Exhaust Mathemagic

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Mar 17, 2012 | 10:26 AM

Tugly

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Joined: Jan 2011

Posts: 18,849

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From: Puget Sound

Exhaust Mathemagic

I've been doing my research thing, and this topic has my head spinning.

The simple part:

EGT (F) + 460
Divided that by 540
Multiply that by your intake CFM (turbo output)
Equal total exhaust flow in CFM

Step 3 is jamming me up because it takes into account air temp, intercooler efficiency, turbo mapping, RPM, fuel/air ratio, elevation, and possibly planetary alignment. I think I've worked it out where my stock turbo puts out about 300 CFM max with my 80e tune, but I can't be sure. If my research is in planetary orbit of the world of Accuracy, I should figure 1000 CFM max exhaust flow under a worst-case scenario while keeping the EGTs safe. This is all in the interest of figuring the benefits of different exhaust diameters.

Rules of thumb I've gleaned from the "experts": You want less than 1.5 PSI total back pressure from the whole exhaust system after the turbo (no sensor there). Figure 2.2 CFM per HP output on a normally asperated engine, but I'm too brainfried to look into the rule for a turbocharged engine. The right muffler is as good as a straight pipe - there is no need to go loud to get straight-pipe performance.

Questions: Does anybody know the CFM out of our stock turbos at sea level on a 60-degree day? Does this post lead anybody astray on working the math on an exhaust system? Has anybody worked out the back pressure of a straight-pipe system for 3.5" vs. 4" vs. 5"?

My hypothisis: 3 1/2" (stock) does just fine with just about everything except modding and/or heavy towing. 4" takes care of everything but mega-modding. 5" is needed for mega-modding, but is typically installed by the "If 4" is better, 5" should be great" crowd.

Oh.... This is my 500th post on FTE