RPIpower

Its both. Google all you want. If you read it closely, the "backpressure" issue is merely a lack of velocity.

How do you know it works? Because someone told you it "might" work? How do you know the testing is accurate? How was it tested? ARe you going to build a custom exhaust, test it, make sure it was tested accurately, then make it again, then test it, etc? The reason we didnt make a quad exhaust was because of 2 things. 1. Cost. To make a quad system would require you to change the rear diffuser and also you would need to make some changes since your battery housing is on the passenger side. We dont like making stuff where you have to modify your car to make it fit. The reason people are losing power on their quad system is that guys are building the systems wrong. How do I know? Because I test/dyno more exhaust systems then any of the guys who build custom systems here. There is no possible way CRMD or anyone else has tested ANY system as extensively as I have. So the two things are cost and having to modify your car to get it to work. The cost part goes way up because we looked at the car and had to design a system that required the least amount of modification to fit. Because of this, more r&d would have to take place to see if the design that fit well ALSO performed well.

keyword here, "should". There are even times when I try things and it "should" do something but it doesnt. There are times when we dyno test something and it shows a gain, then on another day it doesnt. How the car is strapped down, the conditions, load, ramp rate, etc all effect the results of the dyno testing. This is why you have to test it over and over and over again over a period of time to ensure that your initial test results are accurate. This is not to say what you tested is not true.

In the end, do you want a quad system that you KNOW for sure works? Or do you want a cheap alternative based one someone suggesting that something works? I went through all this with Joe at Trinity. He had a quad system on his 545 which he knew lost power. I gave him suggestions and he took what I suggested and had changes made. When the car was dynoed, it made better power but it was still down compared to stock or with a single sided exhaust upgrade. If you want it to work, its gonna cost time and money. The metal core cats dont change much on the performance of the exhaust. Magnaflow uses Carsound cats which are the best cats you can buy (imo), but the performance difference between those and the secondaries found in the BMWs are 1-2hp? Is it worth it? I also do not suggest going with the sizes that were recommended to you.

crmb

I think the biggest benefit of a true quad system is you could actually reduce the noise levels using a pair of mufflers below whats physically possible with just using a single muffler without generating a bunch of extra back pressure with an extremely restrictive single muffler to get it under control. My 545i is still a bit loud for me. I'm spending a lot of time making sure the M5 sound is right. I may go back and fix my 545i with a dual system here in the future.

RPIpower

The mufflers, perforations, perf diameter, gauge steel, tubing diameter and length all can keep the noise levels down. The cool thing about having your own shop is you can play around with different designs for your own car. As a musician, I find that tuning the exhaust sound isnt that much different than on an instrument. It all comes down to how true do you want the results and what your time is worth.

socale39

You state that "backpressure" is merely the lack of velocity yet this lack of velocity plays a large role in developing performance gains at different ranges of the vehicle's powerband.

The whole point of this thread is to help people understand that just slapping together 3" piping and a muffler with dual outlets and dual tips may not be the best performing exhaust system. With people getting together testing their exhaust setups we will be able to formulate a general solution towards what components to use and what setups to avoid. Sure the testing isn't going to be as extensive, but we are trying to maximize power here, just prevent a lose.

This isn't rocket science here. I'm not trying to make claims that these setups will perform like RPI's but rather eliminate the worry that if you want a quad setup, you lose power. Quite honestly if I retain stock power on my quad setup, I'm happy cause the dual exhaust just looks so much better. Additionally, there are plenty ways in which you can have a quad exhaust setup without modifying the car.

One last thing, if cost was such an issue, why is it that you have a $2400 exhaust for the 535i which according to your posts, you hardly make a profit?

RPIpower

So are you getting this info from google, yahoo, etc? or are you speeking from experience?

If you paid attention, you would know that the 535i exhaust doesnt sell for $2400? Hence, why we have discontinued the production of this system.

We should have a new post for people to post their poffessions. Afterall, this is the new America and capitalism is going down the drain. lol

west

My thoughts as well. Heck, if I lose a few horsepower, that's OK too. Hopefully it's just not on the order of 20 or more hp.

