Hello everyone, I'm planning on building a set of tuned headers for a 261 Chevy in the near future, and i have some questions some of the more experienced folks may be able to help me with. Since 261s have siamese ports on cylinders 2-3 and 4-5, i can't make a tuned header as easily as a normal 6 into 2 setup. I've come across this setup used on MG inline fours, the way they do it is make the center siamese port the calculated length, with the individual ports joining together at half of that, and eventually the collector. Has anyone tried a setup like that on a 6? It would be a 4-3-1 setup, (1-6, 2-3, 4-5) similar to the 3-2-1 on the british inline 4s. The only issue i can see is the pulses on the exhaust, as the way this is paired makes the 1-6 pair pulses every 360° while both 2-3 and 4-5 pulse in alternating 240/480° intervals. If no one has tried it, what are the opinions? Should i try it or just stick with a tried and proven 4 into 2 for my 261? Sent from my ASUS_X00DDA using The H.A.M.B. mobile app
Took me a while to figure out everything you were saying. It seems to me that the 4-3-1 setup you propose would be better than the 4-1. Each pipe in the collector would deliver 2 pulses for every 720 degrees of crank rotation and the collector could be a little smaller with only 3 pipes which would aid in the scavenging effect. The fact that the pulses in the pipes from the siamesed ports are not 360 deg. apart should not be a big factor. That being said, I don't know how much if any gain there would be. Those are my thoughts and I am just an amateur.
I made a cast aluminum equal length runner intake manifold and tuned headers for my 230cuin Chevy 6 many years ago. I divided the ports by brazing in dividers and epoxy dams in the intake passages to get rid of the Siameze ports. Luckily the exhausts were not conjoined.
Years ago when Colin Chapman first started racing he had a 'special' powered by a 4 cylinder English Ford engine. This had siamesed ports and the rules forbade modifying the head. So he welded a fin on the header to divide the port. Wonder if you could do something similar. Or maybe just a free flowing header would be practically as good. I've heard the best setup for a straight six is to group the 3 front and 3 back ports, make the distance to the collector as long as possible then run 2 pipes all the way to the back of the car. The longer the pipe the lower the speed at which it resonates and your six is not a high RPM engine.
The MG setup sounds like what is called an LCB (long centre branch) manifold on Minis and other applications of the BMC A-series engine: I understand that Brazil has a long tradition of building killer Chevy sixes!
Thanks for your input Ol' Blue Interesting take on the later L6, most people do Lump ports or just leave them alone i thought about it, but the ports are not too big to begin with, and while i have not looked at my ports or found a good pic online, the valve position on the other side of the head indicates that the fin would be over sized and unpractical . All i have seen are like that indeed, but i wanted to know if there were different options for the siamese ported engines. Yes, it's exactly that setup, i forgot to mention the Long centre branch name on my original post Yes we do! but with the later 250s... while with those we have guys on drag racing making 2200+ hp with stock blocks and heads (on big turbos and nitromethane) the older 261 is just a truck engine with not many aftermarket parts...
You might be interested in this article from 1954 on the then new 261 and its hop up potential.http://www.earlycorvettes.com/linda/mcgurkhotrod/mcgurkhotrod001.pdf Speed parts for the 235 chev six will fit the 261.
Can someone explain this long center branch business? Is there some advantage to making a siamesed exhaust pipe twice as long as a single pipe?
A little O/T, but it might be of interest. I have a vintage dirt modified that was the Minnesota State Champion "Stock Car" in the mid sixties running an Olds Rocket. While doing research on the car, I was able to get in touch with one of the original owner/builders (the Thurmer brothers from Jackson, MN). He told me that one of the things that they did to the engine to improve performance was to modify the siamesed center exhaust ports. He said that they added a divider in the head and enlarged the resulting individual ports to the max and built a set of four pipe headers. These guys were serious about the car and had an excellent winning record in southern Minnesota and Iowa in the sixties. He said his brother even flew to California an couple of time to meet with Ed Iskendarian, who made them a special cam grind designed specifically for their needs. He also told me that the two changes that they made to the engine that had the greatest effect were the exhaust modifications and adding a magneto. The heads are still supposed to be floating around southern Minnesota some place. I have been trying to track them down for years. I was in contact with the second owner of the car, who said he still had them, but wouldn't part with them. I have since lost track of him, and given his age, I expect he has passed on. I have no reason to doubt the information I have from the original builder, given the reputation of this particular car.
I wish I could add somerhing worthy, but I just wanted to say I was also beginning to look into what to do with with siamese centre exhaust ports on my Ford flathead V8.
Do you have access to an engine dyno? If you do, then you can build a few different designs, and see what works best. If not, then you can build the system you are planning, and tell everyone that it's based on sound design principles, and is the best way to make a set of headers for this engine. In other words, don't ask for approval here...just build it, and show us pictures when it's done. It sounds like a great idea.
I read that article and the way they figure it, you should treat the 2 siamesed cylinders like 1 cylinder running twice as fast, therefore that runner should be half the length of the others not twice as long. I'm still puzzled.
GraeffSS...This is about the best your ever going to do. It was designed by Burns Stainless in Costa Mesa Ca. They are covered in this photo and the stinger is also not on that I run. This is to protect my neighbors. 1 and 6 are 3 step and 2-3 and 4-5 are 2 step. The 4 are blended into 2 then the 2 into a tapered stinger. Upped every record I had by the addition of this system. This is a Siamese exhaust of GMC not a later 194-292 GM cylinder head no Matter what you think you are seeing. There in no reason to give you lengths because it was engineer for my cam, compression, valve lift, etc As a note I did not race for a year for the cost of it.
I really don't understand everything you said, but generally, I like it (except for that funky little muffler at the end).
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