I think I'd rather spend the money on an Alta downpipe than potential EPA fines for messing with emissions equipment. Not to mention the ever increasing noise abatement laws. You wouldn't want to drive through New Hope, PA with this rig installed.

Gotta respect OP for trying new things, though

 

I wouldn't want to, period

 

Drive it? Or go through New Hope? Don't know where Chief stir-potter is. On a reservation somewhere?

 

It's about pulse velocity, and yes, there is a difference (Think about it, you can make a 4" exhaust, crimp the tip at the exahust pipe, get any back pressure you want, and it will be a crappy exhaust. You can run a straight 2.5" exhaust, no bends, much lower back-pressure, and have a screamin exaust that makes lots of power...) And for turbos, all that stuff goes out the window. To get the turbo to work well, you want the largest pressure delta across the turbo you can get.

Matt

 

Well, the lack of a muffler is not gonna affect pulse velocity any more than the presence of the turbo and cats will. In fact, at that late stage, I would imagine its effect on it is almost none. The way I see it, once the pulse is past the turbo, its life as a pulse is pretty much gone as far as affecting combustion through turbulence or scavenging. You would like it to proceed to the nearest exit as quickly as possible in order to free up that side of the turbo. That is where size does come in, you want his brothers to suck him along, with as little turbulence as possible. Size your pipe in accordance with what each cylinder will flow. That's why a 5.9 I-6 needs a much larger pipe than a 6.3 V-8.

Why would you even suggest putting a 4-inch exhaust on an 1.6 I-4?

 

If it has a 42r, yep.

 

it was claimed that backpressure is needed, that's just BS, what is needed is proper sizing for the gas flows that happen at peak HP, the example was an illustration of how to make backpressure in such a way to not make power. That would be known as an "existance proof" that it's not backpressure that you need to make power!

Don't need to listen to me, just go to the high end exhaust manufacurers about the issue. It's because most quote the backpressure myth don't really understand ressonant system or fulid dynamics. For what it's worth, I did a lot of reading on the subject to write the last tech article for MC Squared, and I go deeper into the subject there. You can also go back a few issues of Hot Rod to read a tech article about the same effects. There's a great write up on exhausts on Dinan's web site about exhaust sizing for power as well.

What it comes down to is there are lots of ways to increase backpressure that degrade power, and there are lots of ways to reduce it to create power. Exhausts systems are tuned mass flow items, where ressonance has lots to do about how they make power. Simple saying like "backpressure makes torque" just are too simplistic to even come close to capturing any of the truth in exhaust sytem design.

Matt

 

Lots of interesting posts.. I have not looked much into sizing yet.. I believe the Alta is a 3", which is similar to what I was leaning towards..

I don't know too much about backpressure, but what I do know is when the stock exhaust is on, the car starts up great and takes off great from a stop (especially when cold)...When I had the catback off (just an open dump) The car felt like it was cam'd and would sometimes struggle to take off, if there wasn't enough gas pedal pressed, unless the motor was warmed up.

 

a 3" exhaust will flow fine up past 400hp. A 2.5" has worked well for 2.0L WRX guys for years....those cars come stock with more HP than 99.99% of the MINI people on this forum will ever have. A 3" system is marketing hype for MINI owners

 

I'm no automotive engineer, but I was under the impression that the reason to have backpressure is that there is not just a simple situation of intake closes, mixture is compressed, detonation occurs, exhaust opens and spent mixture is expelled out of exhaust. But there is overlap in valve timing, so without back pressure unburnt fuel exits open exhaust valve before it closes and fuel is detonated.

It's all to confusing for me, but if BMW designed valvetronic engineering to use backpressure, you are least going to lose fuel efficiency and unnecessarily waste fuel for what would likely be minimal power gain. At least in the absence of completely different fuel and timing mapping of the ECU as well as redesigned camshafts.

At least in the Harley world the quickest way to lose power and waste fuel is to put wide straight pipes on a bike with a stock camshaft and timing.

And I don't think I'm way off base because the R56 is getting a massive torque curve from 1750-5000 rpms and getting 28mpg in stock clothing.

If it were easy to do the JCW Stage I would be providing a lot more power than it does. And anyone who thinks they're going to plug in an Alta PnP ECU and get 230hp and 28mpg at the same time is in dreamland.

 

The turbo does a lot to the pulses, and also somewhat decouples the header from the rest of the system. Not completely, but more than standard exhausts.

So, for "normal" exhausts, one of the things you have to remember is that the whole system, from intake to exhaust tips, is a resonant system. Valve overlap, timing, etc is tuned as a system, and if you really change something without retuning the system, you end up with something that may be non-optimal.

