BoOm 146

I just got my exhaust completely done on my 2000 Tiburon:
OBX 4-1 header> Carsound Hi flow cat> 2.5 catback> Apex'i N1 Turbo muffler. i noticed a fair decrease in low end power- what can I now do to increase low end power? I also have an Injen CAI.

Red

It has been proven over several different occasions that the perceived loss of low end power is exactly that: Perception, see also Placebo effect.

The car's much increased ability to haul itself from 3K to the redline makes your mind readjust to the car's abilities... Yet, the low end of the RPM spectrum seems "doggy". The result isn't necessarily that the car lost power, it is that it gained that much more power on the top end and didn't ADD to the bottom.

Eons ago when my car was a much younger little shark, I slapped it on the dyno with the full gammut of N/A parts. The car made 125 pounds of torque at 2500 RPM's. If you drove it, yeah it felt gutless at the lower RPM's. But in actuality, it was doing just slightly better than stock. The problem was that the high end picked up substantially, without any equivalent increase at the bottom.

The short answer? You didn't lose any low end, your mind is just overreacting a bit. smile.gif

Relic

full exhaust just got put on my 99 (HVE ceramic coated 4-1, cat, then mbrp catback) and it was a full gain across the board. From 1000-6500 the car feels stronger faster and although the top end feels more aggressive the whole power range feels good to me. I think he's right, that you are just thinking its slower otherwise i would look into changing out some parts in your exhaust or looking for answers somewhere else.

kane moser

I always thought that backpressure helped low end power in small displacment motors, and freeing up the exhaust flow eliminated any back pressure, thus robbing bottom end hp. EX my cousin took the cat out of his s/c civic si and his 60' time suffered, he reinstalled it and things were fine?

sharkey9883

i suppose this is on topic, im_with_ but since i'm going to have a T3/T4 turbo setup, which i know will be capable of making gobbs of power at high RPM, what can i do to beef up the low end so it is a little more smooth and consistent, rather than stock feeling torque on the lower end of the powerband, and a sudden surge of power at the boost threshold. i'll have a fidanza flywheel to get me to that point sooner, but any suggestions to make my low end more formidable till it gets there would be welcome. help

Red

"Backpressure" does nothing of the sort, velocity of exhaust gasses is what makes a smaller pipe work better at low RPM's on a small pipe.

If you truly have backpressure, your engine has to WORK just that much more at getting the exhaust out. Back pressure in the true sense of the phrase would immediately result in LESS horsepower due to pumping losses on the exhaust stroke.

Exhaust gas velocity is dictated by the displacement of your engine, the speed at which it's turning, valve size and quanity, cam profile, header/manifold design, the heat of the gas and the size of the pipe it's going through.

Let's say you make the exhaust pipe 4" in diameter. At low RPM's on our small motors, the total amount of exhaust gas is very small. The exhaust gasses will flow into this huge pipe, and will basically "stall" -- the pipe is so large, there's no real urgency for them to continue moving. When they stall, they will also cool off, making a much more dense cross-section of exhaust gas. That cold, non-moving exhaust gas starts actually creating a blockage in your exhaust pipe, which will end up losing your horsepower.

Of course, at high RPM's, the exhaust is more able to continue pushing outward without obstruction, and as such the bigger pipe may end up performing better at the top. However, with a 4" pipe on one of these motors, it might still be just too big to notice your full gains.

Now the flipside: an exhaust pipe that's 1.5" in diameter. At low RPM's, the pipe is so small that the relatively small amount of exhaust gas is able to stay moving quickly through the pipe. Because it stays moving, it also retains it's heat, which keeps it "lighter" and prevents the gasses from stalling. This will provide better power because the gasses are getting out easier.

Of course, this is at the cost of higher RPM power... At higher speed, the engine will be trying to exhume a LOT more exhaust, and trying to cram it down a 1.5" diameter pipe isn't going to be the most efficient method of doing it.

What we've neglected in both of these scenarios is a proper manifold/header design. A well-built manifold can keep the exhaust gasses moving away from the engine at relatively good pace. A good header is also "tuned", so that overlapping exhaust pulses can actually serve to create a small artificial vacuum to "suck" the other exhaust pulses out of the engine.

With a good header design and a reasonable pipe size (and 2.5" is definately within reasonable) you should not notice ANY drawback at low RPM's -- but yet should be able to find another handful of horsepower at higher RPM's.

Turbochargers work quite a bit differently because of the inherent obstruction that a turbo causes to the exhaust stream.

FlyRyde

imagine a "smart exhaust". That would be soooo crazy. Someone will or has designed this I bet. Could you imagine, an exhaust pipe that stays small (1.5) at low RPMs and then Opens up (3.5)when at higher RPMs. Sounds like something on a James Bond car or something that is controlled by the ECU. Think we'd ever see somethin like that in the future. Or is that a bad fantasy?

[ April 22, 2003, 08:45 AM: Message edited by: BlackTIbs ]

Red

What you propose would require that the pipe is smoothly able to transition it's diameter... How exactly do you do such a thing, and still have it mate to a fixed size collector?

The "smartest" exhaust uses staged sizing, starting at a smaller size and working it's way to bigger size as it transverses to the back of the car. The size stepping would prohibit flow reversion, as well as acommodating the ever-expanding exhaust gasses as they cool off in transit. Thus keeping velocity up, flow inertia up, and still having the potential to be quiet...

Maxx

Thanks Red, excellent points....I have suggested to people that a lot of the supposed torque loss with larger diameter pipe may be, just as you suggested, a perception thing. I don't think my low end changed much at all after the 2.5 cat-back, but it did SEEM weaker compared with the stronger top end. From the dyno results I've seen PEAK hp and torque improve with larger
pipe. But, at some point the law of diminishing returns would kick in, and the increases in diameter would no longer be a benefit. Exactly where that is on our cars I don't know.

ElantraGT2001

v8 people actually have different diameter headers to choose from, depending if they want more low end grunt or top end hp, or perhaps even a happy medium.