Originally Posted by Flexref
Not exa tly sure how convertor slippage works wht73..can you explain in simple man
Torque converters, non lockup, are not a true solid coupling between the motor crank and the input shaft of the transmission.
The converter is nothing more than a simple water wheel, but with trans fluid being flung from one set of turbine blades into another one making a coupling of sorts between the two turbine, which are inside the converter.
There are not a 100% coupling though. The amount of slippage to some extent will depend on the stall speed of the torque converter. The stall speed will be the rpm that for all practical purposes the converter couples up. If you brake torque your car, the rpm that starts to spin the rear tires is "close" to the stall speed. The lower the stall speed the positive the coupling is due to the straighter angle of the turbine fins inside the converter.
As an example in my '40 Willys couple with a blown 392 hemi, I ran a 4:11 rear and a Chrysler 518 31% O/D.
With a 3500 rpm stall converter I could cruise down the road in O/D 70 mph at 2700 rpms. I had numerous R&R's of the trans due to more hp than it could handle and the converter going out a few times.
The trans guy gave me a stock 6cyl converter while he was fighting to get one made for me. I used that one and drove to Indy Good Guys and it would pop rubber on every shift and I could cruise at around 2500 rpms at 70 mph. So this converter was more efficient than the stall converter.
When the trans guy got me another stall converter he said this is the one now to use. I took it out and was blown away with its performance, but it was VERY impractical for the street. I hit the go pedal and it went to the MSD rev limiter at 6000 rpms and never left the rev limiter the full time until I was over 100 mph and let up on it. With this converter I was at 3200 rpm at 55 mph. WOW!
So the very high stall converter was so inefficient that I was loosing all that speed at cruise. I ended up driving the car down to another Good Guys event in MI with that very high stall converter. I fried my trans fluid because of all the slippage and heat build up. That was good for the go quick and stop, but not great for cruising.
Again, sorry for the rambling, but sometimes I might be able to share experiences that are appropriate for the thread.
Anyway, a stock converter will slip about 3 to 5% and the higher the stall speed the more it will slip. As was said above, the ratio you pick will also depend on your converter efficiency; low or high stall speed.
If you're running a 10" or smaller converter you're going to be slipping on the high side and you should allow for the extra rpms when you pick your ratio.
Sorry again, but I just had another memory that I thought of earlier and forgot to say. Cut me some slack as my grey hair causes my memory to fade from time to time.
I mentioned about chirping the tires on every shift. When I finally got the right converter, about a 3500 stall, even though I had a stage II trans kit setup, the shifts did not produce any shift tire chirp. The reason is that the converter is slippery inside and when the trans would shift the converter would slip and not provide enough coupling to produce a rear tire spin. The stock 6cyl converter was amazing on how HARD is shifted in each gear and on the 5000 stall converter I could not feel one shift at any time on full throttle.
Power was not an issue. I could light the tires up at 45 mph just by nailing mashing the treadle to the floor; although the tires were not that grippy, BF T/A's, 285/70-15's 29" tall.
I hope I have not bored you with my ramblings.
Hope my explanation makes sense to you.