Typical converter slippage?
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
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From: Finland
Typical converter slippage?
When using those MPH calculators one should know the converter slippage %. I´m playing with changing the rear gear ratio to optimum on 1/4 mile. Is there a typical % that could be used as good questimate
So what have you been using as % or have you even tested yours on track so that you know exactly the truth?
I have an ATI 10" 3500rpm Unit. Did send the ATI guys an email yesterday, but no answer yet...
So what have you been using as % or have you even tested yours on track so that you know exactly the truth?
I have an ATI 10" 3500rpm Unit. Did send the ATI guys an email yesterday, but no answer yet...
Race Director
Joined: Jan 2003
Posts: 15,176
Likes: 87
From: State of Confusion
..... in a performance-converter for pre-A/OD transmissions, I've always been told, and have always used, a slippage factor of 10% as a baseline, and have foiund it to be pretty accurate:
I have a 9" ATI and an 8" Transmission Specialities spragless, and I believe the spragless piece to be even-looser.
I have a 5800-RPM chip in the shift-lite of my Z28, and with a THM400, 4.11s and 28" tires, I trap just before the light blinks in high-gear:
the Z28 runs 12.80s @ 105 with the ATI-converter, and the formula that I have used, and backs-up my findings, is -
RPM x .9 (converter slippage ) x .006 x 1/2 TH (Tire Height ) div/by GR (Gear Ratio ) = MPH
5750 x .9 x .006 x 14 div/by 4.11 = 105.76 MPH
I believe the above formula will give you a good, ball-park idea of what gear to use for your particular application.
Additionally, I think the gear-ratios chosen for cars in the late-'70s, when fuel-mileage became critical, backs-up the 10% slippage factor:
automatic Z28s came with 3.42s, while stick-shift cars had 3.73 (3.42 x 1.1 = 3.77 )
I have a 9" ATI and an 8" Transmission Specialities spragless, and I believe the spragless piece to be even-looser.
I have a 5800-RPM chip in the shift-lite of my Z28, and with a THM400, 4.11s and 28" tires, I trap just before the light blinks in high-gear:
the Z28 runs 12.80s @ 105 with the ATI-converter, and the formula that I have used, and backs-up my findings, is -
RPM x .9 (converter slippage ) x .006 x 1/2 TH (Tire Height ) div/by GR (Gear Ratio ) = MPH
5750 x .9 x .006 x 14 div/by 4.11 = 105.76 MPH
I believe the above formula will give you a good, ball-park idea of what gear to use for your particular application.
Additionally, I think the gear-ratios chosen for cars in the late-'70s, when fuel-mileage became critical, backs-up the 10% slippage factor:
automatic Z28s came with 3.42s, while stick-shift cars had 3.73 (3.42 x 1.1 = 3.77 )
Thread Starter
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
..... in a performance-converter for pre-A/OD transmissions, I've always been told, and have always used, a slippage factor of 10% as a baseline, and have foiund it to be pretty accurate:
I have a 9" ATI and an 8" Transmission Specialities spragless, and I believe the spragless piece to be even-looser.
I have a 5800-RPM chip in the shift-lite of my Z28, and with a THM400, 4.11s and 28" tires, I trap just before the light blinks in high-gear:
the Z28 runs 12.80s @ 105 with the ATI-converter, and the formula that I have used, and backs-up my findings, is -
RPM x .9 (converter slippage ) x .006 x 1/2 TH (Tire Height ) div/by GR (Gear Ratio ) = MPH
5750 x .9 x .006 x 14 div/by 4.11 = 105.76 MPH
I believe the above formula will give you a good, ball-park idea of what gear to use for your particular application.
Additionally, I think the gear-ratios chosen for cars in the late-'70s, when fuel-mileage became critical, backs-up the 10% slippage factor:
automatic Z28s came with 3.42s, while stick-shift cars had 3.73 (3.42 x 1.1 = 3.77 )
I have a 9" ATI and an 8" Transmission Specialities spragless, and I believe the spragless piece to be even-looser.
I have a 5800-RPM chip in the shift-lite of my Z28, and with a THM400, 4.11s and 28" tires, I trap just before the light blinks in high-gear:
the Z28 runs 12.80s @ 105 with the ATI-converter, and the formula that I have used, and backs-up my findings, is -
RPM x .9 (converter slippage ) x .006 x 1/2 TH (Tire Height ) div/by GR (Gear Ratio ) = MPH
5750 x .9 x .006 x 14 div/by 4.11 = 105.76 MPH
I believe the above formula will give you a good, ball-park idea of what gear to use for your particular application.
Additionally, I think the gear-ratios chosen for cars in the late-'70s, when fuel-mileage became critical, backs-up the 10% slippage factor:
automatic Z28s came with 3.42s, while stick-shift cars had 3.73 (3.42 x 1.1 = 3.77 )
One more question...i think I have read or heard this somewhere, but can not recall where:
Is it so that exactly same converter in same car will slip different amount if changing rear end ratio (for example so that changing from 3.08 to 4.11 would subtantially reduce slippage %)? BS or true?
Race Director
Joined: Jan 2003
Posts: 15,176
Likes: 87
From: State of Confusion
One more question...i think I have read or heard this somewhere, but can not recall where:
Is it so that exactly same converter in same car will slip different amount if changing rear end ratio (for example so that changing from 3.08 to 4.11 would subtantially reduce slippage %)? BS or true?
when I went from the as-delivered 3.42 in my Z28 to 4.11s, I couldn't feel any 'seat-of-the-pants' difference, in-slippage, on the open-road with the OEM converter.
If you have an idea of what RPM you want to trap-at, know your tire-diameter, and have a possible MPH/trap-speed in-mind, you can use this formula to help choose a gear-ratio
(this is an inverted formula from above )
RPM x .9 x.006 x 1/2 TH div/by MPH = GR
Let's say you believe you'll make enough power at 6400 RPM to push your car to 110 MPH trap-speeds, and you will use a 30" tall-slick
6400 x .9 x .006 x 15" div/by 110 = 4.71 gear-ratio
Because nobody makes a 4.71 ring-and-pinion for a Corvette, you would choose between a 4.56:1 (and make the motor 'grunt' down-track, possibly giving-up a few .01 ), or a 4.88:1 (and let the motor wheeze the last few hundred feet )
Race Director
Joined: Jan 2003
Posts: 15,176
Likes: 87
From: State of Confusion
the ET & MPH seem to jive, so it appears your car is working very-well off the line for traction & 60'-times..... adding another 22% gear may effect it's consistency.....
