Un-plugged and Flying!
Having gone the way of overdrive pulleys, cracked heads
and blown head gaskets the assertion by Bill Hull that the stock SC exhaust
system causes cracked heads and blown head gaskets caught my attention.
Tending not to believe everything I hear I decided to do the research required
to understand the problem and let physics provide the facts. So, armed
with a great deal of motivation (my cracked head episode set me back two
grand) I set out to determine if the SC's exhaust system is really as bad
as Bill claims.
The test platform was my modified 91 SC which sports ported
and chambered heads, high lift cam, overdrive pulley, ported stock exhaust
manifolds, large fuel injectors and a few other goodies. I figure the car
is a good test platform to determine if a better exhaust system is useful
for Super Coupes on their way to making big time horsepower.
I started out by tapping into the stock exhaust system
at several points and measured the back pressure at wide-open-throttle
(WOT) over a RPM range from 3000 RPM to 5000 RPM. The results are a real
eye opener, see the plot below. I had a hard time believing what I saw,
15.5 PSI back pressure at 5000 RPM's!! The pressure readings were so off-the-wall
I double checked the readings with another gage, same results.
1.) During the overlap period (the time when the piston
is at or near top-dead-center and both the intake and exhaust valves are
open) there is more pressure at the exhaust valve then at the intake valve.
Which way will air move? Hmmm, exhaust gases are flowing into the intake
manifold! Race engines with long tube headers create a vacuum at the exhaust
valve during this period to purge the combustion chamber of exhaust gases.
Exhaust pressures so high that exhaust gas wants to flow into the intake
manifold is obscured.
2.) After the exhaust stroke the combustion chamber is
still charged with hot exhaust gases at 15 PSI. Assuming exhaust gas temperatures
are 1500 degrees and accounting for the cylinder to combustion chamber
volume ratio the intake air temperature is raised by over 1-50 degrees!
The tendency of a fuel/air mixture to detonate has been studied for decades,
the primary factors are:
There are other factors such as combustion chamber shape, flame pattern,
etc. however, temperature is one of the significant factors. In fact, some
of the Buick Grand National racers run 160 degree thermostats to reduce
engine temperatures 30 degrees in an attempt to reduce detonation while
running high boost pressures.
- A.) Pressure (compression ratio)
- B.) Temperature of the mixture
- C.) Time (the duration the gas mixture is held at the first two conditions)
- D.) Fuel chemistry (octane rating)
3.) When the piston is moving up on the exhaust stroke it
is doing work (pumping air) against high pressure. To move enough air to
produce 350 HP (about 600 CFM) at 15 PSI requires around 30 horsepower.
I can think of better uses for this energy, like turning wheels.
I'm convinced it was designed that way to make the Super
Coupe just a tad bit slower then Ford's dear little pony car.
Considering the above I set out to build the lowest back
pressure exhaust system possible. I calculated that two 2 1/2 exhaust pipes
would have about 1/2 PSI back pressure, but upon inspection of the SC under
carriage I could see it was next to impossible to route two 2 1/2 inch
pipe through the channel provided for the exhaust pipe. After pondering
the situation I soon realized that one 3 1/2 inch pipe has the same cross
sectional area as two 2 1/2 inch pipes and would fit the routing channel
nicely. It would also have the same back pressure. Well, several weekends
and several hundred dollars later I had built a 3 1/2 inch system from
mandrel bends that fit like a glove and even used that original mounting
brackets. The design is the same as the original except the pipe sizes
were increased. The 2 inch pipes were replace with 2 1/2 inch and the 2
1/2 inch pipe and front muffler were replace with 3 1/2 inch. The system
was aluminized and fitted with pressure measurement fittings at the same
points as the stock system. The same series of pressure measurements were
ran on this system, the graph below tells the story.
The back pressure at the manifold (Before Cat) was reduced
from 15.5 PSI to 6.3 PSI, a big improvement. Notice there is only .6 PSI
drop between the rear muffler (Before Rear Muff.) and the catalytic converter
(After Cat). Things are getting better, but the system still has 6.3 PSI
The next two areas to improve are catalytic converters
and mufflers. The mufflers I chose were inexpensive or I should say cheap
Thrush California Boss Turbo Mufflers with 2 1/2 inch inlet and exit, about
$20.00 each. These mufflers didn't improve the back pressure at all, they
had the same pressure drop as the original rear mufflers, about 2 1/4 PSI.
They were so loud that I could hardly hear the stereo over the roar. I
shopped around a bit but couldn't convince myself the mufflers I looked
at were both low back pressure and low noise, so I decided to design and
build my own. The experimental mufflers I designed are nearly as quiet
as the stock system and have a pressure drop of only .6 PSI. So I proved,
if only to myself, it is possible to have both low noise and low back pressure
at the same time. It would be interesting if someone made a side-by-side
comparison of available mufflers to determine which ones really work on
blower birds. In any case the back pressure is continuing to drop as can
be see in the chart and the power levels are going up a bunch.
My SC with the original exhaust system accelerated from 20
MPH to 55 MPH in 4.17 seconds. With the 3 1/2 inch exhaust system this
time was reduced to 3.80 seconds, an improvement you can really feel. I
made several 0 to 60 MPH attempts but wheel spin hampered performance,
the 245/50 tires don't even come close to holding the power this motor
is capable of producing. The best 0 to 60 time was 5.7 seconds, part throttle
through most of first gear then nailing it ,after 40 MPH. On the exhaust
side I still have high flow cats and headers to install. I am sure both
will improve performance since the CAT back pressure is 3.5 PSI (I think
this can be reduce to 1/2 PSI) and the exhaust manifold's ability to move
air with its small 1 3/4 inch diameter is poor. It is interesting to note
that the blower (even with an overdrive pulley) could only build 12 PSI
boost after the heads were ported and now with the high flow exhaust the
manifold pressure has dropped to 10 PSI. I think it is time for a modified
blower cap, perhaps a S blower, larger throttle body, high flow intercooler
and a pay raise. All kidding aside, the SC responds very well to an improved
exhaust system and makes other modifications possible. Just think how high
the back pressure must be if you have a S Blower, larger throttle body,
etc. and a stock exhaust system.
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