Im curious to see what the actual backpressure of an exhaust is both with, and without, a cat. Do they really hurt performance? Or are they OK? The first test I did was a long time ago with hooker 2055 headers, y-pipe, catco 3" TPI cat, and a 3" exhaust with a dynomax bullet muffler for a resonator and a flowmaster 30 series. The engine, a 350, had TBI with an RPM airgap, vortec heads, and a small flat tappet (something like 208/216 with ~.460").

The measured backpressure there (from my old notes) was 7 PSI @ 5000 RPM

I ran the test again recently with my current setup, which is the same engine, but with trickflow 195's, edelbrock victor-e intake, and a comp cams xtreme energy 4x4 cam (206/210, .459"/.459", 111 LSA). The exhasut was the same, but this time with a hooker maxflow instead of the flowmaster. This time I also measured the airflow from the MAF sensor, which was around 530 CFM at 5500 RPM.

The backpressure this time was 5 PSI @ 5500 RPM

Better than the flowmaster, but still kind of high. The last test I want to run is with a straight-pipe in place of the cat. Wonder what the backpressure will be then? I suspect a good bit lower...

Heres the test setup. Basically the gauge is plumbed into the AIR manifold on the farthest header on the drivers side. The gauge is then taped to the windshield for a couple WOT runs. Next time I do it, Ill see if I can get some video as well. Kind of neat to see the gauge at part throttle. The gauge rapidly oscillates between no pressure and vacuum when the car is just driving. Shows that even with a cat, a performance exhaust produces little or no restriction when driving.

 

I ran the test without the cat. Big difference, but not as big as I thought it would be.

The backpressure @ 5500 RPM: 2.5 PSI

The cat is definitely a restriction, but on the other hand, a 3" single exhaust with shorties is as well. Without the cat, though, is a big difference. The engine pulls much faster, and I get wheel hop the whole time as well. Also made it harder to get an accurate airflow reading as the OBD-II scantool I have is kinda slow. It only registered a max of 470 CFM as the readings where lagging.

Anyway, I think we can conclude that if you want to run emissions legal in a peformance application, you need to run 2 cats and not just one.

 

Interesting findings. I have a backpressure tester laying around somewhere that I bought for checking for clogged cats, now I think I might just have to stick it on my car for s**ts and giggles.

7psi seems like ALOT.

 

The 7 PSI reading was with a different muffler (flowmaster 30 series). The current one is basically just a perferated tube with some fiberglass around it.

Id be willing to be that stock exhausts are probably even higher. Probably around 10 PSI at WOT to get proper silencing. The flowmaster with a cat was almost as quiet as a stock exhaust.

 

Hi,
after reading this topic I made also some backpressure measurements at collector.
It has 355sbc(~400fwhp).Exhaust system is coated SLP tri-y headers,no cat ,3" intermediate pipe(SLP) and dynomax ultraflow muffler(3" in 2x2.5" out)
Backpressure readings are:
at idle->0.3kPa(0.04psi)
cruise(~2000rpm at fourth gear)->0.75kPa(0.1psi)
Made also few 3rd gear wot accelrations.At 5500rpm backpressure was ~12kPa(1.74psi),and 6500rpm it was ~15kPa(2.17psi)

I read at speedtalk that when backpressure exceeds 0.2psi it starts to hurt power.Can someone say how much is power gap between zero backpressure and 2psi backpressure?