The Stovebolt Site Forums The Shop Area Engine Shop Manifolds and Carb suggestions.

I have a 1949 GMC one ton and my GMC 302 engine is finally back from the machinist and we are doing the assembly. I would appreciate opinions on whether split exhaust manifolds create power or mostly modify the sound of the engine. Patrick's son says 10% increase in power from the split exhaust alone.
I have a 302 military intake manifold with four hole carb mount and a civilian intake manifold with two hole mount. For carbs I have a Zenith 2 bbl and a Bendix Stromberg 2 bbl (four hole mount). I am willing to go with a Holley 4bbl or Edelbrock 4bbl provided I can find an intake to match (Patricks is currently out of their GMC intake with four bolt mount and Holley adaptor plate).
My goal is additional performance as I hope to build a camper for the flatbed and anticipate trips to the mountains where the power will be needed. Any advice is appreciated.

There are experts who will answer your question about dual exhaust manifolds resulting in 10% power increase. I doubt it.

The 270 and 302 exhaust pipe diameters are larger than on a 228 (and maybe on a 248).

What diameter dual exhaust pipes would you be using?

Tim, it would all depend. I'm guessing the split exhaust manifolds, new intake for a Holley and the Holley itself would be nearly 800-$1000. I would have to have a good reason to spend that much without a significant gain in performance. So I would go all in with all the pieces to establish no restriction in flow (larger exhaust-2"?) or go stock.

Call Buffalo Ent. had repro Nicson GMC headers some time ago.
Ph 360 652-7684

The advantage to any split-type manifold or header on an inline six is the separation of exhaust pulses from adjacent cylinders in the firing order. 1-5-3-6-2-4 separates into pulses going front-rear-front-rear- - - - -etc. There is no "stacking" of exhaust impulses in the same pipe, so there is better cylinder scavenging with a dual setup. A measureable 10% power increase? Maybe in the land of unicorns and the Easter Bunny!
Cast iron split manifolds are fine- - - -RUN, do not walk away from tube steel headers! Don't go too big on exhaust pipe diameter on a split system because it will slow down the velocity of the gas in the pipes and reduce the scavenge effect.
Jerry

Nice explanations, Jerry. Thanks

I've read/heard that there's some advantage to having an "X pipe" or some type of crossover connection on a stovebolt with dual exhaust.

What is the explanation for that?

Bovine scatology! Just more pipe to rust out and much more complicated plumbing.
Jerry

Crossovers do not work on inline sixes due to the firing orders, thats what Doctor Gas told me and my brother when my brother was still building custom mufflers. They will tone down some noise, but not really help in power like they will on a V8.

I appreciate the merged post-helpful information. Surely someone here has made these improvements and knows what's real and what's pretty. I would think that someone has dyno information to document what they gained.

He also makes a neat adapter plate for using a modern Chevy bellhousing to a 302, or Buick or Cad., for that matter, makes it easier to use some modern trans, also, easier to bolt bellhousing to block. I have one on mine.

Ed

I had reams of dyno documentation on a Ford 300 6 cylinder until some pea-brained school administrator decided to do some housecleaning in my shop during summer break one year. He emptied a bunch of filing cabinets with dozens of dyno run sheets on about 50 different engines, street machines and round track race engines! He also trashed Motor service manuals covering vehicles back into the 1940's- - - -"They were dirty and looked bad in the book shelf". He couldn't understand why I never had time to get the students to do a tune-up or a brake job on his car after that little incident!
Jerry

The conventional L6 (1-5-3-6-2-4 firing order) engine, and its performance use in side-valve, pushrod OHV, and OHC variants, is about 100 years old. The whole "what should the exhaust look like?" had already be resolved before WW2:
1. front 3 and rear 3 grouped together
2. keep the 2 pipes separate as far back as possible

This describes 1920 Grand Prix engines, Jaguar XK-E, and Toyota Supra.

The Austin-Healey 3000 with a 3-liter inline six had split manifolds and exhaust pipes that entered a siamesed-pipe muffler at a distance that became a tuned-length header when the muffler was removed. In the days of SCCA and other club sponsored races, this was no accident. On a series of dyno runs we did with the Ford I mentioned previously the best low end torque performance was recorded with manifold and header pipe volume that matched, or was a multiple of the volume of one cylinder. This allows the entire cylinder volume to be contained in the pipe to provide the maximum scavenge effect from the volume and speed of the exhaust gases. At higher speeds, best horsepower was recorded with relatively short exhaust runners. Like every other modification I can think of, there's a trade-off between HP and torque. Increasing one reduces the other. "There ain't no free lunch!"
Jerry

