Thinking of putting a CAT back dual 2.5" exhaust (quad tips) on my 4.0 6 cylinder BF II YT auto (sedan, but same issue applies to XR6s), possibly a Herrod system. One exhaust place I tried noted I might get a loss of power. Any thoughts out there?

thats a very broad statment.

I had a hemi with triples on it and it went better with a single 2 1/2 on it than twin 2 1/4's. If you do go twin make sure they put a balance pipe in it.

a dual 2.5" system is a equivalant to around 3.5". its WAY oversized for your application, if its just for looks maybe try looking at getting something made for you that suits the motor...

2 x 2.5" or 3.5" is what an xr6 turbo upgrades to when they want to push 350-400rwkw

I used to work at an exhuast fitting centre, and yes twin 2 1/2's is way to big, if you were after a twin system, i'd say twin 2" 2 1/4 might still be too big, you need that pressure in there.

You could even go the single 2 1/2" and just split it at the tailpipe (behind the muffler), with two 2 1/4" tailpipes instead of catback, few places do those sort of custom jobs, depends where you go really.

FFS when are you exhaust shop d**k heads going to get it through your thick heads that exhaust systems don't need pressure and pressure hurts power.


Yes there is such thing as too big, NO pressure is not the issue. You need HIGH GAS VELOCITY in the system. the right exhaust is a balancing act between velocity and restriction.


Add to that, If you are after out right power and you have fitted a proper set of extractors anything after the end of the collector should pose no restriction EVER.
Note: off the shelf extractors don't even come close to be a proper set of extractors.

think before you type. just another rubbish post to add to your 4000 odd posts of dribble.

back pressure is caused by restriction, so if you want air speed through your exhaust your going to introduce turbulance, which is going to add back pressure.... one is related to the other

So before calling people d***head, take a quick look in the mirror and you will know what one looks like....

how about you stop and think, or perhaps read properly. back pressure does not make power.

You don't want that much speed that you create pressure. in the same way you don't get over do it on the pipe size and go huge to the point of poor gas speed in an effort to reduce pressure.

Like i've said many times before YOU DON'T AIM FOR BACK PRESSURE. YOU AIM FOR OPTIMUM GAS SPEED IN THE RPM RANGE THAT SUITS YOUR ENGINE. back pressure above this point is a by product, the same way that poor gas speed below that point is a by product.


Do you not find it strange that people will happily put a 3inch system on BA engine to make less the 200rwkw. yet EVERY ONE jump up and down when people want to the put the same system on a SOHC engine to try and make the SAME power.

OR that people will happily put a duel 2.5inch system on 5L V8, think about that thats one pipe per bank, with each bank only 2.5L in capacity. yet people jump up and down about fitting anything bigger then 2.5 system on a SOHC 4L I6.

Take a look at street commodores magazines VN V6 Vengeance project. when from a 2.5 to a 3inch system on a basicly stock engine and made MORE power and 1/4mile times dropped and 1/4mile MPH lifted.

From my experiance low back pressure = poor idle, but does little above this. However, it does need to be balanced. You can't have air going in faster than it can get out.

id like to see a 100rwkw v6 improve its performace by adding a 3" exhaust

but.... if it came out of street commodores it must be true, so case closed

I said near stock. and the drags strip doesn't lie. but hey believe what ever you want.

I've got my bible, it's a bommadore magazine

Exhaust shop has no idea, if u go by my case.
Previously i had a 2.5 system hi flow cat and pacies car was making 174 rwkw.
Last Feb I changed the system over from behind the cat to a 2.5 dual system, you can put whatever tips you like. personally i put the quad tips aswell. - looks mean. new dyno is 182rw. Doesnt necesarilly mean u will loose power... Results speak for themselves.

Yeah good one. i would type haha after that, but you weren't that funny.

back pressure is way better for power.

If its a 2-Stroke