Convertor slip and gear change 1/4 to 1/8

[DailyDriverSst]

I originally intended for my truck to run Super Street (hence the name), but I'm still gunshy on boost and fueling. Right now it runs 12.0's (7.70's) in fair weather with a 4" pulley which is a good spot for that index class.

With a 3.70 pulley it runs 11.60's (7.40's) and I plan on getting a 3.55 pulley as well - that will be my fastest pulley as a 3.40 will spin the impeller past the rating.

I want to be able to run 7.0 index as well. The truck has a full float Ford 9" so gear changes are easy. But I don't know what gear ratio I want to go to.

Right now I'm running 4.10 with DOT 3354R's (28"/88").
I already have a set of FSS 3060R (30"/93") slicks. The taller tire will help combat spinning off the transbrake while making up for the short gear at the other end.

Right now I'm slipping 15% at the 1/8 and 5% at the 1/4. In February I went 7.41 and this weekend I went 7.49, both on the 3.70 pulley. So weather doesn't seem to affect it much with the EFI.
Based on my math, 5.13's would compliment the taller tire if the slip stays at 5%, but 4.71's if it stays at 15%.

How do converters typically react after a gear change? In my mind, as long as the driveshaft RPM and power output are the same, then engine RPM should be the same, regardless of track length. But does reality follow this or another direction?

Guidance is appreciated.

For the table, the left stack is driveshaft RPM and the right stack is engine RPM. I used the truth numbers from the logger for 4.10/3354R and extrapolated the rest of the data.

[Stan Weiss]

I have always run a manual trans.


Based on one person I know. Making only a change to a lower gear ratio reduced his convertor slippage.



Stan

[Tom Goldman]

Making a change from a 4.29 rear gear to a 4.56 gear reduced slip on my converter from 12% down to 5 1/2% . of course the converter flash stall dropped about 300 rpm .
overall trap rpm increased 200 rpm and MPH increased by 2 MPH.

[Jeff Stout]

My bracket car had 4.30 gears and went to 4.88 converter is 500 tighter on leave and 10% slip at 1/8 mile. Down from 13% car runs 6.50 in 1/8

[DailyDriverSst]

Originally Posted by Tom Goldman

Making a change from a 4.29 rear gear to a 4.56 gear reduced slip on my converter from 12% down to 5 1/2% . of course the converter flash stall dropped about 300 rpm .
overall trap rpm increased 200 rpm and MPH increased by 2 MPH.

Originally Posted by Jeff Stout

My bracket car had 4.30 gears and went to 4.88 converter is 500 tighter on leave and 10% slip at 1/8 mile. Down from 13% car runs 6.50 in 1/8

This is exactly the info I was hoping to hear. I'd feel better having more-common 4.88 instead of 4.71. It sounds like the slip difference will be sufficient enough to make the former work.

Thank you for the feedback.