Testing the Mac Prochamber Exhaust Crossover for Late Model Mustangs

Mac ProChamber (left) and Mac off-road H-pipe right. Both in 2.5" pipe diameter.
When Ford first designed the fuel injected 5 liter Mustang, the exhaust system was a study in compromises. With the need to meet federal and California emissions standards as well as federal drive-by noise standards, the stock exhaust does not qualify as a high flow system. At least it was a true dual exhaust with two, 2 ¼ inch pipes, two mufflers (unlike the Camaro/Firebird twins) a cross-over pipe and tubular, stainless steel headers. The sound was vintage V8, albeit a rather subdued one.

Round One - Off Road "H pipe"
In our last installment Green Machine II had solidly hit the 12's at over 106mph. This was thru the stock h-pipe and mufflers. The need for speed is never satisfied and the next mod would be to lose the "POS" stock h-pipe, a restrictive unit due to the four catalytic converters. We procured an offroad h-pipe (no cats) in 2 ½" diameter. The piece manufactured by MAC products comes in two pieces, with a slip fit at the crossover. This is a good idea as it took some time to get the exhaust system to line up and seal properly. By setting each side in place individually and gently bending the hangers we were able to achieve a good fit. The 5.0 sounded a little louder and deeper, still exhaling thru the Ford mufflers. Since the cat-back system was still stock we debated on the possible gains.

The previous week we had run high 12's and had a best mph of 106.7. Less than one week later at a Wednesday night bracket race, in almost identical weather conditions, we tested the off road H pipe with the stock mufflers. The resulting 12.66 at 107.73 confimed the generally accepted thougts that the factory four cat H-pipe is a restrictive piece.

Stock cat-pipe, stock mufflers: 12.81 106.7 1.82
Mac H pipe, stock mufflers: 12.66 107.73 1.78
Net Gain: -1.5 tenths, 1 mph.

Mac FlowPath 2.5" mufflers.

A better crossover?
The next step in our exhaust system testing was to obtain a true cat-back muffler system in 2 1/2 " pipe. We contacted Mac for their FlowPath system, and they also recommend we replace the "old techology" Mac h-pipe with their latest and greatest inventon, the Prochamber crossover unit.

Unlike "X" type crossovers, Mac claims the pipes merging into a small rectangular chamber not only helps exhaust pulses scavenge the opposite bank (the pulses are 180 degrees out of phase in a 4 cycle V8) but the gases in the chamber become hotter as the pulses merge, increasing flow. Sounds plausible, but is it worth anything? We told them we'd be happy to test one out along with their Mac 2.5" FlowPath muffler and tail-pipe kit.

Just like Ford, MAC uses ball and socket flanges for easy alignment and a leak-free fit. The flanges come with studs for one side to insure good clamping force and no stripped bolt holes. Even the nuts have directional teeth on the undersides to prevent them from loosening after many heat cycles. The Prochamber comes in two pieces and the chrome tips are separate with ball and socket flanges so you can install the pipes and align the flanges in your driveway and have a leak free system. We know because that was how we installed it.

The new exhaust sounds much healthier. It is louder than stock and definitely deeper in tone that becomes a satisfying roar at full throttle. We did notice a strong drone at 1800 to 2100 rpm. Since Green Machine II has 3.73 gears we are rarely in this rpm range during cruise.

Round Two - Prochamber and FlowPath Mufflers
After bolting on the Prochamber and FlowPath cat back system, we grabbed the sticky tires and headed back to the track.

The first few runs resulted in bogs at the start as the track was well prepped and the tires were biting hard. We raised the tire pressure from 14 to 15psi and increased the launch rpm to just over 5000. The following table shows the gains made for the best three runs. Needless to say, we were pleased with the results. The Prochamber and FlowPath combo netted 109.5 mph, beating the open H-pipe combo by almost a mile per hour.

Mac H pipe, stock mufflers:
12.66 107.73 1.78
ProChamber, FlowPath mufflers:
12.51 109.45 1.78
Net Gain: -1.5 tenths, 1.7mph

The factory exhaust system was good for 12.80's and over 106 mph. However it's restrictions began showing as we added better breathing capability to the 5.0L motor. With the addition of a 2.5" Prochamber and FlowPath cat back exhaust we were able to cut the ET down three tenths and gain as three mile per hour!

After running this fast we had to know what kind of power GM II is now putting to the ground. The car weighs 3200lbs in race trim and the driver is right around 185lbs. Using several different horsepower calculators on the Internet, including our own, the results are astounding. Using a conservative 3350lbs, Green Machine II is producing approximately 350HP at the flywheel! Using 15% as your drivetrain loss, that puts the rear wheel output at 297.5, just shy of 300rwhp. Proof positive of Ford's long runner intake design. We hope to get on a chassis dyno in the next couple of months to confirm this number and see just how big and flat the torque curve is.

We have a goal to reach 110mph on the stock cam and injectors. Installation of an electric fan should free up enough power to reach it. Once that milestone is reached we will look into a cam swap to push the GMII project deeper into the 12's! F/M

Mac Performance
MAC Products, Inc.
43214 Blackdeer Loop
Temecula, Ca. 92590-3473


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