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Tunabomber
02-19-2007, 06:37 PM
ok so im used to haveing big motors that seem to do better with less back pressure ie. freeflow type thing.

but now i got a jeep with 4.0 ho streight 6 and as always first thing i did was hollow the cat and put on a flowmaster .
but i didnt seem to make any difference in fact now it kinda pops almost like a backfire on the decel but only when i go over 3000 rpm.

whats your opinion

to much flow ?:dontknow:

Andrew Goll
02-19-2007, 06:49 PM
im callin the dot thats illegal

MetalMan
02-19-2007, 06:55 PM
Sounds like an exhaust leak, or you might have an O2 after the cat that you have to bypass. One way or another, putting a length of pipe in place of the cat is always a better idea. If you need the cat for aircare, make it as a test pipe, when airscare comes around you just unbolt the pipe and bolt in the cat.

epic3
02-19-2007, 06:58 PM
the cat is supposed to burnup unburnt fuel in the exhaust so without it if your a touch on the rich during initial decel then you may get a backfire or popping sound from your exhaust try advancing your timing a little more to give a bit more burn time

Moondog
02-19-2007, 08:15 PM
I have a k&n on mine flowmaster with 3" pipe and a high flow cat I have had no problems

DirtyComanche
02-19-2007, 08:32 PM
Mine makes a pop when I come off the throttle... Straight through, just a glasspack. Oh, and a cracked header. But what 4.0 header isn't.




Anyways, if you want more or less back-pressure with a 4-oh is pretty much entirely up to you. What works better? Who knows. People swear one side, others swear the other.

Dieseljeep
02-19-2007, 08:41 PM
I think that back pressure for those motors is around 3 pounds.I think some exhaust shops have a guage they plug the pipe with and take a reading . The back pressure is a balance from the intake of air.

MetalMan
02-20-2007, 05:11 PM
the cat is supposed to burnup unburnt fuel in the exhaust so without it if your a touch on the rich during initial decel then you may get a backfire or popping sound from your exhaust try advancing your timing a little more to give a bit more burn time

It won't pop from a rich condition, the popping is the ignition of unburnt gasses. But it doesn't just ignite itself, it needs oxygen, hence air is getting into the exhaust at some point where the gasses are still hot enough to ignite. Unless the vehicle has an active AIR system, a leak is the only way for oxygen to get in.

Tunabomber
02-20-2007, 06:39 PM
:confused0006:hmmm i like poping noise on decel its very suttle and i have to give it a little gas to make it happen .

its when i acel up past 2900 rpm and hit the clutch that it makes the slight backfire noise

Trooper
02-20-2007, 06:59 PM
If your exhaust is to large and you dont have enough back pressure it will pop too wont it?

I have a little MG that pops and flames out the back but thats a whole other story

u2slow
02-20-2007, 07:26 PM
When I upgraded to ram's horn manifolds and dual exhaust with dumps (350 TBI) I got lots of popping and banging when riding a gear down a hill. So bad it actually slit the seams on the muffler. Still no idea why it did it. :dontknow:

MetalMan
02-20-2007, 07:26 PM
I had a Chevette that popped and shot flames out the exhaust, that was an extreme case mind you. I'm fairly certain the cause was a shot cat coupled to an active AIR system. It's pretty tough to totally seal an exhaust system, so it almost always happens to some degree. Check to make sure you don't have a smog pump, I'm not on the up and up with those Jeeps.

MetalMan
02-20-2007, 07:27 PM
When I upgraded to ram's horn manifolds and dual exhaust with dumps (350 TBI) I got lots of popping and banging when riding a gear down a hill. So bad it actually slit the seams on the muffler. Still no idea why it did it. :dontknow:


Refer to everything I've said so far...

Tunabomber
02-20-2007, 07:44 PM
thanks for the input guys [cheers]

Trooper
02-20-2007, 09:27 PM
Wont an intake leak cause popping too? Do the old intake cleaner test to see if its a intake leak

MetalMan
02-20-2007, 09:32 PM
I'm just going off the fact that the last thing he did was toy with the exhaust. There are a lot of things that CAN cause popping out the exhaust, but they usually come with the engine suddenly running like a bag of smashed assholes, and often people overlook the simplest things.