Exhaust Decision Time

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H

Heartworm

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Dec 7, 2002
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My engine rebuild on my 1986 FJ60 will soon be complete; only one oil leak to deal with. Now, I need advice on the exhaust system. I have de-smogged it according to the IH8MUD itech nformation (removed air pump, EGR, HAC, had Jim C rebuild the carb for desmog). I have plugged the hole in the exhaust pipe and at the intake manifold according to directions. It has a stock exhaust manifold and exhaust pipe. What should be done from the exhaust pipe back?
Remove the CAT or replace with a high-flow CAT? I have heard that CATs will clog up when the air pump has been removed (any truth?). Also, what diameter tail pipe beyond the exhaust pipe (same size, smaller? larger?). I read that too large a pipe will actually cut horsepower (Any truth?). Which muffler would you recommend?
 
2.5" exhaust (or so) with a high flow cat
 
Cats won't clog with the air pump removed.

Here's an article and dyno tests regarding exh pipe and tailpipe size
http://www.boyleworks.com/ta400/psp/exhaust2.html

See page 2. It does confirm you can use smaller dia tailpipes without any loss in power.

I guess my 3" tailpipe is just for show then :D
 
Unless you have a turbo the 3" exhaust is actually a bad idea (loses low end)
 
FL cruiser said:
Cats won't clog with the air pump removed.

Here's an article and dyno tests regarding exh pipe and tailpipe size
http://www.boyleworks.com/ta400/psp/exhaust2.html

See page 2. It does confirm you can use smaller dia tailpipes without any loss in power.

I guess my 3" tailpipe is just for show then :D
Click to expand...

My reading seems to indicate that the bigger pipe helps in the power end up to a point where larger pipes do not offer significant payback. I think you can also read between the lines to say that a lack of muffler is also a good thing in the power department. Fortunately for me the turbo allows me to eliminate the muffler entirely.
 
Don't fall for the "back presure" myth....Any "back presure" is a bad thing, what you want is for your exhuast to flow and for it not to lose velocity with a 4.2L motor what only spins up to 4,000 rpm pretty much anything over 2.5" is going to reduce the speed at witch your exhuast flows enough to effect performance(unless your going forced induction and in that case the amount of air flowing though your system will me increased so your need for a larger diameter exhuast will also be increased..

Find a good local muffler shop and have them build you a custom exhaust with as little turns in it as possible without affecting clearance , using a 2.5" pipe and a good flowing muffler and you should be golden.... :beer:
 
What are the advantages/disadvantages with the cat? If the engine is in good tune, is it really necessary? Will it really stink more if it's removed? On the flip side, do you really gain much power by removing it? Or do I just go with a high-flow cat for some middle ground?

Thanks,
Cameron
 
The sole purpose of the cat as I understand is to burn any remaining unburned fuel. There is a power penalty in having a cat as it is one more item to restrict exhaust flow. If removed you should theoretically get additional power.

You'll note that the cat is generally very close to the engine and always before the muffler so it benefits from the hottest exhaust gasses to heat it up and burn the unburned fuel.
 
Check out the latest info in:

4Wheel & Off-road, page 48, October 2005

which does a nice job covering this subject.

...
 
Please summarize

Dmorse. Don't have magazine. Could you summarize? Also, where is best source of high flow CAT? (anyone)
 
Basically; no sharp bends in pipe and use a muffler with low restriction and low noise. Pipe ID should be close to stock, maybe a little bigger for high end power; but who cares on a tractor. The cat can be any modern style since it makes no back pressure that can be measured. I do not include the original TOY cat in this. U can do a google for suppliers; I don't have one my FJ40 (1980). All my smog stuff is on the engine, but it doesn't do much and never did.
 
Loose the CAT...its an extra expense that has no purpose with the pump removed and an extra place for condensation to sit and eventually rust out, as far as larger diameter pipe, not nessesary. Its not a dragster and horse power is not the objective, torque is what you want in a 4 x 4 and the OEM pipe is perfectly sufficient. Additionally the old Cat I took out of mine was an extra 25 lbs due to a pile of carbon build up inside, weather it was affecting flow...no way prove or dis prove that one but its in the trash. I went up to the local muffler shop and bought a couple flanges and a short length of pipe ($20) bolted it up and then tack welded it into position, took it out and finished the welding and reinstalled, took a half hour. I'm fairly sure that was one of the contrubuting factors (along with a few other mods) that is getting me 18mpg fuel milage.
 
#1 If you remove all the smog stuff why keep the cat?

#2 Contrary to what FL says, the air pump supplies oxygen so the cat can burn unspent fuel. Remember Chem 101? Burning is combining molecules (of hydrocarbons, i.e. fuel, here) with oxygen, & the result is other molecules (CO, CO2, etc.) + heat (delta, remember?).

#3 You're not getting any scavenging effect without a tuned header.

#4) Turbos don't like back pressure: it's the pressure differential between the header & the tailpipe that powers 'em.

#5) If you think any back pressure is bad, try running the enging under load without header - you'll burn the valves.

#6) You can remove the cat, but a commercial shop won't 'cause it's against federal law, regardless whether you're in a smog-test area or not. Nobody will come after you, though.
 
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