My car has been running kinda sluggish lately and i just did a full tune up including o2 sensors and it's still feels "slower" than usual, i suspect the magnaflow cats but what are the symptons?
A test for a clogged exhaust system is to hold the RPM's at 3K rock steady. Have a vac gauge hooked up and watch the reading. For a stockish motor, you should see about 20" hg (the absolute number isnt important, just note the baseline reading). If the reading starts to drop after a little bit, that suggests an exhaust restriction. If after about 30 seconds you are still reading the same vac you started at, that sounds alright.
If the reading drops, the cats are where I would normally look first for an exhaust restriction.
One can also drill a tiny hole before the cat. If air is pouring out of there like a banshee, that can indicate a restriction (otherwise, the air really should mostly continue along through the free flowing exhaust system). This test is subjective and something I might only do if the vac gauge reading drops.
A test for a clogged exhaust system is to hold the RPM's at 3K rock steady. Have a vac gauge hooked up and watch the reading. For a stockish motor, you should see about 20" hg (the absolute number isnt important, just note the baseline reading). If the reading starts to drop after a little bit, that suggests an exhaust restriction. If after about 30 seconds you are still reading the same vac you started at, that sounds alright.
If the reading drops, the cats are where I would normally look first for an exhaust restriction.
John, you're the engineer with the scholarly background, so feel free to comment upon my thoughts.
Manifold vac is basically a measure (of sorts) of the restriction of air flow to the air pump (engine). Manifold vac is (crudely stated) inversely proportionate to throttle opening with respect to time.
When one is in gear churning some revs and letting the engine brake, manifold vac can actually exceed the idle reading (the engine, or air pump, is pumping, but the throttle is closed). Now along that line of thinking about the dynamics, if there is an exhaust restriction, there is a back-up of exhaust gasses. This starts to simply bog the motor down (one sign of a clogged exhaust is that under WOT, the car bogs or even dies, but if one backs way off the gas, where the exhaust gasses are not bottlenecking, the car runs alright). As the motor bogs, not as much air is needed, so even if the throttle-blade position doesnt change, not as much air is moving across the throttle blade, and the manifold vac reading decreases.
I know that was hard to follow. That is how I have thought about it in any case.
Vacuum & pressure go hand in hand, pressure being on the positive or + side and vacuum being the negative - side. So atomosheric pressure is 720 mm hg or there about, therefore the lower the # in mm hg the better your vacuum. To pull a vacuum you need to remove air from a system whether it be a your car engine, a distillation colume or a vacuum cleaner. The faster the air can be removed the better the vacuum or lower # in mm hg. So look at it like this your vacuum cleaner is not pulling or sucking hard enough, you open it and the bag is full (causing reduced flow or back pressure) you replace the bag and presto full vacuum again. Your car exhaust being pinched off due to a blocked cat increase the back pressure on the system and therefore reduces the systems ability to pull and hold a vacuum.
So when you said the vacuum should drop off after a little bit, did you mean as you open the TB some more or while you held it steady. The thing that got me wondering was that you said to hold the rpm's steady which would mean, hold the TB steady. The laws of fluid mechanics (or just by the definition of the term for that matter) say that at steady state, nothing changes per time.
Bernoulli's equation for incompressible steady state flow has no time variable. In the case of the car, the flow through the TB plate is obviously just a function of only the pressure difference and velocity since 'z' term drops off.
Then I thought; well, if you have a restriction and you are reaching a point where you can't force much more through it, you will get a situation where the huge built up exhuast pressure will cause reversion and prevent air flow into the cylinder. If no more air can flow into the cylinder then the rpms don't increase and thus no more air demand. Killing the air demand in-turn flatlines the vacuum and if you open the TB more, without changing the flow, you get a vacuum drop.
So intuitively I want to say you would have to mess with the throttle body to see that effect rather than just hold it steady. So that's what got me wondering, but since I've never tried it, I can't say what exactly would happen, but I just had to inquire
You should have seen how the car ran once my driver side pre-cat broke off inside the pipe and clogged that bank I was shocked it ran at all and I didn't blow a headgasket or something.