How do u calculate?...Can it be measured thru hptuners?

 

engine rpm vs drive shaft rpm

 

what's a good number? Prior to my trans and convertor swap I had a difference of 780rpm between the crank and driveline.

 

It would vary according to the converter -stall-str etc. For instance a stock converter above stall could be expected to slip 100-300 rpm in fact that what GM says is the drop to look for when the converter locks up . A higher stall of course would be more and a high STR converter even more.
Trouble is most want both worlds and generally unless you lock the converter that is not gonna happen.
But the trade off is generally not so bad, What you loose in efficency you gain by spreading much more usable power the entire trip down the track.
A good example IMO wouldd be a 6 speed car makes 450 rwhp on a dyno. The Auto car makes 300 rwhp unlocked on the dyno.
Whos gonna be faster and why?
The M6 has to use its power in the curve its starts at A lower rpm and just like the dyno graph as the rpm goes up so does the hp and torque. While at the end of the track and top of each gear he may be able to put the whole 450 hp to the ground much of the time he cant So it may be that if you averaged HIS HP throughout the run he might actually have alot less than the auto that dynoed lower.
Now the auto with a stall lets say a 3500 well he dosent have to climb to his power the minute he hits it he is at his power so while hes not putting 450 hp to the road at any time he is putting at least his 300 hp ALL THE TIME from begining to end of the track, He does not have to wait on the curve.

Hope this makes sense to someone lol

 

ok so i did some math and im hating what i see...lol...but i suck at math so hopefully im wrong....i cross the finish line right at about 5500....but according to my math should be turning only 5013rpm to make the trap speed that i get...kind of a big gap yes?

 

I asked alot of questions about a similiar subject, talking with all the convertor sponsors on this forum it became clear that the convertor world travels around a different sun. I was really surprised to find out that a dyno does not accurately decrease load similiar to a car accelerating so a high stall convertor never hits it's sweet spot in efficiency.

Some of the companies went as far as to say their convertors on a dyno will only accelerate the rollers so fast and anymore horsepower will actually show lower numbers. Similiar to a slipping clutch and adding rpm was how one company explained it. It was explained as the same convertor in a 400crankhp car might show 330rwhp a 500crankhp car might show 325-360rwhp. You run both cars at the track the 500hp car will pull away.

 

GT Griller it sounds like your calculation also takes in tire size, rear gears? Or is it crank rpm and driveline rpm?

 

Well went off of this...1engine rpm is = to 3.73 turns of the drive shaft...then threw in tire size, trap speed and tranny gear ratio and what not and got to where my rpms should be in a perfect world with no slip. Then just looked at the diffrence to what i actually cross the finishline at.

 

Efficiency is not strictly the mechanical slip ratio,
because while slipping you are also doing some
torque multiplication. But the slip ratio can give you
an "at least XX% efficient" number.

If you ran locked and unlocked pulls, took the slip
ratio for RPM and the acceleration by MPH ratio for
delivered torque out the back, you could get a fair
assessment of efficiencies at various MPH. Going
vs RPM, you're going to have a lot of stacked-up
datapoints with very different output RPM, which
is a factor in the multiplication. You'd be paddling
around in Excel for a while trying to fudge up a
decent approximation. And you'd need real time
acceleration (or dyno RWT) vs MPH data to do it.

My Fuddle 3500/2.0 still multiplies torque out past
5000RPM. My old TCI 3000/2.2 was pretty much
fully coupled by that point and showed 97% slip
ratio.

You're really interested in efficiency across the band,
not just peak. The efficiency is liable to be worse
at higher slip RPM but getting to the torque factor,
is the bit of work.

I wish more vendors had efficiency vs input and/or
output RPM plots to show. For me this was a key
build parameter (99.92% street driven) but vendors
are all about the racing, seems like.

 

you must measure driveshaft RPM and engine RPM at the stripe to determine converter slip often refered to as efficiency!!!. This is the only accurate measurement. We use Racepak data to moniter this in our test vehicles. Let me throw this out there too. Not always do youy want a converter that is 96-98% efficient. It all depends on the application. Gear ,tire,weight, ect. We have converters in cars that run 6.70's @202 mph in the 1/4 with less than 1% slip and we have cars that run 6.60's @205 mph in the 1/4 with 6.5% slip. Difference being dragster-door car. The point I am making is it all depends on the combo.

Greg

 

FTI if you buy a high quality convertor with a 3200 stall and have 450 crank horsepower, if you increase horsepower to 550-600 crank horsepower the stall increases? But on the top end of the 1/4 mile will the efficiency be similiar to the 450 or do you loose efficiency?

 

You also have to remember the MPH on your timeslip is calculated over the last 10 feet of the track, so it may be 1-2 mph low from your real mph. Log output shaft rpm and engine rpm with HPTuners to find out the real slip.

 

being you have a set distance then chances are the converter will go up in slip. If you could travel a longer distance before measurement then it would be tha same. Its all relevant to distance, gear and weight.

Greg

 

<---In for the read. Some good info to learn from in here.