was told NOT to go with 2.5" exhaust

[2001XJ]

So, my Golen Stroker is about to be installed and I wanted to go with a new Clifford Performance header utilizing 2.5" exhaust all the way out. I called clifford and spoke directly to a tech and was asking some details about their 57-2015 and asked if they could make up a y-pipe with 2.25" input and a single 2.5" output instead of their 2.25/2.25 y-pipe.

He immediately said, " 2.5" is too big" and I told him it was going on a stroker. He said, " i don't care its too big and not enough back pressure to get the best performance". I told him there seemed to be a huge amount of guys running 2.5" exhaust. His reply was," I don't care, there is a reason we win race after race, maybe all of these guys running 2.5" exhaust are the ones we are beating all the time...haha"

What is your opinion on 2.25 vs 2.5, I want to upgrade but now am wondering if I should just stick with stock size....

Thanks!

[FlyinRyan]

I disagree. I insist on running 3"!

[wjtom]

At least 2 1/2 for sure.Check any exhaust company 3 inch is marginal for a single exhaust at 300+hp.Dont forget the muffler is part of the system they dont all flow enough to support any kind of real hp in a single system so check the flow.No point in putting a good exhaust on only to have the muffler be the restriction.

[Cheromaniac]

He said, " i don't care its too big and not enough back pressure to get the best performance".

That statement right there tells me he doesn't know what the hell he's talking about. Engines don't "need" backpressure. The optimum backpressure is as close to zero as possible. That said, going too big on the pipe size can reduce exhaust gas velocity resulting in a loss of performance and excessive noise, so the idea is to strike the right balance between maximizing exhaust gas velocity and minimizing backpressure.
On a mild 4.6L stroker producing somewhere around 250hp, you need at least a 2.5" pipe size at the front. As exhaust gases pass down the system and cool down, their volume reduces and you could get away with a smaller pipe size (say 2.25") at the rear. Then the flow limiting factors will be the cat and the muffler, so choose low restriction versions of each.

[SilverXJ]

That statement right there tells me he doesn't know what the hell he's talking about. Engines don't "need" backpressure. The optimum backpressure is as close to zero as possible. That said, going too big on the pipe size can reduce exhaust gas velocity resulting in a loss of performance and excessive noise, so the idea is to strike the right balance between maximizing exhaust gas velocity and minimizing backpressure.

x2. Dino beat me to it.

Isn't the stock exhaust size 2.25" on earlier ones? And I believe they enlarged the down pipe on later models to 2.5" (XJ at least).

[vashxj]

theres no such thing as not enough backpressure, just look at a nhra dragster race they just run like foot long pipes off the head, pretty sure they don't have power issues

[Cheromaniac]

He immediately said, " 2.5" is too big

I don't think his wife would agree.

[caveman]

I've dealt with Clifford performance in the past. I wouldn't rely much on there opinion. I believe the teck guy you talked to is the only guy left working there and he's the owner. Handed down to him buy his fatherinlaw. The last product i got from them were very bad quality. Worse than i would expect from a company from china and took 9 weeks to recieve.

[6TIME]

That guy at Clifford is blowing smoke. A 4.6 liter motor breathing thru a single 2.5" pipe seems a bit on the conservative side....2.25" way to small! It's all about displacement and horsepower needs... Engines are basically air pumps. Most exhaust makers will have charts showing these things in relation to primary pipe diameter.

[5-90]

So, my Golen Stroker is about to be installed and I wanted to go with a new Clifford Performance header utilizing 2.5" exhaust all the way out. I called clifford and spoke directly to a tech and was asking some details about their 57-2015 and asked if they could make up a y-pipe with 2.25" input and a single 2.5" output instead of their 2.25/2.25 y-pipe.

Makes sense - when you plumb banks together, you usually need to step up in size for the junction. It helps with pulse scavenging.

He immediately said, " 2.5" is too big" and I told him it was going on a stroker. He said, " i don't care its too big and not enough back pressure to get the best performance". I told him there seemed to be a huge amount of guys running 2.5" exhaust. His reply was," I don't care, there is a reason we win race after race, maybe all of these guys running 2.5" exhaust are the ones we are beating all the time...haha"

It is /not/ too big! In fact, it's about right for a mid-grade stroker, and shouldn't cause turbulent backpressure.

If backpressure helps performance, then Top Fuel would run rather more than a straight 30" stack on each port - that's just enough pipe to keep the port from drawing in cold air and warping a valve - which would screw up everything...

What is your opinion on 2.25 vs 2.5, I want to upgrade but now am wondering if I should just stick with stock size....

Thanks!

It's been a while since I've run the numbers, but 2.25" will handle OEM, mild overbores, or maybe a stroker crank with no boring.

2.50" ID would be good for a 6-242 with a 6-258 crank, 0.030"-0.090" over, and running typical crankshaft speeds.

2.75" would be for something really exotic - like a 4.000" bore and an offset-ground crankshaft, or a mild- to mid-grade stroker balanced to run at high speeds.

Running an exhaust that is too large is almost as bad as running one too small - just for different reasons. I shan't go into them right now, but I recommend reading The Scientific Design of Intake & Exhaust Systems (I don't recall the author offhand, Morrison?) Design and Simulation of the Four-Stroke Engine by Gordon Blair is also useful.

[the_wrench116]

im running a customized header and a 3in setup on my internally stock motor its faster then it was stock. once I get a stroker from atk im going to doctor the header some more for clearance and be happy with it as is.

[the_wrench116]

http://www.ebay.com/itm/aFe-2-5-Mach-Fo ... 2199130%26


bolt on and go. this is stupid easy. just make sure that the header is at least a 2.5 inch outlet.

[beiwulf]

The Scientific Design of Intake & Exhaust Systems (I don't recall the author offhand, Morrison?)

close enough for horse shoes and hand grenades.

http://www.amazon.com/Scientific-Exhaus ... ke+systems

informative and technical.

[6TIME]

When I put a full 2.5" mandrel bent setup on my 4.0 it responded very well at all rpm ranges. There's no drawbacks to running 2.5" On a stroker.......