 I began searching old postings for head gasket recommendations. Most who expressed opinions preferred steel shim over composite head gaskets. I have built Big Block engines using both. Given most of the discussions I reviewed were a decade (or more) old, I thought I would ask the question again.

The block was finish honed (with torque plates) using composite head gaskets, but considering the block was not decked I am now concerned about quench, given the top of the pistons (Speed Pro L2242) will probably sit .015-.020 below deck height at TDC. Cam is a Howards enhanced L78 (.539 lift - 242 @.050 duration - 114 degrees lobe separation). Heads are 840 castings with one light surfacing.

1. Should I be using steel shim head gaskets in order to achieve (approx) .035-.045 quench?

2. Am I OK switching to steel shim gaskets after torque plate honing with composite gaskets?

3. Which steel shim head gasket shall I use? (manufacturer and P/N)

Thanks!
You don't mention "fuel", but if your counting on straight pump you may want to regroup and check all your numbers. If you're planning race gas/E85 then no problem!

I wouldn't be overly concerned with the "quench" just yet!

Thanks, Gary in N.Y.

P.S. Without some very accurate numbers I believe you may around 12:1 with that unit?? This is based on 106 cc head chamber (840 castings) and a .022" gasket!
 

·
Premium Member
Joined
·
2,173 Posts
Discussion Starter · #4 ·
Thanks for the reply, Gary. GM listed the factory compression at 11:1 and I have changed nothing other than a "clean-up" surfacing on the heads. Many believed (back in the day) the 11:1 factory rating was actually more in the neighborhood of 10.6:1.

Yes, I can re-check the numbers but many with experience with such engines can probably tell me what to expect. That said, I certainly don't want to end up with 12:1 in any case.
 

·
Registered
Joined
·
2,864 Posts
Just throwing a few numbers in the calculator, with a .039 head gasket and .015 deck and 37.10 cc dome and 106 head chambers that's around the 11.3 to 1 range.

Steel shim .018" all the ones I ever measured, would put you at 11.95 to 1

That being said I would go with composite gasket.

I ran a 396 with 11.8 to 1 compression, but I had a huge 262/273 @ .050" 1969 ZL-1 factory cam. that gave me 150 or so cranking compression and ran on 87 octane with my timing locked out at 36 degrees off the trailer ran mid 12's then low 12's then finally 11.60 range with getting the car dialed in.

One of my current engine is 10.4 to 1 with a 240/[email protected] .050" cam and it's at 185 psi runs fine on 91 octane.

I think your cam is to small to "get away" say 91 octane fuel, I would try pushing the cam up 10 more degrees to 250 something @ .050" on that 11.3 to 1 motor.

Or clean up the combustion chamber and open them up to the gasket, every 1cc is about a .1 drop in compression so if you could get your heads to say 112 cc chambers should bring your compression down to ~10.7 which might be a little more manageable on pump gas.

I know for me I run my junk on 87 octane mostly and anything really below 185 psi cranking compression I have not had any issues. So I build accordingly to what I know.

If you stick with 11.3 to 1 for me I would just cam up, your still going to have plenty of torque to do what you want, just saying.

Or change heads, or run e85 or different pistons...........so many options

See what others say.
 

·
Premium Member
Joined
·
2,173 Posts
Discussion Starter · #6 ·
Thanks, Dennis, for your input. The numbers matching L78 that is in my car at present runs pretty strong on 91 octane with a bottle of 104 dumped in. No pinging whatsoever. I believe the cam to be stock and the engine has 210-215 lbs of cranking pressure. With that kind of cylinder pressure I assume the pistons are of the L2242 (38 cc dome) variety. Timing is 12 degrees static with 25 degrees mechanical advance, all in at 2,500 RPM. Distributor is an MSD Pro Billet. Heads are '66 858's. I also have a set of 291's I could fit out and put on the new engine, although they are date-matched to the original block so I was saving them for when I rebuild that engine. I believe 858 and 291 heads have 3cc larger chambers than 840's, if I remember right.

I hate to go more radical on the cam as the stock L78 cam is so mellow and tractable.

Thanks again.
 

·
Super Moderator
Joined
·
13,143 Posts
Ole,
I tend to lean towards reliability more than anything else. That being the case I would recommend the factory type composition head gasket from Fel Pro. Unless this is a race engine looking to get every ounce of power I think you are over thinking the whole deal.

Just my opinion,
 

·
Premium Member
Joined
·
2,173 Posts
Discussion Starter · #8 ·
Thanks, Mr. Bill. I agree and am now planning on using the set of Fel Pro composite gaskets that I purchased awhile back for this engine build. I also found out yesterday that one of my 840 heads has a slight crack, one that (according to the machinist) probably would not cause any problems, but a crack nonetheless. So it is time to dust off the 291 heads I was saving for the numbers matching engine and assemble them for the replacement engine. That should help a bit with the compression, as 291 heads are supposed to have 109 cc chambers, whereas 840's are around 106 and change. This should put the new engine in the 11.0:1 range.

Thanks again! I always appreciate help from an experienced hand.
 
1 - 8 of 8 Posts
Top