Hey all,

I have a question about running header collector extensions. I have read where performance can be gained by using collector extensions to "fine tune" the exhaust setup.

I was using a header "calculator" program that one member on this board had posted on a separate website. For my 327 and it's intended use,(street driving), it said that I needed 36" long primaries in 1.625" diameter and 2.5" collectors about 14" long.

I will most likely buy a set of Dynomax Ceramic coated 1&5/8" headers with the standard 3" collectors. This is going in front of a 2.25" dual exhaust with a set of unknown name generic turbo mufflers.

Ok,sorry for rambling,but since the collectors are a little too large(according to the calculator)will adding collector extentions help or hurt low & midrange torque?


Thanks,
Mark P.

Collector extensions help the lower end of the power band by aiding in matching the flow (read Volume) of gas entering the atmosphere. Also provides a longer length tube for the velocity spike to bouce back for cylinder evacuation.

This is one of the easiest tools for tuning. Hooker makes set, plus you can have a set made.

Install them, paint a contrasting stripe down the length, make a couple of runs to discolor the same paint. The point of the paint strip showing the most discoloration, cut there with cutting tool, hacksaw w/32 teeth/inch blade or a tubing cutter. Now you have a tuned collector.

Ok,sorry for rambling,but since the collectors are a little too large(according to the calculator)will adding collector extentions help or hurt low & midrange torque?

A little too large? How about a 46% increase in area from 2-1/2" to 3", assuming an 18 gauge tube? Ed Henniman refers to this as a "2 tube size error". Once you expand the gas and the velocity goes down, you're SOL on the scavenging side.
If we were talking carbs here, it would be the difference between a 750cfm and an 1100cfm.

I've often wondered what happens if you paint the collector a second time?

Originally posted by onovakind67:
Ok,sorry for rambling,but since the collectors are a little too large(according to the calculator)will adding collector extentions help or hurt low & midrange torque?

A little too large? How about a 46% increase in area from 2-1/2" to 3", assuming an 18 gauge tube? Ed Henniman refers to this as a "2 tube size error". Once you expand the gas and the velocity goes down, you're SOL on the scavenging side.
If we were talking carbs here, it would be the difference between a 750cfm and an 1100cfm.

I've often wondered what happens if you paint the collector a second time? Well said.

Why not get the right size collectors for your application? I would almost rather you got 1-1/2" diameter primary pipes with the right collector size if it came down to that choice. Your 327 is going to be mild, right?

Yes, my 327 will be on the mild side (somewhere between 325 to 350hp). The reason that I mentioned the Dynomax headers is that sometimes one has to strike a balance between what might be optimal for performance and one's budget. At least while you are trying to save money for your kids to go to college.... ;)

Doug Thorley Tri-Y's have 1&5/8" primaries (not sure on primary tube length) and 2.5" collectors. Not that I don't want the Thorleys', but they are fairly spendy. My thought was, if I have to make a compromise, the Dynomax headers would be a good second choice. Especially since many here give them a good report on fit & finish.

-Mark

Most headers have too big a collector. And running a 3" collector into 2 1/4" pipes
is the reverse of what you want - you need to run a 2 1/4" collector into larger pipes,
this is a merge collector. You want the primary pipes coming together at the smallest
cross section area of the collector, this is what makes for a good scavenging effect,
then you want the pipe to go back up in size. Kinda the reverse of whats mostly
seen on most street cars. Most headers also have the right size primary tubes, but are
too short for the engines rpm band, and have too big a collector - or really are lacking
a merge collector.

How about buying 1-5/8" uncoated headers if money is an issue then cutoff the collectors and install your own 2-1/2" ones. Flowmaster, Hedman, Hooker or Dynatech might have a 2-1/2" collector you could buy separate (you'd have to weld though). Then you could have them coated locally, or just paint them and save the cash.

Just an idea, its not like a big collector is going to completely kill your engine or anything . . .

Great thread. Last winter I made an exhaust system that was 3" in/2.5" out Dr. Gas X-pipe and will be looking at headers this winter. Is it viable to go with a 2.5" collector going into the system I just described? That header calculator program suggested 2.5" collectors for my 355.

RickD
You'd have to run the primaries into a 2 1/2" collector, go right back up to 3" and run
3" to your X-pipe, then its ok to run 2 1/2" - after the X-pipe.

LC Engineering in Lake Havasu City, AZ makes a very nice 1-5/8" x 2-1/4" merge collector for about $50 each. I've used them on several cars and they are awesome. I take them to the muffler shop and have them swedged up to 2-1/2" at the outlet. Our 327 Vette made 380 hp at the rear wheels @ 6000 rpm using 1-5/8" x 34" Stahl primaries, 2-1/4" merge collector, 2-1/2" Dr. Gas XCO, and full mufflers.

Where do we find the header calculator program?

Header calculator at:

http://www.headerdesign.com

If you can swing it get the tri-ys. They work super on mild small blocks and the Thorleys are a very well made header that will give years of service with no problems.