M3 at the drag strip

[Bruce Augenstein <Bruce.Augenstein@xxxxxxxxxxx>]

James, I'll open on a slightly down note, by mentioning that your message on
the digest seemed to me to be somewhat rude and unkind - pretty much the
behavior you accuse others of........

I expect you actually didn't intend that, but the "spare us the M3's aren't
drag racer's stuff....etc", along with the accusation of rudeness, made it
read that way.

That said (and no flame intended - just an observation), let's talk about
the quarter mile.

My '95 went 13.99 at 98.41, at about 99% air density, while factory stock,
except for Mobil 1 in the crankcase. Mobil 1 is not an actual modification,
of course, but it's been my experience that sythetics can lead to minor
power gains (i.e. - on the order of perhaps one to three hp). As an aside,
figure an M3 needs about eight hp to gain one mph in the quarter mile, from
which it could potentially gain about 14 hundredths of a second in elapsed
time.

I made no runs with the stock vehicle after that, since I had run a couple
of 14.0s before then, and wanted my "rightful" 13.9. Subsequent events have
shown me that the car probably had about one more full tenth in it while
stock, assuming 100% air density and an optimum launch. The 60 foot time on
the 13.99 run was a 2.03, and I was using 3500 rpm and feathered clutch at
launch.

With Dinan chip and airbox, the car went a best of 13.76 @ 99.68, again in
about 99% air, with a nearly perfect launch at 4500 rpm. The 60 foot time on
that pass was a 1.94, which was as good a time as I am capable of under
those conditions, I believe. 4500 rpm and slight clutch feathering was (and
is) the hot ticket for launching the car, assuming good traction.

The car has gone a best speed of 100.44 mph, at about 102% air density, but
a botched launch (meaning some wheelspin and a 2.08 60 foot time) inflated
the ET to a 13.86 on that pass.

I now have the Euro HFM kit from Turner Motorsports in the car, and it is
clearly quicker than it was, although I haven't run it at the track in this
condition. These days, I spend more time on road courses than at the drag
strip.

By the way, for information purposes, I'm defining "100% air density" as 60
degrees fahrenheit, with an observed 29.92" barometer, and dry air, which is
the old SAE "standard day" for horsepower measurements. Subtract 1% in power
for every 7.5 degrees over 60 (or add if below 60), substract 1.25% in power
for every 1% drop in the barometer (or add if above), and subtract 1% power
for 50% humidity at 60 degrees, or 2% for 50% humidity at 90 degrees.
Extrapolate from there.

The effect of power in terms of quarter mile speeds and ETs is that they
potentially change by the cube root of the power to weight delta. That is to
say, if you have a 3% increase in power, you can "expect" an increase in
trap speed of about 1%, with a decrease in ET of about 1%. (The cube root of
1.03 is about 1.01.) A 33% power increase (or decrease in weight) gets you
potentially about 10%, with a corresponding decrease in ET. (The cube root
of 1.33 is about 1.10.) 

The obvious key to a good quarter mile ET is the start, and there are a
couple of rules to keep in mind with street radials:

1. Avoid water as if you were the wicked witch that Dorothy killed. Even
after a burnout, water trapped between the tread blocks will drip onto the
track at the starting line, and you will spin wildly. Drive around the
water, and just pop the clutch at part thottle for a half second or so (not
more) just before the starting line. This will clean the tires of any dust,
pebbles etc.

2. Avoid *any* wheelspin. Those MXX3s have a ton of traction, but like any
of today's street radials, they are optimized around very low slip rates.
Once you break them loose, the cf goes to hell in a handbasket, and it's as
if you're driving on ice. You should start almost exactly the way you do on
the street in traffic (except on steroids), using clutch slip for about one
and one half seconds while feeding gas and holding rpm nearly constant with
clutch feathering. You probably start out at around 12-1500 rpm on the
street, slipping the clutch as you add gas for around a second and a half,
after which the clutch is all the way out and you motor away. At the track,
try starting at 3000 rpm and holding it right there with the clutch as you
add gas, ending up on the floor about 1 1/2 seconds later, with clutch all
the way out at that point. You can go up from there as conditions permit,
but *always* be gentle in terms of smooth feeding of torque to the drive
wheels via clutch feathering. The other characteristic of today's radials is
that they are very intolerant of torque spikes, because the short sidewalls
won't soften the blow, so to speak. *Never* pop the clutch, because you'll
break loose 10 times out of ten, unless your starting rpm is so low that the
resulting torque spike is weak enough for the tires to hold, in which case
you'll bog the motor down to very low rpm, and take forever to get back into
the power band.

On a good starting line, you can launch at 4500 rpm, going instantly to the
floor with the gas, and feathering the clutch at more or less constant rpm
for slightly less than one second. This will get you the potential of
sub-two-second 60 foot times, which is how you'll easily get into the 13s
with a stock 3.2 liter M3, at 100% air density.

Remember that if you break the tires loose, you've blown the run, every
time.

As a tip, try to get behind a real race car (with big slicks) in the staging
lanes. Then just follow him (or her) out onto the track, and line up right
where (s)he did. You can be sure that there are two wide, clean, warm spots
on the starting line if you do this, and if you line up right in those
tracks, you'll get the best traction available.

Good luck,

Bruce 

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