Converter issue ?

I just got back to the track this last weekend after a few changes to the car. The main one being a rear ratio change. I'm thinking my converter is having issues or is just mismatched. Prior to the ratio change (4:10's to the current 3.73's) the stall was logged at approx 5200, now it's easily 6000. We have a 350 rpm drop in gear change which has remained the same. The converter slip works out a 16-17% going through the top end (RPM / DS ratio) don't know what it was previously. Early days and still sorta testing, it would appear the combo now likes a higher shift point since upping it 100rpm improved et.
The current 8" converter was built specific for an old 780 (ish) HP 528ci build but seemed to work great on my 572 (now 580)
This combo ran a best of 9.68 @ 139.95 (1.37 60ft) crossing the line at approx 7000rpm, in similar conditions with the 4:10 ratio, we should have run a 10th quicker in the 1/4, 0.05 better in the 60 and around 141. and 7100 rpm (ish) (Wallace states anywhere between 880 - 920 hp for the 580's best 9.51 @ 142.50)
I have an ATi 9.5" that stalled @ 5000 behind a 1000 + HP 950ft lb 655 motor, I'm tempted to try it but I would be disappointed if it didn't improve things.
Is it worth a try ? TIA

With the 3.73 the converter doesn't have enough track distance to get coupled up. Similar to running 1/8 mile. I'm surprised you run an 8" I would think that combo would need a 9.5" 350 rpm drop isn't much.
Doug

16-17% slip??

At that point (finish line) on the drive shaft and RPM curves anyway.

Have you tried shifting it higher to see if the motor it likes it or not?
If not, I would, maybe 300 and then 500 higher RPM shift point.
Please try that and let us know your results

how much RPM have you lost at the stripe or any at all with the slip? Interesting the shift drop RPM has not changed, with the slip I would think oyu are blowing through the converter or you just do not have enough rear gear..

That's not alot of rear gear for sure, I would be leery of having it there for such an engine. With a 655ish deal likely be not as big of a concern. At least on our 655 that dont seem to care so much where its shifted

My converter guy didn't even consider an 8" for my combo...similar to yours.

I have had very good luckwith 8" converters in such applications. To be honest the 3-4 times we have tred a 9.5" we could never get them loose enoough. Our dragster has a super tight 9.5" that drops 1400 on the shift..A turd to be sure but its been 1.0x 60'ad runs 7.0's here. I tried a few in the Stratus ended up with an FTI 9" that was a tad tight still behind a 604. New car will have an 8" and should make 1100+

Well that hit me like a bucket of bricks…. got me thinking how my car reacted to the 4.30 gear change.

Thanks for the replies
I don't appear to have lost too much rpm at the stripe, maybe 200 rpm, unfortunately we never logged DS rpm or indeed used a Racepak prior to the gear change. So it's really a bit unknown as the tach (also changed) may have been inaccurate, though IIRC wasn't too bad compared to the Grid RPM. However with the previous logging tach, the same converter would flash to 5200 consistently with the 4:10 rear.
We changed the rear ratio as we just couldn't control the wheelstands, one weekend we'd be struggling to tune out wheelspin, the next time out car would dead hook and we'd bend stuff at both ends, it got expensive and after the 4th time we decided to do something drastic. The lack of available ratio's for a Dana 60 forced our hand somewhat and we ended with 3.73's (we would have preferred 3.90's), we use a radial slick BTW.
Car seems more consistent but slower, which I feel is a backward step We're back out in 9 days for our last race meeting but I don't think I'll have time to change the T/C as we are in the process of a house move and next season we'll likely have the 655 back in .



We upped the shift from 6700 to 6900, picked up a 10th. motor seems to start to nose over after 7000

I use the theory to retest everything I've done after making any changes to the motor or tune up
Changing carbs. with jetting maxed out like different total timing than it did with the other carb. maxed out

We had a play with stuff, took some hi speed retard out of it and stepped up a size in jet (102 -103 !! ) car was happier at the top end.