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Old 05-13-2019, 10:14 AM   #1
PDXPeter
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Automatic upgrade

I have a 91 auto cab and it feels like the clutch is slipping on the 4th gear. Now this car is a fair weather cruiser and I have no interest in dropping in a manual gearbox. I'm thinking that rebuilding the tranny would be a good winter project. However, I'm also thinking that it would make more sense to rebuild a newer tranny and get an upgrade out of it. I can get a tranny from a junk yard for like $100.

From what I can tell the E46 had a 5HP19 ZH tranny that should bolt right up to the M20. Does anyone know what else might need to happen to make this work? I figure the drive shaft will need to be customized.
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Old 05-17-2019, 05:36 AM   #2
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I know absolutely nothing about the automatics in the e30 and E46, but you might have to do some sort of control mechanism for the shift solenoids, assuming management is not the same. Also, you'll have to confirm that the torque converter will work on the E46 trans, or flex disc on the m20 and test the starter. I would honestly think that the driveshaft will be the easiest problem to solve, but that's speculation.
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Old 05-17-2019, 06:35 AM   #3
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Making a modern transmission work will be a lot more hassle and cost a lot more than, say, a manual swap. If you really want to keep it automatic, I would look into rebuilding the 4hp22 with a set of gears that’s better matched to the engine.

The big problem with the 4hp22 in an E30 is that you’re effectively starting in second gear all the time and just leaning on the torque converter to make that feel OK.

You can make anything work (we’re starting to see F80 M3 DCT transmission swaps into older cars), but this won’t be a plug and play drop in.

Also, I suspect that if you’re looking for a 5spd auto upgrade, you *might* have an easier time with the E36 M3 automatic. Without having done any actual research on this, I strongly suspect that the TCU will require far less input to work correctly.
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Old 05-17-2019, 06:52 AM   #4
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any newer transmissions are going to have the wrong bellhousing angle for an M20. You can deal with that somewhat on a manual, but I don't think that will work at all with an auto, unless you can fab up an adapter plate.

If it's just a cruiser I don't get why a 5mt swap wouldn't work. It would certainly be easier. Just use a stock clutch and DMF. Or, simply rebuild the original transmission (or just get a good used one).
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Old 05-21-2019, 08:03 AM   #5
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Rebuild or good used 4HP22 would be by far the most straightforward way to go.

If you want a project, swap in the 4HP22-EH from an E23 735i or E24 635Csi (or E34 535i?), as the electronic controls gives you more options for controlling the transmission while you drive. Those controllers are designed to interface with Motronic 1.0... I'm not sure what's necessary to adapt to Motronic 1.3, which I think is what's in your '91.

ALL newer transmissions will require electronic controls, and getting those controls all the inputs they require from the older engine management may be difficult.

My dad's '87 325i auto with LSD will turn BOTH rear tires in a brake stand, so I don't think the ratios are a problem, although a slightly looser converter may give the car a little more punch. A stock M20 only has so much punch to give, though.
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Old 05-21-2019, 08:11 AM   #6
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Quote:
Originally Posted by The Dark Side of Will View Post
Rebuild or good used 4HP22 would be by far the most straightforward way to go.

If you want a project, swap in the 4HP22-EH from an E23 735i or E24 635Csi (or E34 535i?), as the electronic controls gives you more options for controlling the transmission while you drive. Those controllers are designed to interface with Motronic 1.0... I'm not sure what's necessary to adapt to Motronic 1.3, which I think is what's in your '91.

ALL newer transmissions will require electronic controls, and getting those controls all the inputs they require from the older engine management may be difficult.

My dad's '87 325i auto with LSD will turn BOTH rear tires in a brake stand, so I don't think the ratios are a problem, although a slightly looser converter may give the car a little more punch. A stock M20 only has so much punch to give, though.
I’ve been playing with an iX auto recently (until we get around to fixing that), and the gear ratios are problematic.

Adjusting for the rear end, the 4HP22 is basically geared like a stick E30 if you deleted first gear. Yes, the torque converter makes that tolerable in normal driving, but the lack of a real first gear crushes acceleration performance.

I’m honestly not sure what the best solution to that is, whether it’s more, more aggressive 1-3, with a tall 4, or if the real answer is that everything sucks with only 4 gears, and stock is the best compromise. But the gearing is terrible.
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Old 05-22-2019, 12:36 PM   #7
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Then a looser converter may help a lot. Modern torque converter tech can almost work magic.

The 4 cylinder cars had more aggressive 1st & 2nd gears. You could theoretically build one of those transmissions, although calibrating the governor to get it to shift at the right RPM may take a try or two.

A 3.0 stroker is another alternative to improve off the line performance.

But like I said... fundamentally an M20 only has so much punch.
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Old 05-22-2019, 12:39 PM   #8
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Quote:
Originally Posted by The Dark Side of Will View Post
Then a looser converter may help a lot. Modern torque converter tech can almost work magic.

The 4 cylinder cars had more aggressive 1st & 2nd gears. You could theoretically build one of those transmissions, although calibrating the governor to get it to shift at the right RPM may take a try or two.

A 3.0 stroker is another alternative to improve off the line performance.

But like I said... fundamentally an M20 only has so much punch.
Sure. But an M20 isn’t that slow, either. There’s a reason why the auto M20 cars are 2-3 seconds slower to 60, and it isn’t because the manual cars are more powerful. The auto feels fine as long as your ask is modest, but it really falls over when you want acceleration from rest.
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Old 05-25-2019, 06:50 AM   #9
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Interestingly its pretty torquey around town but relatively slow to reach cruising speeds on the highway. I feel like a taller 4th gear and better TC would be helpful. I basically only know how a TC works in general, otherwise Im pretty lost on those. Does anyone know if there are any worthwhile options that fit between the M20 and 4hp22?
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Old 05-29-2019, 01:00 PM   #10
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Google "zf4hp22 performance torque converter"... quite a few hits and vendors.

Here's a myE28 thread that popped up describing the benefits of tuning the TC. If it can give a 524td some pickup, it'll work wonders for an M20 car.
http://www.mye28.com/viewtopic.php?t=29435

Back in 2014 I replaced the TC in my girlfriend's 2002 Audi A6 3.0. The job sucked, but we used a Precision of New Hampton converter. Price was great and it worked fine. We went with a stock replacement rather than a high stall, though.

Here are their standard replacement offerings for the 4HP22. If you want a higher stall, you'll have to call them to discuss, but they make a lot of high stall units as well.

http://www.gopnh.com/Automotive-Tran...ission=ZF4HP22
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