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Annoyingly high exhaust resonance

M

mattbred

NAXJA Forum User
Location
Canada
On my '89 Cherokee 4.0 I have an annoying exhaust resonance. It happens at 1000 RPMs, 1500 RPMs, and gets VERY loud at 3000 RPMs. As soon as I get to 3700 RPMs and above, the exhaust gets much quieter but more agressive as I can actually hear the exhaust and not the resonance.

I have as far as I know the factory manifold and downpipe up to the catalytic converter, with 2.5" piping to the flowmaster 50 and the dynomax tailpipe.

I had tried this setup with the Dynomax Super Turbo and it was just as bad if not worse, and it was a hell of a lot worse straight piped. That really annoying resonace.

What would cause this? It's obviously not a cat or muffler setup as it's always the same RPM's. The resosnace is a very high pitched sound, and sounds like a honda fart can. It comes from the end of the tailpipe, as well.
 
you sure your cat is good? not partially blocked or maybe the ceramic is busted up on the inside, causing more resistance coming out of it...?
 
JNickel101 said:
you sure your cat is good? not partially blocked or maybe the ceramic is busted up on the inside, causing more resistance coming out of it...?
Click to expand...

No, I just had a cat installed today to see if it would effect the resonance. It helped a little bit but it's still definitely there.
 
Tail pipe from muffler back has less resonance if it is 2.25" compeared to 2.5"
I asked Flowmaster about that after putting on a Rusty's cat back consisting of a Flowmaster 50 and 2.5" pipe all the way. It may be the reason the OEM is 2.25"
 
i have the super 40 and have very little at all, check my sig for vids.
 
tiamat said:
Welcome to the world of FlowMaster resonance - that trademark FlowMaster sound. I took mine off after two weeks.
Click to expand...

If you read my post you would have known that it's not the muffler causing the resonance.
 
techno1154 said:
Tail pipe from muffler back has less resonance if it is 2.25" compeared to 2.5"
I asked Flowmaster about that after putting on a Rusty's cat back consisting of a Flowmaster 50 and 2.5" pipe all the way. It may be the reason the OEM is 2.25"
Click to expand...

It still happend with my old 2.25" setup, and happens straight piped as well (nothing after the down pipe).
 
There is a differrence between a loud exhaust and an exhaust that resonates. On my XJ, the resonation occours between 1000 and 1500 RPM, the area the engine runs when on city streets. At highway speeds of 2000 or more RPM's the resonationis cancled by the exhaust flow. The solution was a Hushpower 2 resonator from Flowmaster. The annoying low RPM resonance is all but eliminated but the loud exhaust note remains;... a little too much for my likeing but I learn to live with it.
 
I know there is a difference, but this resonation you can feel. As soon as it passes the resonation, the exhaust gets quieter. Which wouldn't be right if it was just exhaust. IE at 3000 RPMs its INCREDIBLY loud and annoying and shaking, then at 4000 RPMs its much quieter yet more agressive.
 
like you said...you can feel it...so its almost like a vibration....

how are your engine and tranny mounts? something isnt tight somewhere in your system....
 
I have an oem-style replacement (not performance) downpipe, tailpipe and muffler, with the stock catalytic convertor. I have loud resonance around 2000-2200rpm, I have been wondering if it was the catalytic converter, but I'm getting great mileage and pressure from the tailpipe so I don't think it's broken or restricted in any way.
 
JNickel101 said:
like you said...you can feel it...so its almost like a vibration....

how are your engine and tranny mounts? something isnt tight somewhere in your system....
Click to expand...
Well, you know, I have a feeling that my motor mounts are shot. Whenever I let off the gas (I'm driving a manual) the whole jeep jerks. Same thing when I get on the gas. I revved it in neutral and looked at the engine mounts and the whole motor moves, but I imagine that's normal. The bolts don't have excessive play in them, either. The tranny mount looks fine too.

Would a loose engine or tranny mount really cause a high pitched noise though? What about a cracked manifold?

You guys are running up to 4000 rpm?

Wow!
Click to expand...
I once ran it to 4600 RPM, and the whole jeep started vibrating really bad, the shifter, pedals, everything, so I let off the gas. :p
 
JNickel101 said:
like you said...you can feel it...so its almost like a vibration....

how are your engine and tranny mounts? something isnt tight somewhere in your system....
Click to expand...

Vibration is not resonance. Exhaust vibration usually come when the exhaust is touching the body of the XJ and in my case a few times the gas tank guard. A simple re-position of the exhaust and tightning the hanger takkes care of that. It could be mistaken for exhaust (gas) resonance since it could occours at certain RPM's. Check the entire exhaust pipe for contact with the body or cross member if that is the problem.
 
techno1154 said:
Vibration is not resonance. Exhaust vibration usually come when the exhaust is touching the body of the XJ and in my case a few times the gas tank guard. A simple re-position of the exhaust and tightning the hanger takkes care of that. It could be mistaken for exhaust (gas) resonance since it could occours at certain RPM's. Check the entire exhaust pipe for contact with the body or cross member if that is the problem.
Click to expand...

It's not vibration, but it contains a very high frequency vibration since it is a very high pitched sound. If I can get a video recorder I'll try and make a video of it.
 
techno1154 said:
Vibration is not resonance. Exhaust vibration usually come when the exhaust is touching the body of the XJ and in my case a few times the gas tank guard. A simple re-position of the exhaust and tightning the hanger takkes care of that. It could be mistaken for exhaust (gas) resonance since it could occours at certain RPM's. Check the entire exhaust pipe for contact with the body or cross member if that is the problem.
Click to expand...

What I'm saying is - if his mounts are shot, theres a chance his exhaust could be hitting something - the vibration causing the resonating sound - such as with a tuning fork.
 
Do you have carpet?
 
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