How do you calculate slippage?

[turbojimmy]

So. What is the universal answer so that everyone is comparing apples to apples?

I don't think there is one. It would depend on the data logging mechanisms available to each individual. Least common would be a pickup on the driveshaft.

Most common (easiest) would be MPH data collected from the VSS that any data logger can get. You just have to be careful with that to make sure you're adjusting for different tire sizes and/or growth.

Kind of in the middle would be using the VSS signal to monitor driveshaft revolutions and compare that to engine RPM (though I don't know what kind of information the VSS spits out and how you would convert that to driveshaft revolutions - it sounds like others know how to do this).

Jim

 

[Blazer406]

I'm not sure it really matters as long as you use the same math every time.

Let me explain: (bear with me... this is long)

If you end up with 3.42:1 as the ratio programmed in DS and 28.5" as tire height programmed in DS.... you can calculate theoretical driveshaft RPM. Let's assume for simplicity that your trap speed in DS was 100 MPH. You are using a 3.42:1 gear... and a 28.5" tire. a 28.5" tire has a circumference of Pi x dia .... 3.1416 x 28.5 = 89.54 in traveled in 1 revolution of the tire. 100 miles converted to inches is 100 (mi) x 5280 (ft/mi) x 12 (in/ft) = 6,336,000 inches in 100 miles. Now to figure out how many tire revolutions that would take..... well... you traveled 89.54" in one tire revolution..... so it would take 6,336,000 / 89.54" = 70,761.67 tire revolutions to travel 100 miles. Because you are running 3.42 gears.... the driveshaft had to turn 70,761.67 x 3.42 = 242,005 revolutions to make the car travel 100 miles. Up to this point... we have been talking about hours... mi/hr....tire rev/hr.... and driveshaft rev/hr.... now to convert that to minutes to get revolutions per minute..... 242,005 / 60 (min/hr)..... = 4033.41 driveshaft RPM.

Now..... if I hadn't lost anybody..... you can make a direct comparison to what DS said was the engine RPM at that 100 MPH..... for example if it were 4700 RPM... you would take the larger number (4700) divide by the smaller number (4033.41) = 1.165... subtract 1 ..... = .165..... then multiply by 100 to convert to percent..... so.... 16.5%.... this is the slip % you had at 100 MPH.

This all works at WOT only..... slip percentages will be all over the place at different loads... so only do the analysis at WOT.

Anyone feel free to jump in and correct my math if I made a mistake.

 

[Alky V6]

I would add that for tire size in your calculation, you should make an actual measurement of the circumference of the tire you're using with a tape measurer with the tire pressure you expect to be using. The labeled diameter size on the tire is usually off by quite a bit. As for the rest of your math,... more pondering.

 

[bison]

I'm not sure it really matters as long as you use the same math every time.

Let me explain: (bear with me... this is long)

If you end up with 3.42:1 as the ratio programmed in DS and 28.5" as tire height programmed in DS.... you can calculate theoretical driveshaft RPM. Let's assume for simplicity that your trap speed in DS was 100 MPH. You are using a 3.42:1 gear... and a 28.5" tire. a 28.5" tire has a circumference of Pi x dia .... 3.1416 x 28.5 = 89.54 in traveled in 1 revolution of the tire. 100 miles converted to inches is 100 (mi) x 5280 (ft/mi) x 12 (in/ft) = 6,336,000 inches in 100 miles. Now to figure out how many tire revolutions that would take..... well... you traveled 89.54" in one tire revolution..... so it would take 6,336,000 / 89.54" = 70,761.67 tire revolutions to travel 100 miles. Because you are running 3.42 gears.... the driveshaft had to turn 70,761.67 x 3.42 = 242,005 revolutions to make the car travel 100 miles. Up to this point... we have been talking about hours... mi/hr....tire rev/hr.... and driveshaft rev/hr.... now to convert that to minutes to get revolutions per minute..... 242,005 / 60 (min/hr)..... = 4033.41 driveshaft RPM.

Now..... if I hadn't lost anybody..... you can make a direct comparison to what DS said was the engine RPM at that 100 MPH..... for example if it were 4700 RPM... you would take the larger number (4700) divide by the smaller number (4033.41) = 1.165... subtract 1 ..... = .165..... then multiply by 100 to convert to percent..... so.... 16.5%.... this is the slip % you had at 100 MPH.

This all works at WOT only..... slip percentages will be all over the place at different loads... so only do the analysis at WOT.

Anyone feel free to jump in and correct my math if I made a mistake.

