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Old 01-11-2018, 10:31 AM   #1
BigIrish
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Default 3.73 rear axle in a 4spd?

I searched but did not find anything on this exact question.

I have an '84 NA 4spd OD with the original 3.31 (I guess) rear end. I also have a 3.73 axle out of an auto car, rebuilt with a Tru-trac installed.

Has anyone made this swap? I'm looking for more acceleration, don't care about mpg. I'm wondering if this will turn 1st gear into an unusable granny gear? +T is also part of the plans for this car.
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Old 01-11-2018, 10:35 AM   #2
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I had an M47 in mine for a while, with the car's original automatic 3.73 rear end. First gear was really short, like get the first 50 feet short. I practically never got into boost in that gear, before anything spooled I was already into second.

It might be better in an NA car. 5th gear on the M47 was a bit annoyingly short at freeway speed (70 - 75) as well, that would be amplified with a 4 spd.
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Old 01-11-2018, 11:06 AM   #3
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http://forums.turbobricks.com/showth...ht=ratio+chart

Look at the m46, and compare 3.31 to 3.73 in the "final drive ratio" charts.
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Old 01-11-2018, 12:19 PM   #4
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I have a M47 with an 3.73 due to the auto to manual swap. First gear is really short, which is sometimes nice for adventuring on borderline 4x4 roads and stupid steep hillstarts. Everywhere else, though, it's too short. If I'm stopped I put it back in 1st to start moving, but anything above 3-5 mph there is no problem getting going in 2nd. 4th is good for 40-55 mph and anything above and I'm in 5th.

The shortness/closeness of 2-4 is nice for acceleration, though.
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Old 01-11-2018, 12:47 PM   #5
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My car was just this combo. First is really short. 4th gear freeway speeds will buzz your teeth out. Cruising for hours at 3000-4000 rpm is not fun on the ears. With 3.73/3.91 an overdrive is crucial at todays highway speeds.
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Old 01-11-2018, 12:51 PM   #6
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Are the first gears of an M46 and M47 the same? I can see how the gearing would be too short to allow boost before shifting, unless you are really winding it out.

For clarification, this will be mostly used around town, lots of stoplights, etc. Maybe the occasional short highway trip

Charts are very helpful - thanks!
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Old 01-11-2018, 01:07 PM   #7
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When I was dealing with OD issues on an m46, I did about 6-months of driving with no OD and a 3.73 axle. Not great at 70mph, but doable for a few hours at a time.

1st would build ~5psi before it would hit the rev limiter (b230ft and ipd turbo cam), and 2-5th was a blast.
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Old 01-11-2018, 01:09 PM   #8
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Yes, the first gears are the same. If your car is being used mostly as you describe. You will like the change very much. Although I would also suggest you install an A or B cam to take better advantage of the setup.

What you are basically doing is the stock setup for a 240 turbo. It works fine you just have to be happy with more rpms at any speed than you are used to now.
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Old 01-11-2018, 01:09 PM   #9
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AFAIK 1 - 4th gear are the same, 5th/OD is not quite the same.
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Old 01-11-2018, 01:21 PM   #10
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The early 240s with an M46 had lower rear axle gearing, 3.91 with all transmissions in 1978. In 1979, 3.91 with manual transmissions and 3.73 with an automatic. I think a 3.73 is just about the perfect gearing for an NA 240 with an M46. The car accelerates noticeably quicker and you don't run out of gearing at highway speeds.
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Old 01-11-2018, 01:31 PM   #11
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It'll be fine and the car will accelerate quicker.

http://forums.turbobricks.com/showpo...9&postcount=13

Quote:
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Cruising for hours at 3000-4000 rpm is not fun on the ears. With 3.73/3.91 an overdrive is crucial at todays highway speeds.
3000 is a lot different than 4000. M47/3.31, 3000rpm is high 70s (3100 at ~80). That's fine. M47/3.73 and short tires, I recall being close to 3500+ at cruise. 1:1 and 3.73....slower cruise or higher RPM.

Won't be great on the highway, but around town, 3.73 is the way to go. 3.31 was always a dog.
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Old 01-11-2018, 03:35 PM   #12
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Thanks for the input. The car already has a B cam in it, so hopefully that helps somewhat.
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Old 01-11-2018, 04:04 PM   #13
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It's gon be buzzy, might as well drive a BW35 140.
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Old 01-11-2018, 05:17 PM   #14
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Don't be ridiculous. That is a 12% difference. The 83-93 240s were geared way too high to begin with. A 240 can't even pull fifth gear if you aren't doing over 60 miles an hour.
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Old 01-11-2018, 05:51 PM   #15
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Every time I am on the freeway I comment about wanting 1 more gear in my m45 (ask my better half..). I also run out of 1st gear before the other side of the intersection, and I complain about this occasionally. This is more than balanced by not having to burn the clutch and rev the piss out of it when starting out on a steep incline, which are plentiful here in the mountains. The low gears are much more forgiving of 2foot/3pedal issues with hills and an anemic engine.
No question when the new engine goes in, I am going to seriously look at tranny/rearend ratios. I like the idea of a t5 with 2.95 1st, 0.73/0.80 5th with a 3.73/3.91 rear. Final decision for me will depend on engine powerband and tire size.
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Old 01-11-2018, 07:52 PM   #16
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Taller tires will help a bit.
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Old 01-12-2018, 05:17 AM   #17
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It's gon be buzzy, might as well drive a M46 240 Turbo.
A 240 Turbo with a M46 has a 3.73 rear axle. As does a 1981 non-turbo 240 with Bosch ignition. So, no big deal.

