At 12:01 AM 6/4/01 +0000, you wrote:
>i'm shifting right before red line, i don't remember rpm across the line,
>but i shift into 3rd around 70 i think and i cross the finish line around 80.
>Daniel
Going by the factory specs for the 3.9L, red line is at 4800 rpm.
So, let's say that you are shifting at 4700 rpm.
A hp curve for a 3.9L stock engine shows that peak hp happens at
about 4200 rpm. Your peak hp rpm will depend on what mods have
been done to your truck. Ideally, you want to cross the line near
that peak hp rpm so that you making full use of your available power.
Again, going by the stock figures that I have seen, shifting around
4700 rpm is probably too high when you factor in the tranny and
diff. gear ratios, i.e. the truck is decelerating (not humanly noticeable)
at the shifts. Also, according to specs, the 3.9L peak torque happens
at 3200 rpm (yours will be different depending on mods).
If you can, try and get a chasis dyno to get these rpms figures.
In meantime, I would suggest the following the next time that you are
at the track.
On the first t&t run, do exactly what you have done previously
with regards to launch and shift points but remember to see what
your rpm is when you cross the line (write it down on your timeslip).
On your next run, do the 1-2 shift at 4200 and your 2-3 at 4600.
Again, remember to see what your rpm is crossing the line. Write
these on your timeslip. I'm guessing at these shift points but I'm
using observations from personal experience with regards to slippage
on the shifts. Ideally, based on stock figures, when a shift has been
completed, you don't want the rpm to drop below 2800-3000 when the
next gear engages.
Experiment from there based on your et result but change only one
factor on the next run (just as an example, do the 1-2 at 4300
but keep the 2-3 at 4600), and keep records of them.
Basically, what I'm saying is that you need to know what you
are doing in terms of rpms, not mph.
Don't know if I need to mention this but red line rpm is an indicator
of much you can push the engine parts without breaking and bending
them. Usually about 300-500 rpm safety margin but don't count on it.
The rpm crossing the line will provide you info. about gear ratio selection
if you don't do any more mods. At one stage in my mods, I was crossing
the line at 4000 rpm. I had not found a dyno shop then but everything
that I had read and what I could feel on the track indicated that there
was more power there i.e. I had not reached peak hp at the end of the
1/4. I chose to change my effective gear ratio by going to smaller
diameter tire. It gave me 'more' track and I was now crossing the line
at near 4500 rpm. It also got me into the 14.9s. The next year I
put in the MPI intake manifold, found a dyno shop, and my peak hp
was at 5000 rpm. I could not go to a even smaller tire so I change gears
and now was crossing the line at 5000 rpm.
Sorry for the length. HTH and good luck.
Bob.
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