So, what's happening in the header? The gas pulses (the two, explosion out as the valve opens, and the slow push of the piston going up), are really like "bullets" of gas that come out of the exhaust port. As they leave, they expand and cool (such is the nature of gasses), but they have momentum and energy, and one can think of them as little plugs that pass down the primary. They compress what's in front (and if there's a lot in front, it slows them down), and they pull what's behind. The pulling what's behind is called "scavaging".

Now, this pulse goes down the pipe, and gets to a collector. This is a rather traumatic transition for a pulse, and some of it gets reflected (in electronics terms this is called an "impedance mismatch" and we're all very familiar with it in real life. This is the small, but noticable reflection that happens on the surface of glass. While pressure waves and light waves are different, you'd be shocked how similar the math is that describes the behaviour. So this leads us to two concepts. A good collector design minimizes this reflection, as it's really a small percentage of the pressure wave going backwards (and this can lead to something called reversion, that I may get to later). the second item is that if you think about how long it takes the pulse to get to the collector, and the reflected pulse to get back to the valve, it's pretty easy to imaging that you can "tune" the length of the primary such that the pulse going back doesn't screw with the gases coming out of the exhaust valve. But it's never so easy, and this only happens at some RPMs, as the time between exhaust valve events is dependant or RPM, to first order, the time for the reflected pulse to get back isn't. This is why you tune the length of the primary to give the best effect at peak HP, and you take a bit of a hit at other RPMs. (you choose the gear you're in based on staying near the HP peak... At least you should.) So this covers the length of the header, but not the diameter, so what's up there?

Now, the speed that the pulses go down the primary are dependant on the primary diameter. There's a "magic number" for pulse velocity, and I'm not really sure where it comes from, but it exists (I don't know everything!) And if you open up the pipe too much, the speed of the pulse is less than this number, and while the momentum of the pulse is somewhat constant (it's really a funcion of mass flow, and that's determined by the amount of gas and air burned at a given RPM), but the energy of the pulse (and the amount of work it can do) isn't conserved. So you want the speed to be high, and this comes from smaller primaries, not larger ones! (This is the root of the backpressure myth, as the primaries get very small, velocity skyrockets, as does the wall-gas interaction, leading to backpressure. BUT you can also get backpressure from bend and the like, that have little to do with pulse velocity, and this is where the confusion lies, as backpressure and velocity are related a bit, but in all but the best race only exhaust, are really not that coupled at all.) This is really why too big looses power, it's not because it gives up backpressure, but because the exhaust pulses are going too slow, don't contain much energy, and can't pull a partial vacuum behind them, that helps the next pulse go down the pipe and on and on....

So, now let's get back to reversion. If you have a crappy collector, or run at rpms way off peak, (usually a combination of both), a large amplitude reflected pressure wave is showing up at the exhaust valve right as it starts to open, and this actually pushes gasses BACK INTO the combustion chamber! This is exactly what you don't want to happen. This is also where cam timing can come into play. Variable valve timing cams can change the opening and closing to minimize this effect somewhat, and that's why a VVT engine can have quite a surprisingly wide torque plateau compared to the lumpy, non variable cams of the past. It's a way to have a car have a low RPM (used to be called a torque or truck) cam, and a high RPM (race cam, in engine slang) all in the same package, and why the new engines with VVT are really a very good thing for performance freaks (another reason to love computer controlled engines). But there's more to reversion than just that. Turns out that all the stuff above makes best sense if you think of the gas pug as kind of a cork that fills the pipe, but mother nature isn't quite so kind, and the center of the pusle moves faster than the edges, and in some cases, gas flow can actually be backwards at the edges of the pipes while the pulse goes down the center! this is why stepped primaries exist. The step in the primary blocks more of the reversion at the walls than it screws with scavaging by having an increase in primary diamater. (Remember, everything is always a trade off). Now there's more to reversion than just this. This is really thinking about just one primary in isolation. Truth is that the collector gives the pulses the opportunity to couple, or revert, up other primaries. Evil, aren't they? Anyway, you want the pulses sequenced in the collector such that the next cylinder to have the exhaust valve open gets the smallest reflected pulse from the primary that fires just before! If you subscribe to MC Squared, you'll see a photo of the exhaust headers from an honest to god race GT40 that I took at the Monterey Historics. If you look closely, you'll see that some of the primaries from one side of the engine go to the collector for the other header! This is to get the pulse phasing to work best when the firing order of the big nasty 427 V-8 is taken into account.

