I'm playing around with ideas for my pontiac. I'm trying to judge the tradeoffs of deck height vs compression ratio based on the parts I have laying around.
I've got a set of custom Ross pistons with 21cc dish, 1.54 pin height and 4.155" bore, a pontiac 400 block with 10.225" deck that fits those pistons, and a set of ported 670 70CC heads. I'm thinking about using a 4" stroker crank with BBC rod journals and a set of 6.66" or 6.7" BBC rods. Basically a 434 stroker built with parts I have, to keep cost and machine work down.
Would it be better to have a 9.65:1 compression engine with the pistons .025" down in the hole, or a 10:1 compression with the pistons .015" out of the hole? 91 octane corn-free or 93 octane with 10% corn are available locally.
If there was a 0.016" to 0.020" embossed shim steel head gasket available, I would go with the short rods and shoot for a 0.040" to 0.045" squish/quench with the 0.025" piston deck height. I would want the lower static compression ratio because of the iron heads. General recommendations among us wrench twisters is that you should limit static compression ratio to no more than 9.5:1 with iron heads and 10.5:1 with aluminum heads for use with today's unleaded pump fuels. Using your numbers and a 4.250" x 0.018" shim gasket (I just picked this thickness out of thin air, but it is an educated guess), I get 9.82:1 SCR. I could live with that using the tight squish/quench.
I was researching and found a part number 3494VC under the Clevite banner and also under the Victor Reinz banner. I have entered a query with both Summit Racing and the Mahle Corporation, asking them what the compressed thickness of that gasket is. I should get a reply within 1 to 3 days and will re-address your question then. I strongly suspect that it is a thin steel gasket simply because of the cost, under 20 bucks.
You and the rest of the fellows and gals on this board should understand that a thinner squish/quench is much more important than a lower static compression ratio. In other words, if you increased the squish/quench measurement by using a thicker head gasket to lower static compression ratio, then the motor would be more inclined to detonate than it would with a higher static compression ratio and a thinner squish/quench by using a thinner gasket.
Thanks for looking into it. Cometic makes several thicknesses of head gaskets for pontiac engines, they're just expensive. I'll find the paperwork and confirm the piston compression height and block deck height. I'm working from memory. I did have the block decked, but that was for 6.8" rods and these custom pistons with 3.75" stroke. With the 400 crank, the compression is 9.2. I'd like more cubes and a bit more compression.
The goal is a strong street/strip motor in a 3600lb LeMans. Pump gas with a 125-175 shot of nitrous.
A larger quench distance will make the engine run hotter than normal. That is why you will get more preignition, not more detonation. High octane racing gasoline will not prevent preignition.
I built my 389 (400) with some of what Tech has suggested.
I zero Decked the block (actual .005) and used a .041 head gasket.
With my 7H1 heads,milled .045 ..... YES .045 they ended up at 80 CC.
That gives me a static of 9.49 to one with the forged slugs and a 6CC eyebrow.
The 670 heads you have IIRC can be open or closed chamber.Not 100 % sure about that. Maybe mousefink knows for sure.
It would be better if they were closer to 80 CC.
Have you CCed them to be sure?
You absolutely have to CC them. No guessing there.
Pontiac 670 heads are 72 cc closed chamber heads. The 670 heads have great flow rate, even comparable to the famous RA4 heads. Pontiac machined the spring pockets .125" on the 670 heads and used them with longer valves and taller valve springs (1.719") on the first 1967 Ram Air engines with the 744 camshaft. The 744 cam regularily broke the standard 1.586" valve springs. I used a pair of 1967 Ram Air heads on my 1966 GTO with 1.719" valve springs. However, 1968-1979 Pontiac 72 cc open chamber heads will make more overall power because they will tolerate more initial and total timing advance. I traded the 1967 Ram Air heads, casting #97, for a pair of 1968 Firebird 400 / GTO #48 heads and improved performance.
The engine was rebuilt as a long-rod 400 in 2004. The heads were totally rebuilt, new valves, hardened seats, viton seals on cut down guides, Comp 995 springs, ARP screw-in studs, springs pads cut to 1.7", etc etc.
The heads are closed chambers, they were CC'd at 70CCs and ported to flow 236CFM@.500 lift intake, 185@.500 exhaust. So I know exactly what the head specs are. The Ross pistons were selected to work with those heads and give 9.3 compression with 6.8" rods and a 400 crank cut to BBC rod journals. I sprayed the engine with a 125 shot and it loved it, shifting at 5800. Best it ever ran was a 11.70 at 111 MPH.
The engine ran strong from 04 to 13, when it ate three lobes of the HFT cam. I pulled it out for the autopsy. The valvetrain is shot but the rest is still good. So instead of just going roller, I figured why not stroke it and go roller too? The piston's CH is custom, so it gives me the chance to play with stroke as long as deck height is manageable.
I never got into stroking Pontiac engines. There were plenty of 428 and 455 engines, crankshafts and various cylinder heads available in salvage yards by the time I quit racing. I bought a good pair of 64 heads for $200 in 1985.
In 1988, I found several pairs of good 670, 48 and 64 cylinder heads and a complete Ram Air 4 engine in local a salvage yard. I purchased a complete 1970 Ram Air 4 engine for $200 from a salvage yard and the salvage yard pulled it and delivered it to my garage. I went back to that salvage yard two days later to get the 3.90:1 rear end but they had already crushed the 1970 GTO that the Ram Air 4 engine came out of.
I had a 1969 Firebird 400 in 1972. It was factory equipped with a 335 HP engine with #48 heads.I rebuilt the motor with a Crane Z-300 solid lifter cam, forged Pistons, Ram Air 4 valves, Chevrolet 427 valve springs, 4.33:1 Saf-T-Track, 428 HO exhaust manifolds, hood tach, Muncie 4-speed, and 12" Goodyear wrinkle wall slicks.
In 1973, I matched and grudge raced a white 1969 AMC AMX 390/340 HP and was beaten badly, 3 put of 3 races at Dallas International NHRA sanctioned dragstrip. The AMX turned a 10.90 -11 flat ET and I was running mid 11 ET. I was informed that the AMX 390/340 in factory trim, actually had over 400 HP. Three months later I wrecked my 1969 Firebird 400 in a outlaw grudge race on a rural county road against a 1969 Plymouth 440 Roadrunner.
In 1988, I won DAPA Fastest Full Size Pontiac award at Dallas International in my 1963 Pontiac Catalina 421 HO, 4-speed, 3.90 Saf-T-Track. Best ET /. Speed was 14.20 / 97 MPH. Eliminated by a 1963 Chevrolet 409 with a 4-speed.
With 428 and 455 blocks drying up, the 400 stroker is becoming the go-to combo for pontiacs. Most guys are building 460-474 inch combos with stock 400 blocks and 4.21"-4.5" strokes. There are aftermarket blocks that can go up to 535". Rumor has it that Smokey Yunick tried to tell Pontiac engineering in the early 60s that they didnt need the 3.25" main journal to get the 4" and larger strokes to live. Looks like he was right.
I found my build sheets. The piston CH is 1.54" and dish is 22CCs. I calculate that the piston would be .020" out of the hole with a 6.7" rod and 4" stroke. I could use a .060" thick Cometic head gasket, which would give me .040" quench and 9.78:1 compression.
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