I am a mechanical engineer and am pretty familiar with the workings of an automobile. As for the header being of a specified length, i dont believe that to be correct. What will make the biggest difference is the shaping of the pipes, as this is why the best headers are the most expensive, it take a lot of time to make the headers right. A perfect header would have larger tubing, and each peice of tubing would be the same exact length (exactly the same) from engine to the single pipe. The reason for this is that if the pipe is the exact same length, then each cylinder will fire and push its exhaust out at a different time and therefore collect into a single tube at a different moment, and creat very low back pressure.

 

As a ME you will appreciate this book: The Scientific Design of Intake and Exhaust Systems. Pick up a copy, it's well worth the read if you are into performance engines.

Primary tube and collector length is VERY critical to exhaust tuning as is the dia. of both. The RPM range you are tuning for determines the required primary tube length. The current Scion headers use a very short primary tube length so they can bolt to the stock exhaust flange but those short primary tube is tuned for about 11,000 rpm on our little engines. I don't think anyone is twisting their engines quite that high..... The collector is WAY to short to be effective.

You want the primary tube dia. to be large enough to handle the volume of exhaust gases but small enough to maintain high velocity- without restriction, it's a balancing act for sure. A 1.5" dia. primary tube will handle up to 130-150hp in a 4 cyl., depending on a few variables.

Yes, more bends make a header slightly less effecient but having the correct length is much more critical. You want the primary tubes as equal in length as practical but within an inch or so is considered close enough.

This info just scratches the surface of what's involved in header design.

As I have said many times before, the currently available headers are "convienence" headers that bolt on to the factory exhaust. They used absolutly NO science whey they were designed. The fact that they make little power (though noticable) is a result of that. Here's proof: I spoke to DC (himself) and he told me he did not measure the exhaust port dia. when he designed the prototype header! Port dia. is pretty important consider when designing a performance header....

ken

 

Damn I started all this?!? All I want to do is have braggin rights for shows that I'm gonna enter in. Plus If i manage to get ahold of TRD headers Toyota will still waranty my car. :D

 

If you really want bragging rights, find a custom header designer/fabricator and have a one off header built with matching exhaust and you will have more to brag about than a TRD header.

ken

 

I could but that would equal a lot of R&D and possible engine downtime, and No Toyota Warranty coverage.