need a tech on this question....

[snooper0069]

I HAVE A 1993 EXPLORER WITH THE STANDARD 4.0 ENGINE . I KNOW ALL THE FEDERAL LAWS REGARDING THE REMOVAL OF THE CAT. FOR THE SAKE OF CURIOSITY , IF THE CAT AND RESONATOR WHERE TO BE REMOVED ONLY LEAVING THE MUFFLER, HOW MUCH HORSEPOWER WOULD BE GAINED AND WOULD ANY OTHER BENEFITS BE INCLUDED ,SUCH AS BETTER FUEL ECONOMY, TORQUE ETC. ETC.

 

[Alec]

you may actually lose torque if you totally remove the cat. the 4.0 engine needs a certain amount of backpressure to maintain good torque. if your cat is bad, i recommend replacing it with a high flow unit, get a good high flow muffler, and a 2.25" tailpipe. You don't need a resonator, unless you want it for the sound.

Besides the fact that a working cat does help the environment. I'm no tree hugger, but I think having a vehicle without a cat is irresponsible.

 

[MyExpWork]

I have wondered about this for a while now so educate me.

How exactly does having less backpressure in your exhaust system reduce torque? If it is easier to push that air out of the combustion chamer, doesn't that free up power (torque) on the firing cylinder which would no go to the crankshaft instead of pushing that air out.


I don't get it.

Brian

 

[spiper]

There are 2 cats. I had the them plugged up. I removed one and knocked the guts out of the other one. No substantial gain in hp though. Upon inspection it still appears to have a cat. No problems due to loss of back pressure either. Plus dont have to worry about them plugging up again either!

 

[mrboyle]

The reason you loose torque by reducing the back pressure is somewhat simple. Assuming (I know that can get me in to a lot of trouble) you understand how an 4 stroke, internal combustion, engine works, the easiest way to understand how back pressure affects the engine is this. Every time the engine fires and exhales there is a short lag between the time the exhaust gas enters the manifold and the next cylinder fires and exhales. The time between exhaust strokes creates low pressure pockets in the exhaust system. Having a slight amount of back pressure prevents these low pockets from forming making the end of the tail pipe the low. Since pressure will force itself to the lowest point, the exhaust will actually force itself out the tail pipe quicker, thus increasing torque. At higher RPM's, where torque is not needed as much, the lag between exhaust strokes is much shorter, there fore the need for back pressure is overcome by the physical force of the exhaust.

By the way, complete removal of your cats will give you the enormous gain of ZERO horse power. You may not believe me, but I have whitnessed a dyno test that proved it.

 

[Alec]

thanks for the explanation, Mike.

What Mike says feeds into this: you want to promote the exhaust flow, not just "free" everything up. For this you need the right amount of backpressure, and exhaust components that helps the scavenging effect help too. You also don't want the exhaust pipes to be too large, as this actually slows down the velocity of the exhaust gases and also contributes to the lawnmower/fart sound factor.

so for mild OHV motors, I recommend a single exit muffler (I like Magnaflow) and a 2.25" tailpipe. Later years may go to a 2.5" tailpipe as they flow exhaust a bit better. For the SOHC: the same, with a 2.25 or 2.5; whichever is a 1/4" larger than your stock pipe.

Granted, every vehicle is different, and these are safe suggestions. That is, they should not lose torque, but may not yield the max hp gain. And of course if you're highly modded most rules go out the window. My exhaust consists of JBA headers, and exhaust components (hiflo cat, magnaflow muffler, resonator x 2) that are basically a free flowing 3" pipe to a 2.5" tailpipe.

 

[MyExp]

Thanks for the explanation Mrboyle, that makes sense. So can you now exactly define what backpressure is?

My idea is that it is the pressure in the exhaust system going toward the engine. Therefore, wouldn't there be a low pressure area in the exhaust manifold anyway? But, by your defenition back pressure stops low pressure pockets from forming in the manifold anyway. Isnt that a double negative or something? As you can tell I have no idea how exhaust affects the engines performance, but I do know exactly how the 4-stroke engine works.

Thanks for the class lesson.

Brian

 

[Robb]

Back pressure is simply pressure created by the exhaust flowing through a contricted area (the pipe). Smaller pipes, stock cat, muffler, manifold, all create a resistance to the flow called back pressure.

Robb

 

[MyExp]

Oh, DUHHHH!!!!

Thanks.