besides robbing power what would the affect of having about 7-10psi + backpressure(i tested it by putting a fuel pressure guage in the O2 bung and reving it in park, less than a second to the floor)? would it hurt gas milage much?

also, i have a 283 in my 84 and when you down shift into 1st at about 10mph it almost spins the tires, if i went from about 10psi ot about 2 would it be the difference of spinning them? just trying to get an idea of what i should expect.

 

Thats not how you check exhaust back-pressure

 

Lots of backpressure is bad for everything... lower hp, tq, no top end, bad mileage... the list goes on.

You want to have as LITTLE backpressure as possible, while getting the HIGHEST exhaust gas velocity as possible... that combo will make the most power at any rpm.

 

Well I don't think that would be an accurate reading. You're probably reading most of the pressure from the exhaust quickly exiting the exhaust ports and heading down the primaries. How you would go about measuring it, I'm not exactly sure though.

Other than robbing power it will also get lower mileage.

 

Where you checked the pressure was an OK spot. Maybe not the best spot, but OK.

A better place would be in the pipe a couple of inches down from the manifold.

A better test would be to go for a drive also. Exhaust flow is not only related to RPM, but engine load also.

There's a company that sells a kit for ~$50 that you can use to measure backpressure.

Their website is www.JDSDiagnostic.com

Unfortunately their website is down right now. Here's their addy & phone #.

JDS Diagnostics
6625 Coneflower Ct.
Maineville, OH 45039
877 - 660 - 1832

There was a right up in CHP magazine about their product. They did a test on a car but didn't say much about it except it had Flowmasters and had a peak reading of less that 2 psi.

They broke it down a little further like this...

0-2psi ......... Acceptable exhaust backpressure

3-6psi ......... Mild backpressure; power improvement possible

7 or higher psi ....... Excessive backpressure; significant power improvement possible

"1psi of exhaust back pressure = 2.5% loss of horsepower (approx.) This means if you have a vehicle that has 170 horsepower stock, and have 0 psi back pressure-You will not gain a single amount of horsepower by spending hundreds or thousands of dollars on a performance exhaust system. If you have 5 psi back pressure, you are losing approx. 21 horsepower."

 

I would have checked it while driving it but the connector i used has a rubber o-ring i would have burned out.

as for the kit, i've checked into it and its about the same thing as i used but it has an 8" metal peice so you don't burn the hose. i'l have to get a conector and stuff so i can check it while driving it.