Proper exhaust size for stroker

[BREEZE1]

Well I haven’t seen any threads on the website about exhaust! From what I have picked up stock 4.OL need to have a little back pressure but when you have a stroker it needs to be free. I recollect seeing something about 4.6L & 4.7L need 2.75” exhaust but no one makes that size exhaust pipe, that’s why folks go with 2.50” or 3.00”. Honestly I can’t remember. So please correct me if I’m wrong.

I plan on starting to build a 4.7L at the end of this month for my 98 Cherokee (xj), and I was wondering what the best size exhaust it’s going to be for it. For my personal application it’s my dd (lots of highway miles), so there is not going to be much off-roading going on in this one. I already have a jeep for that. I miss the power my mustang had but I love my jeep (that’s why im building a stroker). So to cut to the point whats the best size exhaust to run for a “freeway flyer / street sleeper” should I go with the 2.5” or 3”? whats the pro’s and con’s of it?? Again looking back at my 4.6L mustang I ran duel 2.5” x-pipe exhaust with out cats and it ran like a top, now I building a 4.7L jeep engine and folks are putting one puny 2.5” exhaust on it.

[lafrad]

When it comes down to it, there is no "correct" size for exhaust... just "optimal for your use"


In my case, the stroker is being built up as a "rebuild" that can gain more "torque" for everyday use. Since I'm not going to be revving the crap out of it, am keeping the cam small, and want to be able to use as many off the shelf parts as possible... an aftermarket header and hi flow cat are all I feel are necessary. Low RPM/Crawling moves very little exhaust through the system, no matter what size engine you have... you only really start running into issues at high RPM/High volumetric effiencies.... these AMC I6's are very averse to those running conditions without a LOT of cash dumped into them.

[nukfyrsq]

Take a look at some of Dino's tricks, here's a link to a paper he wrote regarding ehauast relative to horsepower http://www.angelfire.com/my/fan/Exhaust.html. I spoke with my exhaust guy about this and he agreed with some back pressure helps on the torque side while free flowing is better for peak horsepower. Kinda a no brainer when you look at mid to high performance race set ups. I'm planning on running 3" from my Gibson header rerouted to behind the bell housing into the CAT basically the route Dino has taken. After that the exhaust will remain as is; Borla CAT back system. Additionally you could run into emission issues. My exhaust guy seems to believe there is a possibility the 'bulge' in the stock pipe is an addition CAT. Anyone have any insight on this? If it is we intend to carefully resize it up to the 3" and weld it back inline. If it can't be reused I may be looking at upwards of $300 for a 'quality' inline CAT to replace it.

[SilverXJ]

What bulge are you speaking of in the stock pipe? Also, you can get a good high flow cat for around $70

[Maddawg]

I'm not very knowledgable about exhaust, but my plan on my 2000 Grand Cheroke is running a dual. I bought a Rugged Ridge header and the muffler shop is bending two down tubes. I am then dropping the pipes off in front of the rear tires with chrome turn outs. I have found that Advance has nice Thrush Turbo muffles for $25 ea. The are round so will tuck up nice. So hence a "true Dual"

[lafrad]

Best to throw an X or H pipe in there... i have never heard of a street driven situation that has done better without one... also, it will sound better with 6 cyl outta each side...

[Maddawg]

Thanks not a bad idea. Gonna have to check for space.

[nukfyrsq]

On my TJ when the down pipe wraps around the front of the oil pan, in front of the oil pan there is a distinct increase ~3/4”-1” in diameter of the exhaust pipe for a length of ~6”. I believe diagrams in my FSM also have this increase in size apparent; however I have not read in detail the section regarding the exhaust system. Is the hi flow inline (compact) cat you’re speaking of also stainless steel? I think the catch here is the unit I was looking at is very compact relative to a standard cat and was supposedly capable of working its magic to the same emissions standards.

[1bolt]

There's no such thing as good back pressure, unless one is misusing the word pressure. There's scavenging which could be termed "back depression" it is the opposite of "back pressure".

