you guys talk alot...shiet...

 

You've never been in an engineering class, I see

 

This is to answer that question it is written by Road Race Engineering concerning their downpipes. Yes it is for FI but I am sure some of these principles still apply to NA as well.

Our DPs have a 2 1/2" stainless steel first bend with a 2 1/2" stainless steel flex section. They then expand to 3" right after the flex section right under the oil pan area. Why start out at 2 1/2"? If you are running on the stock O2 sensor housing, even ported it is 2 1/2" inside diameter. When the exhaust gasses are hot, they flow better, 2 1/2" is plenty for the first 12". As the gasses cool , bigger helps. By going to 3" after the flex section, we allow this expansion and also allow for better clearance at the front where room is tight.

By the way...anyone doing any testing yet? hint hint

 

Well, I got a PDA last night. BUt it's like the ONLY one you can still buy that's not supported. ****! Oh well, it'll make a nice gift for the wife It's the Tungsten E for those of you who are interested. So I just ordered a ClieTJ35. This effing thing better work!! Then after the new year it's on to the dyno program and the DPs. Trust you guys will hear first.

I was doing some math in my head. Anybody remember chemistry?

p1=v1/t1 and p2=v2/t2 (for a changing system)

Does this apply? How can we make it work? Gott run back to the job. We'll kick it around some more later.`

 

why wouldnt cooler, denser air flow better?

 

Because it's less excited, meaning the molecules are moving less rapidly. Think of it like this: why do tea pots whistle when they're hot, not when they're cold?

 

One major reason to stay nearer to 2.5 inches is the simple fact that any diameter change unless done properly will hurt you significantly. If you go from a small pipe to a larger pipe with them just mated to one another (like through a flat plate...) you will get recirculation regions due to the flow expansion. This actually induces a pretty significant amount of head loss and restriction even though you are going from big to small.

Going to 3" is fine for forced induction in the case described above. But the problem with NA is that none of us should be running 3" pipe, so that means we'd end up going 2.5" 3" 2.5" and now you've introduced two more major restrictions. Also, you can induce other possible detrimental flow effects from the 3" section before your 2.5" catback etc. Just think about a bullet resonator... same difference. the 3" will help quiet the exhaust gases some, but quieting them means you have to absorb or deplete energy from them in some way (aka slowing them down or other methods) and thus you've hurt yourself again. So while the 3" expansion is reasonable for FI, it's not a great idea for NA.

 

I'm not following your equation. Are you saying pressure equals veloctiy over time? Your units don't work out at all, so I'm assuming that's not the case.... I used to really like chemistry so I may remember what you are talking about once I know what equation you are referring to!

Otherwise I'm wondering if you are referring to rho1*area1*velocity1 = rho2*area2*velocity2 which is the conservation of mass. Which then yes would be applicable to our pipe sizing as you'd want to maintain a certain velocity based upon your area being adjusted to compensate against your changing density.

Keep in mind though as well that you can generate enough of a nozzle effect to generate negative pressures by the exit, which results in shock waves at exit, and interesting things upstream. A lot of the exhaust characteristics and velocities if nozzling is present will have to do with the pressure ratios between the header pressure and the exit pressure, or any throat pressure if you converge/diverge. You can look up the behaviors of the densities and so forth independent of temperature in isentropic flow tables for ideal gases.

Also, you can model the exhaust as a converging diverging nozzle by assuming it to be a single throat CD nozzle, or a simple converging nozzle. I believe then you can get away with things by setting the throat pressure to be atmospheric and the throat area as being the exit diameter.

 

You are mis-reading my post. This was posted in rebuttle to you saying that no one makes a downpipe that goes from small to big. I was just pointing out that yes manufactures do, do this. Again, no one is saying to put a 3 inch cat back on a NA car, those werent my words (I don't see where you got that from) . The quote was talking about a FI car so of course there will be different diameters concerning NA. I also pointed out that although this was for FI some of the same reasoning could be applied to the lancer. I also dont understand where you are getting this 2.5 to 3 to 2.5 idea either. The setup up that RRE is talking about is just the downpipe. It starts at 2.5 goes to 3 inches and is meant to be mated with a 3" catback. So in keeping the same principles of this in mind.... I could see an exhaust set up for the lancer as follows: 2.25 downpipe that expands to 2.5, high flow cat testpipe or DP with cat-eliminator) mated to a 2.5 catback.

