mrg

If memory serves me correct, I recall GM had some developmental hemi engines in it's R&D stable, but these never reached the production stage .. Seems the hemi designed engine (read Chrysler) made gobs of power, but the BB Chevy wasn't no slouch, either .. I was wondering if anyone might share some insight into the how's and why's of those days gone by when "Hemi was king" .. Well, that's what the Dodge boys said .. :D .. Is the hemi design better, perhaps in some ways, than conventional combustion chambers .. ? .. Was the hemi design better suited to the high octane gas available way back when .. ? .. Just wonderin' ..
mrg

SWCDuke

Part of the Grand Sport program was a twin plug hemi head for the SB, but that engine was stillborn after GM management cracked down on internal racing programs. It just wasn't far enough along in development for Duntov to sneak them out the back door, so the first Grand Sport went out the door to Grady Davis in the summer of '63 with a production L-84 to get a little development time in SCCA C-modified. Dick Thompson won at Watkins Glen in August, and when the car was returned to GM for Nassau Speed Weeks preparation, they received 377 CID SBs with a quarter of side draft Webers - basically off the shelf stuff - along with the flairs, scoops, and vents. There is a picture of Duntov's stillborn hemi in Ludvisgen's book.

In the fifties and sixties GM made a lot of money and spent a lot of money on pet projects. There were numerous OHC and DOHC experimental engines, no to mention Buicks with de Dion rear axles. When I was in high school, circa 63/64 Hot Rod had a story on a bunch of Pontiac experimental OHC and DOHC engines and had a picture of them along with McKeller, their engine guy, on the cover.

When I went to work for Pontiac as a production engineer in '68 I asked about those engines and one of the guys took me down to the basement of the engineering building and we found them in a heap in a corner, covered with dust. I was shell shocked! At that time Pontiac had some tunnel port heads on the dyno, but they never made production, either. After '68 all the engineering resources had to be concentrated on emissions and safety.

Hemis typically have better high speed volumetric efficiency because you can get more valve area for a given bore and there is minimal valve shrouding, but because their combustion chambers are "open" with little if any quench area they typically can't handle as much compression on a given octane fuel as a quench chamber typlified by the SB or early "closed chamber" BBs. Also, you can't get an optimal plug location at the center of the chamber with a two valve hemi design. I figure that's why Duntov's SB hemi heads had two plugs per cylinder.

Modern four valve heads can be viewed as a development of two valve hemi heads eventhough the basic four valve architecture dates to 1905. With four valves and their resultant "pent roof" chamber, you can design small quench areas on each side of the chamber adjacent to the valves and with the spark plug located dead center, flame travel is minimal, so detonation resistance is good and they can handle fairly high CRs for a given fuel octane rating.

My Cosworth Vega is an excellent example of a modern four-valve design. Because of the central spark plug, it only needs about 32* of WOT spark advance versus about 38* for a SB and 40* for a closed chamber BB. The less timing an engine needs to develop peak torque/power the more efficient it is because the combustion process is closer to the ideal thermodynamic cycle of heat addition at constant volume.

Duke




[Modified by SWCDuke, 10:37 PM 8/16/2002]

67HEAVEN

How about a semi-hemi.

All Chevy big blocks 396-502 were/are semi-hemis and make lots of the "thing" we all love............power :D





[Modified by 67HEAVEN, 8:15 AM 8/17/2002]

JL66REDCPE

Duke

Great explanation -- as usual.

thanks

john lolli

mrg

Thanks for sharing your experience and insight, Duke .. Thanks 67 HEAVEN for posting that website .. I recently had to pull the head off my daughter's Saturn after it was found that one cylinder had no compression ... WHHhhaaa .. ?... A major leak, to be sure .. You wouldn't expect it on a 70K mile engine .. ? .. It was a burned valve; looked like a cutting torch burned away about 1/4 of that valve away .. ! .. Another valve showed signs of pitting and was replaced too .. It was interesting to check out the combustion chamber and valve layout on this head, as Duke described ... A mini "hemi" ... ! .. All is well now but I have to wonder what made that valve burn in the first place. . ? .. Maybe it overheated .. ?.. I dunno ..

joe58

Chevy and Ford were in a very heated battle for Trans Am bragging rights in 67-69. Chevy developed a semi Hemi style head for the Z/28 that many people call the Porcupine 302. A number of sets were made and were sent to Smokey Yunick for final development but the program was stopped after SCCA banned the 2x4 cross ram intakes. Smokey sold off some of them and there is a guy in CA. with a running Porcupine 302 Z/28.

67HEAVEN

http://www.camarogenerations.com/mem...es/yunick.html
http://www.yenko.org/Yunick/68yunick.html

Plasticman

Read recently that DC is bringing back the Hemi (all new engine design with Hemi combustion chamber and 2 valve head, but will use 2 spark plugs per cylinder) for trucks and perhaps the Chrysler 300.

Plasticman

SWCDuke

Modern engines are set up to achieve higher EGT in the pursuit of low emissions, but modern electronic control and three-way converters don't need as high an EGT as seventies vintage engines. There are a number of widely used exhaust valve materials and the selection is made based upon temperature resistance - the higher the EGT or the longer you want the valve to last, the better the required alloy. Of course, as materials improve, cost goes up.

I don't know what material Saturn uses, but if an exhaust valve failed at only 70K it is either an inferior material or there was something amiss with the engine. I have a friend who put 200K on a '91 Saturn before the engine gave up. Suggest you check the timing and consider advancing it a few degrees as this should slightly cool the EGT, and also keep track of the valve adjustment and maintain it to spec.

