gtm130

here is what im thinking...
as exhaust gasses get further from the headers, it cools and needs less pipe diameter to flow at the same rate, therefore wouldnt i experience better performance if i did 2.5 from headers to the cat and then 2.25 the rest of the way? and what if i did a flowmaster with single in and 2 out. (in that case, 40 or 50 series?) and for headers i am debating nwor, downey, and doug thorley

i have a 3.0

anyone have any experience with any of this and sound clips would be great for any of the headers and flowmasters
thanks ahead of time

jimbo74

your logic sounds correct, but in real world, the temp at teh tailpipe is still pretty hot.... 2.25 all the way through sounds fine... be warned though, if you go with dual outs, you may lose backpressure as dual 2.25 outs is a lot more flow than a single 2.25

gtm130

yes but is dual 2.25 out of a single muffler much more than 2.5?

what if i tapered the exhaust from 2.25 out of the muffler to 2.0 and then put on nice looking tips?

yotafool

Your really set on gettin this 3.0 pepped up. Good luck man. As for the exhaust I would just go with a single pipe. Seems like dual would be just for looks.

gtm130

lol i want to do something im just doing it in stages

gtm130

i think im gonna stay away from dual. so....

2.5 from headers back
2.25 from headers back
2.5 to cat, 2.25 cat back
2.25 to cat, 2.0 cat back?

also... do i run it in the stock location or run it out the back next to the hitch? i dont do serious wheeling so is there any performance for strait as opposed to bent out the side?

jimbo74

i would do 2.5 to cat then 2.25 out.... single pipe.. there is not real performance gains for duals on a v6

and as far as duals go.. exiting duals with 2,25" will try and flow a lot more air than a single 2.5" will provide, thus losing backpressure... especially with a high performance exhuast which is designed to scavenge gasses

gtm130

thats what i was thinking, but the nwor header's y pipe is 2.25 so it would be 2.25 all the way through if i went with that.

still waiting to see if anyone has used these headers other than downey.

Jason B

You can do what I did and have the best of both worlds.



More info and a write-up on my 4Runner site.

gtm130

that looks nice but you have forced induction and no backpressure problems, i need the backpressure. you could run 3" without a problem where i would loose power there.

also, in the winter, one pipe will have have "smoke" coming out and the other wont so your little secret is given away. id rather not pay for something not functional as well.

mastacox

Hot exhaust would be less dense, requiring (thoretically) less surface area to flow than a similar volume of much colder (and denser) gas. However, the amount of cooling from the exhaust port to the exhaust tip is a negligeable amount, especially once the engine is at running temp.

I'm going to go ahead and say a full 2.5" system would be awesome, and sound really good too. 2.25" is also good as long as you have a good cat and muffler, but if your header has a 2.5" collector, why not just stay there?

mastacox

2* 3.141 * (2.25/2)^2 = 7.951 in^2 area for dual 2.25"

3.141 * (2.5/2)^2 = 4.908 in^2 area for single 2.5"

3.141 * (3.0/2)^2 = 7.06 in^2 area for single 3"

In essence, to get flow <similar> to a dual 2.25", you would need something like 3.125".

gtm130

thanks for that technical data.

but i was under the impression from my chem classes in highschool way back when tha the hotter the gas, the less dense, meaning more volume, cooler gasses = more dense and less volume? correct me if im wrong but i think thats right.

heres the problem. nwor headers have a 2.25 y pipe. downeys i think is 2.5 and thorley i dont know anything about.

mastacox

The thing is, you have a [volume per second] coming out of your engine, and it is cooling down (barely) as it exits the exhaust. If you had a fixed volume and increased it's temp, it would expand, but your gases are being constantly replaced by more gas from the engine. Basically, if you had two flows of gas, one cold and one hot, to have the same back pressure the hot one would have a smaller surface area pipe than the cold flow.

That being said, I would go for the downey (2.5" collector) and keep 2.5" all the way back. This will be especially beneficial for mid to top end gain (about 2500 to 5500), but you may lose a tiny bit in low end (under about 1500 rpms), probably not noticeable.

The 2.5" will be a good choice for exhaust size IMO.

gtm130

thanks, i see you have a flowmaster as well, how does it sound? and do you have the headers as well?

sutlTL

I'm running headers and complete exhaust system from Downey and there is a huge difference in performance! 2.5's all the way!

mastacox

I don't have the headers...yet... but it's high on the list

The flowmaster is a good choice, it sounds deep and powerful, with little to no interior reverb (I feel like it has less obnoxious backfiring or high pitch noise, better than a magnaflow or the like IMO). I got center in/offset out and it fit quite well.

elripster

First off, you should be talking about cross sectional area, not surface area when dealing with flowrates. (flow rate, or M-dot as we call it, is the densityXvelocityX cross-sect area)

The amount of gasses as far as number of molecules going into the pipe equal that going out. As the gasses cool, and they DO cool, they will take up less space and cause less pressure. (pv=nrt) The pressure will affect scavenging. Whether or not it's in a good or bad way is another story.

Unless you want to get on a dyno and try some different sizes, you are probably fine just sticking with the 2.25" all the way out.

Another thing to consider, since you are modding in stages, what might be the optimum diameters now, might not be after say some intake work. Since the exhaust is probably the largest bottle neck, it's a good way to start. After all is said and done, if you are curious and can find some shop to accomodate you, maybe you can run the truck ona dyno with the cat back, off, a 2" mock up, and your existing set up. Then you'll know for sure.

Frank

mastacox

Calm down buddy, I MEANT cross sectional area but I retarded , hence the A=pi*r^2 equations above, I wasn't calculating the outer surface area of the pipe or anything.

I stand with 2.5"...

Flamedx4

gtm130, the reduction in pipe size over distance would be correct if you were building a fireplace flue, where you need to accellerate the flow over distance and at the same time compensate for the gas cooling (othewise smoke up the house...) But here the gasses are being pumped out, and only a little backpressure is needed. I would guess that if stepping down the pipe size worked better in a car, all the aftemarket systems (Borlas etc) would come that way...

jason B - that looks cool with those big tips, but I'd smash em to pulp in minutes the first trail I hit... <grin>