RPIpower

If you want to do it right, come out with an idea of how you want the exhaust to fit. Make it, then use a pressure gauge, egt gauge or specific temp coating. Dyno the car and take measurements of where the temps change along the exhaust system. Measure and note the strap down tension of the car, strapping points, tire pressure of the tires before/after dyno, car position on the dyno and atmospheric conditions. Do it the right way and you cant go wrong. Part of doing this is the adventure in building this system and working to get a system that works for you.

west

Can't argue with that. Depending on the results I get back today, it's my hope that somebody on the forum will be able to leverage the data to carry out further experiments.

socale39

Actually speaking from experience. Specifically Yoshimura R&D of America where we developed exhaust systems for the Yoshimura Suzuki Team that raced in AMA Superbike, Superstock and stock applications. You can check them out online or maybe "google" the race proven results we helped contribute to. Not working there anymore and since you seem to be interested in my profession, I got into marketing working for a large Mortgage Wholesaler and Retailer.

Bimmer32

We discussed this in length several years ago on this board. If you are looking for analytical and concrete information, you will not find it. It is about economy of scale, feasibility, cost/benefit, or whatever other terms you want to call it. BMW engineered their engine to performance at maximum output throughout the rpm range, and they decide that x amount of cfm at x rpm will produce x decibels and x HP/torque. You can rest assure that these same guys/gals didn't tune the exhaust for Camry performance. Since exhaust is only half of the power generation equation, the ECU adjusts other components to maintain peak performance as well (like fuel-air/mixture).

We go back and forth about "back pressure" and high flow. We all know that racing gurus want high flow because they don't hang around the low rpm. That's pure and simple physics. So high flow works well for racing applications. It's loud, it drones, and it guarantees little or no flow restriction on the engine. Naturally, straight pipes or high flow aftermarket system (custom or not) are going to be loud, free flowing, and produce higher performance output assuming the computer (ECU) does not tweak the frontside and the engine to accommodate for environmental/safety regulations. This as been proven over and over in racing circles. It is also physics 101.


With these cars, you drive at low speed and high speed. The goal to produce or replicate the stock exhaust performance into a different configuration (x-pipe, different pipe size, different mufflers, different routing of gas flow) would inevitably cost a lot for a single individual, but the gain in performance output at all range of the RPM is minimum due to other factors mentioned above. For some individuals, it is about cost/hp gain and the look.

Does anyone measure CFM rather than customizing something and dynoing it? For exhaust flow, it is about CFM. You will find that the more CFM an exhaust can produce, the greater the change of higher performance at high RPM. So what about low RPM, and low gas pressurization (what most of us loosely call "back pressure)? Stop and go traffic requires such (not in racing). Most aftermarket exhaust are louder than stock. Sound is relative to air flow of the same source. You can argue about noise canceling tubing and mufflers, but in the end, per decibel is relative to per cubic feet of air flow in automotive application (HVAC is different). The reason is that air flow changes over the RPM range with very limited room to design every effective noise cancel designs.

We do not know if the assumption that BMW engines require higher pressurization at low rpm is true, but we do know that no matter what you do, the performance gain is either at the high rpm range, none at all, or possibly a performance loss at the low rpm. Nonetheless, the gain/loss is insignificant. Many aftermarket people will tell you that -+10 hp gain is nothing for a 325 HP engine when it comes to street racing at low or high speed. There are many other factors such as gearing, driver reaction time, tires, weight, etc. that +-3% is not going to do anything except intimidate the inexperience opponent with the loud exhaust note and/or boost the drivers ego (confidence) level when he accelerates.

In the end, it is about looks and sound. PERIOD. You don't need exact numbers to prove or disprove anything. Don't listen to marketing hypes about performance gain with just an exhaust upgrade. Torque loss at low rpms with a high flow exhaust? Possible, but it could be that the driver sense a loud sounder but not a quicker picker upper at low rpms. If you do experience performance loss at low rpm, it is also possible that your engine does require higher pressurization at low rpms.

Use your deductive reasoning, save your money on test, and put it toward your exhaust mod. I have stock two cats, two stock resonators, two Jones 9" long resonators, a magnaflow x-pipe, stock box resonator, to a left and right side Jones 12" x 4" turbo mufflers, out to quad 3.5" staggered and slanted tips. All with 2.25" piping. The sound is louder than stock, but it does not drone in the cabin. The sound is louder after a hard drive or when the exhaust gas is hotter from a hard drive. Wife and children can sleep in a long trip, or converse in a slow traffic with no problem. The "loud" point is around 1700-1800 rpm, which with my very sensitive ears, would rate it as close to borderline drone. The note at 3000 rpm and higher is notably louder than stock as well.