Explain resetting shift points to me, please
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Tech Contributor
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From: Should this thoughtful, valuable contribution meet with no acknowledgement or 'thanks' this post----
Explain resetting shift points to me, please
Finally picked up a 3:42 dif for my 97 A4 with 2:73 diff. I have tried in my head to resolve this condition, but I get hung up. Exactly what is the situation with rpms, etc? There are 3 parts that are revolving per minute...the trans, the engine, and the diff. I understand the newer diff will turn faster, but after that I get confused....thanks
Melting Slicks
Joined: Feb 2007
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From: Indian Rocks Beach FL
St. Jude Donor '08-'09-'10
Actually the new differential (3.42) will turn SLOWER per engine rpm than the old (2.73). 3.42 means that for every one revolution of the tire, the input to the differential must turn 3.42 times. To get the same one revolution of the tire with the 2.73 the input only has to turn 2.73 times.
Another way, if the transmission is in a direct drive (1:1) gear, which I believe is 3rd in the auto, the engine will turn 3.42 times to get the tire to revolve one full revolution.
Another way, if the transmission is in a direct drive (1:1) gear, which I believe is 3rd in the auto, the engine will turn 3.42 times to get the tire to revolve one full revolution.
Team Owner
Joined: May 2003
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From: East Meadow NY
St. Jude Donor '12
The short answer is that for the same engine RPM, the cars speed will be lower using 3.42 gears compared to 2.73.
Given that fact, the transmission needs to be given new instructions on shifting. I believe the computer has tables and for a given gear, there is a speed and engine RPM range that dictates when to shift.
Given that fact, the transmission needs to be given new instructions on shifting. I believe the computer has tables and for a given gear, there is a speed and engine RPM range that dictates when to shift.
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From: Should this thoughtful, valuable contribution meet with no acknowledgement or 'thanks' this post----
Actually the new differential (3.42) will turn SLOWER per engine rpm than the old (2.73). 3.42 means that for every one revolution of the tire, the input to the differential must turn 3.42 times. To get the same one revolution of the tire with the 2.73 the input only has to turn 2.73 times.
Another way, if the transmission is in a direct drive (1:1) gear, which I believe is 3rd in the auto, the engine will turn 3.42 times to get the tire to revolve one full revolution.
Another way, if the transmission is in a direct drive (1:1) gear, which I believe is 3rd in the auto, the engine will turn 3.42 times to get the tire to revolve one full revolution.
Melting Slicks
Joined: Feb 2007
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From: Indian Rocks Beach FL
St. Jude Donor '08-'09-'10
The computer takes more into consideration than rpm for determining shift points. It uses vacuum, vehicle speed, throttle position and rpm. As said before, most tuners have tables for different gear ratios.
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From: Should this thoughtful, valuable contribution meet with no acknowledgement or 'thanks' this post----
No, speed will be LOWER for a given rpm. The engine must turn a higher rpm to get the tires to turn the same.
The computer takes more into consideration than rpm for determining shift points. It uses vacuum, vehicle speed, throttle position and rpm. As said before, most tuners have tables for different gear ratios.
The computer takes more into consideration than rpm for determining shift points. It uses vacuum, vehicle speed, throttle position and rpm. As said before, most tuners have tables for different gear ratios.
I am aware computers can deal with it, I just want to understand the mechanical engineering of it all....so, if the engine turns at a higher RPM to get the same revolution for the rear wheels, what accounts for the shorter times in the quarter mile?
Drifting
Joined: Aug 2009
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From: Richmond Virginia
This seems more complicated than it should. If a 3.42 has a lower top speed than a 2.73 it seems to me that the engine would over rev between shifts not under rev. The 3.42 require less motion to acheive a given speen than the 2.73. To put it simply your 0-60 time will decrease with the 3.42. So I don't see how the engine has to turn more times to rotate the axle if it makes the car faster.
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Melting Slicks
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From: Indian Rocks Beach FL
St. Jude Donor '08-'09-'10
It's about torque multiplication. The 3.42 requires less torque to attain a given rpm than the 2.73, therefore the engine will attain a given rpm quicker with the 3.42 than the 2.73 resulting in a faster acceleration for a given distance. A higher (numerically) ratio will out accelerate a lower (numerically) ratio. It's all about acceleration, not top speed.
