How does exhaust create more Hp?

[Jeenyus]

How does installing a new exhaust system increase hp? does anyone have the techinal answer, not just an increase in exhaust flow. i know it has something to do with backpressure?

[sw20>>s14]

scavenging...

[DJ_Sunrise]

umm a bigger exhaust NEVER increases horsepower. stickers, mufflers, 14" spinners, and altezzas increase horsepower..but seriously.. it is backpressure that is involved with more HP. when you upgrade your exhaust, its mandrel bent, and has less curves in it.. therefore reducing backpressure. scavenging is the effect of air somewhat sticking to itself, and pulling more out... 3" piping on an NA KA would see less HP than 2.5.. if the piping is too big, the air stops sucking itself out, per se. on a turbo charged vehicle backpressure helps the boost build faster.. but also reduces peak horsepower. on an SR that spends most of its time on the track above 5.5k constantly, 90mm piping would be pure pimp :) on a street driven SR, 80mm would be the best for power, and enough to still help the turbo spool. i have a 2.5 inch cat delete on my full 3 inch exhaust.. i had 3 inch, but switched to 2.5 because the boost response was actually much better :) little bit less top end power, but the shorter spool up from the 2.5 actually gave me faster quarter mile times...

-Bart

[S13 Charlie]

Actually..

Scavenging is the effect of the exhaust gas flow assisting in pulling the air/fuel mixture into the combustion chamber during the time when both intake and exhaust valves are open. There is a segment of the 4-stroke cycle during which the intake and exhaust valves are open at the same time. So, the velocity of the exiting exhaust gas creates a vacuum in the combustion chamber that helps suck the air/fuel mixture into the cylinder. I believe a diagram of this can be found on howstuffworks.com.

[Taniguchi_Is_#1]

scavenging...


ditto. read about it. google. engine theory.

[cmcdougle]

Back pressure is not what creates horsepower, nor is it what causes its loss when the exhaust flows too freely. When you have too small of an exhaust, the engine gets too hot, and therefore runs poorly. When you have too large of an exhaust, not only are you compensating for something else, but your engine runs poorly due to hyperventilation, like breathing in a little bit of air, then exhaling more than you can.

:aw:

[orion::S14]

^^^ Huh?

And scavenging is due to header design...NOT exhaust size. At least 90% of it's header design.

...
SO...why does an exhaust make more power????

Think of it this way: You motor is nothing but a large air pump.

The easier you make it to move air in and out, the more power that can be spent turning the flywheel, and not forcing air (exhaust) out.

SO...you put on a 3" exhaust. It's now easier for your KA to move air (less restriction). The result is a higher RWHP b/c more of the combustion energy can go towards spinning the crank/flywheel...rather than pumping out the exhaust.

You're freeing energy that's otherwise wasted forcing air OUT...that's why you seen higher RWHP and faster accel.

- Brian

[orion::S14]

stupid double posting...

[gotta240]

and to the dude who said "3 inch will make more power on a ka than 2 1/2"....

WRONG....search

[DR566799]

and to the dude who said "3 inch will make more power on a ka than 2 1/2"....

WRONG....search

with a t3/4 i bet it would...anyway...

I'm not sure why the OP asked...it's like saying how does a straw work...you suck shit through it...the larger the straw the greater the flow until it's so large that it's harder to suck and then you learned why an exhaust is only a little larger than a factory exhaust rather than 6"...

scavenging is important, but that's a little more advanced...and like others have said based more on parts closer to the combustion chamber not several feet which is where your c/b is...

[mrmephistopheles]

yeah this isn't rocket surgery.

internal combustion engines need 3 things to work: fuel, air, and spark.
The more air flow you have, the more fuel you can throw at it (and eventually, the more spark you'll need). More fuel + more air = more power.

Anyone who says you lose power by going to a larger exhaust is dumb.

[downshift_sideways]

Loser thread............

[!Zar!]

and to the dude who said "3 inch will make more power on a ka than 2 1/2"....

WRONG....search
Actually, YOU are wrong. If you searched, you would've known that.

[Kn1ves]

Actually, YOU are wrong. If you searched, you would've known that.

"3" piping on an NA KA would see less HP than 2.5"

NA KA can take 3" and make more power. if we all searched (http://www.zilvia.net/f/showpost.php?p=607300&postcount=9) we would have known that

[!Zar!]

"3" piping on an NA KA would see less HP than 2.5"

NA KA can take 3" and make more power. if we all searched (http://www.zilvia.net/f/showpost.php?p=607300&postcount=9) we would have known that
Wtf are you talking about? You just contradicted yourself.

[FaLKoN240]

3" exhaust on NA KA gains power.

More than 2.5" It's been proven. By a dyno. ThatGuy will search it and post it if he has to, I've seen it done.

