| Qualifying the Graph
Quarter mile figures are taken mostly from "Motor Trend" magazine.
I believe they are realistic.
We've intentionally left rear engine and front wheel drive cars off
the chart since rear engine cars, putting a lot of weight to the rear
wheels when they leave the starting line, tend to fall to the right
of the curve.
Also, tire size is not figured in, simply because I had no way to measure
the tire diameter of all the cars.
Qualifying the Equation
Vehicle weight is figured from the test weight of the vehicle without
the driver. If only curb weights are given, add in approximately 150
lbs.
Engine torque is torque at the flywheel.
Final drive is the rear end gear ratio (assuming 4th gear is a direct
drive 1:1 ratio). If 4th gear is other than 1:1, then multiply it times
the rear end ratio to figure the correct final drive in 4th gear. This
chart works well because, in accelerating through the 1/4 mile, the
gear spacing in a transmission is such that your engine will always
be in its power band. Once in the power band (point of maximum torque
to maximum hp) the torque to the axle will be fairly constant.
The BMW M3 versus the Ford Probe GT & the Pontiac Firebird
TransAm
Now, after taking a few moments to familiarize yourself with the graph,
let's look at some examples of cars shown on the graph. Three cars that
accelerate about the same are the BMW M3, the Ford Probe and the Pontiac
Firebird TransAm. According to the specifications, the M3 has the best
power to weight ratio (14.25) and the most HP per liter (83.5). But
since all these cars accelerate about the same, this method for performance
doesn't seem too accurate. Now, if we take the vehicle weight and divide
it by the amount of torque applied to the rear axles, we see that the
weight to axle torque ratio is almost identical on all three cars.
Captain Says
Early 325e's and 528e's came with a 2.79 rear gear and late models had
a 2.93 rear end. Many people don't realize that what makes a 325i quick
is that BMW changed the rear gear to a 3.73. This is a big jump from
earlier 3 and 5 series BMW's. With this gear ratio, high gear would
feel like something between a stock 3rd and 4th gear, giving the ETA
engine the sensation of a buzz bomb. Also, an ETA engine has about 10%
longer stroke, so piston speed would be increased by the same amount.
In our opinion, the best and most overlooked gear choice is the 3.25
used in late 5, 6, and 7 series Bimmers. This gear ratio offers a nice
compromise. Also, this differential should be much easier to find in
salvage yards. Salvage price ought to be about half the price of a used
325i rear end. For this swap, the rear aluminum cover and axle flanges
need to be changed. Use a large screwdriver or pry bar to pop out the
axle flanges.
Horsepower
- Introduction
To gain a clearer understanding of HP let's first look at the formula
for HP...
From this formula, we can see that any increase in torque or RPM will
increase HP...we can also see that HP and torque will equal each other
at 5252 RPMs. Let's try to give some meaning to the two variables
in this equation, torque and RPM.
- RPM
Two things dictate the RPM potential of an engine (less intake and
exhaust systems); the cam and the breathing ability of the head. Let's
start out by looking at the cam first and head later.
- Cams
Many people believe that adding a hot cam is the quickest way to get
more HP. In a way, this is true because a more radical cam increases
the valve timing and raises the upper RPM limit of the engine. From
the HP formula, it becomes quite obvious that an increase in RPM will
raise HP. This all sounds excessively simple and too good to be true.
Well, it is and let me explain. It is quite easy to increase HP 30-40%
by using a hot cam to raise the RPM limit of the engine. Now, many
people are under the illusion that a 30-40% HP gain means that they
will be pushed back in their seats 30-40% harder. The truth is that
there will be little to no gain in acceleration. This happens because
a cam does very little to increase torque and torque is the push you
feel when you accelerate. The only way to get high RPM HP to move
a car is to change the rear gear to something numerically higher.
Remember in Part 1 of this article where I explained that when it
comes accelerating a car, what counts is how much torque you can apply
to the rear axles? So, when you don't have a torque gain at the engine
but you do have an RPM increase, then you need to multiply torque
by changing the rear gear.
Let's get back to cams and more specifically their function. The cam
controls the opening and closing of the valves or "valve timing".
When selecting a cam, there are mainly three important functions to
look at:
- Duration - how long the valve stays open. (Measured in degrees
of crankshaft rotation.)
- Lift - how far the valve comes off the seat.
- Lobe Centers - how much the intake and exhaust valve overlap. (The
angle, measured in camshaft degrees, between the intake and exhaust
valve.)
The following diagram explains commonly used terms
in describing the cam.
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- Lobe Center Line (measured in cam degrees)
- Nose
- Duration (cam degrees x 2)
- Overlap
- Cam Lift (cam lift x roller arm ratio = valve lift)
- Ramp
- Base Circle
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In conclusion
When increasing the upper RPM limit of the engine, HP goes right up
even if very little is done to change the torque out-put of the engine.
By adding a hotter cam, we're increasing valve timing (cam duration)
and valve lift. This will keep increasing HP until the flow potential
of the intake track is hit. This all sounds great except that peak torque
occurs at approximately 70% - 75% of the upper RPM limit of the cam.
So, in adding the hotter cam, we're simply moving the point of peak
torque right up the RPM scale with peak HP. Simply put, there's a tradeoff
with a hotter cam since everything done to add power to the upper RPM
limit of the engine, subtracts power from the lower RPM range of the
engine. On the street, this often translates as a power loss in the
very RPM range you drive in most often. For example, most German Sport
cams that are about 300° duration (at .011"lift) will come
on the cam (peak torque) at roughly 4,800 - 5,000 RPMs and top out at
6,500 - 6,800 RPM (peak HP). Now, ask yourself, how often does one drive
up in this range? Think about your driving range before adding a hot
cam!
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