low-end loss w/ O/R pipe?

[jaymac]

I have BBK headers, O/R H pipe, and a 2.5" MAC cat-back on an otherwise stock engine. I've noticed a loss of power in the low-end. Is this due to the lack of back-pressure?
If so, since catalytic pipe assemblies are so much $$, would welding on two high-flow cats restore some low-end power?
What brands/ prices should I be looking at?
Thanks for your time,
Jeremy

 

[Michael Yount]

The high flow cats have very little pressure drop associated with them Jeremy - I don't think that's gonna solve it for you. What's probably doing it is the 2.5" diameter on the relatively stock engine; I guess no one warned you that you'd likely see a loss of bottom end with the larger system. A set of long tubes with appropriately sized primary tubes (1.5" if you're looking for help on the bottom) should help restore the bottom, and may improve peak a bit. I'm assuming your BBK's are shorties.

 

[04sleeper]

I have experimented with the header and h/x O/R pipe and found the headers to be the culprit. I tried 1 5/8" FRPP shorties and even som MAC 1 1/2" equal length shorties. Both lost low end power and torque. I ended up putting the stock headers back on after I cleaned up the welds on the inside tubes a little. I will put the FRPP's back on when I get some heads.

 

[90pony302]

Your theory is right on the money. But this loss will eventually be made up through future modifications. If you keep modding your car then you'll end up getting more power out of those mods because of the exhaust. more air in more air out.

 

[Michael Yount]

90pony - it's not quite that simple. Many, many exhaust systems out there are oversized resulting in low end torque losses. And they don't result in any peak gain over a properly sized, smaller system. People tend to forget, the amount of flow you're dealing with is LESS than the amount of air that flows into the engine through a single 3 or 3.5" tube. Many oem applications exist where between 330-350 HP is flowing through factory catted and heavily muffled factory SINGLE systems that are 2.75 or 3" in diameter (LS1 Camaro/Firebird, Nissan Titan pickup, Dodge Hemi pickup, etc.). It doesn't take nearly as big a system to handle the exhaust flow without limiting it as people think. The single 3" offers just over 7 sq. inches of flow section; dual 2.25" systems offer just under 8 sq. inches of flow section. For naturally aspirated street 302's, 2.25" duals will handle just about as much power as you want to flow; not any bigger than 2.5" duals. And the closer you are to stock in the motor mods, the more the larger sizes hurt you on the bottom without any benefit on top. Bigger is simply not always better.

 

[jaymac]

Hmmm.... interesting. So you're saying I should get long-tubes w/ a shorty pipe (cats or no?), and keep the 2.5 cat-back? Sounds like you have alot of know-how in the subject!
Thanks,
Jeremy

 

[Michael Yount]

Jeremy - you have to decide if you're gonna run cats or not; can't help you with that. High flow cats aren't going to offer much flow restriction - so high flows or no high flows won't affect flow much. The cats will noticeably quiet the system down. For a mostly stock engine if it were mine - if I wanted to put headers on it, I'd only put long tubes on - 1.5" primary tubes. That will help bottom end torque, but it won't add much peak power - because the stock headers aren't a bottleneck to flow until you get up around 270-280HP. Putting shorty headers on a mostly stock engine doensn't help peak power much for the same reason - and since most have larger primary tubes, folks often complain of a loss in bottom end with the larger primary shorties. Same issue on the diameter of the rest of the system -- the larger diameter systems on low power engines often have folks complaining of loss of bottom end.

If I had a mostly stock motor - I'd run small primary long tubes, and a 2.25 system the rest of the way back. That should manage peak flow with no problem, and the long tubes should enhance bottom end.

 

[The Shape]

As for a mostly stock engine when I went from my stock 17 year old h-pipe and stock mufflers to the offroad pipe and flows I did notice a small seat of the pants difference in low end. It was made up for though by the car seeming to have more power on the highway in OD (AOD). It was a fair trade to me and the car sounds a thousand times better.

 

[MustangMulch]

When i went from stock 4-cat h-pipe to a o/r prochamber, I noticed a little gain in low end. Then when I went from the prochamber to a 2-cat x-pipe i noticed a little loss in low end but a little gain in the top end.

That being said, none of the gains/losses were very noticeable at all. Exhaust isn't great for performance on a stock motor, but it sure does make some sweet ear candy.

 

[jaymac]

I haven't been able to find any 1.5" long tubes, only 1.75"...any ideas?
Aren't there also inherent installation/fitment problems w/ longtubes?

 

[Michael Yount]

Most longtubes for the stangs are 1 5/8" - not sure where you're looking, but they're available. You might have a hard time finding 1.5's in long tube. Of course, things are more crowded with long tubes, and installation is at best a chore; at worst a real pain in the ass. But, you get what you pay for - in money and time.

 

[cleanLX]

Hooker make 1-1/2" long tubes in both the comp, and super comp lines.

I went with 1-5/8" long tubes and 2-1/2" dual exhaust from there back on my stock motor.
Michael Y. is correct, it's a little asleep below 2500, starts to come on around 2800, and pulls much harder than stock abover 3000... especially up over 4000.
I wanted strip performance, and was willing to give up some around town punch.

If you want the 'around town punch' type power, I'd go with the 1-1/2" long tubes, but, I'd leave the 2-1/2" exhaust, I really don't 'think' there is any performance advantage one way of the other between 2-1/4" vs 2-1/2" on a stocker.
With the bends up over the axle and pushing thru mufflers, my guess is both systems are going to create the same restrictions at those sizes.
It's my belief that the headers are going to make or break you, not 2-1/4" vs 2-1/2" exhaust.

Good Luck.