Now the post collector exhaust pipe is a tuned pipe just like the primaries, and on real race cars, the considerations of length and the like are also modelled or varied to get best performance as well, as everything is coupled. Street cars, not so much.

One other thing to mention before I go off to beddy by tonight. Remember that we want the pulses to stay fast to keep up the velocity so they keep high kinetic energy so they can scavenge well? This is the world of coatings and wraps. Thermal barrier coatings can help keep the heat in the pipe, so the pulse stays fast. This is cheep power, but you have to take your header out of the car to get it done. Lynn Gleason (who owns Motion Minis in Santa Cruz) will wrap anything (he's an old gear head who cut his teeth in the V8 days of the sixties and the seventies) and you get two benefits from this: 1) Better scavenging by keeping the heat in the exhaust pulse, and 2) lower underhood temps because all that heat isn't making it out through the metal of the pipes into your engine bay. (As an aside, there are tons who say wrapping headers is a no-no, but there are tons who do it as well! There was a build up in European Car where they turbo'd a 3 series BMW, and even with the turbo, using a turbo blanket and header wraps kept under hood temps down to about 10 degrees above ambient!Take that, heat soak!) Now, there may have been some truth to the "don't wrap headers" long ago, but it's been a very, very long time since I've heard of exhaust systems rusting through, and really, racers have thier cars apart so often that it's a non-issue. FWIW, I just had my Stahl header coated and wrapped. When it's on, I'll let you all know how cool the engine bay has become.

So, what's to be learned from all this?

1) Simple statement about exhaust really don't mean sh*t! Exhaust are far more complicated than simple statement can ever convey.
2) Length tunes peak RPM.
3) Primary diameter should be selected to get the optimal exhaust velocity at peak HP.
4) If you keep the heat in the gas, it scaveges better, and keeps the engine compartment cooler.

So, what about turbos? Well, here the packaging really screws things up, as there is pretty much no way to get proper length primaries. But all you R56 guys and gals? You know that you have something called a "dual scroll" turbo? IF you look into your exhaust manifold, it's really two separate 2-1 manifolds, each feeding one "scroll" of the turbo. This was done to keep pulse velocity high so each pulse can kick the turbine blades harder, and to reduce reversion into other primaries, effectivly eliminating cross talk between each primaries pulse and at least two of the other primaries. So not all of the above is wasted. Now this is where the similarity really diverges.... The amount that each pusle can push the turbine is dependant on lots of stuff, one biggie is the pressure drop across the turbine. This is why small turbo engines really do have huge downtubes and exhaust system, as the act of scaveging is somewhat less important than getting the energy of each pusle to couple to the turbine.

So there you have it! The basics (the very basics) of how exhaust systems work, and why I hate the phrase "you need back pressure to make torque"!

Matt

 

Well, actually it's called "scavenging" but I'm assuming that's a grammatical error.

I would agree more with your latest post than the earlier one that declared backpressure as BS.

The turbo is in essence acting in the same way as a restriction in the exhaust. And I would assume that much of the required backpressure and scavenging is provided by the turbo as opposed to the pipe behind it. Put a wide enough pipe on and the exhaust gas can't accelerate enough to provide any scavenging or back pressure. So if the turbo doesn't provide it, it won't be there and power gains and efficiency are overrun by poor overall design.

Without the scavenging, one may think the turbo is spooling up quicker, but probably isn't. Imagine a tiny pinwheel (say 1/2") at the end of a 1/2" tube. Suck on the end of the tube and the pinwheel spins. Now replace the 1/2" tube with a 2" tube. When you suck on that you have to overcome the larger volume in the tube and greater effort is required to spin the pinwheel. Not a great example, but you should get my point.

Normal disclaimers apply (as well as Holiday Inn Express comments).

 

Ahh, I see what you are earning your name over, Obnxs, that word!

Ok, I realize some _restriction_ is necessary, happy?

Seriously, I appreciate the correction of my semantics, but that was the long way to go about it, since I never was arguing for either a jet nozzle or a bendy straw to be affixed to his downpipe. What did you think you were arguing against?

 

I thought selling my Honda would get me away from this kind of crap.

No offense, Junior, but from experience- do NOT do anything half-way. You will regret it. And what you're doing is less than half-way.

