A&Q about 350Z
Q:
Alright, I have a 98' Escort ZX2 and I just got in a set of headers. Would it be safe (legality notwithstanding) to go WITHOUT a cat at all? My friend was telling me it may cause compression problems from a lack of backpressure. As it is my car runs really well with a perf. muffler, and I don't want to mess it up.
If not, I'm guessing I should invest in a high-flow cat?
A:
This thread is split from another, old thread.
A:
compression problems?
uh no, it won't cause compression problems.
It'll make the check engine light come on, and if you have emission testing in your area, you'll fail it most likely, but thats the only real disadvantages to removing the cat.
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it'll make your car faster for one thing. you'll also pollute more, won't pass emissions tests, get better gas mileage if you can keep your foot off of it. also if a police officer somehow finds out that you don't have one, i think you can get a fine and i think your car can get impounded.
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It actually won't make your car any faster at all. At best it will give you 1-2 hp, maybe. But the cat on that car is the same core that they use on everything up through the Taurus V6, so it flows MORE than enough.
For the amount of dolphins you'll kill by taking it off, its wiser to just leave it on.
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i thought the cat was more of a resistance than that. so about how much hp could you expect from removing a cat that was properly sized for an I4
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old car cats flowed a lot less, so there was more to be gained by removing them. modern cats are a lot better.
to the original poster...
a way to look like you still have a cat, but get a few extra hp is to remove it, hollow it out, and put it back on.
as long as the car is running fine, you should be able to pass emissions.
if not, take it to some small place that does a visual inspection b/c they cant afford one of the probes to test.
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If its properly sized, then 0 hp will be gained from removing it. In fact, many cats after about 91 are capable of supporting more HP than stock. The cats on my 96 Impala SS are fine up to about 400hp even though the engine only put out 260 stock.
My guess (since they use the same cores for many applications,) the cat on that escort is probably good to about 160hp... if its not clogged
A:
not sure what you mean by "capable" of supporting X horse. A restrictive exhaust causes positive pressure. Any positive pressure reduces the pressure differential when the exhaust valve opens to expel combustion gases. Reduced flow out the exhaust port will reduce horse power.
Its quite normal to see as much as 3psi on modern cars at WOT at the front O2 bung (common test to see if the cat/exhaust is plugged.) Thats 3 psi worth of hp sucking pressure. Although there's no reason you couldn't make more hp with that exhaust system, you're still losing power to that backpressure.
I personally don't think you'll see large power gains by ditching the cat either, but I wouldn't say there's 0 hp gain.
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I've heard that the hollowed out chamber can cause turbulence in the exhaust flow, creating more restriction than the cat did to begin with....don't have concrete evidence for this, but I did notice this sort of effect on my own car. Mind you, I did not "remove" my cat, the ceramic bits decided to evacuate my exhaust system on their own, breaking apart and flying out the tailpipe. After the cat left, the engine seemed to have more torque, but power rolls off quicker in the higher RPMs indicating a flow restriction. It may simply be that parts of the cat are still blocking something, but I thought I'd share. It's something to think about.
p.s. it is a bit louder now, with more of a droning (read: annoying) resonance in the cabin.
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that seems to go along with what i've heard and know about torque vs hp.
i've heard that hollowing out the cat allows a place for the exhaust to cool and expand. somehow (this part is a mystery, but it came from a semi-reputable source) this allows the engine to breathe better and make more low torque.
if the torque curve in any engine is shifted lower and bumped up slightly at the bottom end, it should translate into a lower peak hp, with a lower peak hp rpm.
of course like when the muffler on my dads old 83 S-10 blew open in a massive backfire, you may be FEELING faster due to the extra noise but nothing is really happening differently.
thats the reasoning (i guess) behind the big coffee can mufflers and resonators on a 1.3L I4.
makes you FEEL faster.
i'm guessing a little of both though.
of course a restriction puts more of a strain the higher the rpms so that could be the cause too.
A:
Right, all I'm saying is that cats flow a certain CFM. If the amount of horsepower you are creating requires fewer CFM than the cats flow, then they aren't a restriction.
In my example, removing the cats on our Impalas shows absolutely no gains up to the 400 hp level.
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"Right, all I'm saying is that cats flow a certain CFM"....at a given pressure drop
raise the pressure drop, and the CFM will increase
flow thru a restriction isn't so easy to simplify.
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True, but you're assuming that cats are a restriction in the case of the escort. We're talking an unmeasurable pressure drop in this case. If you measure my cats (for instance) before and after the cat, you'll find almost no pressure drop because there is less than 1 psi before and less than 1 psi after. If you take it all the way to 28" H2O, you'll have different numbers. Pressure differentials create exponential restriction, not linear. You can rate flows at any pressure drop you wish, but the fact remains that in the real world things like heads, intakes, carbs, exhaust, cats, and valve sizes become restrictive as their flow potential is reached... at whatever pressure drop calibration you wish to use. As they start creating more of a pressure drop, the increase in flow exponentially falls as the pressure drop increases. You could measure flow at 5000" H2O, but it won't be much more numerically than flow than flow measured at 1000" H2O
Disregarding exhaust velocity, VE, and torque production from scavenging, consider this: If you put a 3" exhaust on a stock Escort, it wouldn't neither cost nor gain you any hp. If you approach 300 hp out of that escort motor (hypothetically speaking) you start approaching the point where the increased flow demands placed on the now small-ish exhaust will begin to hamper further increases. It doesn't matter if you calculate flow at 28" of H2O or 28" of Hg, One might be 400 cfm and the other might be 900 cfm, but the net result is that the same number of molecules are flowing, just a different measurement.
Please don't think I'm being confrontational, but this is a pretty common topic and I'm not sure where you're going with your argument. You can measure it however you like, but you have to take into consideration that no matter how you measure it, it still flows the same amount of gas with the same restriction, just different units used to describe it.
Its like engine displacement. You can measure it in liters, cc, ci, whatever. A 350 cubic inch engine sounds much larger than a 5.7L, but it doesn't change its actual displacement.
In the case of modern "honeycomb" style cats that started being used in about 1989 and almost exclusively by 1992, very little pressure drop is measured at stock power levels. Most even will support much greater HP without increasing pressure drop measureably.
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I agree with almost all of that. I think we're just dancing around the exact numbers.
Saying that modern honeycomb cat's have no pressure drop worth noting....as I just said, most cars have around 3psi at WOT.
That, is a farely sizable pressure drop on a NA engine (IMO)! It would be one thing if I said .5 psi. 3 psi is a whole nother ball park.
No granted, thats the entire exhaust system added together. Not just the cat. But the cat is involved, I can promise.
Its an extreme case, but turbo'd engines are a good example on this. They can create very high flow in the exhaust, and create very high psi in the exhaust. Many car builders believe that dual 2.5" exhaust on a turbo car is good to 1000 hp. And they are correct. It is possible to get 1000 hp out of that. BUT!!! If you put a very short 5 inch dump pipe on it, you would gain 100hp. Why? Same thing. The 2.5" dual might flow "enough" to make that 1000HP, but its not optimal, therefore, its costing HP.