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Old 02-14-2019, 02:02 PM
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48 gmc

Hello. I am working on a 48 GMC. So I have a 327 built to 325hp. I am running 3.42 rearend with a TH350 tranny and 1800 torque converter. The tranny has been built and tuned for 300hp. The engine comes into its torque band at 3000 rpm.

So this is my deal, the truck runs at about 2000 rpm at 50mph. If you put your foot into it it does not shift down into a lower gear. The mechanical kick down and vacuum are working, or should I say set probably. It takes to 3000 rpm to really get after it but then Iím going 75mph. It is a nice driving truck but I am not getting 325hp performance out of it.

Wondering where to go next, manual shifter, 4:10 gears maybe a 6 speed tranny with manual shifter.

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Old 02-14-2019, 02:19 PM
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One change will make you think you have two motors under the hood, a 10 inch, 3000 stall converter. Don't get a cheezy cheapo 12" unit with the fins bent over, get a real converter, 10 inch diameter, 3000 stall.
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Old 02-14-2019, 02:36 PM
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Have you done any adjusting to the vacuum modulator on the transmission??

Most aftermarket modulators are adjustable, reaching in through the hose nipple with a small flat blade screwdriver, turn the adjustment screw in to cause earlier drop down shift and later upshift timing.

This may help, or may just not be enough.

Are you sure the mechanical kick-down cable is properly adjusted?? The length of the cable casing is adjustable, there is a clip you release to allow the cable to self adjust.
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Old 02-14-2019, 03:03 PM
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If you can describe the cam to me, I will be able to dial you in closer.
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