Did you forget about velocities? Plus hotter air flows faster.

Heat loss is more the material and the coatings/wraps on the exhaust not the size.


Header Main and Secondary sizing is important cause you want to create a scavenge effect to pull the exhaust out of the cylinders. Making it flow faster then an open head. To small and it will restrict. To big and the gases will stall out negating any gain over open head.

After the header collector, bigger is better. As long as the exhaust stays the same size or steps bigger as it gets further. You should see gains up to a point. should never loose anything.


The gains from 2.5 to 3" for me was 4~5hp. Tq was the same. I also replaced the factory "fake" cat with a resonated test pipe. I think this is a bigger problem on the S14's cause the fake cat is a hollow case 6" across and 4" high. Big area for exhaust to stall out and possibly cause backpressure.

The gains being negligible 2.5" vs 3" on the KA. It comes down to there being more 3" exhausts on the market for the 240sx. IMHO and others that have heard my car with the 2.5" and the 3" said the 3" is lower toned and better sounding overall. I dunno about DBL arguments cause I don't care down here and both my exhausts were JASMA cert for sound level.


Exhaust leaks and obstructions will make you lose power. Make sure you get new gaskets and use coper sealant on the flanges. I would rip out S14 fake cats and replace them with (resonated) test pipes. Think it was stupid of them to place that there when the real cat is in the DP. Odd shape allows for turbulence and going from a larger diameter to a smaller creates a pressure differential which results in back pressure.

Cliffs: It's all about flow. You want to get the exhaust out as fast as possible.

Correction.. It's called pulse scavaging. Back pressure is for emissions only and has nothing to do with increasing performance. The least amount of back pressure the faster the car will accelerate. The larger the pipe diameter being to big the more the pulse scavging becomes disrupted. Pulse scavaging is what draws in a positive vacuum to allow flow to increase at a faster velocity though pipe. During valve overlap in high rev this helps increase the intake velocity as well.
Basically there is a lower velocity travelling through the exhaust which is disrupting intake velocity in the higher RPM range.

In refrence o KA's using a 3" exhaust pipe. It would be more for KA24de's rather than KA24e's. KA24e's have one less exhaust valve per cylinder and the CFM rating on the SOHC head is about a 1/5th less in flow to the DOHC head. Timing on the KA24e is slow too, you negate power losses with a large pipe and that kind of timing, with CFM rate.

I blame it on the old muscle car guys that created this idea that back pressure in awesome. No. It's all about flow. Picking the proper diameter for the exhaust gases to travel while still maintaining a good velocity will give you the best results.

http://en.wikipedia.org/wiki/Backpressure
http://auto.howstuffworks.com/muffler5.htm

and the best so far:
http://www.flowmastermufflers.com/backpressure.html


Maybe that might help?

I do need a little backpressure for my go-ped, (a 2-stroke though.)

This will put this issue to rest definitively.

I have a Chevy 350 V8 with carb. I'm running custom headers, Y pipe, Magnaflow cat, and stock S13 catback.

The red torque curve is on stock exhaust. The Edelbrock was dialed to the leanest setting and AFR was only 11, which is extremely rich for an NA.

The blue torque curve is on open catback. Notice the huge jump on power. AFR was at a healthier 13.2, not perfect but almost ideal.


Now I realize that this doesn't apply to KAs, but it shows how restrictive the stock exhaust is. There is too much back pressure and the cylinders can't clear exhaust gas = incomplete combustion = rich.

http://img95.imageshack.us/img95/407...compareix0.jpg

kay so I heard i should also now change the octane i live in so cal so its 87 89 91 sohc?

i like living in texas where 93 is premium :)

so are you gonna keep the 3 inch? or go back to stock?

I also have a 3" exhaust on my KA. Right off the down pipe, cat deleted, through a magnaflow resonator and an a-spec exhaust.

Its fucking crazy deep but also crazy fucking loud when I stomp on it.
I went from stock exhaust to this one and could noticibly feel a difference down low. Butt dyno ftw.

Tons of people have shown that the KA loves a 3" exhaust. Cops, your mom and old people may not.

Theres more to finding the "perfect" exhaust than most people even want to think about. I can get pretty technical with this if somebody really wants me to. I have formulas and in depth explanations of the exhaust pulses, blowdown, Header size, collector size, between unequal length and equal length headers, optimizing valve events, reversion, etc, etc, etc. I can go on. We can figure out the exact size a stock 240 would like but even if we do theres still gonna be the jag offs that say "Oh i got X inch exhaust and i can go sideways or beat a civic on the freeway or i made more power with this size and blah blah blah" I can do that all in my frickin Ford Contour. So i am gonna see if there is somebody who would actually like to find it out let me know and we can get into it. Gonna need some info as far as when the exhaust valve lift reaches .050" and some dyno sheets to give us a good idea of stock max power RPM n shiz.
So if we want to make a nice research project out of this or we can keep arguing between 3 inch cat backs or not where most thought shouldn't even be put if you want to make a true "Tuned" exhaust system.



Oh and G6civcx. What are you trying to show that that graph. The graph doesn't make any sense at all. Maybe that was the point?

