BesideTheBox

Anyone know what I should see for exhaust backpressure on a stock 1997 C4S exhaust with the motorsound option?

I plugged the Tool Aid 33600 Exhaust Back Pressure Tester into the upstream Oxygen sensor location and at idle the tool measures essentially 0 psi (needle bouncing slightly off the stop).
At 2500 rpm, it measured maybe .5 to .75 psi. At 4000 rpm, it measured less pressure than at 2500.

Looks to me like the stock exhaust flows quite freely. With my hand, I can feel slightly less air out the right side exhaust and I was hoping that I would be able to measure this difference. But the pressures are so low, it is difficult to draw any conclusions. Unless I should be seeing -more- backpressure.

Stealth 993

I think you are using the tool wrong. You measure one point and then compare it to the other point.

For what you want to measure is exhaust flow rate and not back pressure.

BesideTheBox

Yes, quite possible that I'm using the tool wrong. There are two points to measure - pre-cat and post-cat. The tool is designed to use the oxygen sensor bungs.

Since the pressure observed pre-cat was so low, I figured there was no point in measuring the pressure post-cat. The assumption being that the pressure would have to be less post-cat since only the muffler could be restricting at that point and any muffler restriction would be part of what was observed in the pre-cat measurement.

Why I'm even bothering to check backpressure:
My hypothesis is (was?) that the bank 2 catalytic converter, or possibly the muffler, is slightly restricting exhaust flow, thus allowing less oxygen flowing into bank 2, and possibly explaining the more negative (by 2.5%) long term fuel trims reported on that bank. For example, bank 1 currently reports a 0% LTFT while bank 2 reports -2.3% LTFT.

chaoscreature

You have 4 O2 ports to test and since you have the tool why don't you get reading from all 4 locations under as identical conditions as you can manage. I am very curious about this from a purely intellectual standpoint.

2.3% does not seem like enough to throw up red flags to me.

Here is a good link regarding fuel trims:
http://www.obd-codes.com/faq/fuel-trims.php

993inNC

I don't know the BP numbers but I believe Steve W told me years ago when we were making pipes that these motors flow something like 300cfm's. Not sure if that's any help but now you have a possibly useless piece of trivia to impress your friends with

4X4SCHE

Wide open at 6,000 rpm it is 1,800 cfm.

BesideTheBox

I called Fabspeed to see if they could tell me how much backpressure to expect.

Their response was, "You should see a couple of psi".

So, it's not like there is a lot of backpressure in a stock 993 exhaust system.

Perhaps CFM would be a better way to measure any restriction. Certainly the tool that I have makes it nearly impossible to distinguish any side to side difference.

Thanks for the input guys.

Wolfk

If you are measuring this in your garage, you do not have any load on the engine, back pressure needs to be measured when engine is doing some work. Reving to 4 grand in neutral does not influence back pressure all that much.

4X4SCHE

"If you are measuring this in your garage, you do not have any load on the engine, back pressure needs to be measured when engine is doing some work. Reving to 4 grand in neutral does not influence back pressure all that much."

That's a statement that I disagree with, please explain?

John

BesideTheBox

I think Wolfk is right. More work is accomplished when the car is moving at 4K rpm versus just revving in neutral to 4K rpm.

Also look at it this way. The throttle position is open further when more work is being done. Larger throttle opening equals more air (and fuel) flowing through the engine.

I'm not likely to pay for dyno time to get my question answered!
.

4X4SCHE

OK, I have changed my mind, the engine must be under load for the throttle to be fully open and only then will the intake air be at atmospheric pressure.

Wolfk

Exactly, throttle is only open maybe five-six percent at 4000 rpm in neutral. Small amount of air, small amount of fuel = small amount of exhaust flow, even though the engine is spinning rather high. A good example is a small gasoline generator. They spin at constant rpm independent of load. Place your hand behind the exhaust at no load, then start loading the generator up with electric load, you will notice quite a difference in exhaust flow.
BesideTheBox, how long is your sensing tube? can you place it inside the car then take the car out on the road?

BesideTheBox

The sensing tube is 41" long. If it was long enough, it would be an interesting challenge to thread it safely into the cabin.

When I first measured, I setup a camera on a tripod to monitor the gauge while I tried various RPMs.

Now I'm thinking what I should have done was blip the throttle, as this would result in higher, and perhaps more measurable, pressures. And in this way, perhaps I might be able to see a difference between the right and left sides.

Not sure that I would be able to blip the throttle consistently enough to make a side to side comparison.