I'll get right to the point--recently I was able to buy a used C5 coupe to be my daily driver. I actually found a three-year-old C5 that was absolutely stock. The wheels were standard silver finish, it still had an AC Delco paper element air filter, and it didn't even have custom tips on its original equipment exhaust system. That's right, the funky little black tips were still there, gracing the factory's meek and mild sounding pipes and mufflers.

A few people may like the sound of a stock C5 exhaust. I haven't met one yet, but that doesn't mean they're not out there. Personally, I think non-Z06 C5s should sound a little more muscular; Z06s don't need a lot of help in that--or any other--department. I wasn't real excited about my C5 sounding almost as sedate as the late-model Cadillac De Ville owned and occasionally driven by the old folks down the street.

On the other hand, I didn't want my everyday ride to be too loud, or to suffer the dreaded interior resonance syndrome that's so commonplace on some cars with modified or custom exhausts. What to do...

I wanted to do something different; something we haven't done previously. That meant not just the de rigueur cat-backs. Almost every company involved in the C5 exhaust aftermarket is building systems from stainless steel Continued on Page 52 couple are now going way-high-bucks with titanium, a la the Z06. Titanium is excessive for my needs and for this car. After all, it's an automatic, it's a coupe, and it's a commuter--the 12-18 pound weight savings doesn't matter on this sort of car. All I wanted was something that looks good, sounds muscular, and might produce a few more ponies.

Anything forward of the catalytic converters is considered emissions related by the EPA types. I don't want headers on this car and I don't want the hassles when it comes time for smog inspections. That leaves trying an "X" type crossover pipe, which replaces the behind-the-cats "H" style factory crossover, in addition to some sort of cat-back system.

I know of three manufacturers of Corvette exhaust systems that also offer X crossover pipes for C5s; there may be others. All three of these companies make very high-quality products. The systems from all three are stainless steel and one also offers at least one system in titanium. One company is in the Midwest, and we've done C5 exhaust articles with them. The other two are located within a 50-mile radius of our headquarters here in Southern California. I made my decision based on convenience. One of them--MagnaFlow--has an in-house tech center with several installation bays and a Dynojet chassis dynamometer for testing purposes, and the other does not.

I wanted to find out exactly what difference, if any, an X crossover would make. Thus our agenda was to take baseline noise level readings on the C5 then establish "stock" horsepower and torque output on MagnaFlow's Dynojet. Then we'd install the Muffler Assembly (PN 15713), test drive the car and take noise level readings, followed by more pulls on the dyno. Finally, the Tech Center crew would install the X pipe (PN 15437) followed by another drive test, more noise measurements, and final dyno tests. The X pipe and Muffler Assembly are available as a set under PN 15660.

Crossover pipes on dual exhaust systems are supposed to balance and smooth out exhaust pulses between the two sides of the system. Theoretically, the crossover should also give a slight improvement on exhaust gas scavenging, which should offer some modest gains in power output. On C5s, the intermediate pipes (forward of the rear axle and aft of the catalytic converters) have a crossover, a straight section of pipe running perpendicular between and connected to the intermediate pipes. Many people refer to this as an H pipe or an H crossover.

An H section is inexpensive to mass produce. It's better than no crossover or balance tube whatsoever, but it is fairly inefficient. Exhaust gases want to follow the path of least resistance--that's why smooth mandrel bends produce better power than crimped and uneven (i.e. the corner muffler shop) bends on exhaust systems. Unless forced, exhaust gases are not going to make the hard 90-degree turns you see on an H pipe. An H pipe will offer some benefit, but not a lot.

An X section, with gentle beds and an open center chamber, offers substantially better balancing or equalizing of the exhaust gas pulses than are possible with an H crossover. Ergo, an X crossover should, with no other changes, offer enhanced power production. It should also eliminate the burbling and popping on deceleration that's not especially uncommon on aftermarket C5 cat-backs. We'll see.

Mother Nature put a crimp in our plans with a multi-day downpour that straddled our scheduled installation. The inclement weather precluded our intended exhaust noise measurements (wet roads would severely skew the interior sound levels) and also meant that we'd have to cut way back on the "vigorous" driving. We weren't going to let a little bit of liquid sunshine interfere with things since MagnaFlow's Tech Center and dyno are well protected from the elements. And we had a semi-adequate fallback plan for the sound level measurements.

