anyone who knows about loudspeaker tuning/loading ... it is basically the same. the size and shape of the cabinet and ports (if the box is ported) determine the resonant freq (most efficient) of the "system".... the exhaust is the same.....(both are air pumps) when I had a Harley, I would see these guys modify their systems to make the louder but lose power......useing a baseball analogy.... if they made the ball too light you would throw your arm out in an inning.... to heavy and you could not get it to home..... we take for granted the range of diameters we are working with but imagine the exaust manifold 4" or 1/4 of an inch..... both would radically change the performance .... the dif between 2"-2 1/2" and 3" is just a refinement/manipulation of the tune..... as someone said you are playing with the torque curve and/or moving it up and down the rpm scale (at best)...... this is why some headers are better for torque and others better for peak power.....
(edit)
btw.. when working with the WMS exhaust which had a 3" path when open,.... at the dyno we saw almost 10 whp difference from open and closed position... so these diameters do make a difference.... and again, it is a system ie header/cat/pips/muffler etc....

 

i'm no engineer like you guys seem to be but i know that back pressure plays a part too. You can get a great HP number from an open exhaust in the hi end but with no back pressure you have no HP on the low end. You are actually slower. That is doubly true with an engine as a small as a 1.6l.

Guaging from the HP numbers i have seen on the exhaust systems currently available i can not see much if any difference between a dual pipe system and a single pipe system. 1-2 hp at most over the alta system with the magnaflow and i am not sure how many people have actually compared hp numbers on the same dyno on the same day. Even if you do 1-2 hp is not conclusive. You might as well assume they are the same in hp and save the weight.

 

I understand the volicity thing, kinda like when you put your thumb over the end of a water hose, it causes alot of back pressure and makes the water come out very fast. But doesnt this build up of backpressure hurt "top-end" HP? I know the backpressure works to create lower end torque, (which i dont care much for) but it seems to me that the backpressure would cause the engine to work harder, as the exhaust gasses build up within the system. Isnt this why a free flowing/open header/straight pipe works alot better for "max hp" (as in top end power) ?

Yes this is extreme, but it basically covers what im talking about:

If two cars had the same diameter pipe, one had 10 mufflers in the system and the other had a straight pipe; which one would you think would create the max amount of HP? I would think the straight pipe with very little back pressure would make more HP, If i am wrong then why arnt race cars doing this

I only said this to show an extreme example of why restriction is the enemy.

As for a not so extreme example, the mini "one ball" exhaust system shows exactly what im speaking of. Isnt the "one ball" the same as the stock exhaust except it has 1 muffler removed with a little bit of re-routed pipe due to the missing muffler? This in turn reduces back pressure/restriction due to one less muffler, and increases HP/TQ along with noise level

This is a link to the "one ball" diagraph.
http://www.ross-tech.net/andy/mini/oneball.gif


And to comment about companies making 5 inch catback exhaust systems:

I dont think it is practical for one, it would be expensive, hard to fit on a car, makes the car loud. (which alot of people do not like) The company has to build a product that will please the masses, not everyone wants an ear bleeding exhaust system that only makes max HP/TQ at high RPM's. Ive seen alot of posts about exhausts and two of the questions that is always asked is "does it drone? is it too loud?" Me personally, i could care less about drone/noise, Ive had alot of cars with very loud exhausts (VW beetle with header and stinger, and a Nova SS street pounder) both were very loud. But this isnt for everyone and the companies know this. The exhaust systems that are made by aftermarket are a well rounded system, Not too loud, not too quiet, decent gains with HP/Torque considering they are trying to keep the noise down to a tolerable level.

As for the pipe diameter for our mini's:

I believe that the mini is capable of "maxing out" the stock exhaust system. Thats why i believe most aftermarket exhausts built for the mini all have larger diameter piping. Which helps these systems gain more HP/torque over the stock system. Im not saying its only the pipe diameter (because there are alot of variables) but it most certianly helps. Otherwise all after market exhaust systems would keep the same diameter piping as stock.

Just so everyone knows, im not an engineer and i dont know everything, but i have built a few exhaust systems for previous cars i have owned. Im not trying to bust anyones chops, or offend anyone, this is just a friendly discussion aboust exhaust, and the theories behind them. Technical talk like this is how we learn and i enjoy it very much. But if i sound like a "know it all", i dont intend it to be that way, I can tell you, that i most certainly do not "know it all" im just bringing up questions and opinions.

 

sure looking at equivalent areas tells you something about what diameter of two pipes is the same as one. Thank god for beginning geometry! BUT, there are flow boundary layers, a bunch of stuff that happens at the y, and on and on.....

There is no a priori way to say that a dual is better than a single! It really depends on the details of each and every part that goes into it. Start looking at wieghts and the difference is even less so.

Matt

 

i didn't take it like that at all, just a friendly discussion As i said in my last post, I'd imagine that a free flowing exhaust would do wonders for high-rpm max power.