I dont see any problems with your math. But the tire circumference will change with a slicked car. Especially when it is going nearly 150 mph. A reluctor that picks up ds speed is the most accurate way to get the numbers. Then simply compare the engine rpm and mph to the ds speed and you can get all the slippage numbers at any mph you would ever want.

 

[turbobuick]

Just to add to what Bison said. You can run with near 0% slip but there are several things to consider. You have to look at rpm drop on the shift. It's tough to have great efficiency yet keep the motor in it's rpm range without excessive rpm drop. And in Dick's case he had been fighting tire spin which was caused by the extemely tight torque converter. When you make a lot of power on a small tire the converter is also key to making it down the track. The VSC enabled him to spool very quickly with a super tight converter but the car would never 60 foot like it should. It was spinning the tires when the fluid filled. With our PTC converter the car started leaving like it should, smashing the oil pan in it's first pass after a wheelstand. So in other words. VSC and a super tight converter is not the best way to try to achieve near 0% slip. I haven't seen the logs but I expect Dick's efficiency to be in the 3-4% range, spool without VSC if he wants and 60 foot well.

Lonnie DIers makes a nice boost builder that we spent ALOT of time developing on my car and IMO works better than anyother design out there since we tested a number of them.

What Dusty posted above is simply not true in my case and from what I've seen. We are achieving 2-3% slip with only a 600rpm drop on the shifts have zero issues with controlling the launch or the boost builder/vsc/converter dump.

It should also be mentioned how well you converter couples is going to depend on how high you spin your motor. I highly doubt you are going to end up twisting your stuff as hard as Dick or Dave FIscus. When you have a car that needs a 4000 stall conv and shifts at 5500 rpm you will have a difficult time getting it to couple with under 10% slip with only a 1500 rpm operating window. My converter will footbrake to 3200 but I spin my motor to 7300 in the traps; big difference. We had an 8" AC conv in the car originally that spooled the turbo great and we were seeing 5% int he traps at 7300rpm and 156mph. But the car was nosing over bad after the 1000' mark. We changed the rear gear from a 3.70 to a 3.50 and went from a 28.5 to a 29.5 tire and the conv. efficiency went to 18% since we weren't spinning the conv. high enough, make sense?

If you are looking to achieve super tight conv slip numbers a VSC is a nice option but it is also another obstacle in tuning the car so i would try to get it right without it. 6-8% is a good number to shoot for and is achievable. You probably won't see much gain with a conv. that is tighter than 6% and the cost to get there can be sufficient. If you are shooting for super tight conv. I would buy a bolt together to begin with becasue you will be swapping stuff around to get it right.

BTW the only reason we looked into the super tight conv and using a VSC is due to the high trap speeds we run. At over 150mph you are going to be over 7000rpm and out of the efficiency range of the cam and in our case the production style heads. Again, stuff you probably won't have to contend with and if you are I hope to see you in the staging lanes for the TSO point series.

 

[Blazer406]

I would add that for tire size in your calculation, you should make an actual measurement of the circumference of the tire you're using with a tape measurer with the tire pressure you expect to be using. The labeled diameter size on the tire is usually off by quite a bit. As for the rest of your math,... more pondering.

Assuming you are fairly close..... i.e. 28" programmed in DS for a 275/60 tire.... the point is that you do all the math... and get a slip % at a certain speed (mph...and corresponding engine RPM)... and that is one data point.... that you use to calculate the driveshaft RPM... and consequently... slip % of the converter. After all this math... you end up with a % slip at a certain RPM in high gear..... you could then set up other data points during that same run to plot the converter slip % at the other RPM's..... cause you will have a miles per hour data point that will have a corresponding engine RPM datapoint...... one datalog examined from shift recovery point in 3rd until you let off could be turned into a plot showing torque converter slippage thru ought that RPM range.

In 88ZNX's case in Reynolds... the RPM range would have been 5000 RPM - 5700 RPM.

I would think that the converter slip numbers would be different in each gear for corresponding RPM's.... i.e 5000 RPM (at WOT) in 1st would show a different slip % than high gear.... cause the load is different....


With that said... IMHO the FAST is no more accurate than the stock ECU when trying to get driveshaft RPM's.... they both are logging driveshaft RPM's... one usually on the tailshaft of the tranny... and one usually on the yoke of the rear end..... both places will yield the same data....

 

[Dusty Bradford]

Are you using the boost builder now Chris? When I watched the video I took it as you weren't using it.

The way Lonnie described the boost builder I understood you are bringing the fluid in over time which is a great way to do it with a tight converter. We found a super tight converter didn't flash enough on launch to keep the tires from spinning if you fill it too quickly.

