How to Measure Exaust System Backpressure, LOOK WITHIN!!!!

[12secondv6]

Well, one of the biggest questions here is what size exhaust do I want???
2.5 inch or 3 inch??

As of right now we do not know which is better.
I'm a fan of 2.5 inch most of the way, there are other members who swear that 3 inch all the way is better and there are members that say no exhaust is much better.

In April 2003 issue of Chevy High Performance (page 104) there is a great article on this. I'm just going to copy it all here:

"There's only one way to know for sure- you have to test it. Any good enthusiast knows that a quality exhaust system improves performance by reducing back pressure and allowing the engine to breathe more effeciently. But until you measure it, there's no way to know just how much better your new system is than your old one. So lets take a look at how to do this.
JDS Diagnostic Equipement and Supplies has come up with an inexpensive test kit that allows you to tap into your exhaust system and physically measure the exhaust back pressure. The kit consists of a 0-15 psi pressure guage, an 8 foot length of plastic hose, a special adapter, a drill bit, 1/4 inch pipe tap, and a collection of pipe plugs. We decided to use the system on one of CHP's small block street machines. We sauntered over to Tim Moore's shop where we drilled a simple hole in the exhaust pipe just aft of the header collector. It took about 10 minutes to drill and tap the hole and bolt the special adapter into the head pipe. The adapter offers a length of 1/4 inch tubing that seperates the plastic tubing from the exhaust pipe to prevent the tubing from melting. Once it was hooked up, we tested the system during a full throttle, second gear blast and recorded the results. Our 2.5 inch dual exhaust system equipped with a pair of Flowmaster mufflers saw a peak reading of slightly less than 2 psi, which is acceptable backpressure. Lower numbers are possible, but you would have to expend serious effort and money, and it's probably not worth the price of admission.
The JDS system sells for $50 and includes several packages of pipe plugs to allow you to use this system on several vehicles. You could also use it to check for a bad catalytic converter, or just to evaluate those new mufflers. Once you measure it, then you'll know for sure."

Here is a link to them:

http://www.jdsdiagnostic.com/about%20us.htm

I really think I will order one of these kits. I know there is more in my car on the bottle. I'd love to see the benifit/ lose thru a cat, and also if changing a muffler/ cat back would benifit me. I'd also love to see how it react's n/a vs nitroused.

You get 20 plugs so you can mess around.
There are 3 kinds of plugs:
Steel Inexpensive; Great for the majority of vehicles on the road today

Brass Relatively inexpensive; Great for the majority of vehicles on the road today with the added benefit of easy future removal; Won't rust

Stainless Steel Great for all vehicles; Easy future removal; Can be re-used many times; Polishable; Won't rust

Under pressure:
Backpressure..............Description
0-2 psi...................Acceptable exhaust
..........................backpressure
3-6 psi...................Mild backpressure, power
..........................improvement possible
7 or higher psi...........Excessive backpressure
..........................significant power
..........................improvement possible

Any thoughts?
Interests?
I'll keep you all updated on this if/ whn I do it

[pvtschultz]

There is more to exhaust flow than just simple backpressure. Granted, it is that pressure that opposes the flow of exhaust gasses, there are other phenominon that exhist in low power cars.

A major one is the effect of the muffler as an expansion chamber. What the muffler does is effectively absorb the exhaust pulse travelling down the exhaust as a shock wave. Then, in between the pulses, it actually creates a negative pressure area sucking exhaust into the muffler.

Another is non-favorable pressure gradients. When you expand the size of the exhaust piping, there is a good chance that you are creating negative pressure gradients, which actually oppose the flow of exhaust. A good rule of thumb is 10 degrees of expansion for minimal losses.

The overriding factor is if the new exhaust is superiour to the factory, you will always see gains.

As for the placement of the pressure ports, I would think that ideally just after the Y pipe would show the total back pressure of your exhaust. If you want to test sections, place a port before and after the section, ie. muffler, equally spaced, and measure the differential pressure between each section. There are equations to solve for the losses in the system, but you'll need to know things like temperature and such...I'll look them up at school today if I get time.

[MustangEater8251]

where exactly woudl it be testing, up towards the front by the y-pipe or at the tail end of the tail pipe?


I might be interested...


As for your car, I would go 2.75, or 3 in. if oyu are spraying [img]smile.gif[/img]

[nikon]

funny this only comes up now....My grandpa was doing this since the 50's and 60's when he started racing....he showed me the whole equation...rather mind boggling, anyways he always had his exhaust tuned perfectly, compared to the guys that just had open headers...he'd fly right by em...this is on V8's though...heh, sorry just thought Id share, I thought it was pretty cool...anyone else ever heard about that?

[Cole]

backpresure = +torque same HP
no back = +high end hp )(-torque
firebirds are heavy i would stick with torque