The Stovebolt Site Forums The Shop Area Engine Shop Offy with Fenton headers on 235

I know there is some issue with using an Offy intake with Fenton headers on a 235, but it can definitely be done. Anyone know what the issue is? I assume some kind of clearance problem?

Here is a picture of a 261 with an Offy and Fentons that I took in Hershey. (maybe it was one of you guys and I didn't know it?)

Ron

I wrote a long reply, and fat-fingered a key and erased it. Drop me a private message and I'll be glad to discuss some of the alleged advantages and a LOT of disadvantages involved in that modification. Bottom line- - - -it's mostly "eye candy" to impress the mouth-breathers at a "show and shine" without any real improvement in power.
Jerry

I had an Offy manifold with Fenton headers on my 235 and didn't encounter any issues. Look at this post.

Hi baldeagle ................ just saying Hello - as I see that you are from "Texas" .

A long time ago - - - I worked in Richardson - (Richardson Paint & Body , over on Texas Street) . I grew up over in Garland and went to GHS - I moved out 'west in th' summer of 1980 .

I even remember when Plano was - - "plain - o" , and the last time I was there; We drove through Sachse .............. WOW ; like a hugh 'growth- pill' .

Jim Partridge

Everything you mentioned is still there. You're welcome back any time.

I put the Offy on my truck which has a 235. I did not use the Fenton headers, I did some stainless steel version of headers. The issue I had was that the longer of the 2 shifter arms from the 3 on the tree would hit the side of the intake so i shortened it just enough to clear. No other issues. Hot Rod Lincoln is right no real performance gains but it does look good. I probably already have a picture in here somewhere on my signature line.

I think the writers at Hot Rod (and Frank McGurk) would disagree with Hot Rod. Just adding headers to a 261 increased horsepower by almost 4 percent, and adding a dual intake manifold with two 1 bbls added 16% more horsepower. The combination of the two should yield (based on these tests) about a 20% increase in horsepower.

I never had my truck tested, but it certainly didn't hurt performance.

It’s all relative. These early chevy motors respond to being hot rodded just like any other motor. More carb, more cam, more compression, more spark, more gear, more rpm, it all adds up to more power. But it’s all relative. These are relatively small motors so the gains are relatively small. You just can’t bolt an offy 2x1 on a 235 and turn it into a screaming high hp power V8 killer, lol. For those of us who like to mess with our inline motors we measure success more modestly.

My first modification, I bolted a Weiand 2X1 on a 235 in 82. The car it was in ran a little faster with the two carbs than the one carb. It was not a lot faster, but a little faster. Then it was a rebuild on the motor, then a cam, Clifford headers, head work.

Never very fast but it was fun tinkering with it all.

Put your offy on and go have fun. Add a little water heat to the bottom of the intake (the fentons have provision for copper tubes to get exhaust heat but I have never found those tubes to work all that well.)

Alternatives to the single bbl. Rochester carb, 1974 Ford Pinto Holley Weber 5200 progressive 2bbls. Small primaries, larger secondaries, almost perfect for a 2x2 install. The Rochesters can be a little too big and tend to run fat. Smaller carter w-1 carbs would be a choice.

But if you want to use the Rochesters, and get them right, here is a link to Pat Smith’s excellent article on how to do it. Pat was Dyson Racing’s IMSA crew chief for many years and one of the best minds I knew when it came to this stuff.

https://www.inliners.org/tech/tech4.html

Bigger 261s with all the mods can handle bigger carbs. One of my 261s uses a pair of Holley 2300 350 cfm carbs and runs great.

But it’s relative. Small motor small gains.

Go have fun my friend.

Give it 3/8" more stroke and a 60 overbore- - - -which adds 40 cubic inches and a massive amount of low end torque, instead of trying to make that "El-Choko" cylinder head breathe better. Then you won't need all the eye candy.
Jerry

Thanks to all those who responded, and especially thanks for the links with more information.

Just to bring this back to the original question, the picture I've attached is the reason I asked about what issues there would be with Fenton headers. This picture is from the Classic Parts catalog. There is a clear warning about not working with the headers, yet people have done it. Just trying to figure out why they put this warning, in capital letters no less. Maybe it's just the lack of a heat riser on the headers??

Also, horsepower gains really aren't my priority. In fact, my concern would be more of a loss of horsepower from improperly tuned/synched carbs than anything else. That's not to say I wouldn't welcome more power if I could get it, but I'll openly admit that I'm really after the eye candy cool factor! (much to some of your dismay!)

Anyway, please continue the horsepower conversation, and if anyone knows why they put the disclaimer in the catalog, let me know.

Ron

Again, I don't know why they put that warning there. BTW, there were several different versions of Fenton headers. I had the originals. Maybe later versions didn't work with the Offy manifold. Mine worked with no problems, but I got my Offy and Fentons from a 1955 235 that had them mounted when I bought it, so they were both from "back in the day". Maybe latter versions didn't work with them both installed. Mine worked with no issues.

