comments please..

i have an answer ready- it seems that engineering principle differs from common practice, now im just curious to see everyones opinion.

please tell me how you feel - good or bad? nec? why?
remember - backpressure and ex scavenging are two different things..(as im sure you all see from the wonderful header designs from SMSP and Hytech.. )

thx!!

 

Backpressure is bad. Smooth exhaust flow is good. Too small exhaust diameter is bad. Too large exhaust diameter is bad. Good luck finding the optimal medium.

 

That is until you go turbo. In that case, you just want to remove your exhaust.

 

good answer

optimal diameter is always changing though, huh?

 

think of your exhaust as a build your own vacume cleaner. you want it to suck all that poo poo out of yur motor. too big of tubing kills your vacume effect. too little doesnt suck enough. backpressure = bad vacume effect = good b/c of reduced back pressure

 

really? how?
how does a small(er) pipe 'suck' out gases?
once they are flowing, you need an exhuast that changes diameter to meet the need of the velocity at each length of pipe..
closer to motor = faster gases = larger pipe
further down from motor = slower gases (im guessing b/c they are cooler now, plus less 'push' from pulses) = smaller pipe.
keep in mind this is also all rpm dependant.

im not sure if your agreeing or disagreeing with me..

 

you want to maintain lower pressure at the end of your exhaust so it gets sucked out. if tubing is too big you have lessening pressure and your exhaust doesnt flow

 

The smaller the pipe, the faster the air will travel. Like if you blow air out of a coffee stirrer, and a drinking straw, the speed of the coffee stirer will be quite a faster. Thus, when the air is not being "pushed" out of the pipe, it is being sucked, because the speed of the air is creating a vaccuum behind it.

Lately I've become quite interested in Fluid Dynamics. It really does apply quite a bit to automotives.

Everyone seems to have thier own opinion about backpressure and whether or not it is bad. IMO, Backpressure is bad. NA, TC, SC, anything, it's bad. I think people get confused by the difference between backpressure, air speed, and air volume. and therein lies the problem.

In a perfect world, exhaust pipes would change with the RPM. A smaller, longer pipe for the low RPMs, and a bigger, shorter pipe for high RPMs. But I don't think that's going to happen anytime soon..

 

Exhaust gas velocity will only depend on how big the pipe is. It doesn't matter if it's an inch or a mile from the engine. What goes in must come out.

A smaller pipe would suck out gases just the same way a small straw will let you suck up a drink. Get the straw too big and, well, good luck.

Anyway, whenever I unbolted my exhaust, I would be about .3 sec and a couple mph faster down the track (b16 crx). Hence, I don't believe at all in backpressure. It just doesn't make sense to me why it would help in any situation, even at low rpms.

I can see how the pipe diameter would make a difference, however.
I don't really understand how the length would make a difference, but supposedly it does.


[Modified by Lsos, 1:17 AM 3/2/2002]

 

#1 - yes, i agree, smaller does = faster. simple fluid dynamics formula: Q=AV [flow=area*velocity] flow is typically a constant..
however, take it to an extreme, and dry to drink soda out of that coffee stirrer, damn cheeks will implode! but, your last comment rectifies this first one.

#2 - yup, fluids and other sciences make the auto world go around.. i wish i remember a lot more from school, or knew how to apply it to this back then, it would have made college classes more interesting! and ya, the biggest problem i think is the misconception of the definition for backpressure, which is simply 'resistance to flow'.

#3 - i agree with you.. the question is, tho, where do you stop? personally, i dont think that there is a need for any more piping about a foot or so after the header. a properly designed header will provide all the scavenging you need. (try to drive around without your header, no power, and loud as hell, lol). the section of exhaust following the header will just allow the gases to properly collect, merge, and exhaust out..

also, lets say we take a gsr intake manifold, throw it on the exhuast, and presto, we've got VEC (lol, variable exhaust control)... well, suprise surpirse, it exists.. bike perf companies have been doing this since the 80's.


Tortex - you stated you want low pressure at the end of the pipe, but not too low. well, ideally, you dont want low pressure, you want a large change in pressure. maybe a funnel shaped pipe (think airplane turbines here) would be more ideal??

Lsos - it totally matters where in comparison to the engine it is! imagine having to wait a half hour b/c the exhaust from when you first started your engine hasnt made its way out yet? (this is the mile long pipe ) kinda like one of those looong crazy straws, you can only suck so long, thats why you could never attach two of them for even more straw twisties of your drink!
i dont think the function is to suck gas out, i think its to design something that will allow the pulses to push one another out, and then, due to wave refraction, and a properly designed header, the sucking will be an added benefit..

thx for the comments, all.
im lost now. where are we going with this? i need some sleep.

 

I would think the length would effect it because the longer the exhaust gasses stay in the tubing, the cooler they get. And cooler means denser, so the denser air would be harder to push out of the pipe.

 

depends on what you have done to your motor. the higher compression ratio you have youll need to have some back pressure. but if you went FI like turboed or blown, youll want a big 3" exhaust, thats mainly for turbo though. helps the turbo spool up faster.

 

Backpresssure, to me, is the resistance to flow. Do I think it is a bad thing, Yes...
To me, I do not think you can totally get around it. The adjustable exhaust on the bikes is a great idea, and works well for them. But I am not sure that it can adjust fast enough to compensate for ALL of the backpressure ALL of the time...


Jason

 

Gasoline - I agree, good point.