Having said that there are many vehicles that will run shitty without their exhaust on but this is usually due to the exhaust actually being a good design that is tuned to create "scavenging" (helping pull the exhaust out of the cylinders) and/or fuel and spark timing that aren't flexible enough to compensate when the balance is upset by the lack of plumbing.

I'm not sure how back pressure fighting against the escape of exhaust gas can lead to better low torque. back pressure would increase pumping friction, which fights the rise of the cylinder, and resists the rotation of the crank... This should lead to torque losses only. Perhaps there's some fundamental I'm missing. I'm pretty sure I've read Dino talking about smaller tubing promoting better SCAVENGING at lower RPMs, but this isn't because there's more back pressure, it's because there's a better siphon effect from the smaller diameter tubing at low RPM's. Put a 1" exhaust all the way back and I'm sure the low torque would be increased, and I'm sure the engine wouldn't rev past a couple thousand RPM, the Peaked torque is from the scavenging effect... the "back pressure" only creates the lack of revs...

Now there is the case of high overlap cam timing (a so called lumpy cam) where back pressure could potentially keep more of the intake charge from sucking out the exhaust at low RPM's thus making the high overlap cam choice appear to work a little better at idle or low speeds, or it could just push exhaust up into the intake tract (reversion) and make the thing run like crap at any speed....

At any rate A high overlap cam (and any backpressure at all) is not the ideal choice for a low/mid torque "RV and Towing" engine build, so having back pressure or a high overlap cam is simply a case of poor build choices (if low and mid torque were the goal).

On the topic at hand, the best stroker exhaust will start with the longest tube headers you can get (or have made) up to somewhere around 48" where diminishing returns really sets in, the longer the primary the better the scavenging.

As we've JUST found out from Rick Mudge who apparently did the heads for Garth Hills 8 second 4.0 based NHRA racer and the 0331 head/intake/exhaust. The exhaust should be split into duals 123 into one and 456 into the other with no H pipe or X pipe. I'm still trying to pin down why an I6 will benefit from this setup. A V8 has a much more "odd" firing order and benefits from the scavenging of one bank to another, thus the H and X pipe. At any rate I'm prepared to believe anyone who's made a 4.0 push an AMC to 8 second quarter miles.

As for the less important part of the exhaust, I think 2" duals would be entirely sufficient for even a strip bound streetable Jeep, smaller tubing may have some benficial "tuning" effect (again scavenging) for lower and mid range power, and bigger tubing will present less "back pressure" and tend to help higher RPM power. It's most definitely possible to tune the length of your entire exhaust system to optimise power, it is done in racing, but it is a very small finicky (couple horse power) thing and is not at all easy with an exhaust that must snake around and fit into a set amount of space.

[Cheromaniac]

A single 2.5" exhaust is sufficient for a mild stroker that isn't putting out more than 250hp at the flywheel. If yours is likely to be putting out about 250-275hp, you could copy the front part of my exhaust and upsize the header collector/downpipe to 3" while keeping the rear half at 2.5" diameter to maintain good exhaust gas velocity.
Why complicate things by trying to construct a true dual system when a simple single system works so well? Most of us will seldom rev the engine beyond 4500rpm (where the torque curve starts to drop more steeply) anyway, let alone to the stock 5200rpm rev limiter.

[1bolt]

Well the reason to complicate things with a dual setup depends on application of course, since this is in Performance Tech and not basics so I think the unstated assumption in this forum area is that getting a bit more performance is the goal and worth the complication. I agree though a single 2.5 will suit 99 out of a 100 Jeepers, maybe more. I wouldn't be at all against running a single 2" pipe for that matter, as it will promote torque a little more in the lower RPM's and probably only give up a horse or two off the peak.

[shockcocker04]

i have a banks torque tube header and 2.5 all the way back, sounds great and works great. large exhaust is for high rpms

[the_wrench116]

this seams to be one of those things where you get to choose something and it could work great for one guy and someone else copies it and he gets squat. i've got a renix head and block with a high out put intake manifold i've got an afe stage 2 cold air intake. the exhaust i put together starts with a modded header 2.5 in collector a home built s pipe and a flowmaster muffler 50 series the muffler back is a dyno max 2.5 inch mandrel bent setup. all the pipe in my exhaust is mandrel bent.

