Lockup strategy

[Bjs344]

Im looking for general philosophy but Ill give my specifics thats got me thinking about it.
I bought a modified 2008 Silverado. Stock 4l60 and converter, small/medium sized cam, 35 inch tires, 3.73

I mostly drive in the range of 57 - 63 mph. 3rd gear locked up seems just right at about 57-58. Its anemic below 2000 rpm so won?t hold OD locked up below 65 mph. with my current tune (saved to my recent thread on this page although Ive been playing around with different shift points and lockup strategies), I lock 3rd, then shift to 4th and even though shift lock is disabled, it stays locked in 4th. So it decelerates and unlocks.

For the philosophy question. If you can run around at 58 mph and 2000 rpm locked up in 3rd vs unlocked in 4th, which would you prefer? According to the instant mpg readout on the dash, unlocked in 4th is slightly better for mpg. Would your philosophy change if the converter was 3200 instead of stock?

[Bjs344]

Dumb question I guess?

[2xLS1]

35s with 3.73s is going to be the equivalent of about a 3.08-3.23 gear with the stock tire size. So at below 2000rpm in 4th in a big heavy truck it's just lugging around. Not making much torque below 2000 so holding 4th locked is probably not realistic. You should really consider 4.56 gears with 35s. Changing the converter stall speed when locked won't make any difference. Lower rpm is not always better mpg if it is lugging around with high (kPa) map.

[5FDP]

I agree, needs more gearing. 4.56's would be ideal.

[RDF1]

For 35s in a crew cab 4x4 i suggest a 4.88 gear.
Regular cab id do a 4.56.
I tune them to lock in 3rd and hold it there for riding on the back roads. Then when up to speed let it shift into 4th. Keeps trans temps in check and not having it drop out of 4th locked helps that poor transmission last longer.

My buddy thought i was crazy when i told him to go with a 4.88 (he had 3.42s) in his 98 extended cab z71 with 35s. So after trying to convince him for a few weeks he ended up going with 4.10s and regretted it. Went down to a 33 and got it back in the sweet spot.
Dont fear the gear.
I would much rather extend the life of the transmission than worry about 1 or 2 mpg and helps it not build up as much heat.

[Bjs344]

Thanks for the feedback. I?d like to go a little smaller on the tire. Thinking 34s and 4.30s. Eventually I?d like to go 2800 stall but I?d also like to swap to an 80e. I?ll probably tow 5 times a year but when I do I?ll be upwards of 10k

For now I?ll just run 4th unlocked. Trying to run locked it bounces around too much between gears and lock/unlock

[RDF1]

Change your downshift speeds and unlock to see if it will hold the gear better.

On my 2500hd I lock my converter in 2nd gear at 22mph light throttle and it stays locked through 4th. It helps a bunch towing heavy (20,000#) on the back roads and lower speeds.

[Bjs344]

Ya if I tow with the current gear ratio it will have to be locked in 3rd around 60 mph max. It just won?t hold overdrive locked at any reasonable speed even with no trailer. It?s peppy up high. Supposedly it?s a truck cam and it only has blue springs so it can?t be too wild but it obviously has a wide lsa because it idles like stock at 625 rpm and less than 45 kpa