Group,
Attached is some correspondence with a new found crew member we met at
the KOA at Speed Week. Elon is a middle aged drag racer who has taught high
school auto shop in the past and is now an engineer at the Lawrence Livermore
Labs in Livermore, CA (10 miles from my home). He had flown his Cessna 172 to
Wendover Intnl. on Sunday and hitchiked to the KOA, lucked out by renting one
of their cabins, and was walking through the campground when he saw the extra
Ardun/Flatty in the push truck.
In the ensuing conversation we offered him daily transportation to and
from the salt, but he would have to get up at 4:30AM to take advantage of the
ride since we were in Impound a couple of mornings in a row. He ended up
"hanging-out" with our motley crew, hopefully getting hooked on salt.
One of our ongoing deliberations during the meet was the RPM drop we were
experiencing on our shifts with the C-4 in the Ardun/Flatty powered Modified
Roadster. The stock C-4 ratios of 2.46/1.46 were causing the motor to drop
about 2600RPM on the 1-2 shift and about 2000RPM on the 2-3 shift. The nitro
would pull the revs back up pretty quickly on the long course but on the
gas-passes (pardon the pun) the motor needed closer gears. My feeling was
that the car could benefit in general from closer ratios. I did some research
and found that alternative ratios of down to 2:1 low gears and 1.33:1 second
gears are available although pricey ($850-$1300 for the low and second gear
sets).
Any of you using or contemplating the use of the C-4 tranny in an LSR car
might find the following interesting.................Ardun Doug King (1313
XX/FMR)
<<
Hi Doug:
Actually, I think the 2-3 shift with the NEW gears is a 23% drop
(1/1.3 = 0.769) which I am assuming (depending on the cam) is even
better than the 25% drop you mentioned. That means that if you make
the 2-3 shift at 6500 RPM the motor will drop to only 5,000 RPM for
that final, long pull. Mike forwarded me your narrative and you indicated
that the
gas motor had a tough time pulling the taller gear when Brian was
qualifying. With the 23% drop you just might be able to pull that
taller gear on gas since the motor will be starting the 2-3 pull much
higher up on the RPM curve (5,000) and not down around 4,500. It all
depends on the cam and you have the best feel for that.
##############
It might pull the taller gear on gas with a closer ratio setup in the
C-4. You're right, the drop 1-2 with the 2.0:1 first gear will be 34.5%
(1.33/2.00) as opposed to 40.7% (1.46/2.46) as it is now. The drop 2-3 with
the 1.33:1 second gear should be 24.8% as opposed to the 31.5% as it is now.
With the present setup it drops to about 3860RPM 1-2 and to about 4450RPM 2-3
using the stock 2.46:1 first gear and the 1.46:1 second gear. With the 2.0:1
low gear and the 1.33:1 second gear I figure that it'll drop to 4320RPM on
the 1-2 shift (1.33/2.00) and to about 4900RPM on the 2-3 shift (1.0/1.33).
Please check my math out and let me know what you think.
################
I don't know how hard the top-end pull was on the nitro motor but I
think you indicated that you pulled off before the 5 mile. So I would
guess that the closer ratio 2-3 shift, would let the nitro motor pull
a slightly taller gear also. I really, don't know because it all
depends on the cam... and you know that better than anyone. The other
alternative is that the closer ratio may let you experiment with a
more narrow-band cam.
################
The motor seems happiest from 5000RPM to 6500RPM+. As I recall, John DeLong
said that cams power band started somewhere around 3500, but with these
little engines it feels best to me above 4500RPM. I have several DeLong cams
that I'd like to stick with in the short term. When John died all his
machinery and cam masters went to Rod Furtado, the Flatty machinist in San
Jose. Jim Stevens (the guy next to us in impound with the 165MPH Flatty
street roadster) has some ideas for a mushroom lifter cam next year. I'll
probably try his ideas then.
#####################
Another thing to consider (but I don't know much about) is that I have
heard that nitro builds a lot of heat in a motor. If you start the
2-3 pull at a higher 5,000 rpm with a slightly taller gear you will
have been producing more horsepower over a longer period of time
...building a lot more heat. You may need to consider your cooling tank
capacity.
######################
The aluminum Ardun heads dissipate(sp?) a lot of heat. As it is I run a 195
deg. thermostat and that does the job pretty well, indicating 200 deg. at the
end of the long-course.
#####################
Do you think the taller 1st gear will may cause a problem? The higher
starting gear will put a greater demand on the converter (building
heat) or loading up the engine?
########################
I have a tranny cooler under the tonneau cover and right aft of the coolant
tank. Since the aerodynamic resistance in low gear will be much less than in
second and high, plus the fact that it's a fairly tight converter, I feel
that I'll be OK on tranny temp also.
#########################
Another question. What was the final gear you were using on the 208
mph run? I have been playing with a few numbers and with a 28" dia
tire at 6,500 rpm and 208 MPH I get a ratio of 2.6 : 1. (assuming NO
SLIPPAGE or any kind in either the converter or the tires). But I
thought you were running a 2.48 : 1.
##########################
We had a 2.48:1 gear in the Quickie on the fuel runs and Brians 179 pass with
28.5" Goodyear Funny Car fronts. Ed Weldon did a chart for me with the
different ratio's RPM's, and percent slippages. The 208-210 at 6500 with 2.5%
slippage was really close to what we actually did. The final gas pass was
with a 2.7 gear in the Quickie.
#########################
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