Three inch Exhaust

[goodburbon]

As some of you may have read, I have finally run 3 inch pipe.

I went to one shop and the man behind the counter didnt want my business. He informed me that what I wanted was not necessary as several of you on this forum have said.

After some looking I found Custom Muffler in Lafayette, La. I explained what I wanted to do to him, and he too was questioning as to "Why?", but said that he
could do it with a bit of time and patience.

the result:

I have not felt the loss in bottom end torque that I was told that I would feel. "the 4.0 likes a little backpressure" is what I commonly read. I have found the opposite to be true.

I had been getting between 11-12 Mpg for the last several months, and since I put the spare on the roof rack the transmission was downshifting up every overpass.

First tank results with spare on roof, and 3" exhaust
14 Mpg. no longer downshifting up overpasses, and lower underhood temps, no check engine lights, no stuttering, no stumbling and a big roar! So far I love it!

I'll keep you posted on the Mpg to make sure the first tank wasn't a fluke.

 

[czb83]

Small world, I live in New Iberia, LA, but I got to UL in Lafayette. I once had some custom exhaust work, mainly getting a new bigger downpipe made. I went to ed's muffler in new iberia and it turned out pretty good. I'll be keeping an eye out for a cherokee with a 3" exhaust :wave:

My jeep should be fairly easy to recognize. Its red with a "borla" sticker on the back and you can see the borla exhaust tip.

 

[mdl]

As some of you may have read, I have finally run 3 inch pipe.

I went to one shop and the man behind the counter didnt want my business. He informed me that what I wanted was not necessary as several of you on this forum have said.

After some looking I found Custom Muffler in Lafayette, La. I explained what I wanted to do to him, and he too was questioning as to "Why?", but said that he
could do it with a bit of time and patience.

the result:

I have not felt the loss in bottom end torque that I was told that I would feel. "the 4.0 likes a little backpressure" is what I commonly read. I have found the opposite to be true.

I had been getting between 11-12 Mpg for the last several months, and since I put the spare on the roof rack the transmission was downshifting up every overpass.

First tank results with spare on roof, and 3" exhaust
14 Mpg. no longer downshifting up overpasses, and lower underhood temps, no check engine lights, no stuttering, no stumbling and a big roar! So far I love it!

I'll keep you posted on the Mpg to make sure the first tank wasn't a fluke.

back pressure is a myth. I have been thinking about this as well. Keep us posted.

 

[aparke4]

pics? how did you fit the pipe with the shackles? i have a 2.5 and it was a good squeez... any bigger and it wouldnt have fitted...

 

[5-90]

Backpressure isn't really necessary - it hurts exhuast scavenging.

However, a 3" exhuast may still be a little large. I had a discussion with another member with a racing background (Ed A. Stevens - I need to find my notes from that) who told me how to figure an optimax for exhuast pipe size. I think it came out to be about 2.75" - which provided enough cross-section for sufficient exhuast gas flow, but not so much that the outside cooled and caused any turbulence. I need to find those notes again, or maybe just talk to Ed again.)

A 3" exhuast would be potentially useful for Barney Navarro - the guy that ran a turbocharged AMC six at Indy, and ran it up about 7500-8000 rpm or so. However, it's probably a bit much for us. I wish I could remember, but I think it worked out to 2.5 - 2.75" inches as a useful optimax - but it's just not coming in clearly...

5-90

 

[CW]

I'm planning on going to a 3" exhaust with my stroker after I get the sc all set up. But the 2.5" exhaust worked perfectly fine for my stroker alone. I agree that the back presure being good for 4.0L performance is an absolute myth.

 

[karter74]

I'm a skeptic to 3" exhaust systems for the 4.0. THere is no real reason why any of us would require it as we are just not flowing enough exhaust. I would always opt for the smaller diameter exhaust to keep the exhaust gas speeds up. A site I just found that has the theory of what I'm talking about:
http://www.burnsstainless.com/TechArticles/Theory/theory.html

 

[goodburbon]

so has anyone else noted that the majority of performance exhaust systems offered for the chevy 350 are dual 2.5" pipe?

ok so v-8 owners/ manufacturers tend to run a 2.5" pipe for 175 cubic inches (half of the v-8 per pipe), while I-6 owners are content with 2.5" pipe for 242+ cubic inches? I know the v-8 guys are revving higher, but simple math shows

pi X 2.5in = 7.85 sq. in. (cross sectional area for 2.5" pipe)
7.85 sq in. / 175 cubic in. = .045 sq. in. per ci displacement.

and
pi X 3.0in=9.43 sq. in. (cross sectional area for 3" pipe)
9.43 sq in. / 242 cubic in. = .039 sq in per ci displacement

that's 14% less flow or 14% higher exhaust pressure allowed by 3" exhaust on a stock jeep versus 2.5" dual on a chevy V8.

go down to the maximum "reccomended" and its

7.85 sq in. / 242 ci = .032 sq in per ci displacement

thats 29% less flow or 29% higher exhaust pressure than the bolt on performance chevy apps.
BTW all other things equal, higher pressure= higher temp!
NO WONDER OUR JEEPS OVERHEAT SO EASILY stock is even smaller, and the down pipe is crimped even smaller to avoid the Drive shaft during suspension compression and there is an oxy sensor blocking at least 20% of the flow!


