Header size question

[tri-power 250]

I need to build a header to clear the Lakewood block plate on my 250. I am interested in knowing if their would be any noticeable horsepower loss building a 1 1/2" diameter header tube instead of a 1 5/8" diameter tube. The header will be either a 6 to 2 to 1 or configuration 6 to 2.
Thanks,
Ken

 

[bubba22349]

Assuming everything else stays the same ie lenght number of bends etc. a smaller ID. tube should lower the RPM for peak Torque and HP may not be much differance though.

 

[aussie7mains]

Theres a lot of "malaky" about headers, IMO it makes very little difference in a street application with silencers. as long as theyre low restriction and the classic 2 groups of 3 (for inline sixes) then it wont make any perceptable difference, I made a few sets and got good performance despite being suposedly too small and definatley NOT equal length. So make whatever fits and stop worrying about it. Some very good perfomamces have been acheived with well shaped low restriction cast iron manifolds, look at BMWs and Mercs. Our local 4 litre DOHC falcon gives 190kw on a cast iron single outlet manifold.
If your talking about a 200ci six, then the 1.5inch (38mm) will be just fine.
A7M

 

[tri-power 250]

A7M, Thanks for pointing out the cast iron manifolds. After I finish installing the Toploader in my Maverick, I'm going to start putting together a 11/2" dia header. There should be enough room to run 2 header pipes over the starter and in front of the firewall, (foot well), area.

Ken

 

[StrangeRanger]

Length from port to collector is more important in determining header performance than diameter (at least within reasonable limits.) What is important is that the header tubes do not inadvertently create a restriction to flow. Measure the exhaust ports and calculate the crosssectional area and then calculate an equivalent tube inside diameter. Ideally, the ID of the header should match the port as closely as possible without being smaller. Increasing diameter reduces backpressure but it also lowers exhaust velocity.

There are 2 reasons that stepped headers work. The first is that they set up two different tuning lengths and therefore two different RPM ranges for which the headers are tuned(typically HP peak for the short length from the port.) The second is that they maintain high velocities at the exhaust port then reduce the backpresure after the transition.

If you're up to the added complexity, you may want to investigate the possibilities.