 

pressure*volume= n[number of moles] * R [planck's constant?]* Temperature.
n and r are effectively constants for our purposes, and even if they weren't I'm not about to calculate the moles of the different gasses that are present in a given volume of exhaust gas.

anyway, when you have a system that's changing- our exhaust system works well for this example- you have the two equations that share variables, but at different times.
pressure1= volume1/temp 1 and pressure 2=volume2/temp2
of course this really only helps us determine what effect the volume of the ehxaust has on the pressure and temp of the system. we keep talking about exit velocity and I'm not exactly sure how pressure relates.
It also assumes a closed system (now that I think about it the thermo texts always use this to help model a piston-cylinder system). But it might have some simple value for our purposes until I have the cash to order the dyno program and have the time to weld up two DPs.

 

we just aren't reading each other quite straight I guess. I should also mention too, that I don't find any of your posts argumentative, and I'm not trying to be by any means. So I hope you don't feel like I'm sitting here trying to be a jerk.. certainly not! Quite the opposite in fact .

I certainly didn't follow that you were posting a rebuttal to my other comment either . I also initially thought you meant tapered pipe, which I was having a hard time picturing someone spending the money to manufacture a tapered pipe. What you posted though about a 2.5" initial to 3" is all good and makes sense. Especially when mating to a 3" catback.

The only reason I reiterated about not wanting to put a 3" catback on a NA car is because we know that someone will come in and read these posts and think that bigger is better for one reason or another, and they'd think that buying a downpipe that goes 2.5" to 3" and then mates to their 2.5" catback or what not on an NA Lancer would be a good idea... and it wouldn't be. I know that you have no issues with that concept, and a lot of people would understand that, but we also know that there are enough people that read through this that would think that they want a 3" downpipe because it is bigger and that'd be better or something.

I follow and agree with your point (at least I hope I follow for once ) that the principle of that header would be fine on our cars... aka your point about exhaust on a Lancer being 2.25" downpipe, expands to 2.5" and then mates to 2.5" catback.

Hopefully that's all a bit clearer and less asinine sounding on my part this time .

 

HAHAHA.... By no means do I take offense to anything you say. I have actually been monitoring these post and like I said before..it makes good conversation. I do see what you are saying about someone slappingon a 3 inch DP without totally reading our post thru so you pointing this stuff out isnt so bad, now is it?

 

There is R and there is R_bar.. The value you need for air (which isn't really quite exhaust gas.. but maybe close enough) is about 53.3 ft-lbf/lb_m-°R or 287 J/kg-°K at least when used in rho = P/RT which is a different version more or less to the PV=nRT equation.

You can relate exit velocity to pressure by modeling (incorrectly) the system as having a stagnation location at a certain pressure. Then use bernouli's equation to determine the exit velocity. It's an over simplification, and may cause bad estimations because of all the nuances of pipe flow that vastly change the actual flow behavior and velocities. But it'll give you a general idea as to what effect the pressure actually has on the velocity.

Probably a more accurate model would be to look into pipe flow equations based on pressure "head" which would be more equivalent to the exhaust idea.

But theory is only so good... experimentation is the only real result :-) So we'll have to hold our breath until we get your info.. which is incredibly kick *** on your part .

 

so wuts the best exhaust? lol we haven't decided this yet.

Which Headers/DownPipe/Cat(or TestPipe)/Cat-Back setup are we looking at as being the best?

2.25 all the way back... 2.5header/dp/testpipe than a 2.25 cat-back? or 2.5 all the way? i mean...any conclusions yet? lol and crimped or mandrel bent?

 

Honestly... I think the difference between configurations is going to be small 2-5 hp. The cats and the muffler you use will make a bigger diff.

Also depends on how much low end torque you are willing to sacrifice for the sake of top end power.

I'm going to dyno my car in 2 or 3 weeks with my Kamikaze header. I just put them on, and that is the only thing that will have changed from my last dyno (which was with my intake and catback) so at least on my car we'll be able to see what they produce. There are dynos for the RRM and RPW header already. although one of the dynos is suspect as it shows 17-22 hp increase for the RPW which is a little on the unbelievable side.

Personally I have 2.25" crimp bend cat back with a 2.5" header/downpipe. We'd have to get someone with a variation of my exhaust ( I also run a magnaflow resonator and a muffler to keep things quiet) with different piping to say for sure.

I'd say go with whatever is the most effective price wise. I think there will be so little difference between the extremes that you might as well save a few hundred and put on a better throttle body or get a port and polish done (from the savings of the smaller pipe and crimp bends) and you'll be farther ahead and spend the few hundred extra for bigger piping, more expensive mufflers, and fancy headers.