One year ago we ran a series of tests on fellow Primedia staffer Randy Fish's C5 coupe, which is essentially a twin, albeit one model year older, to my "new" Corvette--a base interior and automatic with 3.15:1 gears coupe. Both cars are even shod with OEM run-flats. While no two cars are ever exactly the same, this pair ought to be close enough for comparative purposes. The after part of the sound measurements would be done at our offices (same place we'd tested Randy's car last year) and the only thing we'd be unable to measure would be what effect the X pipe would have on the noise levels from the MagnaFlow cat-backs. I gotta admit that I was a little apprehensive about how loud it might be, thanks to their naming the system "Wide Open" and touting the mufflers as having no baffles, no chambers, no louvers, and no restrictions.

I'm not going to rehash a C5 cat-back installation. The MagnaFlow system fit very well and is a gorgeous piece of work, with mirror-polished mufflers, outlets and quad tips. The guys in the Tech Center offered a suggestion that's simple and good to remember. Unbolt the rear stabilizer bar from the differential carrier and swing it down and out of the way before trying to remove the old pipes and mufflers; also temporarily remove the passenger side gas tank shield. The old system will come out a lot easier and your new setup will slip into place easier, too. Of course, be sure you securely reattach the stabilizer bar and reinstall the shield once the new pipes and mufflers are in place.

To install the X pipe, the original intermediate pipe must be unbolted at the rear, from the cat-backs, and cut off up front, several inches to the rear of the "cats," eliminating the original H crossover. The X pipe slips over the ends of the intermediate pipe and is clamped in place (stainless steel clamps are provided) and bolts up aft to the cat-backs, in the factory location. No welding whatsoever is required to install either the cat-backs or the X pipe.

The finished installation looks and fits great. The four large, round, polished stainless tips really brighten and beef up the back end of a C5. The small billet aluminum nameplate that connects the right and left halves of the system is a nice touch.

I was quite surprised by our test results, both the "numbers" off the Dynojet and in our noise level tests, using my hand-held decibel meter (Radio Shack, PN 33-2055). Baseline horsepower and torque output was about what I expected from a totally stock C5 with an automatic trans. The gains from the MagnaFlow muffler assembly were substantially higher than I expected, and both the horsepower and torque gains were across what portions of the powerband we could measure. The subjective (measureable) results from the X pipe installation fell short of my expectations. (See charts for comparative results.) Objectively, the improvement in throttle response is both noticeable and there at all times. The response is both crisp and more immediate, to part and full throttle inputs.

The Wide Open system proved to make less noise, at least as measured in decibels, than did the stock setup in four of our five tests. (Again, see the charts.) Objective evaluations: The exhaust note, both inside and out, is smoother with the X in place, and the car actually feels like it's running slightly smoother. I think it may also be minutely quieter inside with the X. There's little or no resonance. The exhaust note is deep and, for lack of a better way to explain it, authoritative. It doesn't make a lot of noise, although the volume definitely picks up at wide-open throttle--if you want a deafening roar, this ain't the one. However, the sound that comes out the pipes is very business-like, it has presence. You know just by the sound that it's a car you don't mess with.

Overall, I'm as happy as could be with the new exhaust. I'm still surprised at how sizable a power gain the car got from the MagnaFlow system, and I'm still surprised at the large amount of that gain that came from the mufflers. And I think the sound, both tone and the volume, is great. After all, thunderously loud is cool for Sting Rays and Stingrays with side pipes, or for Harleys, but gets tiresome everyday.

