How much power could an OHC straight six support?
 

Wondering how much power Pontiac's straight six is capable of, strength-wise.. I6's are great for turbocharging and weigh less than a V8, right? I've seen some cool-looking turbo'd I6s online but haven't seen their dyno numbers. If the goal is sub-500hp numbers, wouldn't a high boost, small cube engine be more efficient than a low boost, large cube one?

To my knowledge no one has dynoed a boosted OHC 6. Looking at the way the 230 block was designed and the use of forged rods, there were plans for more horsepower. With that being said, one could build a turbo 6 close to 500 hp with the right parts and controls in place. There are a couple builds planned, but not sure if the builders will Dyno them or not. I would build a 230 if I had a choice.

Actually, several have been boosted with both turbos and centrifugal superchargers. I don't know of any max-effort race builds but I know of at least one that made over 400 horsepower reliably and I would suspect that the bottom end would support quite a bit more with some forged rods and pistons. Chevy small-block internals are easy to come by, making rod and piston choices readily available.

Hello Don. I am aware of the centrifugal supercharged OHC 6 in the 69 FB. I have yet to see the actual dyno data on that boosted OHC 6. Pretty much only bench racing numbers. While there are several that have been built and are currently being built, no one has shared actual data to my knowledge.
Rob
Woodland Motorsports LLC

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This is a max effort 4 cylinder

I misread your statement and I thought that you meant that you hadn't seen any builds, not dyno sheets- my mistake. I did a story on that car in HPP many years ago. I do remember seeing a dyno sheet on that car and IIRC it was 401 horses at the wheels, which would be pretty impressive by any standard. I don't have any notes on that story, as it was all lost in a hard drive crash ages ago.

I am in the process of a hopefully high hp 68 250. I started building it a couple years ago but got sidetracked on a few other car builds, was planning on running a s360 but have since decided to go much bigger and really see what it will do. I am going to run either a really nice magnum 80mm turbo I have or a s488. I think either one will make plenty of power once I get them lit. I have a sheet metal intake I started building that I plan to run and will probably end up making headers. Was going to run a stock exhaust manifold but I am worried about getting it sealed with my intake. Hoping to start back on it now that I am done helping with my brothers twin 85mm 74 gto. This combo is going in my 74 gto that I ran the twin turbos in for years. The ohc bottom ends look very tough and I think it will make some steam if I can get enough air and fuel through the top end. I am very excited to get back on this project, these are very cool engines from an engineering standpoint.

So 400hp+ is doable with a turbo? Would an I6 fit under the hood of a 2nd gen firebird? Reason I ask is my bird has T-tops, and most everyone agrees that, even after frame upgrades and a minimal roll cage, more than 500hp is asking for permanent damage from body twisting. So if I'm keeping the power level below that, a lighter, smaller engine with 12-15psi of boost is a better choice on paper than a full-sized V8 running 5psi. And less weight in the front will make for better handling, too.

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Thanks for the photos, I wasn't sure the timing cover wouldn't be too tall.

I see 600 HP in the article, but don't know if that's from dyno runs. 600 seems optimistic on a little 3-71, 2-port injector, tiny exhaust stacks, etc.

Searching the internet forums for boosted I6 engine builds, I'm seeing a bunch of projects like new manufactured cams, modified Ford roller followers, etc. Most of them seem to have petered out due to cost or lack of interest. Are there sources for aftermarket parts for higher-than stock power outputs? Has anyone successfully adapted parts from other engines? I'm sure a major piston company could make forged pistons for a hefty price, but a durable valvetrain for a high boost, high revving engine is what I'm concerned about more than the bottom end.

Not much in the way of performance hardware out there for the OHC motors, especially the cam and followers.

Jay Leno had some followers made out of tool steel but that seems unnecessary as there are stock one available.

Clifford site mentions a manifold but I don’t know if they are really available.

A few options on getting custom made forged pistons but nothing specifically made and in stock for the 230 or 250.

In my opinion, you won’t be able to order a bunch of stuff online and build a hot OHC .
But you can use stock components (generally pretty stout) and/or try to make or have some custom parts made. Sounds like the definition of old school hot rodding.

Isn't the I-6 bore size the same as a 350 Pontiac V8?

Stan

Wallace Racing lists the I6 bore at 3.87" for the 230 and 3.88 for the 250. Stock 350 bore is 3.875 and I have a set of 3.905 TRW pistons. Would pistons for a V8 work, or does a I6 need a different skirt or something? Pin offset? Piston weight and crank counterweight?

What you need now is to see if any other engine has the bore you need with a compression height around what you need. If one doesn't posts them up I see if I can find my books and post them up.

A couple more which my work are SBC 283 and SBC 307

Stan

And the pistons would need a sizable dish in order to run pump gas.

Something like this? https://www.summitracing.com/parts/u...-020/make/jeep

Since most Pontiac C/R's were overrated from the factory, what are the actual combustion chamber sizes?

Are the rods for the Chevy and Pontiac I6 the same? Scat makes forged rods for the Chevy.

I don't know about "strength-wise". But back when I was on the phone a lot with Fritz Voight (Mickey's crewchief, who gave me a lot of help with my hemi V8) I asked if he and Mickey ever toyed with the OHC six. He said yes, but any attempts at serious power caused head cracking between the valves.