Something that you might want to somehow work into your calculation is tire growth. I've been working with some of my latest data and I'm seeing about 5 to 6% growth at the end of the 1/4 mile. That can drastically affect your answer. Try to keep numbers real world for the newbies.
How do you calculate slippage?
- Thread starter evil666
- Start date
With what kind of tire.... and size.....
I'm sure bias ply slicks grow at a different rate than the heavy @ss MT Drag radials.
I would thing with the radial construction.... that the drag radials would be much less % of growth as compared to racing slicks.
Your absolutely right about the different tire construction aspect. That's what gets me when I see all these slippage reports. The tire is a very important variable. Growth will also change with the attained MPH for a given tire construction.
My tire is M/T slick. 28x10.5-15. My calculated tire growth of 5 to 6 percent is at a MPH of 145.
I'd have to disagree. 12.8% sounds like very high slippage to me, assuming a good lock. I would expect those numbers from a worn out clutch, especially a single disc.
Everytime I make a run, and do the calculations, I get 0.3-0.8% slippage through the traps, locked.
6% unlocked.
These are expeceted numbers on my multi-disc. Not sure what you would consider acceptable on a lockup converter, but 13% would be unacceptable in my book.
I once called Precision to ask them what a good slip number should be for a properly setup racing combination. I was kind of surprised that they gave me a number. A good average number was 8 at the end of the 1/4. This is unlocked of course. This was years ago. With changing technology, that number may be different now.
I think you are confused.... he was referring to slip numbers IF you left the converter unlocked....
88ZNX's car we tested in reynolds had little to 0 slip when locked....but almost 17% out the back door when we left it unlocked.
Don, as TurboBob has pointed out a few times that is the advantage of using the vss for the speed - set up the logger for stock speedo and rear gears and tire height and it completely eliminates tire growth from consideration.
I must have gotten confused. I thought Jimmy was telling Evil that 12.8% slip on a locked converter was acceptable.
My contention is that it is not.
So would that make the amount of tci calculated converter slippage increase about the same percentage as the tire growth using the 28" tire ? Basically what I am asking is your 28" tire now becomes a 29.4" tire at 145 mph ? This info is very important to me as I am in the market for a new converter, but with some of these prices it may be better to buy a set of heads. I think that maybe it is time to find a tool that measures drive shaft rpms, wheel speed as well as brain slippage.
I could see this as removing internal transmission slippage out of the equation.... but tire growth would definately still not be "accounted" for.
A very good point worth pondering.
You could install a driveshaft speed sensor to detemine the exact slippage if you have dataloggers. The first tme i saw a PTC work was in Otto J's car in BG. I saw the actual slip because he had the driveshaft speed and the engine speed. Slip was between 3 and 4% at around 6500 rpm. Right where that engine was in the meat of the power curve. He picked up nearly 5mph. Previous trap rpm was around 7100. 600 rpm less and 5 mph more and it spooled an 80 journal bearing turbo in a couple seconds. Faster than the converter that slipped more up top
.Don't I feel like a dunce.....
I was figuring that the trap speed was still being used somehow to calculate converter slippage........ duh... Sorry guys for the brain fart.
Back under my rock.
turbojimmy
Supporting Member
- Joined
- May 26, 2001
He's saying 12.8% unlocked; 0% locked. I would agree that 12.8% locked would be waaaay too much.
Jim
turbojimmy
Supporting Member
- Joined
- May 26, 2001
No, you're right. The VSS is still fooled by the taller (expanded) tire in what it reports to the data logger. The only way around this using that TCI formula is to have MPH calculated by a source external to the drivetrain.
Or as bison said you can directly compare the driveshaft rotation to the RPM which takes MPH completely out of the equation.
Jim
The vss IS a transmission output shaft speed sensor, the only trick is to get the reading in rpm. The ecm outputs the vss reading as mph but that is based on the stock tire height and gearing, so if you use those you can take the vss reading and back calculate the transmission output shaft rpm. Divide engine rpm by the output shaft rpm and you get converter and transmission slippage, and assuming only converter slippage, there you are. Sorry to be so slow in following up but I had to get some work done
.turbojimmy
Supporting Member
- Joined
- May 26, 2001
The vss IS a transmission output shaft speed sensor, the only trick is to get the reading in rpm. The ecm outputs the vss reading as mph but that is based on the stock tire height and gearing, so if you use those you can take the vss reading and back calculate the transmission output shaft rpm. Divide engine rpm by the output shaft rpm and you get converter and transmission slippage, and assuming only converter slippage, there you are. Sorry to be so slow in following up but I had to get some work done
I got it now. I was talking about using the VSS for vehicle speed, not as an output speed sensor. If you use the VSS for MPH in that TCI formula it's still going to be off. I didn't make the leap in my head to using the VSS output as means of comparing driveshaft rotation to RPM. Thanks for that.
Jim
Actually, you point out one thing I forgot to mention - the vss reading may very well be off as far as actual vehicle speed goes due to all the other stuff, gearing, tire height, tire growth, whatever, but it will be right as far as converter/trans slippage so long as you use the stock gearing and tire height to do the calculation since that's what the ecm uses.
Similar threads
