Testing the Mac Prochamber
Exhaust Crossover for Late Model Mustangs
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.
Mac ProChamber (left) and Mac
off-road H-pipe right. Both in 2.5" pipe diameter.
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
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
Stock cat-pipe, stock mufflers: 12.81
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
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.
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
MAC Products, Inc.
43214 Blackdeer Loop
Temecula, Ca. 92590-3473