Exhaust tuning?

[Scott McLellan]

My hotshot header is on the way and I'm going to build a custom 2-1/4 cat back with a high flow cat, resonator and perf. muffler. Anyways, is the lack of backpressure going to cause my engine to "bah, bah, bah"(you know, the little backfire) when I come to a stop sign or whatever. I know when my dad's car's muffler rusted out it would make those sounds that sounded like little backfires. It just didn't sound healthy. Even if it won't make those sounds is there any other tuning neccessary to get the most out of all of this? Seems like if I have O2 sensors it will keep it from running overly rich, right? Thanks!-Scott

[:: orion ::]

1st, do at least 2.5" cat back...2.25" is OK, but will limit the top end. Not that the KA has great top end, but 2.25" is a little small for a 2.4 liter motor.

And as for the sound, it shouldn't. As long as you have no exhaust leaks, you'll be OK...

- Brian

[gg250cc]

yeah if its a custom exhuast go for 2 1/2" , because that is what the collector size is on the hotshot headers

[corn322]

brain, in theory, wich of these exhausts would flow better.left side, with short, sharp bends, or right side, with long, flowing bends?

[Scott McLellan]

We're talking very very little difference here if any at all. The main thing is to make sure it is mandrel bent. If you didn't have mandrel bends then yeah the long radiused bends would be better because the tight radiused bends would have to compressed really tight to get the tighter bend. Yeah you don't want pointless figure eights in the path but I highly doubt it would make any difference what so ever if all else was equal (muffler, pipe dia., ect.). But yes, in theory, the one with the gradual bends would flow better just as a car through a road course wants the straightest lines for the maximum speed. Oh was this quest. directed to Brian... I thought you were asking me since you said "brain.":)

[corn322]

hah, didn't notice the misspelling of his name.

but yeah, I figured the long bends would be better.so now.... how much of a difference between mandrel bends and crush bends?

[:: orion ::]

Big difference.

Crush bends restrict the flow and reduce velocity.

Mandrel bends help to keep the velocity high and increase torque and HP...

A crush bent 2.5" is beter than stock, but as much as it's better than stcok, a mandrel bent exhaust is better than crush bent.

For the sake of the numbers, say a crush bent gives you 5rwhp increase - I'd wager that the same exhaust, with constant radius bends, will give 10rwhp.

- Brian

[Scott McLellan]

If you think about it, the place where air flow is the most critical is the bends where the air has to change directions. If you use crush bent you are only making the bends even more of a restriction because of the decrease in cross section area. Also, that speeding up and slowing down at every turn just backs the flow up even more.

2.5 in.(if inside dia.) has approx. 4.91 sq. in. cross sect. area2.5 in. crush bent could have (if assumed to be compressed 25%?) 3.68 sq. in. in a bend easily. That's about the equivalent of a little over a 2-1/8 in. dia. pipe(3.54). See why mandrel bent is important? Your exhaust is only as good as it's most restric. pt. so you really just got yourself a 2.125 not a 2.5 exhaust.-Scott

[pr240sx]

Good point Scott

Even you can do a step size tubing, starting form the header at 2.5 getting bigger in steps, working your way until you reach the muffler

[crzycav86]

Good point Scott

Even you can do a step size tubing, starting form the header at 2.5 getting bigger in steps, working your way until you reach the muffler

Actually, wouldn't you want the piping diameter to get smaller as you reach the muffler? The exhaust cools down and densifies as it heads toward the muffler, so if you want to keep the velocity the same, wouldn't you want it to get more narrow?

[PhaneSoul]

that does sound about right, but you also have to think, just how much cooler IS it going to get, i mean its a what, 2 second trip or less correct?

[Scott McLellan]

On Headman's Torkstep headers it gets bigger several times on each primary as it goes. I think the key is to get optimum velocity at the closest place to the actual combustion chamber so that the scavenging is done there and not halfway down the exhaust where the scavenging effect is not needed. I think if you gradually necked down the exhaust you would not get the right effect. Instead you would want like a kind of hour glass type thing on the header collector like if any of you have seen the nascar headers. It goes real small, like what looks like is too small and then back to bigger to really scavange a lot. This is all major, major tuning that the average(like me) home garage builder can't due effectively. If you have some computer simulation software and a dyno and everything then, hey, go for it!