Question How Do Headers "Decrease" Low End?

[MrBeanley]

Here is a question for all of you mechanical engineer / engine experts out there: I've read a ton on TS about folks that add headers to their 4,7L, and then complain that they have lost some "low end".

I consider myself to be modestly smarter than the average bear, but I can't quite seem to understand the physics behind this one. So, we're talking about a 4-cycle engine here: "Suck, push, pop, pewy", as it was explained to me ("intake, compression, detonation, exhaust", if you will). On 3 of those 4 cycles, any particular piston is dead weight to the rest of the engine, plus suffering from the resistance of whatever its pulling into, pushing out of or squeezing into the cylinder. It seems logical to me that, if you can get the exhaust out of and away from the engine more quickly, a given piston would meet with less resistance during the exhaust stroke, thereby reducing the amount of power the rest of the engine needs to use to overcome that resistance. So, reduce the backpressure, right? Headers, larger diameter piping, etc, would allow the exhaust gasses to flow more quickly away from the engine, and reduce the pressure required to expel those gasses from the cylinder.

So, what am I missing? Why would reducing backpressure "too much" cause an engine to loose power at lower RPMs?

Thanks for the education, y'all..

[MrBeanley]

Wow. Plenty of views; but no responses. I wonder if that means now one else knows why this is true, either?

[SE2000]

You think that maybe people are telling you to do a search on the internet and then report back?

I found this with charts that seem to explain it well. And it's a honda site so you get to feel the humility.

Header-Exhaust Design Effects on Engine Power - Team Integra

[ricqik]

all i gotta say is my truck gets up better with my jba's. i felt no low end loss.

[shatto]

My experience;

Installed Gibson Headers on my V6 Dakota.
Smoother power through the performance envelope.
Scary acceleration.
2 MPG improvement.

[bill3508]

No low end loss here. On nearly every V8 you will be hard pressed to find a loss in low end torque with headers. Most gain easily. V6 and 4cyl are another story.

[csawyer]

I am by no means the expert on this, but to my understanding, our vvt-i motors are designed to operate most efficiently at higher rpms and tend to rev higher than the other manufacturer's motors. The design, shape, and size of header tubes has a lot to do with where they produce the most power. A motor with lots of low-end torque benefits better from a long tube header to produce more low-end power. Whereas shorty headers are supposedly better for high-end power. The JBA headers everyone uses with our trucks are technically "shorty" headers but yet they seem to produce more low-end power with our trucks. I don't know if that has anything to do with the vvt-i or not. When I installed JBA headers and dual Dynomax mufflers on my truck, my muffler guy (who builds lots of race systems) recommended that I leave the dual cats in place so as to maintain a little bit of backpressure. He said with the smaller displacement of my 4.7 and the vvt-i, that I would loose low end if I opened it up too much. I definitely gained low-end power, but the truck doesn't work as hard to climb hills and tow loads at high speeds anymore either, so I'd say I gained power throughout the rpm range. All that being said, I can't answer your question with anything more than my own seat-of-the-pants evidence...Don't know if that helps you or not.