I read a few posts awhile ago by Max...Software somebody about modifying off the shelf headers to better benefit your engine. He said that, with an OTS header it's more beneficial to modify the collector to tune in your headers as opposed to altering the lengths of the primaries. Ok...this makes sense to me now. But, how do I modify the collector? And, how do I know how much and where?

I was on HeaderDesign.com reading and calculating...quite fun.

With what I want I need a:
Primary Pipe ID of 1.5"
Primary Length of 34.2"
Collector ID of 2.37"
Collector Length of 14.3"

Now I planned on getting Pacesetter Longtubes, LT1, and they have a
Primary Pipe ID of 1.6304(too big)
Primary Length of (not sure of, not equal)
Collector ID of 2.8804(too big)
Collector Length of (not sure)

The only thing I could really modify would be collector length really...atleast, that's all I'd like to modify. If I need a bigger/smaller collector then I guess I could work on it. As for the rest of the exhaust, I'd like it to be tuned for, having 2.5" piping after collector, into dual 2.5" bullets 'X' length after the collector and then possibly turn downs next to eachother to act as an X-Pipe, or to use a Y-Pipe and go straight to a turn down.

Also, since I'm using mufflers and possibly a Y-Pipe, what should be the length from the collector to atmosphere? If you could show me the formula, that'd be better.

Thanks for any help!

 

I have a book with some of the formulas you might need. I'll try to tag them
on my FTP site and link them tomorrow night.

From David Vizard's columns to, John Morrison and Phillip Smith, I have gathered
the following:

Collector length - you would want to extend the collector by the
same diameter before any reflective surfaces, or abrupt diameter changes
occur.

Pressure waves will reflect back upon hitting the atmosphere, catalytic internals,
or steps in the piping.

I don't think all the math in the world will get you the exact length. I've always
thought about getting a telescopic tube which can be varied on the dyno to
find the correct secondary length. Once you find the best power curve, have
a custom pipe made with a tapered reducer to connect the remainder of the
exhaust system.


Here's something I've been working on, but funds are tight. It will be this winter's
project. Maybe you can some idea from this design concept (fairly standard).

http://gmthunder.com/tino/exhaustcalcs.jpg

3" inch > tapered reducer > 2.5 inch > straight through muffler.

 

There are several manufacturers that build and sell merge collectors. They are between $800.00-$1000.00 a pair. They can be built at home if ya got a college degree in fluid dynamics and a good welder. They work,but the average street build won't see a big change. Changing the length is doable but it is a trial and error. Headers and exhaust takes a lot of dyno time to perfect on a given set up,change the cam,change the headers.Most people who build headers do a pretty good job if ya know what size tube and length to order.

 

lol That calculator told me:

Peak horsepower is unrealistically high
Please Change your values and try again.

Obviously this program isn't really designed for a blower application.

 

Loony,

Perfect Headers are like perfect girlfriends; you don't often run across them on the shelf, and even if you spec them out they don't always perform as you expect or desire.

If your engine is making any kind of power, 1.5 ID (1-5/8 tubing) primaries sound a little small. If it's a basically stock engine, don't get all hung up on leg diameter and length (girfriend thing again) until you've got it (her?) breathing heavy. With a full dress street system, you'll have a tough time getting the collector (skirt?) short enough, and tuning is probably secondary to free, unrestricted flow.

How about gettng an engine designer to suggest a header that you can purchase off the shelf which will work with your engine combination? Cam design should take into consideration the exhaust system you are using. Ask you cam guy.

If you are the engine designer, shop around for headers that are available for your car rather than try to modify. Same advice for girlfriend shopping.

 

There is a major difficulty here that wasn't even mentioned yet. You have to get the headers to fit in the available space, which will in turn make them far less than perfect, at least if we are talking the engine bay of most street cars. Go to the pits at a pro-level race and see what the headers look like. Tough time fitting those on a street car, for sure. And then you need cats (usually) and mufflers. And those may be compromised as well, for various reasons. The best you can do for a street header, IMHO, is to get the right diameter and make them as free flowing as possible (the fewest, most gentle bends needed to get them to fit). Don't worry about length.

Rich