ok, I recall a thread (of course I cant find it now) that had somthing to do with backpressures and negative back pressures , running lean,etc.
Which made me think of a project we have been working on lately on my car.
Heres the dilemna.

I have relocated my battery box off to the left hand side of where it used to be in order to run my exhaust straight back from the headler to my muffler. I am coming straight off of my header into a 2 1/2" pipe and then straight back to a muffler. No cat either I should add.
My thinking in this (actually my mechanics thinking) is that the least amount of bends for the exhaust to have to travel to exit the system the better. But now I am wandering if there is a reason that the mini s has so many exhaust bends to run thru. Am I going to have ngative benifits from some kind of lack of back pressures?
thanks in advance for any replies,
Clint
2003 mini S
mini madness stage 4 car
big head
cam
throttle body
WAIC kit
high flow intake
nology's
high perf. clutch kit
lightweight flywheel
LSD
KW coilovers
stoptech big brakes
15% pulley
madness header
poly bushings
roll cage
adjustable rear sway bar
shortshift
adjustable control arms
h sport camber plates
etc...

 

The theory - the straighter the better. Back pressure can/is be achived via the muffler, of course the degree in generated back pressure will be due to the type of muffler you put in.

If I understand correctly you switched the 2.25" center pipe for a 2.5" correct? What type of muffler?

how does the car drive? Do you percieve any seat of the pants negative?

 

Bends are not bad...
Backpressure is not bad...

Bends themselves do not rob you of horsepower unless there are improper bends. What I mean is that there is whats know as the Ideal Bend Radius for piping/tubing, and as long as the bend is of the ideal bend radius or bigger no significant loss(also known as head loss) is worth major modifications. Bends can be good to help remove gasses from certain places in the exhaust system quickly and reduce exhaust sound.

Backpressure is also not necessariyly a bad thing. When you are designing an exhaust system you have to ask yorself what kind of power you want. Are you looking for more torque or hp. You can create a header designed to increase torque thru the whole range at the sacrifice of huge hp gains.

To design an exhasut system is very complicated.You have to take into such considerations from the shape of the exhaust port to the components of the exhaust system to the length of the pipe. No one component being more important thatn the other because what you gain from one design change you can give back by a bad design change.

SOOOO ask yourself are you looking for more hp or more torque. Keeping in mind when you gain one you will gain the other it is just as to what degree.....

For street use gennerally you would like a better torque curve for the getty up and go that we love but on the track you want more hp because you keep the motor spooled up looking for speed.

As a rule of thumb the less backrpressure will give you better hp that is why drag cars have straight headers(extreme example for the explination purposes.)

When designing an exhaust for yourself since most peple arent looking for head work(me included) a good design header is very important(not necessarily top dollar, I have seen top dollar headers het outperformed by cheaper systems) yo want preferablly a multi step header with proper scavaging characteristics, not too big exhaust pipe the whole length, good flowing cat and muffler, no(or as few as possible) bends not adhearing to the Ideal Bend Radius and proper length exhaust sytem.

What you should be looking for is useable power not peak numbers.

When looking at a dyno sheet you want as much area under the curve not just peak numbers......OR depending on the application you may want more in cetain areas.

There is so much to try to explain without taking up excessive post space but this should get you on the right track.

Determine what you want then design a system for that.

 

now when I say that backpressuree and bends are not bad....

Keep in mind that if you take it to the extreme then they can hold you back but Im assuming you arent going to take what I posted to the extreme literal sense....

 

Check out this white paper by Dinan on the developement of their M3 exhaust. http://dinancars.com/whitepapersFile.asp?ID=3

Here's an excerpt:

I’m fairly certain that many of you have been exposed to a “bench racing legend” that would have you believe that increased backpressure will improve low rpm power and that low backpressure will increase high rpm output. Nothing could be further from the truth. An exhaust system is sized for maximum flow at wide-open-throttle and peak rpm. All exhaust systems are “oversized” for lower engine speeds (rpm), as backpressure is so insignificant that it can’t even be measured. Less back-pressure always results in more power at higher rpm, with no negative effect on lower engine speed performance. The amount of power that can be extracted from an engine at a given rpm as a result of exhaust design is really limited by the exhaust manifold or header. After the header, less backpressure is always better.