DYNO TEST RESULTS

(rear wheel horsepower, approximate 20 percent less than at flywheel with automatic transmission due to drivetrain frictional losses)

BASELINE W/MUFFLERSMUFFLERS & "X" PIPE
RPM HP TORQUE (lb-ft.)HP TORQUE HPTORQUE
3,900 206.5 278.1 220.8 297.4 222.1 299.1
4,000 219.9 288.8 226.2 298.1 227.7 299.7
4,100 225.1 288.3 233.2 298.7 234.6 300.6
4,200 231.7 289.8 239.8 299.8 241.2 301.6
4,300 235.8 288.0 244.3 298.4 246.1 301.1
4,400242.3289.2249.5298.1250.9300.7
4,500246.5287.7255.6297.8257.6300.6
4,600246.9286.4260.5297.4263.0300.3
4,700248.3282.1265.6296.9267.6299.9
4,800249.9279.2270.9296.4274.1299.0
4,900251.6266.6276.0295.8278.2298.2
5,000258.4274.5278.4292.5283.6298.0
5,100264.2272.1277.7290.9285.7294.2
5,200269.3272.0288.0287.1290.6293.6
5,300271.4268.9289.7286.0290.7288.1
5,400273.5266.0290.1281.2291.6283.6
5,500274.9262.6291.6278.5293.9280.7
5,600275.6258.4292.6274.4294.0275.7
5,700277.8256.0293.4270.3294.4271.3
5,800279.0252.6293.7266.0295.6267.6
5,900279.8249.1292.8260.7295.1262.7
6,000278.6243.9290.9254.6293.4256.9

MAX HP
279.8@5,900 rpm

MAX TORQUE (lb-ft.)
289.8@4,200 rpm

293.7@5,800 rpm
(+13.9 over base)

299.8@4,200 rpm
(+10.0 over base)

295.6 @5,800 rpm
(+15.8 over base)

301.6@4,200 rpm
(+11.8 over base)

SOUND LEVEL STOCK MEASUREMENTS, IN DECIBELS(db) STOCK MAGNAFLOW WIDE OPEN W/X PIPE
IDLE, NEUTRAL, INSIDE 83 dB 76 dB
IDLE, NEUTRAL, 50 FEET BEHIND 70 dB 74 dB
3,500 RPM, NEUTRAL, 50 FEET BEHIND 75 dB 75 dB
30 MPH, SECOND GEAR, INSIDE CAR 101 dB 90 dB
30 MPH, SECOND GEAR, DRIVE BY 80 dB 78 dB

How And Why "X"-Pipes Work
By Bruce Wayne, MagnaFlow Performance

There has been an enormous amount of press about getting more out of performance exhaust systems. By incorporating "cross-over" or "X"- pipes in dual pipe systems, to balance out flow, many enthusiasts have found an extra edge. However, the perception is that this is new technology. Nothing could be further from the truth. But, why is an "X" pipe better than an "H"?

The reason an X-Pipe crossover works better than an H-pipe crossover isn't more flow because of volume, but more effective use of exhaust gas velocity. Exhaust gas (or air) has surface tension, and flows much like liquid would through the same pipe system. As the cylinders of each bank on a "V" style motor fire, they create a pulse in the system. The pulses will alternate back and forth from bank to bank as the motor runs. With multiple cylinders, such as a V-8, the eight cylinders alternately fire creates lots of pulses in the system.

If you put your hand behind the tailpipe, it would feel like a constant flow of air, but what it really is a lot of singular pulses giving the impression of constant flow. The idea behind the H and X style crossover is to unite the two banks of cylinders for better exhaust gas scavenging. Instead of two separate banks of four cylinders doing their own work, the crossover uses the pulse created by a firing cylinder of one bank to create a vacuum in the other bank because of surface tension. When a cylinder of the other bank is ready to fire, instead of the piston having to force the exhaust gas out of the cylinder, the vacuum that was created by the other cylinder bank helps suck the exhaust gas out of the cylinder, hence the term "scavenging."

Whenever you can reduce the load on an internal combustion engine, you are likely going to see performance and efficiency benefits. The difference between an H and X crossover is a smoother path for the exhaust gas to follow. Exhaust gas, like a liquid, will follow a path with the least amount of resistance. An H crossover has a path with two sharp 90-degree angles that are close together. An X crossover has a path with a much more gradual bend to allow the exhaust gases to continue their path back out to the ends of the tailpipes, rather then turning sideways for a short distance, then turn again to head out the tailpipes. Smooth flow with high velocity means more power by use of scavenging. Using an exhaust system with too large of piping diameter work against scavenging in the same way that running an exhaust with too small of tubing chokes the motor and forces it to work harder to expel the exhaust gases.

SOURCE
Magnaflow Performance