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Pontiac SOHC three-valve per cylinder V-8

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#1 Gerald Ryan

Gerald Ryan
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Posted 19 January 2008 - 07:01


Pontiac sohc 3-valve V-8 engine

Have any of you come across any information about this engine? http://www.wallacera...c-v8-cover.html

It seems that Pontiac were experimenting with sohc engines from the early 1960s. A sohc six made production but the most interesting prototype was a V-8. That one had sohc and three valves per cylinder. It was 421 cid. What was unusual was that there was a single inlet valve and two exhaust valves per cylinder, the reverse of usual practice. I'd certainly like to find out more about this feature of their design and how it played out in practice. It seems Pontiac thought this gave benefits in flow (possible reason was no interference between paired inlets) and reduced exhaust valve temperatures, hence better resistance to pre-ignition and detonation. The combustion chamber was similar to conventional wedge shape but with the large inlet valve opening onto the centreline of the cylinder. The small exhaust valves were on either side, displaced up the wedge and closer to the spark plug. GM upper management set a cubic inches per unit weight mandate and the project was halted. Apparently someone sold a Pontiac internal report (cylinder head flow data and engine dyno results) on ebay a while back but I've not been able to find anything.

Anyone know any more about it or anything about this engine?




#2 McGuire

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Posted 19 January 2008 - 12:44

Yep. If you read between the lines of the 1968 HOT ROD story the backward three-valve proved inferior to a standard layout... and they surely knew that before they built it. But here is a perfect example of how GM used to do things, and how all OEs still do it to some extent. What are the pros and cons of two divergent approaches? Let's build them both and find out the whole story. And while there are some attractive aspects (chamber layout, intake valve and spark plug position, exhaust valve temps) you can't get around the fact that intake valve area needs to be around 125% (pick a number in that neighborhood) of exhaust valve area.

At least one of the two-valve SOHC V8s escaped into the world and survives. Mac MacKellar (the Pontiac engineer in the above story, long since retired) has one installed in a 63/64 Pontiac B-body and has driven it around to local car shows. I last saw it maybe five years ago in the parking lot at the Meadowbrook Hall Concours. He would be getting on in years -- hope he is doing well.

There was another SOHC Pontiac V8 at about the same time that article was written that was totally unrelated to this program. Pontiac contracted Repco to build a two-valve SOHC V8 on the short deck 303 block, supposedly for use in the Trans Am series. Sort of a shot in the dark slash feasability study, or most likely an excuse to throw some money in a particular direction. And maybe to find out just what Repco had in the Olds, if it was really any good. Basically the two-valve Repco Olds layout adapted to a Pontiac block, there was one known example... which ended up in a store room at the General Motors Insitute, now Kettering University. In the '80s some students and instructors restored it but couldn't make it run. I am sure it is still around somewhere too, but I don't know where.