Fiscus crossed at 7300 and 167 and it comes up around 7%. Are you still crossing at 157 with 3.50 gear and 7300 because that would be much higher than 2-3%?

 

[turbobuick]

Are you using the boost builder now Chris? When I watched the video I took it as you weren't using it.

The way Lonnie described the boost builder I understood you are bringing the fluid in over time which is a great way to do it with a tight converter. We found a super tight converter didn't flash enough on launch to keep the tires from spinning if you fill it too quickly.

Fiscus crossed at 7300 and 167 and it comes up around 7%. Are you still crossing at 157 with 3.50 gear and 7300 because that would be much higher than 2-3%?

Last two passes on car were

7230rpm, 3.70 gear, 1:1 trans, 28.5 tire 161.98mph = 2.32%

run #2
7320rpm, 162.45 mph= 3.3%

I do not need to use the boost builder with this combo but it'll help. In the video I didn't start spooling the car till the light came down and I was trying to control were I left so I didn't kill the tires, 60's were soft BTW. I see about a 600 rpm drop on the shifts. WHen the car was in the mid 150's we were 1-2% with the additional power we are at 2-3%.

 

[Dusty Bradford]

OK. I thought you still had the 3.50 gear.

 

[Blazer406]

OK... I got the datalogs and got the actual RPM's. This run was our quickest pass at a 11.14 @ 117.96 mph. The results are close to what I stated above.

Converter was a 0-pump vigilante. supposedly 3200-3400 stall .... yea right...

Car left the line at 4500 RPM.

1-2 was at 5575..... recovered to 5125.....450 rpm drop.....

2-3 was at 5550..... recovered to 5075.....475 rpm drop.....

timeslip was 117.69 mph ...... DS had 28.4" programmed as tire height... and 3.42:1 as rear end ratio.




TC slippage at shift recovery point on 1-2 ...... 37.7%

TC slippage at shift recovery point on 2-3 ...... 30.3%

TC slippage at traps.................................... 16.4%


Does this come close to what others have seen with the vigilante unlocked?

 

[feotram]

torque convertor slip

I went 118.9 through traps at 5600 with 26 inch drag radials 3.42 gear vig 0 pump 3200 and tci calculator says 6.56 percent slippage. Is this considered good or is there room for improvement?

 

[Blazer406]

I went 118.9 through traps at 5600 with 26 inch drag radials 3.42 gear vig 0 pump 3200 and tci calculator says 6.56 percent slippage. Is this considered good or is there room for improvement?

I assume this was locked..... sounds kinda high to me if it were locked....

We got around 16% unlocked... which is fairly normal for a Vigilante. Locked it should theoretically be 0% unless the lockup clutch isn't holding. I assume you have a single disk?

 

[TurboDave]

That's about what my vigillante goes through the traps (6.5%)unlocked. Locked the slip is about 0.3%

 

[Making Boost]

According to the TCI calculator, I have 14% slippage with my Art Carr 9in. What type of mph and ET gains could I expect with a 9 or 10% slippage convertor?

 

[feotram]

I assume this was locked..... sounds kinda high to me if it were locked....

We got around 16% unlocked... which is fairly normal for a Vigilante. Locked it should theoretically be 0% unless the lockup clutch isn't holding. I assume you have a single disk?

Im sorry I didnt mention , but that was unlocked. Dont care much for locking a single disk. Im actually trying to tune for max HP unlocked to notice improvement in changes made at the track. Sometimes I look at other combos and the MPH and wonder if they were locked or unlocked to get the posted MPH.

 

[feotram]

That's about what my vigillante goes through the traps (6.5%)unlocked. Locked the slip is about 0.3%

So would that be considered good?

 

[brian]

So would that be considered good?

6.5% is real good in my eyes for a l-u converter being used unlocked.
My multi-disc was .66% when locked. According to TCI`s info @ 122mph.

Brian

 

[bison]

So would that be considered good?

If it was that good i wouldnt lock it until i was at the top of 3rd if at all. Ive never seen a LU vig even close to that number unlocked. You should send the unit back to them so they can do the same thing they did to yours to all of them. A lot of vig owners would be very grateful. I usually see 15-20% unlocked with a lockup unit with 3.42 gears and 28.5 inch slicks. A little less if more rear gear and or shorter tire and more if taller tire and 3.23's.

 

[Dusty Bradford]

According to the TCI calculator, I have 14% slippage with my Art Carr 9in. What type of mph and ET gains could I expect with a 9 or 10% slippage convertor?

14% down to a good 4-5% which is common now and the car should pick up 3-4mph or so as long as your motor can pull the extra load (everything's healthy)

 

[Making Boost]

Thanks Dusty!