A transmission is a device that transmits
power from an engine to the drive wheels of a vehicle in a
usable form. Years back two- and three-speed transmissions were
common, fuel was 20 cents a gallon and the corporate average
fuel economy was somewhere around 8 miles per gallon. As fuel
prices increased and emissions regulations were enacted,
transmissions went to four, five, six and seven speeds –
the idea being to keep the engine operating within its power
band and to have the “steps” between gears to be as
narrow as possible for the most seamless and efficient
operation.
Besides writing articles for Transmission
Digest and being heavily involved with the remanufacture and
distribution of manual transmissions and transfer cases, I have
for most of my life been professionally involved in racing.
This year I am driving a Pontiac GTO in the Grand American Cup
race series for the Spirit of Daytona Race Team. Grand American
is the road-racing arm of NASCAR. The Grand Am Cup series is an
endurance series with cars based on
“showroom-stock” configuration. You could not buy
one of these cars at a dealer, but the rules require using as
many stock components as possible. Spirit of Daytona will sell
one of these race-prepared cars for about $185,000.
The race team is owned by Troy and Todd
Fliss and is based in Holly Hills, Fla., a village that is
actually in Daytona. Troy was my crew chief when I raced the C5
Corvettes for three years and when we raced the Cadillac CTS-V
in this same series. When he asked me to drive for this team I
immediately agreed, because they are a top-flight professional
team capable of winning races and championships. Our collective
relationships with GM have brought us to this year’s
Pontiac GTO, and our first race was in Daytona at the beginning
of the year.
What is little understood by the public is
how much work goes into developing a new race car. The platform
of the stock GTO is modified to the maximum the rules allow.
The car will compete at 2,800 pounds and has 260 linear feet of
roll cage in it. A fuel cell, a modified LS6 engine equipped
with Crane cams and producing 420 horsepower at the rear
wheels, adjustable gas shocks, racing brakes, an Exedy racing
clutch and a Tremec T56 six-speed transmission all work to make
this car capable of 160-180 mph.
As I write this article, we’re
preparing for our next race, April 22 at Virginia International
Raceway, Alton, Va. This road course is 3.27 miles long with 18
turns and a variety of elevation changes. As we are still
developing the Pontiac GTO to make it as fast and durable as
possible, I thought it might be interesting for you to view one
piece of the puzzle, which is gear-ratio development for the
Tremec T56 transmission. The rules state that we may use only
transmissions with stock ratios or that are produced by the
manufacturer and available to everyone for purchase. The Tremec
T56 is a six-speed manual transmission that is used in the
Camaro, Firebird, C5 and C6 Corvettes, Dodge Viper, Ford
Mustang, Cadillac CTS-V, Pontiac GTO and Aston Martin. The two
available stock-ratio configurations for this transmission are
listed here:
2.66-1 1st gear
1.78-1 2nd gear
1.30-1 3rd gear
1-1 4th gear
0.74-1 5th gear
0.50-1 6th gear
2.97-1 1st gear
2.07-1 2nd gear
1.43-1 3rd gear
1-1 4th gear
0.80-1 5th gear
0.62-1 6th gear
When we were developing the road-race
version of the C5 Corvette, GM asked Tremec to build a specific
close-ratio transmission for road racing. That unit had the
following ratios:
2.29-1 1st gear
1.61-1 2nd gear
1.22-1 3rd gear
1-1 4th gear
0.85-1 5th gear
0.75-1 6th gear
These are the three ratio configurations
that the rules allow us to use in the Spirit of Daytona GTO. As
you can see there is a pretty large difference between the
steps of the gears. What this translates into is how much
engine speed you will lose during a shift and how long it takes
to get the engine back to the power band after the shift is
completed.
Remember that every engine has both torque
and horsepower curves. The ultimate performance is achieved
when we can keep the engine making maximum power and maximum
torque for the longest period of time. These curves are not
parallel.
This LS6 engine is capable of 7,000 rpm,
with the rev limiter set to 6,500 rpm. The engine will produce
maximum torque in the range of 5,000-6,000 rpm, and we probably
will use a 6,200-rpm shift point for maximum fuel economy. When
you consider that we will make about 12 shifts per lap, it is
easy to see how much time we can lose on each lap if we
don’t use the optimum ratios for this track.
To do this with a minimum of effort we will
chart the ratio performance on a laptop computer. The program
we will use is called Quick Shift and is available online from
www.ed-it.com at very reasonable prices. This is definitely a
program that will save you a lot of R&R labor and help you
to see in graph form the results of any changes you might make
on a customer’s vehicle.
For instance, if a customer asks you to
change the differential ratio on his pickup truck to help him
save on fuel, you could quickly graph what he has now and what
the results would be with a new ratio. The information you need
is the transmission gear ratios, the differential ratio, the
engine speed used and the tire diameter on the vehicle. What we
will do now is to map out the differences in overall
performance using the three available ratio configurations for
the GTO. We have a 3.46-1 differential ratio, a 25-inch tire
diameter, redline of 7,000 rpm and a rev limiter set at 6,500
rpm. See the accompanying graphs for each of the three ratio
configurations we can use.
Looking at the results of our graphing
program, we are now able to review which will work to our
advantage. The first-gear
numbers are not relevant, as first gear
will be used only for leaving the pits; there are no first-gear
turns on this track. Sixth gear is also useless, as we do not
have enough horsepower to use it. This race will be driven in
four gears: second through fifth.
The unit with the 2.66-1 first gear has the
lowest performance loss (4.4%). In looking at the graphs we see
the road speed at each shift point, the engine-speed drop at
each shift point, the engine speed achieved after each shift is
complete, and the percentage of time the engine spends in the
power band in each gear.
One other option is to change the
differential ratio, and we may have to recalculate all the data
if that makes an improvement. The beauty of a computer program
like Quick Shift is that you can see your options with a few
clicks and five minutes of your time.
Now this information will go to the
engineer in charge of engine management, who will make fuel
calculations to further narrow which gearbox will be best for
performance and pit-stop strategies. As this is an endurance
series with the shortest race being three hours, the rules
mandate two drivers in the race. Depending on how the cautions
happen, we have to make one stop under the rules. We can change
drivers, replace four tires and fuel the car in 46 seconds.
This does not account for the time it takes to get into and
leave the pits with a 45-mph pit-lane speed limit. A fuel load
is good for about an hour and 20 minutes under green-flag
conditions. You never want to stop under green, as it is too
easy to go down a lap; this race will be a two-stop event if
all other things work correctly.
Fuel prices are high, and the economy is
under a lot of pressure. There are few conditions, good or bad,
that do not provide opportunities for making more money. With a
simple computer program and some innovative thinking you can
come up with a variety of options to offer your customers to
increase their performance or fuel economy. Every cloud has a
silver lining. 
©2006 Transmission Digest