What makes an exhaust better ?

[woohoo]

Hello!

The aftermarket crowd is all over exhausts...

I can understand if an exhaust is wider, it would allow for more HP, as the air passing though it would flow easier outward allowing more exhaust to escape from the engine, but from what I can see many are the same exact thickness as the original exhaust fitted to the car in the first place.

How can these exhausts be better ?

Do they cool the gases faster ? (and thereby allow for more gas ?)

[xxChrisxx]

Most people buy them because they are shiny and bling.


The technical reason for getting a wider exhaust is that it affects the pressure waves of the exhaust. You don't get more power just becuase it's wider, you get it because the shape of the exhaust refelcts air (in this case exhaust gas) waves.

An exhaust is tuned to provide a low pressure region at the valve just as it opens, to help draw more exhaust gas from the cylcinder. This gives a torque increase at a specific rpm.

A wider shorter pipe gives this effect at higher rpm (higher torque at higher rpm =more power) a narrower longer pipe gives more low down torque.

Going too big or too small will harm performance. When you see these enormous exhausts they becomes too big for an gine and all pulse tuning is eliminated leaving a net loss in performance (although it sounds monsteraously loud). Too small chokesthe flow of gas.

This is why exhuasts can look extremely complicated, they take a spaghetti like path to get the length necessary for tuning to a certain rpm. In road cars the primary exhausts will be different lengths, to flatten and broaden the torque curve.



EDIT: I'm not sure but think I may have got mixed up with the longer/shorter pipe and what rpm range it tunes for.

[SZ]

I can understand if an exhaust is wider, it would allow for more HP

If this were true there'd be big business fitting toilet bowls to engines... huge power!

Seriously, there are some good books on this subject - "Scientific Design of Exhaust & Intake Systems - Engineering and Performance" is old but pretty good, and starts with the really basic theory first. Worth a read if you can get a copy.

[Shrek]

a day at the engine/chassis dyno with multiple lengths of headers, secondaries and collectors will do wonders

[SZ]

a day at the engine/chassis dyno with multiple lengths of headers, secondaries and collectors will do wonders

You don't even need that if you know what you're doing.

[Ciro Pabón]

We've had a couple of threads on the subject. I like pictures, so, there you go: a thousand words in each one!

The first idea: to use a trumpet. You can model the piston and the moving mass of air as a spring-mass system.

Helmholtz resonator: yeah, that's a piston!


The frequency "" for a trumpet length "L" and a speed sound of "a" is:



I've used it (rather naively, I imagine) for a couple of bike tunings. Notice that the frequency you have to use is the one of the piston, same goes for the volume V and the area A.

That's as simple as you can start: saw the damn exhaust to the lenght given by the equation.

Now, if you want to build a better chamber, take a look at this three pictures that show what happens when air goes through the exhaust:







You need a program (there are several available) for the design of an expansion chamber. The one I've used (for bikes) is no longer on line. In a few minutes (lunch time!) I'll post a picture of how it works.

[Carlos]

A 16 page PDF on paper and pencil design from Cycle Magazine, by the esteemed motor journalist/2&4 stroke expert Gordon Jennings

Do You Really Want to Know About Expansion Chambers
http://www.vintagesleds.com/library/man ... really.pdf
Intended for 2 stroke engines

EDIT: For a wider understanding:
Two-Stroke Tuner's Manual by Gordon Jennings
http://www.datafan.com/TunersHandbook/2 ... tered.html

A related system; an 'expansionbox' for four stroke engines:
US Patent 5050378 - Exhaust recharging of a four cycle internal combustion engine.
http://www.patentstorm.us/patents/50503 ... ption.html

[F1_eng]

Chris, we don't generaly aim to have the negative wave arrive at the valve just as the valve opens. As the valve is opening, the flow is already choked so an even greater pressure difference won't make any difference.

There are various methods of pulse tuning, and ofcourse inlet and exhaust methods are also different.

The theoretical equations don't really work that well on their own, you need a good simulation model to be sure. Sometimes you could strike lucky by having two variables which are wrong cancel out to give you what you thought.

There are a lot of reasons why the simple theoretical equations won't work very well, I could explain a few if people want. Its not hugely interesting

[SZ]

Go on - simple resonant theory usually holds up pretty well around the first harmonic. Subsequent resonances involved more complicated maths to predict well in relevant amplitude.

[ernos5]

Im surprised no one has posted this picture, imho it illustrates the 2-stroke exhaust very well.

[xxChrisxx]

Chris, we don't generaly aim to have the negative wave arrive at the valve just as the valve opens. As the valve is opening, the flow is already choked so an even greater pressure difference won't make any difference.

Ah well I knew it arrive at some point, and it's obvious that the flow is choked now you mention it. I even remember this very point coming up when I was learning about it.

I swear my brain has addled whilst i've been looking for a job, i'm starting to overlook and forget even the most simple obvious things. I've tried to keep it active, but its just not the same as having to rigourously use it every day.

[Giblet]

I have an 80cc 2 stroke for my bicycle. Just an 80cc that goes near 60kmh without pedaling. It takes some balls to ride. Then they hurt when you get off after the ride is over.

You can see the exhaust in this pic, and it is definitely all muffler.

What would be involved and what could I expect if I was to have an expansion chamber exhaust?

Insanely, I want more power on my mountain bike.



EDIT: If the exhaust is not a reasonable straight run is is still doable, as this motor the exhaust port is on the front side, and the exhaust points down. It would need two turns to get pointed rearward.

[flynfrog]

Im surprised no one has posted this picture, imho it illustrates the 2-stroke exhaust very well.

I have the 4 stroke animation

[flynfrog]

I have an 80cc 2 stroke for my bicycle. Just an 80cc that goes near 60kmh without pedaling. It takes some balls to ride. Then they hurt when you get off after the ride is over.

You can see the exhaust in this pic, and it is definitely all muffler.

What would be involved and what could I expect if I was to have an expansion chamber exhaust?

Insanely, I want more power on my mountain bike.



EDIT: If the exhaust is not a reasonable straight run, as this motor the exhaust port is on the front side, and the exhaust points down. It would need two turns to get pointed rearward.

Time to step up to the 470 husberg

[Giblet]

When traffic is in my way, say at a light, I just open and lock the clutch, and pedal my bicycle onto the sidewalk.

Albeit a nice ride, the bike you showed me is more of a bona fide motorcycle, and you could just jump over the cars.

But still, I wonder of the exhaust and what I can do other than put a yellow stripe on it somewhere, cuz if I learned anything from boy racers, yellow makes your car faster.