 

Junior, I've run with a disconnected header on several cars for short periods of time, and with a broken exhaust manifold in my truck for several head out the window like a dog months. Trust me when I tell you you are breathing exhaust. You can't smell CO, though if the break is North of the cat you should smell other exhaust byproducts. CO poisoning starts off with a headache or a feeling of being unusually tired (yawning uncontrollably), both of which you could write off as coincidental. If you get to the point where your tongue gets numb, you're a step away from needing medical attention. http://en.wikipedia.org/wiki/Carbon_monoxide_poisoning
According to the Traffic School I just completed, in one case 2 people died in a car with windows fully open from CO poisoning. MINI doesn't have a particularly great way of extracting air out of the cabin, because rear windows don't go down. Next time, do yourself a favor and wait for the exhaust building session before you cannibalize your exhaust. We bash because we don't want others to follow your lead, and we don't want you to hurt yourself. You seemed to be in denial that there was risk to running open exhaust that ends in front of the drivers seat.
-kARL

 

if you don't now see that the backpressure game isn't the game to play, then there's no hope. I hoped that my examples that you can modify backpressure without effecting velocity would make the point. You can still make an exhaust that has crappy velocity, and any backpressure you want, and it's still a crappy exhaust. It's the wrong word, the wrong concept and a backwards way of thinking about it. Every time someone quotes backpressure about making exhausts instead of velocity, they are demostrating that they don't understand that you can vary the two almost completely independantly. To me, that means they don't understand how exhaust systems work.

There's a really good thread over on Mini2 where Roland (of GT Tuning) made some stuff to measure backpressure. Guess what? The stock exhaust with a one-ball mod made lower backpressure than several aftermarket exhaust systems. But I don't think it made more power.

Matt

 

And if you read more of my posts, you'll see I make tons of them!

Matt

 

And as I explained in that post, and in this one, for one last time. I was using the word that is in common usage to be the opposite, or at least a great impedance to exhaust leaving the engine at a beneficial rate. I was unfamiliar with the term "pulse velocity", and was unaware of the possibility of difference in direction of flow between the inside and the outside of the stream as it courses down the pipe, thank you. The rest, I knew already, I had a subscription to Hot Rod from 81-89, after all. I think they touched on the subject of exhaust once or twice. Do you have a link to the thread on Mini2? I would love to read it, as long as I do not have to hear about how I am apparently not understanding it.

 

but the reason that I hate the term or the idea that backpressure makes power or torque or whatever is you can create it just by denting your exhaust. And it won't make more power.

Now I've read tons of mags as well, and there are a large number of authors that get it wrong as well, and never touch on the idea that backpressure and exhaust velocity are close to uncoupled in most exhaust systems for cars.

Backpressure and exhaust velocity aren't synonyms. They aren't even dependant variables. They are coupled, yes, but not to the degree that you can use one term in place of the other, and have a tightly coupled relationship imply an exact behaviour of the other variable (this would be called conjugate variables... but that's another post!)

Anyway, I didn't mean to **** you off, and if I did then I'm sorry for that.

Matt

 

Meh, that would be a strong word, exasperated would be more accurate. I learned something, so it was not all in vain.

Really, if you have a way of locating the thread on Mini2, I would like to read it. Pointing google at their forums has not gleaned it for me.

 

but it's a bit hard for me to navagate there, as I visit it so often, and Roland does TONS of posts, so searching on his name returns pretty much every thread!

Matt

 

Thanks for the info.. I know it's hard to say everyone who says I'm breathing in CO is right..so I did take the advice, I put it back on *doh..forgot to put on the metal gasket? looking thing, but it's still bolted on tight so its ok* but anyways..it's quiet again.. I did notice I *think* I had better gas mileage but a 1mpg when I had the catback off.. I stayed around 23.8 on the trip computer and thats with a lot of hard driving.. I'll see what this tank gets now that I have the catback back on.. So ya, I won't be doing that again.. In a couple weeks I'll post pics, I'll have the catback made and the downpipe made, hopefully, and we'll see how it runs then.. After which I will probably dyno to see where I'm at before I decide if I want to chip it now or not..

Thanks for everyone's comments so far.. it seems there is a LOT to be learned about many things we may or may not know about, and even if you do, it never hurts to expand or be open, and to research what it may be. This is what forums were meant for, open ideas, creativity, advice, bashing, loving, etc ...

 

Dr Obnxs,

Thank you for the write-up on the exhaust systems, back pressure, pulse velocity and torque. Learned something about how exhaust system works as well as what a twin scroll turbo really means. WOW.

Thanks again.

 

I disagree. Look at the picture below. If you blow on the left end of the tube, which propeller will turn faster?

 

The Porsches, Ferraris, and F-1 cars do not have long exhaust system. If it generated more power, I am sure they would have an extra long exhaust pipe coming out the back.