I have heard of issues arising with the thickness of the flanges being different between the intake and exhaust manifolds, causing problems with leakage because the bridge clamps don't hold the parts tight to the cylinder head. Also, the quality of the offshore made "Fenton" clones is poor at best, nothing like the USA made manifolds from the 1950s and 60s. Using coolant to "heat" the Offy manifold is a lost cause- - - -it's cold when the manifold desperately needs exhaust heat- - - - -during warmup. It's possible to plumb exhaust gas heat to the area at the center of the Offy intake manifold with some creative plumbing- - - -right where there isn't a carburetor to benefit from it. Why don't the "It worked fine for me" comments ever include dyno run figures to back up their claims? (Maybe because there aren't any?) "Your mileage may vary!" My dyno tests usually documented no improvement- - - -or a power loss everywhere except wide open- - - -high RPM operation.
Jerry

Many may not know this but offy has a new owner as the last offenhauser with any interest retired. The new owners name is Jason Curtis. He is a young guy. He has spent the last two years rebuilding the equipment, repairing the tooling, and finding a new USA foundry for the offy products. He has all kinds of offy stuff now available, for all kinds of motors, parts that have not been available for a good number of years. You should see the new small block valve covers and mid rise dual four intakes.

These are USA cast and USA machined parts. Here is the rub. You have to buy from offy direct. Any place else is a licensed product (prior to new ownership) that is overseas product.

Jason is really great to talk to, he frequently answers the phone himself. If you are going to get an offy, spend the extra and get a real one. My guess is better quality than the licensed stuff.

Newly cast chevy valve covers, now with a baffle welded in. Small block ford port injected, valve covers, they now hav a side cover for a 235.

OK, so I'm not crazy for wondering what's the point of using coolant. By the time the coolant is up to tempt, the manifold probably is as well, or at least well on it's way. I really couldn't quite figure out the benefit of that system, but thought maybe I was missing something.

The manifold gets warm but the floor of the intake really needs to get hot to make sure the fuel atomizes. Otherwise it will puddle a little bit and you end up with a sloppy pig of a motor. So, I use the hot water method. I use to use no heat but many years ago now, I was sitting with Tom Langdon at the Carlisle swap meet and he persuaded me to give heat to the bottom of the intake a try.

And the truth is, Tom was still working as an engineer for GM, had a great deal of racing experience both on his own and for other big name folks, and every thing he ever advised me on was spot on. So I converted my Weiand intake to water heat and it ran better with the Carter YF carbs I was running at that time.

So I tend to be in the heat the intake corner, lol!

This is my Weiand intake. I ran this intake from 82 through 2017 or so when I switched to a Clifford 2X2 which has the hot water chamber cast into the intake.

If you look to the right of the intake, you can see my hot water plate.

My 20 something son is building a 261, and it’s a budget build, so I am resurrecting this intake set up for his motor.

GMC lower thermostat housing. They have ports on both sides of the housing, heater side and intake side.

Well then, there's the part I was missing!

I have Tom Langdon's twin progressive weber two-barrel carbs on my Chevy 3600 with a 235, and the Fenton twin carb intake. I also have Fenton split exhaust manifold. I ran hot water to the intake. I re-jetted the primary (smaller idle and main jets) on the weber carbs to my expertise. I have the 6848 head. I have NO dead spots when the engine is cold, it accelerates very well, no lack of power at any speed, I have the SM420 four speed, Gearvenders Over drive and a 4:10 rear end out of a 71 C20 Eaton. It gives me a final of 3.2:1 ratio. Hills are No problem, I can run with normal traffic, and I have gotten as high as 20MPG highway and 15MPG around town. 55mph is 2100rpm and 60mph is 2300rpm. I have the stock cam with NOS hydraulic lifters. It starts instantly at a dead idle, runs quiet and sounds awesome with the dual exhaust all the way back to the rear bumper. Did I need to do this? No, but I enjoy a project that challenges me. My automotive machinist friend who has worked on these engines many decades ago said they used to do all kinds of improvements to the 235 like I did to mine. I could not be happier with my truck.

Go to the effort to plumb the manifold floor with exhaust heat- - - -it's worth it for good performance through the entire operating range. The best way to do this is to use an OEM exhaust manifold WITH A HEAT RISER and do the "split" that people in the know have been doing for 75 years or more. It's not just a matter of adding a second pipe- - - -a baffle is installed that almost (but not quite) separates the manifold into two separate chambers. A small gap is left at the top to allow a small volume of exhaust gas to be shared by both sides. Like P.T. Barnum, "Fenton" knew how to reel in the suckers. An inline six engine needs either ONE, or THREE carburetors- - - - -not two!
Jerry

If one drives the vehicle maybe 15~20 miles when the ambient is between 35 and 45 degrees and the atmosphere is foggy, then one will learn first hand while manifold heat is important. Inline engines tend to have more of an icing issue than Vee engines.

The above temperatures in a light rain will produce the same results.

Jon

What Carbking said!

Icing is a problem for me in the winter even with exhaust heat to the aluminum Offy manifold, at least till the engine warms up. It was a problem with the stock setup on the various Chev sixes I've had. One could take out the heat insulators in the winter but what a hassle. I found much less icing when I used an electric fan but then there were a host of problems with that. You would think that ethanol gas would forestall the icing but that doesn't seem to be the case. Once the thermostat opens the problem goes away in my case.