I am not sure of the exact percentage of torque converter slip Clark, and I'm quite
sure it does vary somewhat with acceleration, etc. As far as your question is
concerned, I do not yet have chapter and verse on this until I raid the library
again, but FWIW, the following is my understanding of transmission gearing.
The transmission ratio is taken from the torque converter input to the transmission
output shaft (thus including the torque converter). Gear ratios vary from about 2.50
to1 for first gear to 1.00 to 1 for third gear. In the example you gave with the
engine at 2400 rpm in third gear on the highway, there would be 600 rpm of slip in
the torque converter, as you stated. In order to provide the designed 1.00 to 1
transmission ratio, the transmission gears actually have a .75 to 1 ratio step up
compensate for torque converter slippage. So the engine turns at 2400 rpm, the
torque converter output shaft turns at 1800 rpm (approx), and the transmission
output shaft turns at 2400 rpm (approx).
BTW, in terms of gas mileage with a lock-up converter configuration, I would expect
something on the order of a 25% improvement over the present setup. Vehicles with
similar power reserve (ei: top speed) have experienced such improvements. Thus if
you are getting 11 mpg then you might reasonably expect nearly 14 mpg with lock-up
overdrive.
Of course up here in The Great White North eh, we have the imperial gallon that is
20% larger (and only costs twice as much), so it would be over 16 mpg. Maybe they
made it that way because up here you have to go so much further to get anywhere :~}
- --Bill (mypentium)
> Date: Sat, 14 Nov 1998 22:58:06 -0500
> From: searlecj (Clark Searle)
> Subject: Re: GMC: GMC Lock-up Torque Converter Clarification
>
> Automatic transmissions are essentially a servo-controlled gearbox
> connected to the engine with a torque converter. The torque converter
> has two great characteristics. It can provide enough slip to substitute
> for a clutch so that the engine can idle in gear, and it can multiply
> engine torque at low rpm's to augment gearing. When you get past lower
> speeds it reverts to being a direct fluid coupling from engine to
> transmission. This coupling has slippage inherent in its design that
> results in about 25% loss in rpm before reaching the mechanical gears of
> the transmission. That percentage of horsepower (and fuel) is turned
> directly into heating your transmission oil. --Bill
>
> I was interested in your comment about the slip ratio of the torque converter.
> You said 25%. That means, for example, on the highway with the engine at 2400
> RPM, there would be 600 RPM slip in the converter and a transmission speed of
> 1800 RPM. I'm surprised that there would be that much slippage going down the
> road. I certainly don't know much about torque converters, but I was estimating
> that the slippage under typical road load would be about 200~300 RPM.
>
> Any substantiation available from anyone?
> Clark
> 78 Kingsley
> Mid-Michigan
>
sure it does vary somewhat with acceleration, etc. As far as your question is
concerned, I do not yet have chapter and verse on this until I raid the library
again, but FWIW, the following is my understanding of transmission gearing.
The transmission ratio is taken from the torque converter input to the transmission
output shaft (thus including the torque converter). Gear ratios vary from about 2.50
to1 for first gear to 1.00 to 1 for third gear. In the example you gave with the
engine at 2400 rpm in third gear on the highway, there would be 600 rpm of slip in
the torque converter, as you stated. In order to provide the designed 1.00 to 1
transmission ratio, the transmission gears actually have a .75 to 1 ratio step up
compensate for torque converter slippage. So the engine turns at 2400 rpm, the
torque converter output shaft turns at 1800 rpm (approx), and the transmission
output shaft turns at 2400 rpm (approx).
BTW, in terms of gas mileage with a lock-up converter configuration, I would expect
something on the order of a 25% improvement over the present setup. Vehicles with
similar power reserve (ei: top speed) have experienced such improvements. Thus if
you are getting 11 mpg then you might reasonably expect nearly 14 mpg with lock-up
overdrive.
Of course up here in The Great White North eh, we have the imperial gallon that is
20% larger (and only costs twice as much), so it would be over 16 mpg. Maybe they
made it that way because up here you have to go so much further to get anywhere :~}
- --Bill (mypentium)
> Date: Sat, 14 Nov 1998 22:58:06 -0500
> From: searlecj (Clark Searle)
> Subject: Re: GMC: GMC Lock-up Torque Converter Clarification
>
> Automatic transmissions are essentially a servo-controlled gearbox
> connected to the engine with a torque converter. The torque converter
> has two great characteristics. It can provide enough slip to substitute
> for a clutch so that the engine can idle in gear, and it can multiply
> engine torque at low rpm's to augment gearing. When you get past lower
> speeds it reverts to being a direct fluid coupling from engine to
> transmission. This coupling has slippage inherent in its design that
> results in about 25% loss in rpm before reaching the mechanical gears of
> the transmission. That percentage of horsepower (and fuel) is turned
> directly into heating your transmission oil. --Bill
>
> I was interested in your comment about the slip ratio of the torque converter.
> You said 25%. That means, for example, on the highway with the engine at 2400
> RPM, there would be 600 RPM slip in the converter and a transmission speed of
> 1800 RPM. I'm surprised that there would be that much slippage going down the
> road. I certainly don't know much about torque converters, but I was estimating
> that the slippage under typical road load would be about 200~300 RPM.
>
> Any substantiation available from anyone?
> Clark
> 78 Kingsley
> Mid-Michigan
>
