Corsa Performance Exhaust Mufflers - Tech Corner

Question: I would like to know what the difference between "straight-through," "chambered," and "conventional" mufflers is.
Larry
Via the Internet

Answer: Great question. Conventional mufflers (above right) use a combination of bends, deflection walls, and packing materials to direct and absorb exhaust flow within the muffler body. This is the most restrictive type of muffler, but it offers the greatest reduction in exhaust sound.

Chambered mufflers utilize separated walls, set at angles, to create "chambers" that reflect sound waves and exhaust flow. While some chambered mufflers are less restrictive than their conventional counterparts, they also tend to be louder.

For maximum performance, straight-through mufflers work best. They don't restrict much exhaust flow and therefore produce little or no power-robbing backpressure. The downside is that they typically offer very little in the way of sound control. Most straight-through mufflers use packing material around a perforated tube in an attempt to muffle the sound. This design tends to trap high-frequency tones while intensifying other ones.

One company, Corsa Performance Exhausts, uses a patented sound-canceling technology (called "Reflective Sound Cancellation," or RSC) in its straight-through mufflers (below right) to eliminate drone without creating any additional backpressure. This design helps Corsa mufflers provide an aggressive exhaust note under heavy acceleration while keeping noise to a minimum during normal driving.

Question: I saw your recent article about the C5 column-lock recall ("Going in Circles," Aug. '08). When I contacted my dealer's service department, however, they knew nothing about it. My Corvette had been working fine until last week, when the column lock failed. Now it doesn't lock at all. Should I try to get GM to do the recall, or just get a column-lock simulator and do the job myself?
Bob
Via the Internet

Answer:I feel your frustration, but I assure you that the recall-specifically, Recall Bulletin No. 04006C-does exist. (You can even view it yourself, by visiting www.wscc.ws/Clo%20Lock%20Recall.pdf.) If you can get your dealer to perform the work, I would recommend doing so.

As we mentioned in the article, the Corvette models affected by the 04006C electronic-column-lock recall include the following:
* 1997-2004 Corvette equipped with a manual transmission
* 1997-2000 Corvette equipped with an automatic transmission
* 2001-2004 Corvette equipped with an automatic transmission (European export only)

Question: I'm looking for the perfect gift for my spouse, who loves driving and racing his Corvette. What would you recommend?
Samantha
Via the Internet

Answer: How about satisfying both loves at once? The Bondurant High Performance Driving School (www.bondurant.com) in Phoenix, Arizona, offers a variety of performance-oriented driving courses, including a four-day Grand Prix Road Racing program that combines race-prepared C6 Corvettes with the open-wheel excitement of Formula racing cars.

All courses take place on Bondurant's purpose-built facility, which utilizes more than 200 GM vehicles and boasts a 1.6-mile road-race course. The school also has a skills pad, where students can practice an accident-avoidance simulator, brake-and-turn exercises, a slalom course, and skid-car training. Even if you never plan to go racing, a class at Bondurant will make you a better, safer driver.

Question: I was sitting around one of the local automotive shops last week, doing some bench racing, when I heard someone mention BMEP. When I asked what the term meant, he gave me a long-winded answer that left me more confused than ever. So what is BMEP, and why not just use "torque" or "horsepower" instead?
Fast Eddie
Via the Internet

Answer: The acronym BMEP is an engineering term that means "Brake Mean Effective Pressure." "Mean" is another word for "average," which in this case means the average effective pressure of all engine-stroke cycles. BMEP is a function of the temperature of the gases in the cylinder. To increase the temperature, you need to either burn more fuel or increase the volumetric efficiency of the engine.

Facts: Torque is a function of BMEP and displacement, while horsepower is a function of torque and rpm.

Getting back to basics, remember that a typical automotive engine converts pressure into torque. The more pressure, the more torque. It's that simple. The hard part has always been trying to make more pressure.

Take a look at the chart and table on this page, which were taken from a training session at Katech. The chart shows the relationship between the piston speeds (PS) of various engines and the BMEP readings those engines generate. Note that the average pressure is highest at peak power and falls off from that point on. The actual BMEP numbers vary between engines, but the overall curve is very predictable.

Two things immediately jump out. First, all of the engines-with the exception of the LS7-are at approximately 200 psi BMEP at peak power. Second, the piston speed is approximately 4,500 fpm (feet per minute) on average. This is roughly the maximum speed a durable street piston can attain, given current technology. This isn't surprising, considering that all of these powerplants were based on solid engineering data.

All of the above engines were designed for maximum power and basically unrestricted in design, with high compression ratios, low friction, and unrestricted inlet and exhaust systems. The exceptions are the Winston Cup V-8, which was required to use a single four-barrel carburetor, and the LS7, which is installed in a mass-produced street car.

Listed below are a few equations you can use for quick power analysis. A few items will be needed for the calculations. They are cid (cubic-inch displacement), the stroke of the engine, and claimed power at a particular rpm. From these numbers you can quickly tell the following:

Piston speed: PS = Stroke x rpm/6 (Anything much higher than 4,500 fpm is suspect.)

Torque required for hp: TQ = hp x 5,252/rpm

BMEP from TQ: BMEP = TQ x 150.8/cid (Anything much higher than 200 psi is suspect.)

BMEP from hp: BMEP = (hp x 5,252/rpm) x 150.8/cid

So the next time someone tells you how much power he made, get out your calculator and keep him honest.

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