2.5" or 3" single exhaust - that is the question.

So - I'm taking the Magnum 360 from my 02 Dakota and tossing it in my '14 JKU. The engine has a mild Comp 280 cam, stock Magnum manifolds, and I'm trying to decide whether or not to go with a single 2.5" exhaust or single 3" exhaust. I can do either, and here's the interesting part...

The Dakota with the 5.9 V8 had a factory 2.5" single exhaust...and...the '14 Wrangler JKU with the smaller 3.6 V6 also has a factory 2.5" single exhaust - same single exhaust size for both chassis'. I'm putting the 360 driveline in my Jeep...and I am VERY tempted to use the 2.5" exhaust that already in the Jeep and save myself a ton of time and $...and I can always install a bigger exhaust down the road. But...I do wonder if the 360 would benefit from a slightly larger exhaust...and why the factory didn't do a single 3" to begin with. I know it's a very subjective question, but it is interesting.

The ole rule of thumb of 'every 100 horsepower needs 1” of total pipe diameter' is ringing in the back of my mind...and while I don't know exactly how much schlitz my engine is making I know it isn't a mega power house - it might be 300 hp - maybe.

My engine will be 'done' by 4500 rpm - this is not a race Jeep I'm buildin'...

I'm thinkin' use the 2.5" exhaust and call it 'good enough'...at least for now. The Jeep already has the nice 2.5" exhaust under is and it's in good shape, and the 'smallish' exhaust might actually aid the engine in scavenging out the flow...and a whole lot less fab time which is always a bonus.

Thoughts?

- Sam

i would think you will be fine with the 2 1/2" pipes, especially since you said it's all in by 4500rpm.
just my opinion, and i have been WRONG many times !

Now...you said "pipes" - I'm looking to run just a single exhaust pipe, not duals. Were you referring to dual 2.5s, or a single?

...and yah I've been wrong many times too...which is why I surround myself with lots of smart people...and why I post questions like these and ask for multiple second opinions

Go big or go slow
Been their, done that, 3.0 way better, especially with a single pipe outlet

Get it running as is. As you say, you can always upgrade later if you're unhappy with it. TBH I think the 2 1/2" single will cope OK if the motor is done by 4500 but I would have thought a 280 duration cam would wake a 360 up a bit more than that

...and apparently Ma Mopar thought the same thing with the OE Dakota's single pipe being a 2.5" unit. And I mistyped - the cam I'm running is a Comp unit #20-745-9: 212-218 (264/270), .480/.480, on a 114* LSA.



ya I dunno - I haven't really had a chance to do any real street time - no full-throttle runs yet...mostly just low rpm trail runs.

Meantime, if anyone else has input pls feel free to keep the conversation going

There was an Engine Masters dyno test on this topic, and many years ago a magazine test in HotRod or CarCraft where they did a similar test.
I think the magazine used a 3rd-gen Z28, probably with a bit less power output.
Gut feeling is if the OP's engine is done @ 4500, 2.5" should be OK - muffler's likely as important in that scenario.
If it spends much time at 4,500 - not likely on the street - then 3.0" would likely help.
3.0" will also be louder...

yah if I didn't already have this nice 360 I'd be going with a turbo'd 4.0 liter, but I do have this nice 360 so that's what I'm using. It's a truck engine - nothing fancy - just a bit bigger cam to help it breathe and be a bit more efficient...not really for more power. I mainly want the engine to be as reasonably powerful and efficient as possible...with the stock heads...when cruising the highway between 2500-2000 rpm. Most of the rest of its time will be spent just off idle crawling along the off-road trails, and with a 134-1 crawl ratio I don't need a lot of power for that.

If you already have it, try it and measure backpressure.

I've had a 3" single on my Dakota for years. Back when everyone was moving heaven and earth to run x-pipes and duals our local exhaust builder said it wasn't worth the extra weight. He said it takes a big HP jump to max out a single 3" system, because velocity is important for the entire system length.

TLDR? A single 2.5 will do fine for now.

If you decide to do the single, I'd suggest buy or fabricate a copy of the flowmaster Y collector for the crossover.

They're a tested/proven torque enhancer on that kind of a deal. I really liked it on my C body.

Nobody talks about them much, because anything other than super exotic race garbage is not cool.
It's difficult enough to even stay relevant, much less be thought of as uncool

If I had room under the truck, I'd love to run a setup like these.
I'd really like to run merge collectors on the headers and y-pipe on a street car to see if any benefits happen

I'm planning to run the factory 2" merge pipes from the Jeep, which are also the same as what the Dakota had. The only thing I'm intending to fabricate is the actual down pipes from the Magnum manifold exits to the primary tubes, and use all of the Jeeps' OE plumbing after that - might even retain the Jeep exhaust 'loop' pipe (just downstream of the cat) to keep the distances equal.