The run don't walk comment about the tube headers is interesting , Is it a fitment issue , a performance issue or something else?
I'm in the market for headers for a 261 and would like to do it just once, so I'll take all the advice I can get

Tube steel headers are notorious for warping the flanges where they attach to the cylinder head, and blowing out gaskets. They also raise the underhood temperature by a huge amount because the pipes ahead of the firewall are so thin, and a lot of that heat finds its way into the cab, after melting wires, etc. Cast iron split manifolds keep the heat under control better and unless the bolts get loose, they don't blow gaskets. The 120-degree separation of exhaust pulses also helps scavenge the cylinders better than the stock exhaust and marginally increases torque.
Jerry

If the headers are coated inside and out, heat is not as much of a problem. Coating is expensive, so the off set price could be comparable to cast headers or even more.

Here's an article on X and H pipes.

http://www.superchevy.com/how-to/ex...sepower-but-which-is-best-for-your-ride/

H pipes quite the exhaust sound noticeably.

Joe H, an inexpensive coating option is flame sprayed aluminum. The flame sprayed aluminum on my '36 Chevy PU exhaust that I restored in the early 1970s is a little bit dirty now from road grime but is still intact and has protected the pipes and muffler from rust.

In the summer of 1967, when I was a college kid, I had a summer job welding at Doug's Headers. Since then I've built several sets of tube headers for myself and others and I've had no leakage or blown gaskets. My present street rod, a sbc powered '32 Ford roadster has block hugger tube headers that have been installed for 13 years and over 21,000 miles with no leaks. The key may be using quality gaskets. The ones I used were in a Fel Pro set. The headers do radiate a lot of heat that makes the under-hood area like an oven. To compensate I built a cold air ducting system to feed the air cleaner cold air from in front of the radiator.

Totally agree about headers and heat. Some coatings are to protect, some are for heat. The heat protection is the expensive type. I have had headers on about everything I have owned and made a fair number of them. Even with just plain old steel headers, heat radiation wasn't much of a problem if the cooling system was working right. I took the set off my '37 and went back to cast iron manifold, guess I am getting old because the noise was bothersome. Thin walled headers make a lot of noise, and it's not out of the tail pipes, you can hear each cylinder fire or ping through the pipes. With the close quarters of the '37 cab to the engine, along with the open cowl vent, it was like setting right on top of the exhaust system.

I made some test run at the local drag strip with and without headers in a two week time frame. With low rpm and measly 110 hp of the 250, headers made zero difference, not in mph or ET. I used the same 2" muffler and tail pipe, head pipe was 2" with cast manifold, 2 1/4" with headers. RPM had little effect on the out come.

On the pull truck engines we built for all the low end torque we could get, a set of equal length header pipes where each pipe held exactly the volume of one exhaust stroke made a measurable difference in torque during a dyno run. On the Super Stock Truck MOPAR 440 engine we ran for several years, each header pipe needed to be 8 feet long. I used a 6" diameter collector pipe and let the headers extend back into the collector to hide the extra length rather than just letting the pipes end at different points under the truck.
Jerry

I am also presently weighing out whether to go with a dual exhaust...

I'm going to run a fenton intake with the dual tiny two barrel carter/webers that Langdon sells (too late, I already bought them- so I don't need opinions regarding these or other carbs!). With a 261 I feel like I ought to be able to run three of those carbs...but this is my first stove bolt, so I'm trying to play it safe by going with setups that others have proven work well.

If I decide to run the stock exhaust manifold on my 261, what diameter pipe would you guys run? It's a mild build- an 848 head milled 060 and pistons bored 060 with a Bulldog cam.

If I go with a fenton exhaust manifold, what diameter pipe would you guys run? I was considering a muffler with 2 inlet/one outlet with that setup as well...would that be a bad idea?

Thanks for any input,

Dave

Pipe size really doesn't matter- - - -the intake and exhaust ports are so tiny and restricted that you could almost run a soda straw as an exhaust system.
Jerry

Yeah Jerry, I know... but there must be an optimum diameter for a single or dual exhaust to get that extra 3% out of my clunker, right?

If you're running duals, there's no point in going bigger than 1 3/4" or so. Each cylinder pulse goes through a different pipe, so there's no way to "stack" two exhaust pulses down the same pipe. Going too big slows the velocity down and reduces scavenging on a 2-pipe system.
Jerry

I use original diameter pipes - around 1 3/4" (maybe 1 7/8" outer) diameter.

I do not know if they got me 3%, but they were easy to find to fit the headers.

Thanks!