 

bahamabart- to answer your questions-

I am running 2 3/8" id tube all the way from my header to my small aftermarket muffler . There is nothing in between, no converter whatsoever.
And, I havent driven the car, it was a severely damaged car I bought from the insurance company that we are just getting finished up.


minimotorman- thanks for your informative reply, but I am still in the dark here for the most part. Are you saying I will have a loss of low end from doing this? I definately dont want that to happen as that is my favorite part of the mini experience.
thanks again!
Clint

 

Although I will agree with you vaguely and you picked out the paragrapgh for your aruguement I was placing more emphasis on the paragraph below....
Although the article didnt go in great detail about exhaust flow and manifold manipulation, the below paragraph is what is important for mp5999

I didnt touch to much on much on how to move the power curve in my post directly. I apologise and should have mentioned that but like I said there is alot to cover with many different variables and really will get complicated which wouldnt benefit anyone...

excerpt from abbetts article. that I feel is imprtant for this post

The exhaust manifold’s length, tubing diameter and the manner in which each cylinder is linked to the other is critical when attempting to maximize an engine’s power output. The manifold configuration can be manipulated in order to generate maximum power at low, middle or high rpm, changing the shape of the power curve accordingly.


Reply to mp5999

Im not 100% as to what the exact dimensions of the factory exhaust are compared to what you have done so im not sure if I could make an accurate comment on what you should expect. Just a rough GUESS making it free flowing and header design isnt there to support such improvement I would GUESS you will loose the bottom end punch BUT if yo didnt increase pipe diameter to far past stock then you might not notice anything but a loud exhaust. being so free flowing I think you would benefit from decreaseing from stock pipe diameter just a little....

What I did want to get across to you is that the relocation of the battery and fighting so hard as to not bend any pipes might be a little over kill as long as the bends fit the proper bend radius bends will not significantly hurt your goal of more power.

And if you go too big on the exhaust pipe you gennerally will loose the bottom end punch from a small motor which the mini has. Many people (not necissarily you) think they have gained power but it is that fact the exhaust is now much louder which tricks you into "feeling" more power. So when you get to drive the car stay objective...

 

Your setup is similar to mine. I run the Minimania header into a Milteck resonator and straight though into a Flowmaster 40 series. Performance has improoved on the track. MPG has also improoved on the road as the car is so loud I just get it into 6th asap so I can hear the radio.

If my car were a street car for the most part I would return to my Miltek catback, but on the track this setup seems to make the car quite happy.
Note, I removed my back-up light and the exhaust now exits through there with a 4" resonated tailpipe.

 

My battery is to the right but I don't have any particular issues.

 

Very nice J! I'd be curious to see a finished photo with tip and rear bumper in place. From my experiences (I too went straight), in order to have the tip be properly situated to exit the valence, it needs to be lowered a couple or so inches. Curious to know what you did... Maybe you did as onasled did, and removed the reverse light as that would be the exit path...

FWIW, I felt no loss in low end grunt, and definite gains in the higher RMP's.

 

Side view:

 

Rear view.

I wouldn't recommend it for the street. It's too close/could hit the body.

Had to shim the center plate.

JP

 

.................................................. ..........................................Edit:tha t wuz quick !!!!

would like to see pix of a finished product

 

Wouldn't want to hijack MP's thread....I can PM you.

 

kool !! **

 

I can appreciate that. Thanks for the quick reply with photos. Again, nice job with that, as well as other aspects on your MCS!

I have a better underside photo somewhere on NAM, but here's one that shows what I did... an offset muffler (Borla), inverted, to get the right elevation for the correct valence exit:

 

Like your tip better

 

It fills the valence nicely... Looks like they sell it... about 2/3 the way down:

http://www.qualitysmart.com/products...haust_tips.htm

 

wow J that looks killer . But that battery looks tiny, what is it? I want to be able to use a battery that will be dependable for street driving in the winter but also want to go as small as possible. Should I even ask how loud your exhaust is ?

thanks

 

looks great!!!

 

The battery is a drycell Odyssey PC680. It's actually good to -40 Celsius. It has 220CCA and up to 680 Cranking amps for 5 seconds. It measures 7 in. x 3 1/16 x 6 5/8 height. $86 USD. You can find it at most specialty battery shop (Batterymart.com, etc.) or most speed shops. Don't forget to order the Automotive terminals. It's actually pretty heavy for the size.

JP

 

Oh and the exhaust is well, loud! But isn't that what makes the beauty of a race car? The helmet+balaclava is actually a pretty good muffler too!

JP