improving exhaust note - Page 2

supersteve · #11 11-09-2006, 06:27 PM

It's a balancing act. Back pressure is inversly proportional to velocity. High back pressure = low velocity = torque. Low back pressure = high velocity = horsepower.

Back pressure actually helps torque. Torque is what will get you moving. Diesel engines live off back pressure. It will hold some of the spent exhaust in the cylinder. This gives the new charge of fuel and air more mass helping it push down on the piston. The draw back is you can't fit as much new fuel and air in the cyclinder which hurts the horsepower output in the high rpm range. Diesels live under 3500 rpm.

Less back pressure hurts the bottom end torque but helps the top end horsepower. Less back pressure = more velocity of the exhaust. High exhaust velocity = burnt exhaust valves. This is why you see so many rice burners with brown exhaust stains all over the back of the car. A 2.0ltr engine with a 3" exhaust has almost no back pressure. No back pressure = burnt valves. Great for racing @ redline for 500 miles. I would like my X5 to last longer than that.

BMW most likely spent many hours on dynos to balance the torque and horsepower curves for these (and all of thier) engines. There is no doubt that manufactuers tend to be a little on the conservetive side, that helps with longevity. If you think sound = power, I think you should reconsider. If you really want to have a loud exhaust note, record a NASCAR race and put the CD in you stereo.

The old racing saying goes something like. . .How fast can you afford to go?

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RGX5 · #12 11-09-2006, 09:09 PM

I was going to post this as a new thread but it's really fitting here. I bought Ekaz' X5. Most thought the exhaust sounded great. It's an 05 4.8is and what he had done was remove/cut the pipes leading to the resonator, and then installed a flowmaster 50, followed by straight pipes out the back. Sounded pretty snarley....everyone apparently really liked it at one of the meets..but it was loud. Fastforward to me purchasing. Dealer couldn't sell the truck due to exhaust. Too loud. Then I had to live w/it and decided even before I bought it that I'd put it back to stock. G a great deal because of it...but to me for a 4.8 it seemed a little flat , especially in the low and mid rpms..you heard every shift etc, but again, felt like it was lacking torque. Well yesterday morning I had a brand new full stock 4.8 system put in and I never really expected as much of a difference, way more torque, estimated 30-40 ft pounds, way smoother in all gears, now this is like what I recalled from a test drive of another 4.8 a year or so ago. Bottom line is torque is what makes a vehicle feel powerful off the line and accelerating, pushes you back in the seat ect., not horsepower. I now have my power back...or should I say torque, and I can't be happier. Just thought I'd share my experience because most people go the other way, from stock to modded...it isn't always what it's cracked up to be....

94502 · #13 11-09-2006, 11:39 PM

this makes me think of going back to stock set up.. I installed the X-pipe and I love the sound but I really should think about longevity of the car. I can't tell if i lost power or gained any..

RGX5 · #14 11-09-2006, 11:53 PM

94502, I'm not sure you're really going to hurt anything...just for me I was amazed and really pleased with the difference going back to the stock system...I'm a performance oriented guy, so I was questioning whether it would even give me less power, but here is a case where it is so noticable. I can't say anything bad about what was on there, nicely done and all...I'm sure the difference went unoticed due to the excitement of the new sound and looks but there is no doubt in my mind..I wasn't expecting much if any real noticeable change..just happy as heck right now! My 4.8 now feels like a 4.8!

noncom23 · #15 11-10-2006, 11:44 AM

Whats interesting though is that my SA pushes the Dinan exhaust for more
power. So what is the difference? Sorry about the highjack!

RGX5 · #16 11-10-2006, 09:37 PM

I guess that depend on how they define "power". There's a formula for it using torque and horsepower. Sure, lots of sytems will give you "power" as in horsepower, but maybe it makes it all above 4500 rpm. then look at the chart and see what happened to the torque, which is the twisting force that defines pretty much what you feel off the line, (stop light etc). Many times the torque is down from an exhaust mod (or in days past a cam mod). For me the difference I feel , and how little I have to hit the throttle now made it clear to me in my specific case. What I had was not tested, or dyno'd etc. It was put together..a muffler and some striaght pipes. Just when your thinking of adding a new exhaust, look for or ask for the dyno chart and look at torque at the lower rpms and horsepower at the higher rpms. I'm sure there are lots of well designed systems out there, but most engines mods have some sort of drawback..Dinan is one I would tend to trust since they are endorsed by BMW.

supersteve · #17 11-11-2006, 01:27 AM

Well said RGX5.

Sometimes the "more power" is what you hear.

otoupalik · #18 11-15-2006, 01:17 PM

take the resonators out - you will love it. Cheap and it will give you the sound you want.

On our dyno on my old 98 540, taking the resonator out gave us 4whp.

Thanks
Brad

supersteve · #19 11-15-2006, 02:04 PM

Brad,
What did it do to your torque?

gordonn · #20 11-15-2006, 03:47 PM

Under the heading of "too much money and time on my hands"

I cannot imagine wanting to improve the exhaust note on my 4.4; it's the knarliest sound as is. BMW must have a harmonics/sound division that employs dozens and spends millions and does nothing but create exhaust notes for their products.

Different story if one has a $30k Jeep Cherokee.

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