Anh
29-08-04, 01:51 AM
I love using the power out of the 7M, but the autobox isnt as strong as the R154 manual box. The problem of the auto trans is 2 fold, and there are quite a number of BPU owners in the states who destroy their autoboxes when boosting over stock psi on a regular basis.
2 issues need to be dealt with regarding the gear box.
1) Fluid overheating
2) Sudden torque load during gear shifts
First, the fluids in the automatic (A340E) are poorly cooled with the stock transmission cooler, it is practically the smallest cooler by any standard. Now with anything above standard HP at normal ambient temps, we're talking about serious heat generated by the box and it will in time be bye bye torque converter and seals.
To tackle this, I'm working out how to install a secondary transmission cooler, something of the size of 10 - 13 rows inline with the stock cooler. I figured this is a good compromise between compactness and effective cooling capacity. To prevent *over-cooled fluid* a thermostat unit will be plumbed in between the stock and new cooler. The typical thermostat unit opens around 80 - 100 degrees C, so the new cooler only becomes useful when the trans fluid really becomes hot (i.e. hard acceleration @ boost). Regular changes to trans fluid might help maintain box life also.
Second problem lies with the shifting mechanism of the MK3 autobox. The MK4 TT box shares the same mechanical gears parts, but it has beefed up shift valves and a more elaborate shifting computer.
The standard MK3 turbo produces torque between the 200 - 270 lbs/ft range STOCK. I can assume that the autobox is strong enough to deal with this stock torque + some tolerance. How much tolerance is the key to prevent "excessive" torque loads to the box. i.e say you were boosting with 12 psi, and upon kick down the box shifts into 3200 rpm @ WOT. This would mean the box shifts directly into 380 lbs/ft of torque generated by the flexplate! That's atleast 100 lbs/ft of torque more than standard.
Dealing with this problem is more tricky and ultimately leads to making LESS torque and therefore LESS potential power to preserve the shifting mechanisms of the box - or atleast preserve it for a period that's acceptable until you find something else to play with.
So I find a sophisticated eletronic boost controller such as the Apexi AVC-R has the solution. This is as far as I know the only EBC that can vary boost automatically based on RPM values. The trick is to kerb boost levels so that the torque curve will never exceed a peak of say 290 lbs/ft in RPM ranges that the box would likely shift into (be it up or downshifting). Boost would then be raised so torque can be as high as 320 - 330 lbs/ft for maximum power in RPM ranges say 5000 - 6500 RPM. I don't think the auto box would ever downshift into these high RPM ranges.
I have read plenty of horror stories about high boost with stock auto transmissions, and their attempts to preserve the gear box failing (i.e. one made 450 lbs/ft of torque and 480 hp, stock auto lasted 2k miles), but none of them have dealt with both problems at the same time.
2 issues need to be dealt with regarding the gear box.
1) Fluid overheating
2) Sudden torque load during gear shifts
First, the fluids in the automatic (A340E) are poorly cooled with the stock transmission cooler, it is practically the smallest cooler by any standard. Now with anything above standard HP at normal ambient temps, we're talking about serious heat generated by the box and it will in time be bye bye torque converter and seals.
To tackle this, I'm working out how to install a secondary transmission cooler, something of the size of 10 - 13 rows inline with the stock cooler. I figured this is a good compromise between compactness and effective cooling capacity. To prevent *over-cooled fluid* a thermostat unit will be plumbed in between the stock and new cooler. The typical thermostat unit opens around 80 - 100 degrees C, so the new cooler only becomes useful when the trans fluid really becomes hot (i.e. hard acceleration @ boost). Regular changes to trans fluid might help maintain box life also.
Second problem lies with the shifting mechanism of the MK3 autobox. The MK4 TT box shares the same mechanical gears parts, but it has beefed up shift valves and a more elaborate shifting computer.
The standard MK3 turbo produces torque between the 200 - 270 lbs/ft range STOCK. I can assume that the autobox is strong enough to deal with this stock torque + some tolerance. How much tolerance is the key to prevent "excessive" torque loads to the box. i.e say you were boosting with 12 psi, and upon kick down the box shifts into 3200 rpm @ WOT. This would mean the box shifts directly into 380 lbs/ft of torque generated by the flexplate! That's atleast 100 lbs/ft of torque more than standard.
Dealing with this problem is more tricky and ultimately leads to making LESS torque and therefore LESS potential power to preserve the shifting mechanisms of the box - or atleast preserve it for a period that's acceptable until you find something else to play with.
So I find a sophisticated eletronic boost controller such as the Apexi AVC-R has the solution. This is as far as I know the only EBC that can vary boost automatically based on RPM values. The trick is to kerb boost levels so that the torque curve will never exceed a peak of say 290 lbs/ft in RPM ranges that the box would likely shift into (be it up or downshifting). Boost would then be raised so torque can be as high as 320 - 330 lbs/ft for maximum power in RPM ranges say 5000 - 6500 RPM. I don't think the auto box would ever downshift into these high RPM ranges.
I have read plenty of horror stories about high boost with stock auto transmissions, and their attempts to preserve the gear box failing (i.e. one made 450 lbs/ft of torque and 480 hp, stock auto lasted 2k miles), but none of them have dealt with both problems at the same time.