When are siamesed center exhaust ports a liability on a V8

[PackardV8]

Most of the Gen I Kettering-style V8s, Olds, Cad, Pontiac, Studebaker and Packard, had the two center exhaust ports paired inside the head about 1" before the manifold flange. A typical firing order is 1-8-4-3-6-5-7-2, with odds left bank, evens right. So 3, 5 and 4, 6 do not directly follow each other; at what cam duration and overlap does the shared port become a major obstacle to power production?

To a lesser extent, the four center exhaust ports come together at the exhaust heat crossover under the intake manifold, but that's seldom a determinant problem and usually blocked anyway in a performance build. We used to melt pistons and fill the exhaust crossover, then grind the fill to match the other ports.

FWIW, back in the day, we used to put a lot of work into contouring the dividing wall face of the shared port and grind a matching divider welded onto the exhaust header flange. Then, we'd make four-port headers. It noticeably changed the sound of the open exhaust, but with the few reground cam profiles available for those engines, we really couldn't find that much more power with the four-port exhaust.

What would we do differently today?

[novadude]

Great question. I am anxious to see the replies. I was looking at a friends Oldsmobile 455 heads wondering about this.

[pdq67]

I am so hoping that bigjoe1 pop's in here and offer's his real world experiences!

pdq67

[falcongeorge]

I have been wondering the same thing, especially with all the talk about Mr. Vizards magic incantation lately. I am building a 324 Olds using all "period" parts, Thomas mag rockers, Isky E-3 (234 on 109 lsa, which is the narrowest I could find in a "period" cam, .465 ish net lift), Edelbrock OL-496 with four 94's, ect. Have been wondering about if, or how the siamese exhaust port affects valve timing decisions and valve seat angles...

[bigjoe1]

One thing that comes to mind along this subject.. Engines that have the two center exhaust ports were never helped very much, or no help at all, from a good set of tubing headers.On the famous W-30 Olds engine, when we built a custom made set for dyno testing, it only picked up FIVE HORSEPOWER. No one could believe it. This was also what I saw later when I put headers on a Trans Am Pontiac, with a 403 OLDS ENGINE==== NO GAIN at the drag strip



JOE SHERMAN RACING

[modok]

A typical firing order is 1-8-4-3-6-5-7-2, with odds left bank, evens right. So 3, 5 and 4, 6 do not directly follow each other

I think you meant "they don't overlap", I would say they DO directly follow each other. They are 180 degrees apart, so both valves will be open at the same time for a short period.
Although the ideal depends on a dozen variables, i have played with this a lot, and find 40degrees overlap to be a good rule of thumb for the ideal overlap.
So, you could say, If the cam duration exceeds 220 @ .050 then they do overlap enough to cause some loss.

[rfoll]

Would it help on the Olds to build a divider on the port? I was never impressed with the 3 tube Pontiac header either, it just looked wrong.

[falcongeorge]

I don't really want to divide the exhaust ports and run 4 tube headers on my deal, due to the appearance. I wanted to run headers basically copied off a NOS drivers side Belond I have. The header I have is for the 303 heads, and I am running the '56 #10 heads, so I have to make the headers, as the original Belonds fit the earlier round-port heads. Retaining the circa late fifties stuff is a MAJOR restriction, I think I could probably pick up 30+ hp just with a similar 230ish modern camshaft.

[modok]

Those olds and pontiac from what I remember it looks like you could divide the port.

What if you use a "center" header primary that is 2x larger, and slit the pipe halfway down and weld a divider into it. It would look like a 3 tube header but the center tube would actually be a tri-y internally! Or maybe not. It'd be entertaining for sure, but off the top of my head I don't know that it would be the best layout.

If you keep the Siamese primary it might be safer to go with half length. 40" tube length the center primary would surely be out of time with the second cylinder, as well as the second cylinder screwing up the firsts scavenge, so you could have NOTHIN goin right.
But at half lenght(20something inches), I don't think the timing would be a problem, and still benefit from a good collector. It's been tradition to make the center primary one tube size larger than the outers, and that seems logical. Yeah, when it doubt use half length.

If you want to confuse the competition further use half length for the center primary and full length for the outers.........big old "Ram's horn" deal. Might work! might not. You like building headers? try it!

[falcongeorge]

NOS driver side Belond "header" for 303. Closer to a glorified exhaust manifold...

belond.jpg

[Rizzle]

Considering gm's modern-ish v6 with fully integrated exhaust port (yes, one per bank) can do 86hp/L, NA. I cant see a single combined exhaust port being an insurmountable issue to a good power figure.

[modok]

Yeah that's good design, they fire 270 degrees apart.
The downside is the big exhaust port junction there is a lot of mass that can get hot, when those overheat it warps the CRAP outa them!

With short cams firing 180 apart is ok, but duration and longtube headers changes the game. Lets use four valve heads! there you go problem solved.

[Truckedup]

What was done on Stone/Woods/Cook supercharged gasser Olds engine back in the 60's?

[Roadknee]

I helped develop a 304 ci Studebaker V-8 in the early nineties for use at Bonneville. Welding a divider wall in the center port was among the many modifications. Cam was a Crower solid roller with high 260's at 0.050" and 0.600+ lift. This 9.8:1 engine made 450 hp on the dyno with around 115% VE. It made just shy of 640 HP with the Paxton supercharger bolted on.

[iadr]

Ed Henneman (Headers by Ed) was adamant the siamese tube had to be of altered diameter. IIRC his very first header was for his split port Pontiac, and that car got more than a dozen sets of headers built for it over the years.