Exhaust flow talk
08-13-2006, 06:34 PM
Post: #1
Exhaust flow talk
LOL, JTA, if you want to slap those riceboys around with NA knowledge you might want to check your facts first. Luckily there is plenty available on the forums.;-)

First tip: the less backpressure the better..

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08-14-2006, 09:17 AM
Post: #2
Exhaust flow talk
NoHachi: Seriously... try running that engine of yours without mufflers... It will kill off any low-end torque that gets you going... And imho you seriously don't want to kill more low-end torque since that little engine doesn't have a lot to begin with. And since there will be rush hour traffic involved with lots of stop and go, even in the states, I heart me some low end torque… I don't want to be revving in first because it bogs down when driving in the low rev range in second.

With turbo's it's different, no backpressure is best since it's the exhaust fumes that rotate the turbo. But I don’t have to tell you that I’m sure. :wink:

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08-14-2006, 09:37 AM
Post: #3
Exhaust flow talk
Well its the turbo that gives the backpressure needed for the engineBig Grin

I guess both are right. No backpressure aint good, to much backpressure aint good. I don't know the theory about no backpressure but I did read it a long long time ago why it is bad, I remember it having to do something with exhaust header design but thats prob noob nonsence hanging around in my head Big Grin. To much backpressure simply meant the exhaust fumes couldn't get out and thus there was less room left for air/fuel mixture in the cylinder the next cycle.

Greetz,

Bastiaan "mux213" Olij

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08-14-2006, 11:17 AM
Post: #4
Exhaust flow talk
Ok Mux, time for a split....cause here I go again...

JTA: you are stating as fact something which is one of the biggest urban myths going round under the new breed of NA fanboys. The "need backpressure myth" is the biggest piece of BS on carforums today.

Just THINK about it for a second..what is an engine? An air pump. When does a pump work best (aka most cfm)? When the pressure difference it has to overcome is the smallest.

Now, all the flow/velocity etc parameters are already set down by the header. Headers do create a "backpressure" kind of effect called scavenging..type it into google and I'm sure someone has written it down in understandable terms..I'm way to deep into the mixture of acoustics and thermodynamics to write down something you guys would understand.
After the header you want as little backpressure as you can get...

So why do some fart can exhaust lose some (very minor, its still 99% BS!!) low end power? The problem lies in the connection of the XX diameter header/midpipe, to the XX+A diameter exhaust..There is a step that is created that creates a lot of drag, especially at lower flow speeds (large eddies)...If you create a smooth transition between the fartcan and in between pipe, the engine will produce more power over the ENTIRE revband..

But don't take my word for it..I'm just another engineer...if you google on any combination of words in the text above it should come up with more then enough interesting information..

Please remember people...I am not a good source for automotive knowledge, and some pimpled benchracing teenager who spends more time on a forum then actually racing certainly isn't(I'm not referring to you JTA, but people all over the world). Make sure to doublecheck your facts on some "known good"sites like How stuff works etc etc...stop this kind of misinformation from being spread!

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08-14-2006, 11:52 AM
Post: #5
Exhaust flow talk
Split time it isBig Grin

Ok scavenging, that was the thing I read about, now I remember!

Okay in my lamemans terms what I remember from it.

You have say 4 cylinders like the hachi, all exhaust ports eventually meet up in the header. If they meet up to quickly or to late the pressure from the exhaust gasses from your first cylinder (which fired a little while back) will hinder the exhaust gasses from the 2nd cylinder (or whichever fires next) to exit which means you loose power, the pressure from the exhaust gasses from the 2nd cylinder then hinder the exhaust gasses from the 3rd, etc.

However if they meet up at the right time the exhaust guesses of that first cylinder have moved so far down the exhaust pipe they have created a vacuum behind them that will help 'pull out' the exhaust gasses from the second cylinder (or whichever fired next), the gasses from the second cylinder then create a vacuum for the third, etc.etc.etc. Increasing the power output. Offcourse there is a certain RPM at which this effect is at an optimum for a certain length at which the header meets up.

This is the whole theory behind equal lenght exhaust headers if I'm not mistaken.

The thing with backpressure was that if you had no backpressure, there was no vaccuum created and you wouldn't have any scavenging effect (or the thing causig the backpressure was the same thing keeping the vaccum intact or sumething like that). But you only needed a little or something because more backpressure means more slowing down of the gasses means less gasses escaping meaning less room for fuel/air mixture meaning less power output.

But again, this is just my lamemans understanding of what I read and I may be repeating it in error.

