What was the trap mph, rpm and slip BEFORE the converter was replaced ?
TCI specs state "up to 4000rpm stall" with the caveat "dependent upon motor combination"for that particular model. Could be the issue is elsewhere in the 'box ?

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Most places will probably recommend a 9.5” in the 4500 range.
I can’t say that’s a bad choice for a street/strip ride.

Hey Tig, this is Jim Smiths red challenger you know the one. He had a hughes 3500 in it before, going 11.5>6's@116>117+'s. 1.65>1.66 60's. Faults found in 727, rebuilt with all good parts and this converter, don't know the rpm prior but picked up .5>6 10ths and 4mph and now doing 1.56 60's. He prob would've picked up some with just the box being rebuilt as rear clutches were toast, how much the verter is helping I don't know but as said near 6800rpm@the stripe is not conducive with 3.91/28" tyre combo and 555fwhp in 3800lbs.

60 Foot E.T. : 1.54 Seconds
1/8 Mile E.T. : 6.98 Seconds
1/8 Mile Trap Speed : 97.40 MPH
1/4 Mile E.T. : 11.06 Seconds
1/4 Mile Trap Speed : 121 MPH
1/4 Mile Trap RPM : 5,689 RPM+20%=6826

Hey Tig, this is Jim Smiths red challenger you know the one. He had a hughes 3500 in it before, going 11.5>6's@116>117+'s. 1.65>1.66 60's. Faults found in 727, rebuilt with all good parts and this converter, don't know the rpm prior but picked up .5>6 10ths and 4mph and now doing 1.56 60's. He prob would've picked up some with just the box being rebuilt as rear clutches were toast, how much the verter is helping I don't know but as said near 6800rpm@the stripe is not conducive with 3.91/28" tyre combo and 555fwhp in 3800lbs.

60 Foot E.T. : 1.54 Seconds
1/8 Mile E.T. : 6.98 Seconds
1/8 Mile Trap Speed : 97.40 MPH
1/4 Mile E.T. : 11.06 Seconds
1/4 Mile Trap Speed : 121 MPH
1/4 Mile Trap RPM : 5,689 RPM+20%=6826

http://etslips.co.uk/Ticket.php?ts=2022-06-04%2020:09:41

Jim ran 119 a couple of times last year and a few 11.4's I thought he was doing well then !! Slip seems excessive.......I'd be dropping the trans pan, looking at the fluid and looking for any excessive build up of clutch fibre in the bottom of the pan. If there is non or it's nominal it may be the converter ???

Its deff worth checking the fluid etc., Duncan built the box so it should all be ok but we'll see, he would've noticed something wrong with the shifts and said. Yes on checking he ran 5 11.4's last year best of 119 at the May event with 1.60 60 being his best and that may have been with an issue in the box as he said 1>2 shift seemed to stop and go to me but when that was I don't know.

I'll have to ask what rpm he was trapping at. Not enough information to make a good judgement on what the issue really is. Its always a lack of that that makes fault finding harder but in this case it is likely to be converter I think albeit its picked up well with it.

A 20% slipping converter is going to show +rpm in high gear/traps but mph would drop off would it not, anyway so from best to best he's picked up 4>4.5 10ths and 2mph (perhaps it should be 4mph?) with that converter and a good box rebuild+ increasing valve lift to somewhere in the .580"'s. To me 6800+rpm was never an issue with my 440, 7500 was though. 6700+ is to Jim I think so the 3500 may go back in but who wants to go slower. I hope not cos the car leaves more like a race car should now. Like most, a 10sec street car is the goal here, I think a proper race converter would do that easy without the excessive revs but that is 6 weeks out as he's found out already and we are in the middle of 3>4 meetings attended, it could be done though if ordered now, depends on how important he thinks it is.