New Converter, Different Shift Points?

[S/ST 3040]

First of all, I do not race S/ST, as my screen name would imply. It was the number on my car when I bought it 15 years ago. I don't race SS either although, I've always been impressed with their performance.


My car is a back-halved 67 Barracuda. It weighs 2620 lbs. with me in it. The engine is +.060" 340 w/9.24:1 compression, OEM 596 castings w/1.88" IN valve, roller cam, Victor 340 manifold and Holley 950 carburetor. I run an aluminum drum, 904 transmission w/2.45 1st gear and trans-brake. Dana 60 w/5.57 gear. Either 31.2" or 32.6" tire.


I have been using an 8" ATI converter that hits about 6250 RPM on the brake and it has worked pretty well for the last few years. In an effort to find more MPH and shave some ET, I thought I'd try a little taller tire. It wasn't a horrible move in ET or MPH, except I was running towards the end of the year and was also getting into better air. On my last outing for the year, I was driving into a quartering headwind which, naturally cost me a little MPH but, I realized I was blowing a lot more RPM into the converter.


Fast forward to this year, I decided to swap to a tighter 8" converter and it hits about 5800 RPM on the brake. I take it out to the track, in 2400'-3000' DA air and the 60 ft. is hurting a little and the MPH is down 1 in the 1/8th mile and probably 1.5 MPH in the 1/4. Amazingly, my split times are better to the 330' by .019 sec. and .015 sec. to the 660' and ETs are pretty close.


So, I'm thinking, "Try the shorter tire!" Well, the track prep was awesome and my 60 fts picked up huge but, MPH is still down and my split times are worse.


Anyway, here's my question: Is it typical to have to alter your shift points after swapping to a "better" converter?


Thanks, in advance.

[Lenny5160]

This isn't a direct answer to your question, but I've always found a looser converter to be quicker and more consistent as long as the finish line RPM is acceptable and not out of the engine's range.

[Adger Smith]

You are trying to do something you want to do. Listen to your car. It is telling you it doesn't like what you want to do.

[S/ST 3040]

Originally Posted by Adger Smith

You are trying to do something you want to do. Listen to your car. It is telling you it doesn't like what you want to do.

I ran the tighter converter with the taller tire and the 60" to 330' was better, the 330' to 660' was better but, loses MPH in 1/8th mile and the 1/4.


Then, I swap to the 1.4" shorter tire and the 60' is better than ever but, shift recovery is worse than the same converter with the taller tire.


I'm not hearing what it's trying to tell me. I would think the shorter tire would recover on the shifts better than the tall tire. Unfortunately, my last race was all 1/8th mile so, I never ran it out the full 1/4 mile. Looking at the MPH at the 660' doesn't indicate it was going to find anything in the last 660' but, I could be wrong.


I called ATI about sending one of them back. They told me they would be guessing if, they tried to make either one of them better and suggested I just use the one the gives the best ET.


Thanks for the responses, so far.

[Adger Smith]

Quote: Then, I swap to the 1.4" shorter tire and the 60' is better than ever but, shift recovery is worse than the same converter with the taller tire.

Did it ever occur to you the taller tire had the car going faster at the shift point than the short tire. Think about the physics that comes in play when that happens. The difference in the load on the stator, maybe, Aye... No wonder it wont recover. BTY: there are subtle changes that can be done to a converter that has no effect on stall speed that helps with recovery and TQ multiplication. You could even drop the pump pressure and make the converter act entirely different. Why do you think that any serious Comp racer or serious SS racer has at least two or three converters that all stall within a few hundred RPM of each other?

[1320racer]

Originally Posted by S/ST 3040

My car is a back-halved 67 Barracuda. It weighs 2620 lbs. with me in it. The engine is +.060" 340 w/9.24:1 compression, OEM 596 castings w/1.88" IN valve, roller cam, Victor 340 manifold and Holley 950 carburetor. I run an aluminum drum, 904 transmission w/2.45 1st gear and trans-brake. Dana 60 w/5.57 gear. Either 31.2" or 32.6" tire.


I have been using an 8" ATI converter that hits about 6250 RPM on the brake and it has worked pretty well for the last few years.

Fast forward to this year, I decided to swap to a tighter 8" converter and it hits about 5800 RPM on the brake.

Anyway, here's my question: Is it typical to have to alter your shift points after swapping to a "better" converter?

What's your criteria that has you believing it's "a better converter"?

The one critical bit of information you omitted and frankly the first and foremost bit of information necessary when discussing converters, in the absence of dyno results is... your cam's duration @ .050?

Without it, there is no meaningful discussion or advice.

Lastly, you have an engine combo that maybe makes 500+HP and 400+ ft/lbs. torque at probably around 5000 RPM. A converter that flashes to 5800 is too loose no mind one that flashes to 6250 IF the goal is to "shave some ET." The fall back RPM is what is most important.

[ss3011]

There is a lot more to a torque converter than just stall speed . There is the torque multiplication, coupling point, and efficiency curve . Maximum torque multiplication happens when there is no output rpm, and decreases to 1 to one at the coupling point . Efficiency is zero at zero output rpm and approaches 100 % when output rpm approaches input rpm (engine rpm) . Since the converter works based on input torque, as you change things like the tire size, or the gear ratios ( both in the trans, or rear end ratio) this effects where on these curves your running . This is why you can see improvements in one part of the run , but lose in other points . As for modifying a particular converter , if you want a higher stall speed, a typical modification is to bend the pump fins towards the negative direction . This will raise the stall speed , but will also increase the multiplication, and the down side will reduce the efficiency . So you may see a better 60 foot time, but may lose mph . There are so many things that can be changed to dial in the "perfect" converter, but as we all know, change the weather, and you may need a different converter combination .

[1320racer]

Bottomline is based on the limited info the OP supplied, both converters are too loose and neither converter is near optimum for the combo. There's no magic required in the year 2016 to get the right converter for a ~ 500HP n/a small block, 1/8th mile bracket car

[S/ST 3040]

Originally Posted by 1320racer

What's your criteria that has you believing it's "a better converter"?

The one critical bit of information you omitted and frankly the first and foremost bit of information necessary when discussing converters, in the absence of dyno results is... your cam's duration @ .050?

Without it, there is no meaningful discussion or advice.

Lastly, you have an engine combo that maybe makes 500+HP and 400+ ft/lbs. torque at probably around 5000 RPM. A converter that flashes to 5800 is too loose no mind one that flashes to 6250 IF the goal is to "shave some ET." The fall back RPM is what is most important.

Maybe I should have not used "better" and I understand there is more to converters than stall speed. I just figured, I'd tried a couple of different converters with two tire sizes. (basically, a gear change) and hoped it was enough data to give the guy at ATI some direction to go.
It looks like, everybody agrees it could pick up with the "right" converter?

[1320racer]

again, what is your cam's duration at .050?