Greetz,

Bastiaan "mux213" Olij

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08-14-2006, 12:10 PM
Post: #6
Exhaust flow talk
I don't know if any of you lot is an engineer (EDIT just reread you are Huh) but for driveability big ass exhaust is pants in an NA engine, calling it an urban myth is just wrong, you can go bigger than stock but not as big as you want (hondas are good example as they are highly engineered regarding flow)... I'm with JTA or better I'm with Graham Bell (read his books he does know what he's talking about)...
I'll give a little rundown on what he says when I can be arsed... but I can tell you (NoHachi) already your not right, 'cause you can't simplify an engine down to an air pump... anyway no offence but have a read of this there's a third edition out in the meanwhile...

EDIT: don't forget that exhaust gas does also cools down on the way out (volume decreases) so to keep gas velocity level it would need to get smaller further too the back (this goes for turbo as well)
I agree with the fact about steps in the exhaust making it rubish but that's not the point we're looking at here...

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08-14-2006, 12:25 PM
Post: #7
Exhaust flow talk
The trick I think is more related to correctly tuning the pressure pulses than with backpressure. I can imagine the exhaust diameter has something to do with the speed of the gasflow, which in turn is important for tuning these pulses.
This is an area that even formula one teams refer to as a bit of black magic to get right, I doubt any of us here will know exactly how this all works.

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08-14-2006, 01:45 PM
Post: #8
Exhaust flow talk
I have read Bell and about every other book out there..

Lets tackle this thing step by step and see if you lot can figure it out by yourself.

Quote:EDIT: don't forget that exhaust gas does also cools down on the way out (volume decreases) so to keep gas velocity level it would need to get smaller further too the back (this goes for turbo as well)

Why do you want to keep up velocity? Massflow stays the same.. In the end u need to dump your gass in 0,0 velocity air at ambient pressure..think about that and then about the "steps" that you siad you believed in..

Maybe I should have been clearer about my formulations...but I still stand by my case..the best exhaust has the biggest diameter possible as soon and smooth as possible...what does this exhaust look like? Weeeelll it would be a trumpet shape right behind the header..Take a look at whats dangling behind an F1 car and then think again abot your "reduce diameter" theory.

The point all of you are missing and the one I should have emphasized is that you have to make the change GRADUAL. Make too abrupt a change and the flow seperates from the walls and creates drag. That what happens in exhausts..the change in diameter is too fast...but the big diameter itself is not the problem.

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08-14-2006, 02:15 PM
Post: #9
Exhaust flow talk
Urban myths... Guess I'll have to question the racing team I am going to be with... Because they seriously noticed power loss in the low rev range with an open exhaust on N/A motors... And I wouldn't call those people inapt or teenagers since they are all around my age (25).

Oh well…

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08-14-2006, 03:07 PM
Post: #10
Exhaust flow talk
NoHachi Wrote:I have read Bell and about every other book out there..

Lets tackle this thing step by step and see if you lot can figure it out by yourself.

Quote:EDIT: don't forget that exhaust gas does also cools down on the way out (volume decreases) so to keep gas velocity level it would need to get smaller further too the back (this goes for turbo as well)

Why do you want to keep up velocity? Massflow stays the same.. In the end u need to dump your gass in 0,0 velocity air at ambient pressure..think about that and then about the "steps" that you siad you believed in..

Maybe I should have been clearer about my formulations...but I still stand by my case..the best exhaust has the biggest diameter possible as soon and smooth as possible...what does this exhaust look like? Weeeelll it would be a trumpet shape right behind the header..Take a look at whats dangling behind an F1 car and then think again abot your "reduce diameter" theory.

The point all of you are missing and the one I should have emphasized is that you have to make the change GRADUAL. Make too abrupt a change and the flow seperates from the walls and creates drag. That what happens in exhausts..the change in diameter is too fast...but the big diameter itself is not the problem.

my last phyics lesson quite a while ago but I'll try to formulate my thoughts/thinking as good as possible...I'm just an enthusiast not a pro

if you read all of it your probably right but IMHO this goes for the case to achieve max power... and thats nothing to do with driveability - this is also were I think (haven't looked at it from a physics point, more of a feeling Wink) that velocity comes into play as while driving you don't get constant airflow as your revs change continously in contrast for a helicopter turbine where rpm is constant (while flying) and you just change the angle of the blades... what I desire is a useable torqueband with 'enough' power... with good emissions as a bonus Huh
PS: with te steps i ment what you called GRADUAL above, miss-understanding Wink but I do occasional shovel my own grave Banging head

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