HELP REQUIRED:exhaust design.

[Mayank]

hi guys...
sorry to put up this very basic question...but I cannot help it...
What are the FORMULAS to calculate header dias and length for a 4-2-1 exhaust system??
I have done quite a lot of research on net..been thru all the links mentioned on the forum.The books that have been mentioned on the forums are not available here in India...and it will take some time to ship them here..
All that I have come across is theory...but no formulas to do the real thing.....
my email id is
mayank.gupta7@gmail.com

[Mayank]

hi guys...
sorry to put up this very basic question...but I cannot help it...
What are the FORMULAS to calculate header dias and length for a 4-2-1 exhaust system??
I have done quite a lot of research on net..been thru all the links mentioned on the forum.The books that have been mentioned on the forums are not available here in India...and it will take some time to ship them here..
All that I have come across is theory...but no formulas to do the real thing.....
my email id is
mayank.gupta7@gmail.com

[Mayank]

we have also made use of burnsstainless.com to get our calculations done.
Can we trust them and go ahead and manufacture the exhaust design proposed by them??

[Mike Claffey]

http://en.wikipedia.org/wiki/Longitudinal_wave & http://en.wikipedia.org/wiki/Helmholtz_resonance is a start. Matlab / c code / excel can be used to create a model of the system. Think of it as a fancy pipe organ.

[James Morris]

Hi i did on our exhaust system last year for my second year project. hope it helps.

Initial calculation

Primary length = (effective cam duration/360) x (60/rpm) x (mean primary temp/4)

so the next step is to measure your cam timing and decide what rpm you want the primarys tuned for. i measured the exhaust temp with thermo bosses last year and at around 8k on an f4i the average temp was 700ish deg C.

But theres a lots more to the design of the exhaust but this will keep you busy for now i guess, just make sure all you primarys are the same lenght tho. Feel free to ask any more questions.

James Morris
Swansea Institute
siracing.co.uk

[Homemade WRX]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Moz22:
Initial calculation

Primary length = (effective cam duration/360) x (60/rpm) x (mean primary temp/4)
</div></BLOCKQUOTE>
have you worked the formula as you wrote it there? I don't recognize it and none of its units/conversions are labeled. Just running some quick numbers for my drag cam to see how this formula compared to my other calc's and I get 0.98x, what?

I'm going to assume this to be meters as I'm using a 240* cam, 840*C and 8500 rpm. If meters, than it does give some slightly longer numbers from my calcs.

[James Morris]

Sorry man, all the units are meters and deg C.
To work out effective cam duration i take a total of 30 deg off the total measured cam timing so that then formula works at the peak of the pressure wave in the exhaust where it most effective.
This part of the equation(mean primary temp/4)the 4 represents the number of pulse refraction in the exhaust primary (or header)so this number can be 2, 4, 8, 16, 32....just depends on how good you are at designing.
One thing that suprised me a little last year is that the relationship between the best tuned lenght of the primary and RPM is not linear, so if you tune your primarys for a higher RPM it will be optimised for a larger range of RPM's AND as its shorter you might perk up at a lower range of RPM's too as that tuned length is likely to work due the refraction doubling. For instance tune your primarys for 10000RPM using 4 refraction waves, the change the formula to make RPM the subject and change the refraction to 8 and for the same tuned lenght a lower RPM works too.

For pules creation i times the tube dia by 1.5 when moving from the primarys to the secondarys and again by 1.5 when going from the secondarys to the tailpipe.