How to Measure and Estimate Exhaust System Backpressure

[kitcarbp]

FYI-
I ran across this ref. on exhaust system applications.

While its written by Caterpillar and focuses on turbo-diesel engines (oooh.... that hurt to type), it offers a good explanation on how to set up backpressure measurements on your exhaust system. There is also an equation that is simple to follow to allow you to estimate (A to B comparisons) exhaust system backpressure if you have muffler pressure drop data, gas flow rate (cfm) for your engine,and exhaust gas density at the measured AFR (lets say AFR of 12.5-13.0:1 @ WOT peak power).

See pages 21-25 for the details at the linked document below.

http://www.gregorypoole.com/products...%20SYSTEMS.pdf

[DocDirk]

LOL Bill, that's too much for me, you must have a calculator built into your brain....I love math .... PM me your email address and I'll send you something mathematical for fun.

Regards!!

Dirk

[Argess]

Well, I can't say that I grasp everything in that article, but I believe the following:

1/ They seem to be talking about exhaust restrictions, such as small dia tubing and sharp bends, ...basically screw-ups in aftermarket, or muffler shop fabrication.

2/ Backpressure is a much touted term for muffler shops to use when trying to convince customers to put the proper mufflr on their car vs the el-cheapo alternatives. "You'll burn your exhaust valves with no back-pressure" they say. I doubt this very much.

3/ An engine can be thought of as an air pump. Anything you can do to reduce restrictions to both the intake and exhaust is good. Not as critical in 4-strokes, 2-strokes (i.e. MX motorbikes) can use this to great advantage, but hence the narrower power band compared to a 4-stroke because you can't tune a fixed exhaust for a wide range of rpms.

Anyway, I wouldn't worry about back-pressure on your Cobra, especially if you stick to your manufacturers recommendations for an exhaust system.

[CHANMADD]

Primary tubes and length make a difference to the backpressure and hence the power and sound. The collector kind of creates a ventury to speed things up and accelerate the gases. A petrol/gas engine needs to scavenge the exhaust gases as completely and quickly as possible........no backpressure being best in a 4 stroke.