At 10:09 PM 8/28/00 -0400, you wrote:
>Should I shift to 4th or let it ride? Also, what do you suggest
>for the topend so that I wont have to lose all the power when
>going past the rpm range in 3rd or losing the time by going into 4th.
>Alex Reed 99' Ext. Cab. 2wd, 318V8, 3.55 limited slip rear, 5spd. manual,
255/7015 Pirelli A/S tires ... best time 16.688@83.05
A generally accepted guideline to find optimal shift points
is to start the shifts about 500 rpm above the peak hp rpm
and to experiment from there. A chasis dyno baselining
your motor's hp and torque curves would speed up and take much
of the guess work out of the process.
Roughly, the stock mag 5.2L has peak hp at around 4500 rpm
and peak torque at about 2500 rpm (from the Magnum Engine book).
Using the guideline, you would make a run with 5000 rpm shift
points then make another run with 4900 rpm shift points.
Your timeslips will supply most of the data to continue the
testing by lowering or raising the shift points but not all.
Also important is the rpm when you cross the line. You are
getting the most the closer this rpm is to the peak hp rpm.
This figure you'll have to note on each test run and write
it on the timeslip.
Once you have zeroed in on same shift rpms for the best time,
you can now experiment with shift points at different rpms
to adjust to observed conditions (traction problems, enough
track to work with, factors unique to your engine/truck, etc.).
For example, let's suppose your testing show 5000 rpm shift
points have given you the best ETs but you notice that when
you make, say, the 3-4 shift, the rpms drop so low when
beginning 4th that it takes a little time before the rpm
needle begins to ascend. This indicates that you have probably
dropped below your power band and may need to do the 3-4 shift
at 5200 rpm to keep it in the power band when 4th begins
(another reason for a chasis dyno).
While my truck is modded and an auto., a chasis dyno shows
that my peak hp is between 4900 to 5000 rpms. I know that,
if I go through the trap in that rpms range, I'll have a
good ET and trap mph run. My torque curve tells me that the
motor has to start 3rd at least at 3300 rpm on completion of the
2-3 shift or I'm going to lose time because I'm too low in
or out of the power band. For my truck, this means that I
have to start the 2-3 shift at no lower than 5200 rpm. Testing
has also shown that I can start the 1-2 shift at 4800
(while still improving my 1/8 times) so that I have enough track
to use the accelerating power that I have in 3rd (from my mods)
to cross the trap at around 5000 rpm. Now, all this may change
whenever I find I can do 5500 rpm shifts without having the tranny
refusing to upshift.
It took me all of last year with nearly 175 runs to get
this worked out mainly because I was still searching for
a local chasis dyno place. Having the dyno figures at the
beginning of this year has shortened my adjustment period
and taken the guesswork out. The dyno has also given me a
pretty accurate indication of what mods I needed to add
to make correct adjustments.
Sorry for the length. Just trying to give you an overall
feel/picture of what was involved for me.
Bob. Ont, Canada.
'97 Dakota CC, 5.2L, 4x2, 3.92 SG, auto., 4265 lb.
PB: 14.737 @ 91.75 9.364 @ 72.95
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