cherokee90

Quick question for all you guys. I read on this site all the time about how you should not remove the cat because you need the back pressure... But then i also read all the time that the bend in the exhaust right after the manifold to clear the front drive shaft is killing everyone's performance. So if you did remove the cat would the bend not supply enough back pressure??? Or if you remove the bend and leave the cat would you have enough back pressure??? Just confused on how the two are different

DFlintstone

I don't think the bend restricts it much. As far as back pressure from the cat, with companies striving/competing to achieve better performance and mileage #'s, I would think they would just make it a little bigger if there was a problem. Course I don't "winde out" my 4.0 hitting the power band!

weebur

I think that an engine "needing back pressure" is a myth. Why would there be a billion dollar industry dedicated to reducing back pressure (free flowing headers/cats/mufflers)? If I'm wrong please show me your reliable source.

cruiser54

Seat of the pants difference with a modded headpipe. More than converter delete IMHO. That crush in the headpipe looks to reduce the volume by about 30 to 40%.

caged

the reason there is high flow exhausts, is to match the added intake if modded.

so in short, if you increase the intake, you have to increase the exhaust.

only opening one and not the other won't do much in the end.

cruiser54

I'll have to disagree in this case based on my experience. On both my MJ and XJ, I had the downpipe replaced with ones that were bent to clear the driveshaft by a muffler shop. Noticeable difference.

Later on, added 60mm throttle bodies on both from www.strokedjeep.com. Another noticeable difference.

While only a high flow muffler may add little in performance or almost unnoticeable gains, removal of the "crush" in the downpipe is a worthwhile mod. Take a look at how restrictive it is sometime.

caged

ok

'Manche

The engine does need some back pressure, however prolly not as much as factory.. Hate to say it, but those engineers that make all that money to design stuff actually make things purposeful in vehicles (although not always in the most idea ways).

cruiser54

Yes they do. Like adding a crush in the downpipe as a band-aid to promote the cat heating up faster, while making it appear that it was necessary for driveshaft clearance..

kissthis66

Engines need back pressure, unless your running a big cam. Perfect example is my '66 Chevelle Malibu. I would run around on weekends with open headers, it killed the performance of the motor but it sounded mean as hell. But when I would run the exhaust (dual flow masters, no cats, x pipe) it would kill at the tracks(12.13 best time). Back pressure is no myth, it's needed for small cams. The motor will run with no back pressure , it's just you will lose tremendous amounts of power unless you got a big 'ol loppy cam in it. Don't believe me, unhook your exhaust and take it around the block. I bet you see a big differance. As far as companies out there , they make high performance exhausts for everyone from daily drivers to get a little more horsepower to drag strip cars who need open headers cause the exhaust restricts the motor. If back pressure is a myth go ahead and put 6" exhaust on your XJ and let us know how it goes...do your homework kid... better to not say anything and appear not to be dumb than to open your mouth and remove all doubt.

cruiser54

Can't wait for 5-90's response............

hankthetank

This was recently covered in another thread. It's already been proven that no, naturally aspired motors don't need back pressure. It's all about flow control and velocity. Google backpressure myths, you'll see.

djb383

Google "back pressure myths" and get WAY MORE FACTS than opinions.....there's just too many Ronnie Racers out there confusing fact with fiction. A IC motor is an air pump, the better the in/out, the more efficient.....but there's got to be some compromise with a DD streetable/legal ride. It doesn't need a cork in either end, but straight pipes do get a little annoying. LOL

Gee oh Dee

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