Melting Slicks
Joined: Feb 2007
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From: Indian Rocks Beach FL
St. Jude Donor '08-'09-'10
This is incorrect. The 3.42 will require a higher input speed (rpm) than the 2.73. Remember, the designation number (ratio) indicates how many times the input shaft must make a full revolution to cause the output shaft to rotate once. A 3.42 ratio differential must have it's input shaft turned 3.42 times for the output to rotate one revolution.
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Thread Starter
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From: Should this thoughtful, valuable contribution meet with no acknowledgement or 'thanks' this post----
Ok, we are getting closer here
So, what about the shift points? Let's put it this way:
If I switch to 3.42 and do not adjust shift points, exactly what will happen as far as MPH at each gear? Versus the illusive adjustment? A chart would really help!
So, what about the shift points? Let's put it this way:If I switch to 3.42 and do not adjust shift points, exactly what will happen as far as MPH at each gear? Versus the illusive adjustment? A chart would really help!
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St. Jude Donor '03
You can change the gears to whatever you want, the PCM still knows how fast the car is moving since it senses axle speed. You do not need speedometer correction for a gear change on a C5.
The A4 shift correction is usually needed because the PCM usually commands upshifts at specific MPH points, not RPM. The gear change obviously does change the RPM/MPH ratio.
So, for example (numbers pulled out of the air, this is just an example)...Say your 2.73 car is programmed to upshift from 1st to 2nd gear at 46mph when WOT, you will find the engine near 6000RPM at this point.
Now, adding the 3.42 gears means your engine is now at a higher RPM for any given vehicle speed. So you now try to accelerate, and all of a sudden you find yourself hitting the 6200 rev limiter and engine power is being cut before reaching the 46mph commanded shift point. Trans never gets the command to shift because the MPH hasn't been reached, and you won't reach the MPH because the rev limit won't allow it. The conflict there becomes quite obvious.
This is a greatly simplified explanation, but is enough to show why shift points need to be adjusted when changing gear ratio on A4 cars.
Thread Starter
Tech Contributor
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From: Should this thoughtful, valuable contribution meet with no acknowledgement or 'thanks' this post----
Wrong, in fact, it's the exact opposite.
You can change the gears to whatever you want, the PCM still knows how fast the car is moving since it senses axle speed. You do not need speedometer correction for a gear change on a C5.
The A4 shift correction is usually needed because the PCM usually commands upshifts at specific MPH points, not RPM. The gear change obviously does change the RPM/MPH ratio.
So, for example (numbers pulled out of the air, this is just an example)...Say your 2.73 car is programmed to upshift from 1st to 2nd gear at 46mph when WOT, you will find the engine near 6000RPM at this point.
Now, adding the 3.42 gears means your engine is now at a higher RPM for any given vehicle speed. So you now try to accelerate, and all of a sudden you find yourself hitting the 6200 rev limiter and engine power is being cut before reaching the 46mph commanded shift point. Trans never gets the command to shift because the MPH hasn't been reached, and you won't reach the MPH because the rev limit won't allow it. The conflict there becomes quite obvious.
This is a greatly simplified explanation, but is enough to show why shift points need to be adjusted when changing gear ratio on A4 cars.
You can change the gears to whatever you want, the PCM still knows how fast the car is moving since it senses axle speed. You do not need speedometer correction for a gear change on a C5.
The A4 shift correction is usually needed because the PCM usually commands upshifts at specific MPH points, not RPM. The gear change obviously does change the RPM/MPH ratio.
So, for example (numbers pulled out of the air, this is just an example)...Say your 2.73 car is programmed to upshift from 1st to 2nd gear at 46mph when WOT, you will find the engine near 6000RPM at this point.
Now, adding the 3.42 gears means your engine is now at a higher RPM for any given vehicle speed. So you now try to accelerate, and all of a sudden you find yourself hitting the 6200 rev limiter and engine power is being cut before reaching the 46mph commanded shift point. Trans never gets the command to shift because the MPH hasn't been reached, and you won't reach the MPH because the rev limit won't allow it. The conflict there becomes quite obvious.
This is a greatly simplified explanation, but is enough to show why shift points need to be adjusted when changing gear ratio on A4 cars.
