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1970 |
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Ranchero Squire |
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429 Cobra Jet |
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1970 |
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Ranchero Squire |
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429 Cobra Jet |
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I decided I might as well go with the biggest tire I could fit.
If I was running a manual transmission, there might be some reasons to
not put on too big a tire, but I've got a C6 with a 3000 RPM stall so I don't
need any tire spinning to keep the engine in the RPM range during launch.
And as I make more horsepower the tires will be ready for it.
Mickey Thompson ET Drag slicks are popular at my track and are available in plenty of sizes. The widest I could fit under my stock fenders was 28.0/10.5-15. At 28.1" tall they are about 2" taller than my street tires. That extra diameter makes a HUGE difference. It is like replacing my 3.5 rear end with a 3.25 gear for the track - not what I'm looking for. If I had it to do over again I'd go with 26/10.0-15 and sacrifice a little traction for the better gear. Life is full of compromises, and racing a street car is full of even more.
At 12.2" of section width (the specs say 11.7" but the tape measure says more) these are wide tires, but with a full 6.5" of backspace on my Weld 15x10 Draglites they just manage to clear the fender. There is about .5" of outside clearance and maybe a whole 1" of inside clearance, but this is the practical limit without getting creative with a hammer.
Note the facts: for the rear wheel openings, whatever tire and wheel combo you use, the total forward spacing must be less than 5.1" and the total backspacing must be 7.1" or less.
Since I'm on the board of directors of our local racing association I try to stay NHRA-legal. Running slicks means longer wheel studs and a drive shaft loop are required at any NHRA sanctioned track.
To
meet NHRA rules, the threaded part of the stud has to extend past the nut
portion of the lug nut. For wheel studs I went with 3" ARP 100-7703 studs.
Although these are listed in ARP's application guide for Ford rear disk
brakes, they were what everyone told me I needed for my 1970 rear drums and
everyone was right. When I run my street wheels the longer studs wouldn't let me
use my closed acorn lug nuts on my AR wheels.
So I use open acorns and then put on my closed acorns on as a second nut
for a more finished appearance.
Installing the drive shaft loop was time consuming, but the final
result was as near perfect as you can imagine.
The first problem became obvious as soon as I got under the car.
My iron drive shaft is HUGE. It
barely fit inside my Summit universal loop and left only about 3/16" of
clearance on each side. The next
problem was the top of the loop bottomed out on my hump an inch before the
brackets reached the chassis. I cut
a couple of inches off each side of the upper loop and that fit perfectly. By cutting an inch or so off the width of each "L"
bracket I was able to get the loop to fit perfectly in a recess on the unibody
cross-member where the seats bolt. Once
I got everything to fit it was off to Midas for some welding.
Several of the mechanics there help us at the race track and one is an
exceptionally neat welder. After
they finished and I used spray on undercoating to complete the installation.
It looked like the factory put it in.
The first impression when I launch the car is "Wow!" With these slicks I don't get ANY wheel spin at all. I power brake to about 1200 RPM - just enough to load the suspension - and flash the converter at the launch. The converter stalls and the car launches with no wheel spin at all.
For burnouts I'm following M/T recommendations: I do a hard burnout at the beginning of the day and then just light burnouts to clean the tires for each run.
After a
half dozen time slips the second "Wow" becomes apparent.
On street tires launching the car is a delicate exercise requiring a part
throttle launch and gradual increase in power through the first 100'. Although my best 60' time of 2.205 wasn't bad, my average on
street tires was only 2.436. And
the worst problem was my inconsistency. Concentrating
as best I could my 60' times on street tires still varied a lot. On slicks the
average 60' time came down to 2.170 but importantly they were very consistent,
ranging from 2.116 to 2.199. And launching the car is a no-brainer, just point and shoot.
Now that I don't have to concentrate so much on the delicate launch, I
can focus my brain on reaction time. My 60' times are limited by HP not
traction. As I increase the HP I expect these times will come down, but I
hope to keep the consistency.
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Page lasted updated April 06, 2008
