Re: Weslake Chevy heads

[Duke W.]

Looks and sounds like they are basically scaled down 427 heads(?).

Thanks for the photo. I've never heard about or seen them before that I can recall.

Reminds me of the stillborn Pontiac tunnel port heads, but recently I found out that a few sets also went out the back door. Same deal - the power improvement was mostly above the rev range where even a race prepared engine had acceptable longevity, and SCCA said no, anyway.

Duke

[Michael H.]

Re: Weslake Chevy heads

Duke,

The combustion chamber looked similar to the chamber on the 69 and later open chamber big block head. Valve sizes were the same as production Z28 @ 2.02 and 1.6. The new head did produce more horsepower at a slightly higher point in the RPM range but the HP range was narrowed in the process. At 1000 RPM below the peak, the HP numbers were actually lower than those of the conventional head. In the end, the AVERAGE HP throughout the expected RPM range for a typical road course was less than the conventional cyl head. The intake ports were too large and velocity was less than desired. GM did prepare hmomlogation papers for the new engine for the 1969 season but I don't know if they were ever submitted to SCCA.

[Duke W.]

Re: Weslake Chevy heads

So is my understanding correct that your photo is the bottom side of the same head in John's photo, and these were designed by Chevrolet?

If so, then that still leaves an open question. Did Harry Westlake ever design a set of SB heads? What was the general layout? Were any ever built? Any photos available?

Duke

[Michael H.]

Re: Weslake Chevy heads

Duke,

Yes, the photo is indeed of the GM designed and mfg'd AL Trans-Am head. It was all "in house", and I know of no assistance, at all, from any out sources. Blue prints are mostly all dated 1968, although some were redrawn in 69, if I remember correctly.

The general layout of chambers, valves and ports was similar to the later Pro Stock big block heads. The intake and exhaust ports were all individual, not siamesed, as the conventional small block head. Every chamber and int/exh ports exactly matched the next in line. Not symetrically opposite as in conventional SB heads.

I have several fair quality photos and I'll try to post later this afternoon. Also think I have a dyno sheet here somewhere.

[Michael H.]

The Correct Pic Of The Semi Hemi Head

Sorry guys, I had an incorrect pic posted. (above) This pic IS the correct semi Hemi Chevrolet head.

[Dick W.]

Re: The Correct Pic Of The Semi Hemi Head

Michael, that photo looks more like what I remember. It has been about 15-20 years since I saw this engine apart.

[Michael H.]

Was Actually The F- -d Cleveland Head

Yup, that's the real one Dick. I didn't notice that I grabbed the incorrect pic the first time around. That was actually a pic of the F--d Cleveland head. I'll try to find a better shot of the Cheby head. I have a shot of the intake side with it's large intake ports.

There was one complete engine at Yunicks and several sets of heads.

[Verne Frantz]

Re: The Correct Pic Of The Semi Hemi Head

Michael,
The fellow who had that engine in a Camaro at the GM Nats also had a head just like that one on display in a case. The combustion chamber is definitely more of a "semi-hemi" design, very much like an open chamber MKIV. The plug entered from the side and not under the valve cover. He had a sign there stating that 302 produced 500ftlbs of torque! I have to wonder about that one....
I believe he acquired most of the parts from one of Smokey's auctions. It definitely was a very impressive looking motor.

Verne

[Duke W.]

Re: The Correct Pic Of The Semi Hemi Head

Peak torque is basically a function of compression ratio and displacement. A racing engine with headers and open exhaust will produce more peak torque than the same displacement street engine (nothwithstanding street engine's lower compression ratio) because the racing engine will make peak volumetric efficiency of say 110 percent versus 90 percent for the street engine. Current F1 engines may be as high as 130 percent, but VE levels were lower in the late sixties than today. Modern CFD computer modeling has done a lot to improve VEs of racing engines in the past 30 years.

Back in that era Keith Duckworth's seminal narrow angle four-valve head was unmatched for specific power output and peak gross torque was a little over 80 lb-ft per liter in max racing tune and peak VE was about 120 percent.

Given the above, a 5-liter racing engine might make something on the order of 400 lb-ft gross torque regardless of head design, but high flow efficiency heads will allow peak torque to occur higher in the rev range with less rapid torque falloff beyond the peak, which means more peak power.

I can't think of any naturally aspirated engine that makes 100 lb-ft per liter - just isn't possible on gasoline without boost unless someone can change the physics of our current universe.

Certainly with those heads and a decent induction and exhaust system that engine would have made well over 500 HP or over 100 HP per liter. Power would have probably been RPM limited due to valvetrain or bottom end stess/durability considerations rather than head flow.

Duke

[Duke W.]

Some other thoughts

High output engines can be compared regardless of displacement or head architecture by their maximum useable mean piston speed - assuming the valve gear and bottom end can take the level of stress at the corresponding level of revs.

It's interesting that the current limit is approaching 5500 FPM, and this range is achieved by F1 engines and... NASCAR engines. Isn't it amazing that a pushrod design can run at nearly the same mean piston speed as four-valve DOHC designs. So much for the common wisdom that pushrod designs are "obsolete".

The LS7 makes peak power (in emission controlled and muffled street trim) at 6300 revs, which is just under 4200 FPM. At the 7000 redline it's running just under 4700 FPM and 4500 FPM was probably the limit in the late sixties.

A 3" stroke engine at 4500 FPM is turning 9000 revs, so it was a matter whether Chevrolet could make the valvetrain live at that speed. NASCAR does today, but I don't think a 9000+ rev valvetrain existed that could live in a circle track or roadracing environment back then; 8000 was about the limit with reasonable reliability.

Mean piston speed in FPM is easy to compute - twice the stroke (in feet) times RPM.

Few actually know the mean piston speeds that F1 engines are running because specs are closely guarded secrets, but most observers estimate bore at something over 3.5" and stroke of no more than about 1.75". With their high compression ratio the combustion chamber is a thin disk, which has poor surface area to volume ratio that rapidly transfers heat energy out of the cylinders. That's one reason why they only make useable power at extremely high revs, when clock time for heat transfer loss is minimum.

Duke

[Dick W.]

Re: Some other thoughts

"but I don't think a 9000+ rev valvetrain existed that could live in a circle track or roadracing environment back then; 8000 was about the limit with reasonable reliability."

No one told Smokey Yunick that in the late '60's. We had a garage stall beside him for the Paul Revere 250 in Daytona. Being inquisitive like I am, I would poke my head in the car when they came in from practice. Every time the tell tale tach would read above 9,700, at least one time past 10,200 (but them again after that session all they salvaged from the engine were the carbs and valve covers )