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

https://l.facebook.com/l.php?u=https...FbmnjhzwB093Lc

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.

...

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.

New thread started in thee OHC Tech section.

Look at the pictures and the caption with them.

https://www.motortrend.com/how-to/ah...-six-build-one

5.7" rod lenght. Don't know what the deck height is? But that makes the bore, stroke and rod length for the 230 ci engine is the same as a 307 ci SBC V-8.

Stan

Found this while I was on Speed Talk

https://www.speed-talk.com/forum/viewtopic.php?t=13927

Talking about 307 SBC Pistons.

Stan

I saw your mention of the OHC 6 being much lighter and that was a factor in your decision to possibly build one. I don't know how much lighter that engine would be than a Pontiac V-8 with aluminum heads which is quite common now.
The OHC 6 has an iron block, iron head and a mile long crankshaft. I would check the weights to be sure. If weight is a major consideration, an aluminum block/head V-8 may be lighter yet and make more HP of course. Lots of decisions to make. A 400+ HP OHC 6 is going to be an expensive build. But really cool.

Hard to argue with someone like Mickey. What kind of power would a pro team like his be trying to wring out of their engine? 400, 500, 600hp? Every last ounce, probably.

https://www.gomog.com/allmorgan/engineweights2.html
About weight, this website lists the I6 as 200lbs lighter than the V8. Aluminum heads save about 50lbs each right?

AS stated a decade ago; my 1969 OHC-6 Sprint had connecting rods superior to the Chevy Pink Rod. And I reckon them GM forgings to be similar to a RA V rod design.

Cameras were not a thing for me in 1982-1985

Being a Pontiac V8 minded fella, i loathed and hated high-rpm engine spinnings, yet found the Sprint to be in its own from 4500 to 7500. 7000-7500 upshifts were a routing thing on the major roads. Mallory Dual-Point dwell beat stock dwell, but the grafted HEI was fantastic.

Maple Grove run fell onit knees at the top of 2nd gear. ran mid 14s or 15 (forget), and never did add a pusher pump.

I dunno why there was no fear of spinning the factory flywheel! yikes.

. YES.

Ought to get to 400 Piston size, and actually the SBC 400 slug from sealed Power can be had 4.18" Bore ( thats 455+030 ) and lighter than the Stock OHC6 Slug.

The 4.18" Overbored OHC6 would retain the 7500 rpm capability.

Has anybody SLEEVED all six OHC6 Bores for 4.15/4.18.4.21"

I've been waiting for him to post an update on his Firebird - I believe he was having the engine work done at Ed Pink's shop, so it will be interesting to see how it turns out.

Do you think hypereutectic pistons are good enough? I'm concerned about them surviving any preignition and detonation I get while trying to get a good tune once the engine's been assembled.

I didn’t read through the thread for other responses but what kills engines N/A or turbocharged or blower etc. Is usually detonation. A good tune will go a long long way to making a set up last.
I don’t know much about the rods in the L6 but the blocks appear pretty stout. Id bet it’s make good HP torque and live if the rods are any good in them . Look for any short comings in oiling system too . Aside from that should live if the tune is good .

Many an engine builder has been blamed when the tuner should have been!

Why let it detonate set it up fat as hell and lazy on timing . Then back into it. Shouldn’t ever go lean if you start wayyyy fat and lazy

Dunno about hypereutectic (high silicon AL, but theses days aka CAST). i always use Forged slugs and they hold up. A breached headgasket (in 2002?) caused a warped slug-top at the leak.

The big thing you probably need to worry more about is head gaskets. You can use the factory rods with some ARP bolts , forged pistons, same as the Chevy 6. Rebuild the head and have a good tune. I so far have only run pump gas to about 15 lbs safely, just over that I did pop the head gasket. Replaced the gasket along with head studs. Car ran 9.42 @ 76.15 in the 1/8 on some old tires I got off Craigslist, spinning too damn much. Using a 1 barrel head will have your compression ratio good for the turbo setup, there is a document somewhere on the forum that shows the different ratio combinations with the heads available.

Having read through this thread, I can't really comment on longevity of the I6 in general, but I don't really think one of the stated goals of a lighter package is really in play here. It's pretty hard to find information on the weight of those things, but one source I've seen puts them in the 450 lb dry area, compared to about 630 dry for a v8.

An aluminum intake and heads v8 should be around 550-560 dry.

Adding a turbocharging system to the I6 is going to add anywhere between 75-130 lbs depending on the size of the turbocharger, whether it's non-intercooled, or is air-air or air-water intercooled etc. Weight wise, I think it's going to be pretty close to a wash.

The cool-factor with a turbocharged OHC 6 under the hood, is undeniable, but at the 400-500 hp performance threshold and very near or equal nose weight, you're going to be money ahead by building a typical street v8.

I think the head gasket situation could be dealt with. It is iron, thank goodness. ARP head studs or similar could be used. Clark, Flattout, Hussey or others could make a copper gasket. You could o-ring the head and block if copper alone won't get it done. Also, Cometic will make a multi-layer steel head gasket for that engine. These are not low cost solutions, but when working with a unique engine platform and trying to make big power with it, the costs do ratchet up exponentially. I build some high HP supercharged Studebaker V-8's for drag racing and endurance road racing and deal with similar issues. High HP, unusual engines are not for the weak of wallet.

Not sure of how much a OHC Boosted 6 cylinder GM engine will support.

I do know from experience that not all 6 cylinder engines are the same.
I did a engine study with a 300 cid @ord engine with a cast iron head and dual
small (correctly sized) Turbos (many years before the ECOBOOST 6 cylinder stuff)
that destroyed the 302 and 351 Naturally Aspirated similar trucks in a 3 vehicle shoot-out.

We are talking 100 foot margins here in the quarter mile racing.
Not once not twice but throughout the whole day.

Nino Novelli, his brother Gino Novelli, and I made a bunch of V-8 guys unhappy that day in front of a large crowd of people at the test track.

We saw almost 500 hp with the 300 cid engine (less than 15 psi boost and a small inter-cooler set-up.) Far more HP vs the V-8 guys.

So could a OHC GM smaller engine with a proper turbocharger make slightly less than 500 hp with todays turbo technology? For SURE.

Charlie66 has made HP much higher than that with his 4 cylinder engine.
But he has a good flowing (not great flowing) E-Head.

Has anyone ever put a OHC 6 cylinder head on a flow bench to see what that OHC head is capable of NA??? That would be the first question.

Tom V.

From this article.

Stan

1968-1969 (only!) Olds 400 was 3.870 bore. 1.75 CH. However, 40 over gets you 3.91 which is 318 mopar, if you can make that work.