I'm shifting right at 5000rpm (even with the slightly raised
powerband)...4800rpm is just about perfect on the V6's (without cam
changes).
- Bernd
-----Original Message-----
From: owner-dakota-truck@buffnet.net
[mailto:owner-dakota-truck@buffnet.net] On Behalf Of Bob Tom
Sent: Sunday, June 03, 2001 8:31 PM
To: dakota-truck@buffnet.net
Subject: Re: DML: New Track Times For V6
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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