I am not telling everyone to run out and put custom 4" exhaust on their Cherokees. I am only saying that sometimes what everyone says won't work......does.

 

[5-90]

True - but V8 engines are easier to balance for high crankshaft speeds. The inline six, however, has a much longer crankshaft, and there are no opposing forces to counterbalance each other.

Exhuast size is not only a function of engine size, but of engine speed - and where in that speed band the engine will be run. While we're running 70 more cubic inches than half of a small block Chevvy, we're also going to run rather slower - most of the 242's run down around 3000rpm, so they'll not be moving quite as much air as the small block Chevvy...

5-90

 

[goodburbon]

1994 chevy silverado 5.7 redlines at 4000 RPM,

while the OBD II Cherokee red lines at 5K.
while I agree that V8 engines are more easily balanced for high crank speeds, not everyone who runs a 5.7 with 2.5 inch exhaust runs it at 7000RPM

 

[5-90]

True, but most people who are serious enough to get a full system are going to be thinking about getting their engine done anyhow, and you can do pretty much anything you want to the SBC. Hell, I've heard of them being run up around 12Krpm (don't know how long they hold up on that - depends upon the builder and the attention to detail they put into it.)

While the average V8 owner is interested in pure horsepower, people like us are interested in torque. Torque, for us, typically happens between 2000 and 3000rpm - so we're still not moving as much air when we want to run our engines. Torque is ALWAYS higher than horsepower below 5252 rpm, the horsepower takes over (which is why trucks tend to have lower redlines - no need to run high. Truck users are also more interested in torque as opposed to horsepower.)

The fact that the inline six has a harmonic issue up around 6000 rpm is yet another reason you don't often see them run fast - not only do we really not want to, we'd like to keep our engines in working order. Granted, you can run an inline six at higher than that, but don't loiter around 6kRPM, or you'll probably break a crank...

5-90

 

[Slo-Sho]

Break a crank? Even with 7 main bearings? Say it isn't so....:tear:

 

[GoliathJeeper]

I honestly don't think staying at a high rpm for an extended period of time would break a crank. Unless you have an oiling problem, which causes friction to build up on the crankshaft, causing it to weaken, breaking the crank. The only time I EVER broke a crank is when my engine locked up at about 20 mph. Damn, if it has never happened to you you don't understand. Feels like it could have flipped the whole friggin truck.

 

[CW]

If you have ever taken the 4.0L up to 6k you never want to do it again. There is an issue with wied noises and vibes plus you aren't accelerating as fast as you would if you shifted up a gear. I did one pull to 6500 on my 4.0L before I dropped the stroker in just to see what would happen, I didn't leave it pegged at that rpm because it was pointless but I could see some internal problems occuring if one did.

 

[5-90]

It's not a matter of oiling - it's harmonics. The 4.0 crank is so damn long, and doesn't have any counterbalancing forces, that it's possible to snap the thing - probably between a main bearing and a rod bearing, or at one of the journal fillets.

If you would be running the engine for a longish time in the neighbourhood of 6000rpm, you'd set up serious vibrations in the crankshaft, and that would result in accelerated metal fatigue, due to the vibrations.

If you didn't lose a crankshaft, I'd have no trouble believing that you'd lose a rod (probably at the big end,) due to the vibrations and fatigue.

If there were balancing forces, or the crankshaft wasn't so damn long, then it wouldn't be an issue. The "natural harmonic frequency" of the Small Block Chevvy is, I'm inclined to think, up around 18000-20000rpm - and you'll never run the engine that fast. I'd need to know more engineering than I do now to figure out how to redesign the crank to eliminate the harminoc issue - but it would probably be something similar to how suspension bridges are "detuned" (ever notice the weight clamped to the cables? Think they put them there at random, and for no reason? Ever see that video of the suspension bridge literally "twisting in the wind?" Same sort of thing here.)

5-90

 

[goodburbon]

the harmonic would be totally dependent upon weight distribution, and thickness, the harmonic would be different for each different type of crank.

 

[Beej]

Here's a link to a page that shows the bridge that 5-90 was talking about - videos at the bottom:
http://www.glendale-h.schools.nsw.edu.au/faculty_pages/ind_arts_web/bridgeweb/Tacoma_page.htm

I destroyed a 350 foot long iron fence when I was about 17 using harmonics. The fence was loose in the soil and when you pushed it in the center, it created a small wave that went down to the ends and back. Each time the wave came back to the center, I pushed it again, amplifying the wave.

After about ten pushes, I started to fear for my life, by about 15 the center section of the fence ripped right out of the ground and twisted in the air about 3 feet from my head before tearing into pieces. It was quite a graphic display of waves and harmonics. I almost became naturally selected..