Shifting Question
11-05-2007, 08:29 AM
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Shifting Question I have a buddy that just did a cam swap, MS4, and a heavy duty tranny build, 3800 stall. He took it to WOT the other day and at the top of third at about 120mph it hit the limiter and acted like it didnt want to shift. It has a mail order tune, is this a tuning issue?
11-05-2007, 09:30 AM
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You need to look at the tune details.
Some tunes have the 3-4 set up with a "lift to shift" MPH in it,
like WOT and high part throttle have 255MPH on the 3-4. Then
again cars with the low speed rated tires have a low MPH limit
too.
The 3800 stall probably has 6-700RPM of slip in it at the upper
end and the shift MPH are just going to be too high, at the
least the motor will sit on the limiter until the body MPH can
catch up.
Trans line pressure may be short for the motor torque output,
it should be "all in" at whatever the observed "delivered torque"
output is but many tunes don't get to full line pressure until
way more than the motor makes; conversely the true torque
output can be a whole lot more than the PCM knows about if
the motor is hevily modified, so the trans may be shortchanged.
This could get ugly, quick, if the clutches are being starved to
the point of full power slip.
It wants you to log this stuff and see what the active problem
really is.
Some tunes have the 3-4 set up with a "lift to shift" MPH in it,
like WOT and high part throttle have 255MPH on the 3-4. Then
again cars with the low speed rated tires have a low MPH limit
too.
The 3800 stall probably has 6-700RPM of slip in it at the upper
end and the shift MPH are just going to be too high, at the
least the motor will sit on the limiter until the body MPH can
catch up.
Trans line pressure may be short for the motor torque output,
it should be "all in" at whatever the observed "delivered torque"
output is but many tunes don't get to full line pressure until
way more than the motor makes; conversely the true torque
output can be a whole lot more than the PCM knows about if
the motor is hevily modified, so the trans may be shortchanged.
This could get ugly, quick, if the clutches are being starved to
the point of full power slip.
It wants you to log this stuff and see what the active problem
really is.