CiviChriSiR2 - what does that mean?! lol. youve been reading too many magazines.. so, b/c you have high compression, you want to have clogged pipes? (ie, resistance to flow)?!?
explain to me WHY and maybe ill buy it.

 

Gasoline,
That is why so many people preach to keep the heat in the exhaust. That is why headers are wrapped or coated. Yes, keeps heat out of the engine bay too, but it also keeps it in the exhaust trail. That is where hot, fast moving molecules come into play...


Jason

 

They exist on cars too...take a guess.

 

[QUOTE]Lsos - it totally matters where in comparison to the engine it is! imagine having to wait a half hour b/c the exhaust from when you first started your engine hasnt made its way out yet? (this is the mile long pipe ) kinda like one of those looong crazy straws, you can only suck so long, thats why you could never attach two of them for even more straw twisties of your drink!
i dont think the function is to suck gas out, i think its to[ QUOTE]

That's true, but once the pipe gets filled, what I said has to be true, unless you take into account gas density, which honestly I don't know how much of a difference would make. Maybe enough of a difference to invalidate what I wrote below such that I make an *** out of myself

Otherwise, lets's say the pipe is ten miles long (constant diameter), filled with exhaust gas. Exhaust is flowing into this pipe, and as you stated it slows down along the way. Let me exaggerate what you said, for more dramatic effect, and assume that eventually the exhaust will stop, perhaps at the ten mile mark. What then? You're pumping exhaust in, but nothing comes out. Where's it go?

If the gas at one end is flowing ANY slower than at the other, then more exhaust comes in then comes out, and that's impossible.


[Modified by Lsos, 6:50 PM 3/2/2002]

 

Good info Lsos- I would love to learn more about this subject- can you recommend any books?

In general- backpressure is not helpful- although it can help to smooth out an idle
(NA) and help the VERY bottom end - I noticed when switching from my greddy back to stock- let's say it had less stallability( i don't want to call it bottom end torque)

 

That's just basic theoretical stuff I picked up in my Fluid Dynamics class.
I guess I suggest you get a textbook, mine is Applied Fluid Mechanics, by Robert L. Mott. That's a lot of theoretical stuff, though.

If you're more interested in how this relates to engines, I suggest Engines, An Introduction by John L. Lumley. I didn't read the whole book, but from what I see it's filled with a lot of good technical info, better than anywhere else I saw.

 

a good thing to remember is the pressure. exhaust gases coming out of your engine are always at high pressure. the end of your exhaust is low pressure. simple physics state that areas of high pressure will always flow to areas of low pressure in an attempt to try to even the difference between the pressures. however, it is hard to apply this to backpressure and the diameter of the exhaust piping.

 

Sound waves play an important part in the flow of exhaust gasses, added to fluid dynamics and all those crazy gas laws. Having fun yet?

Does anyone know why a more restrictive or long exhaust system makes more torque than a free flowing one (given the engine is the same with only exhausts being changed)? Or is this poppycock?

 

Here is my guess- a small diameter exhaust sytem:

exhaust pulses at low rpm travel faster in a smaller exhaust pipe- which would create a vacuum behind each pulse- the faster the exhaust gas pulseses travel, the more vacuum is produced. Each pulse creates a vacuum for the next pulse- which would "pull" the next exhaust pulse behind it. This is effective at low rpms only because as engine speed increases- the exhaust becomes restrictive and the vacuum effect doesnt help because the exhaust pulses are pushing each other instead of pulling. With a bigger exhaust- the opposite happens- at higher RPMs the exhaust begins to create larger vacuum pulses at a certain point- incresing efficiency. At low rpms the exhaust doesnt travel as fast in a larger pipe so the vacuum is not as strong. This is similar to 4-1 and 4-2-1 headers. Does this make sense?? Please correct me if not.


[Modified by fixhondas, 5:06 PM 3/2/2002]

 

god, u guys must b bored. jk, excellent post. specially when im in the middle of designing an exhaust system for my hatch. im goin to try to use 2>5 " EMT w/a ceramic coating. 2.5" EMT in my world is actually 2.75" trade diameter in exhaust terminology. im hopin its not too big if so ill try 2.5" exhaust tubing. any suggestions u guys have will b appreciated.

 

Fixhondas, that sound right.

Low rpm's = less air, which necessitates a smaller pipe for it to actually flow reasonably fast, flowing fast being good for the reasons that you described.

High rpm's = more air, which would try to flow way too fast though a small pipe, creating too much restriction. Therefore you want a larger pipe.

 

fixhondas/Lsos - i agree, but i think we are taking it too far. the wave harmonics and the pull mostly come from the header, and like i said before, im guessing only a very small amount of piping afterwards.. (maybe up to the end of the cat, for visual purposes?)

G - i dont think exhaut length makes any difference at all.. as far as i know, its an old hot rodders tale.


mufflers are the most obvious example that show its mainly a factor of noise control in the whole issue of exhausts, so in my eyes, exhuasts, per se, arent a necessity for max power.

now, relating all this to our street vehicles, i suppose we need exhuasts.

smaller pipes do = more low rpm pull, and the opposite for high rpm.. so, what do we do? guess its a sacrifice. my choice, use a larger pipe (im guessing min of 2.5").

however, i think the main lesson in all this is that backpressure is a bad thing.

bonus lesson - someone develop that VEC

oh, and CivicV10@18000rpm, im guessing your referring to turbos.. the theoretical means of creating efficiency in a otto cycle.. yes?