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[nukfyrsq]

I finished my 4.6 over Christmas and have just over 1100 miles on it. Currently I have a no name SS header to stock pipe to a cat back Borla. Dynoed a week ago with 220ft/lbs 170 hp at the rear wheels. I'll list the details with pics in the build thread later. I'm headed to a custom exhaust shop today to discuss and price a system. I'll post what I find out. I have a Gibson header from my stock engine that has the final collector configured like a box that I think I can remove the down pipe and enlarge the opening for a 2.5" or 3". My plan is to come straight down and cross under the oil pan in front of the sump. This will eliminate the nearly ~120 degree J bend and U bend around the front of the engine. I will run which ever pipe size all the way back replacing the cat and muffler with like size. Here's another question; has anyone tested and posted emission test results after these types of modes? I had an old exhaust guy tell me he used to place a slot and/or holes a short distance before the end of the tail pipe so he could drop in a restricting orifice (big washer) or add air (Kinda like they're doing with some of the diesel exhaust these days) with the holes to trick the emissions sniffer.

[ajmorell]

There's no such thing as good back pressure, unless one is misusing the word pressure. There's scavenging which could be termed "back depression" it is the opposite of "back pressure".

Having said that there are many vehicles that will run shitty without their exhaust on but this is usually due to the exhaust actually being a good design that is tuned to create "scavenging" (helping pull the exhaust out of the cylinders) and/or fuel and spark timing that aren't flexible enough to compensate when the balance is upset by the lack of plumbing.

I'm not sure how back pressure fighting against the escape of exhaust gas can lead to better low torque. back pressure would increase pumping friction, which fights the rise of the cylinder, and resists the rotation of the crank... This should lead to torque losses only. Perhaps there's some fundamental I'm missing. I'm pretty sure I've read Dino talking about smaller tubing promoting better SCAVENGING at lower RPMs, but this isn't because there's more back pressure, it's because there's a better siphon effect from the smaller diameter tubing at low RPM's. Put a 1" exhaust all the way back and I'm sure the low torque would be increased, and I'm sure the engine wouldn't rev past a couple thousand RPM, the Peaked torque is from the scavenging effect... the "back pressure" only creates the lack of revs...

Now there is the case of high overlap cam timing (a so called lumpy cam) where back pressure could potentially keep more of the intake charge from sucking out the exhaust at low RPM's thus making the high overlap cam choice appear to work a little better at idle or low speeds, or it could just push exhaust up into the intake tract (reversion) and make the thing run like crap at any speed....

At any rate A high overlap cam (and any backpressure at all) is not the ideal choice for a low/mid torque "RV and Towing" engine build, so having back pressure or a high overlap cam is simply a case of poor build choices (if low and mid torque were the goal).

On the topic at hand, the best stroker exhaust will start with the longest tube headers you can get (or have made) up to somewhere around 48" where diminishing returns really sets in, the longer the primary the better the scavenging.

As we've JUST found out from Rick Mudge who apparently did the heads for Garth Hills 8 second 4.0 based NHRA racer and the 0331 head/intake/exhaust. The exhaust should be split into duals 123 into one and 456 into the other with no H pipe or X pipe. I'm still trying to pin down why an I6 will benefit from this setup. A V8 has a much more "odd" firing order and benefits from the scavenging of one bank to another, thus the H and X pipe. At any rate I'm prepared to believe anyone who's made a 4.0 push an AMC to 8 second quarter miles.

As for the less important part of the exhaust, I think 2" duals would be entirely sufficient for even a strip bound streetable Jeep, smaller tubing may have some benficial "tuning" effect (again scavenging) for lower and mid range power, and bigger tubing will present less "back pressure" and tend to help higher RPM power. It's most definitely possible to tune the length of your entire exhaust system to optimise power, it is done in racing, but it is a very small finicky (couple horse power) thing and is not at all easy with an exhaust that must snake around and fit into a set amount of space.

You are spot-on about the backpressure. From what reading I have done and talking to people in the fluids realm the sizing of exhaust has a lot to do with maintaining temperature, probably relates to the scavenging thing.