1983-on M46 gearboxes have the same 1st gear as a M47. As do 1981-82 Diesel and Turbo boxes (also known as the M46D boxes). The rest have a 3.71:1 1st gear.

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Old 01-12-2018, 10:29 AM   #18
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I love my car with the 3.73 and m46 (1986). In first you can creep along slowly like an automatic. If you have any momentum at all just skip 1st.

I wouldn't do it though with the 4 speed if the car ever will see the interstate for more than a couple exits
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Old 01-12-2018, 05:48 PM   #19
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With either 3.73 or 3.31 in my old turbo hot rod I only used first second and third at the quarter mile drag strip and I also ran the same time 12.7. I never had to shift into fourth. 108 mph trap speed. 5 speed getrag trans. I could also do 120 in third at 7500 rpms..

Nowdays I'm running a 3.54 ratio rear in my 90 745 factory m46 car but I got getrag 265 5 speed with 5th gear overdrive. Close to all same ratios as m46. It does about 2100 rpm at 55 in 5th. Easy burnouts in first and if I drop the clutch into second in a corner it really busts loose and gets sideways and fun.
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Old 01-18-2018, 02:18 AM   #20
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Quote:
Originally Posted by BigIrish View Post
Are the first gears of an M46 and M47 the same? I can see how the gearing would be too short to allow boost before shifting, unless you are really winding it out.

For clarification, this will be mostly used around town, lots of stoplights, etc. Maybe the occasional short highway trip

Charts are very helpful - thanks!
Everyone's preferences and opinions are slightly different.
I've set up street cars with everything from 3:31 highway flyers to 4:10 stop light to stop light cars. To me all were livable because I built them with what I had in mind for majority usage. They were still plenty usable in the opposite of what I built them for, just not ideal.

If you are always in town, at least 3:73. 3:91 or 4:10's would be even better.

I could hit max boost (14psi) in 1st with m46 and 4:10's. B23FT, K cam, 60/63 Ford Turbo and around 220rwhp. That's 2-3 car lengths if launching hard before you have to shift to 2nd. I'd slam and side step 2nd about half way across an intersection and carry a nice sideways burnout for a few car lengths just for fun.

Quote:
Originally Posted by culberro View Post
When I was dealing with OD issues on an m46, I did about 6-months of driving with no OD and a 3.73 axle. Not great at 70mph, but doable for a few hours at a time.

1st would build ~5psi before it would hit the rev limiter (b230ft and ipd turbo cam), and 2-5th was a blast.
I was leaving for Davis show from PDX one year (8+hrs) in my 83 242 with m46/3:73's. Overdrive went out about 30 minutes out of town. I did the rest of the drive in 4th gear with 205/50-15 Race DOT's. It wasn't fun, but it wasn't terrible either. At least it never slowed down even 1mph over the mountains.


Quote:
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I think a 3.73 is just about the perfect gearing for an NA 240 with an M46. The car accelerates noticeably quicker and you don't run out of gearing at highway speeds.
Best all around compromise gearing.
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Old 01-19-2018, 08:46 PM   #21
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I'm going to do the exact same thing for my NA 83 DD car. Picked up a set of gears from the PnP the other day.

I figure I'll be about 2200 RPM at 60 with the 373. I'm at 2000 RPM now with the 331.
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Old 01-20-2018, 07:05 PM   #22
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Sounds awful with no 5th gear. I have a M46 with a non working OD, so basically a 4 speed and a 3.31 rear end. Around town its fine, but over 50 - 55mph I wish I had a 5th gear. So I imagine a 4 speed with 3.73 would be terrible.
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Old 01-20-2018, 08:11 PM   #23
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65 mph is 3500 rpm. 80 is 4000 rpm. Annoying, but doable. Better than if you had a 4.10 axle. 60 is 3600 rpm. I drove the white 245 for about a year with the Aw70 acting like an Aw55. So, 3-speed with a 3.73.

BTW: Chris, I think the Hollander book may be in error on the 1992-93 cars. The white wagon definitely has a 3.73 rear end.
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Old 01-21-2018, 09:57 AM   #24
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Quote:
Originally Posted by John242Ti View Post
BTW: Chris, I think the Hollander book may be in error on the 1992-93 cars. The white wagon definitely has a 3.73 rear end.
It does show that 92-93 245 AW70 cars are equipped with 3.73 gears.

For my car (with 235/45/17 tires), I would expect that the motor will be about 2200 RPM with the M46 in OD @60.

On 15" wheels with 205/60/15-ish tires, I would expect it would be a couple hundred higher or so in OD.
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Old 01-21-2018, 11:56 AM   #25
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Quote:
Originally Posted by daijoubanai View Post
Sounds awful with no 5th gear. I have a M46 with a non working OD, so basically a 4 speed and a 3.31 rear end. Around town its fine, but over 50 - 55mph I wish I had a 5th gear. So I imagine a 4 speed with 3.73 would be terrible.
Not really before the EPA required hydrocarbon emission testing at 55mph in the early 80's the cars were geared much lower, and weren't flat on their faces at freeway speeds.

4:30's till 70, 4:10's till 75 and 3:91's till 80, 3:54's in 82 then 3:31's in 83+....

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