Of course it's not practical for all sorts of reasons to make a 5" catback, but what i'm saying is bigger isn't always better. There are a lot of variables.

Your second last paragraph hits the nail on the head. Each modded mini pushes out different amounts of exhaust gases, and the more mods, the more gas flow, and perhaps it becomes like blowing through a straw, whereas putting an exhaust on a milltek or similar might be like blowing through a paper towel roll and might lose power

 

I can confirm this as Bob's comments also mirror my dyno results. Randy Webb would say that the exhaust tuning for forced induction cars is a little different than for normally-aspirated ones. I think he said it had to do with scavenging – that it was different.

 

"scavenging" when it comes to exhaust... I have no idea...could someone enlighten me......?

 

Im sure you'll get a better answer than this, but I think it has to do with air being "pulled" out of the cylinder by pulses in the exhaust system in order for the cylinder to fill more completely up with a "fresh" charge.

 

think of each exhaust pulse as a little plug of gas. As it goes past, it creates a little vacuum behind it, this is what sucks more gas out...... Exhaust velocity is very important to get good scavenging, and oversized exhaust (headers or cat back) can drop the velocity (larger cross section = lower velocity) and negativly impact scavaging. I also think this is where the misnomer about increased back pressure = more torque. What's really happening is that you are maintaining velocity... If you make a high velocity low pressure exahust, you have the best.

Also, this high gas velocity is what's behind header coatings and wraps. The hot gas is less dense and has to go faster to move the same number of molecules.

This is what it really is called is a ressonant behaviour. That is, just like pipe organ pipes have a natural tone to them, there are frequencies at which the mass transport are most efficient.

Matt

 

Thanks Matt.

I tried to explain resonance in #26

 

Dr Obnxs

Thanks for the great info! I just learned a little something

Does this scavaging mainly take place in the exhaust runners in the header/manifold? I would think once these pulses hit the collector that they all get jumbled up, (trying to fit 4 different pulses through one hole) and especially if they were to hit a muffler/cat?

What happens to these pulses after the collector/cat in the header?

 

All sorts of things happen at the collector, and beyond. But the "zeroth" order way to understand some parts of header tuning is that everytime you do something drastic to the flow path (think merge 2-1 or 4-1 or whatever at the collectors), you create the possibility of reflecting part of the energy of the pulse back where it was coming from (the exhaust valve). Good collectors minimize this tendancy and pass the pulses with a minimum of distortion, and are very expensive. BTW, it can't be perfect..... Tuning tube length has an effect on the RPMs where the beating of the pulses helps, and you want this point at peak HP to really maximize peak HP. Tube diameter drastically effects velocity.... There really is a lot to play with when tuning exhausts... This is why guys like Don who've put a lot into the design don't want to give the fruits off all thier labors away.

But before evryone jumps all over the errors here, this is really a basic overview of header tuning....

Now while all that stuff was for the header, that's not where all the action is. If you look at V8s with true dual exhaust, they have an H or X crossover pipe to help with scavaging post header.....

Matt

 

Yeah, but the key words there are "true dual exhaust." With only one bank of cylinders I think the closest thing we have to crossover pipe scavenging is appropriate cylinder pairing (and associated primary diameter/length tuning) in a 4-2-1 header.....right?

 

So would it be fair to say that a 4-2-1 header, would be better for top end HP and a 4-1 better for lower end? It seems to me like the 4-2-1 design would actully help these pulses pass more smoothly then a 4-1. Because you are spliting the cylinders and having 2 pulses per "Y" in the header instead of having all 4 go into one hole instantly.

It seems the 4-2-1 would help because in the second section (with the two "Y" splits in the header) it turns 4 pulses into 2? and then those 2 would pass through the end collector? allowing for alot more velocity?

Im just guessing here, as i dont know much about header tuning.

Edit: im trying to find out what is "ideal" for making top-end HP/TQ not too worried about low end.

 

Other way around.
4 into 1= top end
4-2-1= bottom end

Roughly speaking

 

So i guess all the "Y"'s in the header create more pressure then a 4-1 system, creating more low end?

 

I 'think' it also has a lot to do with the EGT. Like someone said there is more to it than just the exhaust pipe itself.

Thanks for starting thread, I have learned some more about exhaust wit this one.

 

You might want to check out this post on header design -- it's the best layman's explanation I've seen so far -- lifted from Sport Compact Car's website tech section. The posts above it cover the exhaust pipe and mufflers. It's a LOT to read but IMHO well worth the time.

 

scavaging does! And to keep gas velocities high, you use a smaller diameter pipe. If there are lots of bend in it, you get backpressure, but that's not what makes the power. It's the price you pay for efficient scavaging. I think the pressure = torque myth is the one that will never, ever die....

If you think about it, why would having more pressure in the header, thus keeping more exhaust gas in the cylinder, ever create power? The answer is obvious... It doesn't.

Matt