perhaps I was over your head with the avg power comment. I was speaking solely of peak (no back pressure) vs avg power (back pressure). and yes I have factual data backing up my theories. hundreds of passes in my own car and logged hundreds more for friends. my car went slowest w open long tubes, no extensions.

and lets face it. street cars , ESP jeeps spend 99% of their time under say 3500 rpm. having more tq avail means everything is better.

 

you guys just love reading books don't ya. reading what others have done and taking it to heart. there is more than one way to skin a cat my friends. and just bc you don't believe it or have no experience with it does not mean it's impossible.

 

surely you must understand laminar flow...lots of guys mistake the addition of a muffler for 'extra power cuz i gots just enuf backpressure', when really the addition of a 'baffle box' is making up for the misproportioned pipe diameter, length and bends in the manifold/exhaust tubing and improper exhaust pulse management... but i'm sure you already know that. backpressure does not give you extra torque or hp. your added muffler was making up for ineffieciencies in the system. sorry to burst your backpressure bubble.

 

I thought the problem with open headers wasn't actually a "lack of backpressure" but "lack of exhaust velocity", in that a properly set up exhaust system, like a performance header, is tuned to essentially "suck" the exhaust gasses away from cylinders, enhancing evacuation within a certain RPM range. This being a totally different phenomenon from exhaust restriction.

(Also don't valveless 2-stroke gas engines have special chambers designed to have enough back pressure, through resonance of the exhaust pulse, to prevent all of the raw fuel from being sucked out? Of course a bike is not a 4-stroke valved engine but this thread has gotten awfully academic anyway)

 

not valveless, reed valves. operated by vacuum and case pressure. and yeah the resonance, or expansion chamber in the exhaust helps keep the mixture in the cylinder.

 

so....mufflers are good or not?

 

it's totally circumstantial. in your case, maybe, maybe not. maybe you missed another variable. you just can't make the blanket statement that backpressure is needed. thats my only point.

i'm guessing fluid dynamics rarely comes up in drag racing circles until big dollars start to get spent anyways.

 

A two-stroke is an entirely different animal - they're subject to crankcase supercharging, while a four-stroke is not.

As far as the "real-world data," what have you got? I'd like to see dyno pull results listed against measured backpressure, please.

Bear in mind that the first two books I cited are considered definitive literature, upon which much of the more recent literature - and work - has been based. Extensive experimentation, data given - Hell, Taylor's work came with very large fold-out charts tucked in the back plotting the data he'd gathered...

Back to my Top Fuel comment - you honestly don't think those guys have experimented with everything possible? The length of the stacks is well-tuned, as is the inside diameter, wall thickness, and any coatings used! (Running too small or too large of an ID for exhaust primaries will create a restriction in the system, just as much as baffling will (cf: Smith & Morrison.)

 

I have no dyno pulls, I have never had any of my cars on a dyno. I had a redwing boot box full of timeslips I gave away w the car. and countless hours at the track.

this restriction in flow we are dubbing back pressure isn't required, but certainly has an effect on outcome - not always negative. that's all I'm saying. I had a million dif exhaust setups on my cars. I actually had exhaust come off at the track, kind of a back to back comparison if u will.

 

Its not backpressure needed. Its exhaust velocity.

 

^ Troof.

"Backpressure" is never a good thing - until you can prove to me that it is (and timeslips won't do it, I'm talking about direct measurement of backpressure and engine output, in a purely objective manner under controlled conditions.)

Exhaust pulse velocity is critical, because the area behind the exhaust pulse is a low-pressure area, which helps with scavenging of the next cylinder. This is why tuning equal-length headers properly takes so much effort, and why they're only good in a limited RPM range - because the length of pipe from the port entry to the collector bell is critical, not to mention all the work that has to go into designing the tubes to have smooth bends without kinks or section changes (which screws with pulse timing and causes turbulence - and, therefore, backpressure) and they're all within about 2% of each other in terms of primary length.

Even then, you're also having to deal with an optimax ID and wall thickness, to prevent rapid or uneven pulse cooling - and turbulence (and therefore backpressure) all over again!

How do I know? I've helped design headers and exhaust systems, complete with backpressure measurements at various points in the system to make sure it's reduced insofar as possible, and dyno pulls were taken showing output at the flywheel and plotted against backpressure readings (and that's a notebook I wish I had with me - but I think it's still in Indiana, and I'm not.)

(I've had my copy of Smith & Morrison for the last 20-odd years, and my copy of Taylor for at least the last 15. All three books - Taylor is two volumes - are VERY well-used!)

 

whatever the case. my car went quicker and faster with some sort of restriction, Brit creating an increase in velocity or whatever. the car had an x pipe and many dif muffler setups. also I swapped header brands, and despite different length primaries, collector location, and general bends, they all performed almost identical.

I'm not trying to argue about back pressure or whatever. you obviously know more than I do about it. all I did was race cars. granted I find the comparisons fairly level given similar da and only changing 1 variable at a time. saying it's not needed is entirely true. but we don't need bacon either...

 

dbl post

 

 

and small *****. bc corvette