Stephen, stall converter question: Do high stall converter rob MPH on top end??
 

I've been kinda perplexed as to why the new 350 in my girlfriends camaro runs almost the same MPH but 7 tenths quicker than the old 305 we had in there. Cam is basically the same but one size up from the 305 (comp 268 extreme, 305 had 262) and heads are similar, mildly ported LG4 heads, but I added a 3,000 stall 10inch converter, which makes the car launch pretty hard I think. Anyways........at the track I could pull 2.0 60 fts on borrowed E.T. streets which still wasn't THAT good considering I was using slicks, but better than I've ever done in any of my cars, and car pulls extremely hard, but whats gives with the ****ty mph???? A friend told me that converter robs 5 mph in the topend, is this true? Someone also said something about slicks robbing some mph. What do you think my mph would have been if I had kept the stock converter? thanx

Oh and 1/8 mile time was 8.9@77........doesn't it sound like the car is slowing down???

how much air do you have in your stickies, the less air the more speed it robs off the top end. an ineficient converter will rob power up top , id it a 700 with a lock-up type converter? actuallly your probably not making a great more amount of power ,but your making more torque.
Perry

Failed to mention................700R4 junk is long gone. TH350 with B@M 10inch converter. Ran 12 psi in the slicks.

It depends on how much torque the engine produces. There will always be converter slip. A good matched converter should slip less than 10%.

My calculations indicate that my 8" 5700 stall converter slips close to 12%. This extra slippage will decrease mph.

When I had a TH350 and a 3800 stall converter it only slipped 8% and I had more MPH than I do now.

To calculate converter slip you need the tire diameter including any tire growth or the tire circumference. This will tell how many revolutions the tire makes in the 1/4 mile. Finish line rpm, finish line mph and gear ratio are also needed.

My car as an example
29" tire that grows to approximately 30.1" (It grew enough to rub the fenders)
6700 rpm at the finish line at 116 mph
4.56 gears

The tire circumference is 94.56194"
Tire revolutions per mile is 670.04
Calculated mph at the 1/4 mile is 131.62
(finish line rpm x tire diameter) / (gear ratio x 336)

Since I'm only going 116 mph this translates into 11.9% converter slip. If my mph was higher then there's less slippage.

How much mph will be absorbed by a high stall converter
depends on a number of factors.
Lock up efficiency.
RPM at the finish line relative to stall rpm
Weight of the car and speed at the end of the quarter. (Load)

As a general statment converters designed for a high
"torque multiplication ratio" at launch, tend to scrub off
more mph at the end (slip) than ones designed with less
torque multiplication ratio. This is independent of stall speed.
A 9" converter built for 4000 rpm stall behind a certain motor with have less slip than a 11" 4000 stall converter.
(More positive fin angle and or closer clearances between elements).

If memory serves me right I recall reading that B&M 10" street converters typically will loose about 1 mph at the end of the quarter on a typical street/ strip car. Compared to the stock converter. And reduce the ET by .3/.4 sec if you have the traction to handle the torque.

Typically if your car isn't geared to finish the quarter mile at or near peak hp rpm, (under-geared) you should expect more loss
then if it was.

I just finished swapping in a Turbo 350/ B&M 10" 3600 Holeshot
in place of my puked TH700/ 2100 stall corvette converter.

I've been breaking it in and really have only had a chance to make 1 "warranty run" to check it out. It seems to be crossing the finish of the quarter about the same rpm. Seems to have a good lockup/ stall / launch comprimise. B&M does not publish
efficientcy or stall ratio specs for their converters.

I'll be going to the track this weekend to check out the car's times with this new trans/ converter and will post the results as a comparison.

Why does everyone keep talking about "lockup stall"??? I wasn't aware that a standard Th350 torque converter locked up. I think I may have discovered more of my MPH crappyness. I was shifting at around 5,900 in 1st, but only 5,300 in 2nd, 3rd. It seems no matter how early i shift the tranny out of 1st, it still shifts real high if I'm flooring it........too high I think to make peak power. Any fixes? Do i need the downshift cable on my TH350 connected in order for the tranny to shift right at the track even though I'm shifting for it? I left it disconneted cuz it looks like a pain in the *** to connect to the edlebrock carb on there.

"Lockup stall" is not mentioned.

Lockup/ stall/launch compromize refers
to the carracteristics of a converters ability to

1. Couple near 1:1 at high rpm

2. Stall at an rpm high enough to allow the motor to be "on the
cam" at launch.

3. Launch with sufficent torque multiplication and transfer.

4. Driveability on the street.

5.$Price$

High stall converters do rob mph at the trap end of the track, but not 5mph for a 3000rpm converter in a street car.

I would think that using a 1800rpm converter as a baseline, you would lose about 1mph for every 500-600rpm extra stall (just a rough estimate). So your 3000rpm converter would be about 2mph slower than the stock one (1800rpm).

Of course there are a few other reasons for mph to drop off:

Gearing is too high
Tires geneating too much drag (slicks or low psi tires)
Car leaning out on top end (fuel pump/injectors/jetting)

Also a lot of times the hp may be exactly the same, but the torque increases, getting you there quicker (time), but traveling at the same speed (mph).

I did hear that a few selected TH350's did get lockup converters, but they never made it into a third gen (maybe a regal or some other everyday RWD car).

New technology is always making it easier and faster to get down the track in a hurry. The aftermarket is starting to catch up to the new OEM tech and good things are happening. Such as, several disk lock up race converters for 4L60Es 700s and 80Es. These get a big stall off the line, lots of torque mult and lock in second to eliminate the slipage that can rob power. Even manually locking the stock converter is good for a couple of tenths, and several MPH. Like it was stated earlier, it's all a matter of having the right combination of parts. Just like matching heads, intake, and cam is important in an engine, it's also important to match trans, converter, and gear in a drivetrain.