Will this piston/head/cam combo work on pump gas?
 

I need advise from the experienced gurus here. I picked up a '73 Lemans with a fresh rebuilt .030 over 400 motor. Has Silvo-lite 1526 pistons ( -10.0cc) and probably .020 down in the block, 6X-8 heads. 204/214 Summit 2800 cam, Fel-pro gaskets and ARP rod bolts. Quadrajet, stock intake, logs, 2" stock exhaust. 3.23 gears, stock 400 Turbo and converter. Engine runs nice but doesn't have much power. I have access to a freshly rebuilt pair of big valve # 13 heads (checked at 73.5 cc). Can I bolt these on with a little more cam ( a Summit 2801 or 2802 or an 068 ) and run this on pump gas on the street ? Or will compression be too high / quench too big and have detonation / pinging ? I would like to give this car some go with a simple head and cam swap. TIA !

I for one would like some information about this engine.

Can you do a cranking compression test and post the numbers?

Also does anyone else think that the exhaust maybe holding this engine back some?

Stan

It’s no wonder the motor runs like crap!
Assuming your 6X-8 heads have not been milled I get a total CC volume by using the rest of your numbers, plus the head gasket volume that your missing of 9.4 CCs, your total is 123.8 CCs!!

Your compression would then be 6.7 to 1, yikes!

Put on those other heads right away if they have been gone thru and are fresh.

If your using a late intake manifold you will need to run the right intake gasket to seal up the bigger exh crossover in these heads as compared to the 6X heads.

Steve,
Just using your total number (123.8 cc's) I get 7.7:1 CR for a 30 over 400. Which is still very low.

But not knowing what he has is why I wanted to see what the cranking compression numbers were.

Stan

The heads have not been cut, they were checked at 100 cc's. I will get cranking compression and post it.

Head 100 Cc
Dish. 10.
Head gasket. 9.4
.020” deck 4.43

Total = 123.8
Do we agree on that CC total?

4.150 x 3.750 = 50.742 cid, or 831.2 CCs

831.2 / 123.8 = 6.7 no?

Shouldn’t it be 831.2 + 123.8 / 123.8 ?

Yes, your right, i screwed it up!
I always seem to do that wrong, sorry.

So your new compression at best would be 9.54 to 1.

Cam wise I would run something with the duration close to the 068, but with more lift and on a 112.

Ooof. Reminds me of my first rebuild, except I had a way too big cam with the 6X-8s. Those 13s and and 068 should really wake it up.

Will the pistons .020 in the hole make it a little more prone to detonation?

That would basically be the Summit 2801.

13s are a great head for a 400 and the 2802 is a fine choice. Run it.

Wallace racing site lists a 400 cid with 6x heads @ 7.6:1 with stock pistons; not sure if they were beveled on the edge:

1975 400 185 HP Y9 A 7.6 255 6X 1-2 7045167 Block Casting # 481988/late 500557?

My 1971 GTO had the ys400 with 96cc heads listed @ 8.2:1 , and i suppose it was 250HP. It Ran pretty good.

I came up with 7.81 using butlers calculator and provided information I did use .035 for head gasket

Using .035” assumes the gasket is round and it’s not, so it’s volume in CCs is bigger then what that .035” factor would come up with.

Back to your question Stan, yes.
If nothing else those 2” head pipes need to go!

So just for you I did it at .050 also 7.62 it’s a estimate but I know you will be dead on. I think the thing is compression is low making the power low but I will let you experts take it from here .

swept vol. (disp. }, plus dead vol. ( the total combustion chamber vol.) divided by the dead vol.= C/R.
I get 7.71 C/R

I use the calculator on S.D. Performance site.

My 77 TA came with the lowest HP engine, 7.6:1, 400 Pontiac engine, 6X-8 heads.

The tune spec was either 16 or 18 degrees, advanced.

Just wondering where your 400 stands, concerning timing?

My car ran pretty well, was peppy to drive. Obviously not a world beater like earlier Pontiac 400's were, but was a great driver.

Compression tested the motor
 

I went into the garage tonight and compression tested the motor. At 35*F cylinder numbers 1 and 3 cranked at 130 and 135 psi with the throttle wide open and a brand new DieHard battery. Ran the car for about 15 minutes until hot and repeated the test. This time cylinders 1 and 3 cranked at 145 and 150 psi. Out of curiosity I walked over to my 67 GP with a stock rebuilt YH 400 (670 heads) and it cranked at 175 psi cold with a 3 year old battery. That car runs well on 93 Octane. I am realizing I will have to upgrade the exhaust also now to 2 1/2 inch and the log manifolds should go too.