Sunset01

I had a dissapointing 34hp loss with my Midwest 3200/2.0 a few weeks back.
(locked/unlocked)
Midwest tweaked it and sent back. I had measured actual mph vs. a gear calculator with the old converter and saw about 15% difference at around 4900rpm. I see about 10% diff/slip now with the redone converter. This is also with them moving the stall up 200-300rpm. The slip %, as measured from actual/calculated speed gets into the 93% area at about 6000rpm.

With guys that have converters in the 3000-3500 range, can you log 2nd gear with rpm & vehicle speed to compare?

I redyno on Wed... based on the slip difference, I estimate the hp loss improving to very low 20s... that would be more like it.

chief455

iTrader: (21)

Do you mean that before the car was taching 15% higher rpm than a gear calcultor?
Now that the converter was done that gap is closer (actual rpm@mph vs. calculated)?
My car reads 6000 rpm at 113 mph in 3rd gear across the trap. Richmondgear.com calculates that with 26" tire(nitto 275/40/17), 3.73, 113mph to be like 5450 rpm!!!
You are saying my example shows a slipping converter?
Mine is a Midwest just restalled to 4000/2.5 str

Sunset01

Kind of, and Yes. I actually calculated exactly actual/theoretical speed and previously got 85%, implying 15% slip. Now I'm about 89% (at 4900rpm) and get to 94% at about 6100rpm.

For your combo, first the listed inflated diameter is more like 25.6"... and then I've always found the effective diameter with weight on the tire is more toward 25.2". This says 113mph is 5600rpm... or my way, at 6000rpm you should be doing 121... 113/121 = 93.3% slip ratio..... almost exactly the same as my rate. Which given your larger stall is pretty good.

Now in theory, the difference can be due to internal trans slip as well... If that's happening, however, usually you know it when your rpms spike and bang rev limiters and such... I suppose you could get a little slip like that and maybe not realize... I'm fairly confident in my case that I'm ok on the trans itself..... and I still short shift, not the other way around.

chief455

iTrader: (21)

I understand. So in my calculation I just plugged in about 1" too short of a tire. The converter does feel tight so I don't think it's slipping much at high revs.
When I calculate usig the Richmond site I used 26", 3.73, 113 mph and 6000 rpm at 1:1 ratio which are known. It said 5450 rpm
I put 70 mph, 26", 3.73 and 2500 rpm at .7:1 (od) and it said 2150 rpm or so.
I tried a bunch of different known rpm vs mph and in 3rd or od locked up both were telling me my tach reading was higher than calculated, regardless of actual diameter since I used the same for all calculations. Is my tach inaccurate?
thanks for explaining your theory.

Ragtop 99

iTrader: (7)

Even under lock-up, there can be some slip. I think GM allows for around 50 rpm differrential at typical highway speeds. If you are calculating over 100 rpm, it is probably tach error unless you have notice actual performance issues.

jimmyblue

iTrader: (11)

34 HP off your 378 locked indicates a 91% efficient
converter. That's not great, but not that far off
what most seem to advertise (look at Yank's site
in the comparisons section and you see most
street-strip converters come in at 94-95%). What
if anything does Midwest advertise for their
efficiency #s?

If you are going by only RPM loss, how do you take
into account any corresponding torque multiplication
that attends slip?

Sunset01

jimmyblue, you're right, I'm not adjusting for any torque mulitplication.. though if I had 15% slip and down to 11% now, I am assuming I've roughly reduced my locked/unlocked loss by 30%.. The dyno will tell the tale on Wednesday. I'll be happy with close to 20hp diff... Midwest doesn't really advertise and effeciency, but they will say that on typical cars that 20hp is about normal... i.e. they agreed that the 34hp I saw was excessive.

Ragtop99, agreed, & under non-locked up conditions, I would expect the best/tightest converter to be no better than 97% or so at high rpm, except maybe for a stock one.

chief455, you could be introuducing error either way with observing your tach. You realy need to a) log your mph/rpm with a scanner and b) use the tire height that the computer is using- either by knowing the stock setting or what you have programmed in there, if using software yourself... the computer only knows how fast (including speedo), assuming you've told it the right tire height. Bottom line though, most of what your seeing is normal converter slip (which is a necessary animal to an extent with a high stall)... My quest was simply that mine was setup with too much... Midwest replaced the stator, tightened up the clearances a bit and actually restalled up a few hundred rpm. (the last bit not related to efficiency, though maybe pushing me a hair more inefficient).

JNorris

iTrader: (4)

Your numbers sound right. My Vig3200 was about 90% efficient at around 5000rpm and 93% efficient from 6000rpm.


John