There is a Cosworth Vega story related to your experience. The 1974 model never went into production because the first test car burned a valve 48K miles into the 50K mile emission durability test and on tear down it was apparent that the second test car was about to suffer a burned valve, too. The '74 configuration had a vacuum RETARD system. What looked like a conventional vacuum advance can actually retarded the spark to increase EGT in all but top gear, and the resulting higher EGT was lethal to the exhaust valves. The emission system was re-engineered for 1975 with a catalytic converter and more timing without the vacuum retard system. I don't know the exhaust valve material spec on the '74 configuration but the '75 and '76 production models use 21-2N, which is a stainless steel (21% chromium, 2% nickel). OEM valves are no longer available, but I can buy 21-4N (and upgrade with 4% nickel) from SI Valves in Simi Valley, CA.

Having properly advanced ignition timing for all operating conditions is important to exhaust valve longevity. Those of you who do not have vacuum advance cans, or do not have a can properly matched to the manifold vacuum characteristics, which are determined by valve timing, are probably running higher than necessary EGT, which can reduce exhaust valve life.

Being as how I added a quick centrifugal advance and a suitable vacuum can (none as built) to the CV back in the late seventies, the valves were in pretty good shape at 70K miles despite 4000 miles of race track hot laps. The exhaust valves did have small pits randomly spaced on the seating surfaces, but they were not deep. After replacing the guides I chose to just power lap the valves, and it took awhile but I removed enough material - probably less than a thou, to eliminate 90 percent of the pits, so I got a nice seat and the valves will probably outlive me at the rate I'm driving the car.

Duke

SWCDuke

This is there first I ever heard of those heads, Joe. Thanks for the pictures. I guess they must have dusted off this old design to produce the "splayed valve" head from the early ninties, and I recall seeing those heads on an IMSA GTP car at San Diego back then.

Duke

threeimpalas

This is my favorite Arkus-Duntov creation:

SWCDuke

I'm about half-way through Jerry Burton's Duntov biography and the Ardun heads apparently suffered a lot of problems due to lack of development. Zora shut down Ardun, but sold the rights to the design and received royalties while he worked for Allard. I guess the revelation for me was the fact that the heads were never fully developed.

BTW, the biography is fascinating, particularly the insight into Duntov's youth and early years before he went to work for GM. I highly recommend the book to all Corvette enthusiasts. It belongs on your bookshelf next to Nolan's books.

I practically rolled on the floor laughing after reading about the incident in a Paris restaurant after his 1100 cc class victory at Le Mans in '54. Duntov expressed to the owner that the meal was less than satisfactory, so the owner refused to accept payment. Duntov insisted on paying anyway. The owner ends up chasing Zora and Elfi out on the street and finally shoves the money down Elfi's clevege, whereupon Zora decked the guy right there in the street. When the resturant owner got back on his feet he declared Duntov an "honorable man" and they were friends ever since. Great story, among many! :)

Duke

achapman

Duke,

There is a lady in our town that has a 1968 Pontiac Firebird convertable that her husband gave to her as a present. She still drives it often.... but what is interesting about the car is that it is a 4 spd attached to an Overhead Cam Six... So some of the OHC's made it into production.... :smash: but what was the fate of this engine.... I guess I was too young to remember it....

67HEAVEN

There were lots of the Firebird OHC 6's sold. Good little engine, but hard to compete for the public's attention during the days of all-out horsepower wars.

SWCDuke

Yeah, I think Pontiac's OHC six (with inline valves) came out in '67, but it actually cost more to build than a V-8, so it was sort lived - maybe three years. The was an A-body (Tempest) package call the "Sprint" (I think) that included the OHC six along with a special suspension and trim. Firebird may also have had a "Sprint" OHC-six package. The OHC six had a tendency to show excessive cam lobe wear that Pontiac never figured out.

The engines that I was refering too on the cover of Hot Rod that I found in a heap in the basement of the engineering building were all V-8s if memory serves.

Duke

achapman

Duke,

Yes, the car does have an option upgrade I remember that it has a 4 bbl carb and I believe a "split" exhaust manifold. One of my many high school jobs was working at a filling station, and once I had to deliver the car after an oil change. I remember that it was quite "spirited" for a 6 cylinder. I never could figure out putting a six in front of a four speed..... In a Firebird.... :jester

Duke you win the prize... here is a link to the Firebird Sprint.....
http://www.bcclassics.com/carsforsal...cainmar02.html

Well the engine was developed under the John DeLoran's watch..... it was 250 cubic inches and put out 230 HP.... :D

http://sprint69.tripod.com/sprint69s...ebird/id8.html


[Modified by achapman, 3:45 PM 8/19/2002]

achapman

Interesting on how some of the GM divisions "bent" the rules....

Quote:

The 400 had a special hood with two non functioning scoops, heavy duty springs, chrome air cleaner and rocker covers. The "Ram Air" which included further engine modifications and a better exhaust, also made the scoops functional. These motors were the same as used in the GTO which was rated at 350hp, however in accordance with GM's policy of a maximum 1hp per 10 pounds a small tab was placed on the secondary throttle linkage, which if removed gave an extra 25hp !.

:lol: :lol: :lol:

Daren Schneider

With respect to four valves per cylinder, I saw that the Germans in 1944 had
V-12 airplane engines with four valves per cylinder.... and get this....steam was used for little short burst of additional power (like a nitrous shot).

:cool:

achapman

My father flew the P-47 which had the big Pratt & Whitney R-2800 turbo charged radial engine... and they used a direct shot of water into the cylinder heads for a power boost.... plus the German planes could never out run it or out dive it.... :flag

colo63sw

The second car I ever owned was a 68 sprint firebird. It had the OHC 6, but with a 4 barrel Qjet, and special extended exhaust manifold, backed up by a heavy duty 3 speed and multi leaf rear springs. Ran good, but im my youthfull quest for hoursepower, I yanked it and put in a 396.