[!Zar!]

The post Knives posted had the link to one of the dyno threads. But I'm still confused about who he is trying to quote.

[mrmephistopheles]

I think he was trying to quote the other guy and fucked up.

[tru_40sx]

no one mentioned design of the catback,muffler...... whatever u wanna call it.

[klohiq]

that's a bit more advanced...he asked how it worked, not the types and parts involved...

there are hundreds if not thousands of different exhausts on the market, if you want to learn more then research each one and try to find an independent dyno chart showing the gains/losses vs another exhaust. to know how it works you dont need to know what a cat does, the different between true dual and psuedo dual exhaust, difference between single and dual piping diameter for a given flow, etc...

[Phlip]

http://auto.howstuffworks.com/engine.htm
see page 8 of that article; "Producing more engine power" for an answer with less bullshit and conjecture.

[spdfreek0o]

yeah but you can have an exhaust that is too big. We want less restriction on flow but we also want the flow to be laminar. Exhaust piping that is too big will cause turbulence in the exhaust flow.

[clutch kick drifter]

dunno if anyones said it but read Corkey Bells book. Good shit. He has a graph for HP to Pipe size and i think anything over 380is needs 3in and below dosent. As most of you know most SR exhaust are 3in or bigger pipe. I run strait 3in with no cats, muffz, or anything and love it.

[wootwoot]

If you are turbo you want the largest possible exhaust size. It would be really nice if you could create vaccum through the exhaust maybe with aerodynamics. Turbo could constantly be "spool'd"

[delphis]

just remember that if you don't have enough back pressure on an NA system (too large of an exhaust) you will lose out on torque which is your acceleration power, but your higher end hp will increase a little bit.

[!Zar!]

just remember that if you don't have enough back pressure on an NA system (too large of an exhaust) you will lose out on torque which is your acceleration power, but your higher end hp will increase a little bit.
That's why it was stated to run 3"...

[aznpoopy]

just remember that if you don't have enough back pressure on an NA system (too large of an exhaust) you will lose out on torque which is your acceleration power, but your higher end hp will increase a little bit.

its not the backpressure that increases low end torque with a small exhaust.

the smaller piping increases exhaust gas velocity, which helps the engine move air. however, the small piping acts as a huge restriction at higher rpms.

when you switch to a larger exhaust, low rpm flow velocity is reduced. conversely, restriction is removed at higher rpms.

[orion::S14]

^^^ You make a good point.

And since this thread has come so far in explaining why a big exhaust is good, and why it frees HP...let's clarify one more thing:

BACKPRESSURE is BAD, ALWAYS!!!!!

Backpressure is a RESULT of smaller piping...NOT a cause of low-end torque. It's a bad side-effect.

See 1 post up for why...

...and then go smack your V8-lovin' uncle/cousin/dad who told you you "need backpressure" for low end TQ.

- Brian

[wootwoot]

Well, one thing I was wondering...Does switching to a larger exhaust on say a ka24 make you produce less tq or just move it higher up in the rpm range?

[orion::S14]

Well, on a KA...it makes more TQ pretty much everywhere...but ESPECIALLY in the higher RPM's.

- Brian

[orion::S14]

See here:

I overlayed this chart (stock 240SX in purple): http://ka24development.com/file_images/240sx_dyno_chart.jpg

...with this chart (cone filter and 3" exhaust in blue): http://ka24development.com/file_images/240sx_dyno_hotshot.jpg

.....................

Stock is purple, with a 3" exhaust and cone filter is light blue.
http://www.ka24development.com/file_images/temp/KA24DE-3-inch-exhaust-dyno-versus-stock-overlay.jpg

...and before someone says anything...the cone filter didn't add ANY power with the stock intake tube, FWIW.

...............

Yah, not the best, I know...but I can;t find Mav1178's old comparison/dyno on FreshAlloy (dead links to his site).

- Brian

[aznpoopy]

Well, on a KA...it makes more TQ pretty much everywhere...but ESPECIALLY in the higher RPM's.

- Brian

yep

i think its pretty much been beaten to death that 3" exhaust makes gains everywhere over stock.

there may have been comparisons of 2.5 vs 3. i've never seen them and i have no idea how that came out.

imo it's silly to try make a generalization as to the 'ideal' diameter. and it's still true that it's completely possible to go too big. i think it's safe to say, however, that 3" is fine.

anyway, this has spawned way too much conversation over very little hp gain. lol. n/a power!

[gotta240]

why don't YOU search zar. 3">2.5" exhaust on stock ka...at least to ONE dyno chart.

[aznpoopy]

because i don't care what the answer is?

if someone is looking buy 3 vs 2.5, they can search for themselves.