3" is large for the single slammer, but its been proven that dual cam's like to breathe and are better off with 3"

:duh: Your analogy disproves your point. It's easier to blow through a paper towel roll than a straw.

edit: just for fun, explain why you need more backpressure

True... However like you said you want as much flow as possible. At a certain point, to much space available will create less flow. Basically, you want to find a happy medium in diameter(based on power, exhuast flow, ect) that will allow a higher rate of flow. In a smaller diameter, your exhaust will be traveling faster. Too small it restricts, too large, exhaust flow slows down as well. This concept changes slightly under boost. NA, you need smaller piping for lower end power, up top however, you wont suffer loss, in fact you will probally see small gains.

All I Want to see is some one in these forums to beat my stock block and stock cam dyno of 162WHP and prove to me that a 3" exhaust is better than a 2.5" exhaust. That way I know for sure that I would want to upgrade from my 2.5" to 3". I'V chosen to stay with a 2.5" exhaust because it seems to prove that equal balance between high power and still allows me to pass my smog emissions.

Torque curves as read by a Dynapack 3000. Blue is open cat. Red is stock exhaust.

What's not to understand?

so about the fake cat, I take it there wont be any raspyness? I like this because my s14 so called fake cat is rusted to oblivion

the curves just don't look right. At 5252 RPM torque and HP will always cross. Doesn't look like it will happen with accurate numbers. Plus in the passed 3 weeks i've physically looked at at least 20 dyno pulls on 350 chevy motors a day. Its what we have been doing at school. Those curves don't even closely resemble what we've been seeing. Not necessarily calling you out, its just they don't look right.

HP isn't on that graph :loco:

Never said it was. I said that it doesn't LOOK like the tq and hp will cross at 5252 with accurate numbers. As in following the curves direction and just estimating the number it would be at at 5252 RPM. Thank you for giving me a red box for no reason. :smash:

My mistake :kiss:. I read the "doesn't look like..." sentence as just a typical internet opinion masquerading as fact.

awww teehee the kiss smiley made it better. lol

Those stupid internet opinions are what i try to correct. Being new on here and having my age show i expect some flame when i post things so don't worry about this. But being ASE certified and in school for this shiz i am pretty knowledgeable and i just want to pass on what i know to people cuz remember kids...KNOWLEDGE IS POWER!!!!

Even if you did loose power, who cares? Bad ass exhaust either way. Looks awesome, sure it sounds pretty beastly too!

I agree. Its not like you were making crazy power before the exhaust anyway.
For KA's I would go with a custom exhaust and a muffler of your choice. I had a custom 2.5 catback made with minimal bends and a tsudo n1 muffler or some bs like that. It looks nice and sounds great.

I hate dynapack, I find it to be a mis calculated dynomometer. But like any dyno at least there is some truth to what is seen in statistical averaging.

The 5252 cross over is just formulaic. NO engine makes a 5252rpm crossover at exactly 5252RPM. It's just a formulaic equation to take any remaining numbers and shift them to HP figures, one reason why dynojet numbers sometimes look over inflated.
My Mustang dyno test report doesn't show the cross over at 5252 (EXACTLY) and I find it to be more accurate. Every engine has a different boreXstroke which will always make the RPM cross over different. 5252RPM is just an AVERAGE that puts all engines into a ballpark figure so to speak.

I think the fake cat gives a hollow resonance to the exhaust tone. I replaced with a resonated test pipe and it went away. Also the area that it gets to expand in stalls out the exhaust velocity.

when has ur car ever been fast?!! slow ass KA!!
and take that FLCL off ur plate!! u dont deserve those letters!! and dont forget! words going around about ur shitty welds!!! i bet this makes u wanna throw a hammer at sum1 huh? o yea.... lol..... idk who said this!!! but u cant beat geo metros? is this true? for sum reason i believe this?! hahahaha

sorry but that post wasnt for u! its for BIGVINNIE who is actually small! lol

Fuck you Tyler keep this up and I will fire you from work.... You even have a snap shot of one of your gay ass video games as your avatar!!!
I am going to kill your MX6 at the track today.

No. No no no.

Yes. Flow is good. Flow, however, is not dependent only on the speed at which the exhaust is moving through the pipe. You want to move volume as quickly as possible.

No. It doesn't. Let's say you have identical cars, one with a 1.5" exhaust and one with a 3". Let's say the air in the 1.5" exhaust is moving at 30 feet per second, and the air in the 3" exhaust is moving half as quickly at 15 feet per second. Even though the air in the 3" is moving more slowly, it still evacuates more exhaust volume because the area of the pipe's cross-section is more than twice as large.

The only thing that matters is the volume of exhaust gasses being moved. Not the straight-line velocity at which they're moving.

Think about it this way. If you have 5 seconds to get the air out of your lungs, are you going to do it faster through a coffee stirrer or a jumbo straw? Or without a straw at all? The air will be moving faster through the coffee stirrer - that is, at a higher straight-line velocity - but it'll take you a LOT more time and your cheeks are gonna be burning.

It's not rocket science.

But you have forgotten all about the scavanging affect. Basically you want your exhaust gasses to move fast enought to create a vaccuum condition that is going to pull more gasses. To slow and you dont get that. This is really helpful in lower rpms where there is not as much flow. According to your theory, everyone should be running open downpipe. Have you ever driven a car with an open down pipe and then put an exhaust on there? A similar concept would be siphoning. Not the same thing, but similar principles. Void created needs to be filled, motion of water keeps creating a void that keeps needing to be filled. Motion of water is aided by gravity.