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M46 overdrive on a M47 = six speed

282Sleeper

282Sleeper

New member
Joined
Nov 1, 2005
Location
Malmo, Sweden
Anyone ever done this? Im thinking of doing it and are looking around for tip and such before I get started. are there any howto or something out there that I have missed? :roll:

Only thing i've found is a picture of such a gearbox...

/Tony
 
:omg: I want one of those...

I seem to remember something about putting the O/D unit on an M45 being not very possible...so I don't know how you'd do that. I'm very interested, though.

It'd leave you with about a .632 final gear, and I'm not sure that it'd even be worth using on a powerful car. It'd be awesome for the N/A '79 buildup I'm working on, though.
 
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I suppose it would be best when used in conjunction with a 4,10:1 back axelratio?
(what's the ratio for the engaged OD anyway?)
Otherwise the gearing in 5th + OD would be to tall? Nice for high speed cruising on the german autobahn ;-) 3000-ish rpm @200km/h? You DO definitely need a turbo for that.

You could put the OD-switch on the steeringwheel so you don't need to take your hand off it at autobahnspeeds. When at 120> OD-off, accelerate to 180 > OD-on and cruise! I can dig that!
 
Well, the T56 has almost as low gearing, at least the one used on the LS1's here, and it's good for cruising, like 1500rpm is 100km/h!
 
The main obstacle to making an M47+O/D is that the mainshaft extends right the way through the gearbox and becomes the output shaft.

Basically you would have to take the tail portion of the M46's shaft from before the O/D drive pump cam rearwards and weld it to the M47's shaft.
 
But if using such tall gear ratios you'd need a lot of torque to make it worthwhile. And m47/m46 can't handle a lot of torque.

Then again you could use a 4,1 or higher rear axle ratio, but then the 1st gear would be completeally useless, and you'd kind of end up with a 5 speed 'box.

So what is the point of this modification, i must ask.
 
Wagner said:
So what is the point of this modification, i must ask.
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On a decent mild to medium build B230 w/ light flywheel and 3.91 rear end, it'd let you accelerate quickly, yet allow you to cruise @ 75mph without sucking gas too quickly. There would be enough power to pull you along, yet not so much that it snaps things.
 
so are we sure a 4.10 or taller gear can handle gear 5 + od? i don't understand gearing, but i know some of you do. what rpm will that put the car at 80mph?
 
Wagner said:
But if using such tall gear ratios you'd need a lot of torque to make it worthwhile. And m47/m46 can't handle a lot of torque.

Then again you could use a 4,1 or higher rear axle ratio, but then the 1st gear would be completeally useless, and you'd kind of end up with a 5 speed 'box.

So what is the point of this modification, i must ask.
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I imagine if you were going to the trouble to extend the shaft you would change the ratios a bit while you're in there, most notably 1st and second. then with something like a 4.88 or shorter rear end you'd ahve a nice close ratio gearset.

however i think it's mostly for cool factor.
 
oo4playmrright said:
so are we sure a 4.10 or taller gear can handle gear 5 + od? i don't understand gearing, but i know some of you do. what rpm will that put the car at 80mph?
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what do you mean can it handle it? think of the rear end in terms of leverage. the higher the number (say 4.1 versus 3.7) the more the engine turns to turn the wheels the same amount. hence, it has more leverage on the rear end (not quite how it works, but it illustrates the point nicely).
 
turboluv said:
what do you mean can it handle it? think of the rear end in terms of leverage. the higher the number (say 4.1 versus 3.7) the more the engine turns to turn the wheels the same amount. hence, it has more leverage on the rear end (not quite how it works, but it illustrates the point nicely).
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Yeah, but the higher the number, the more torque it requires to turn it. For example, the minivan my family has runs about 2200 rpm going about 75 mph (something insane like that, it's been a while since I've driven it seeing as I'm in college 6 hrs away). It's also got a 210 hp VTEC V6 in it. The I4 16v in my Volvo would have to struggle to push the car at that low of an rpm.
 
of course, you could also use the OD as an intermediate gear. i.e. between 1-2 and 2-3. bring the gear ratios real close.:twisted: probably wouldn't notice a difference past the 2-3 shift though. but remember reverse+OD=:help:

the aforementioned comments regarding strength should be taken into serious consideration though. however, maybe if you absolutely loved your b21f and never wanted to mod it, the OD could make it more fun...other than that its not to useful and overall i would say its not practical. do the T56 or something else.
 
turboluv said:
what do you mean can it handle it?
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i mean, will the engine be running at 1,200 rpm's? if so there's no way the engine "can handle" pushing a brick 80mph

and i want this because most highways are 70mph, right now i have an m47 with stock 3.31 rear end. i'd like to go to a shorter rear end, but not run any higher rpm's than i do on the highway. and this is for my NA skinny rod b230f, so i doubt i'll make enough POWA to break something that came on a turbo car with ~ 2X the hp
 
adam_c said:
bad medicine I say.

The OD is by far the weak point of the M46 transmission. I've almost thought about going to an M45 to avoid the damned thing. The OD has it's own "cone clutch" that is usually the first thing to slip under accelleration and causes lots of "bump" type-feelings when you stomp on the gas.

Get a T56 if you really want to drive that fast.
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If you learn to use the clutch when engaging/ disengaging the overdrive you will singularly fail to break it. It's the flicking the switch at full power that kills them.
 
Yeah, but the LS1 puts down 300+ ft lbs at 1500rpm and is why that gearing can be used. On a turbo 4-cylinder it needs to be a little more in the powerband :)
 
Poik said:
Yeah, but the LS1 puts down 300+ ft lbs at 1500rpm and is why that gearing can be used. On a turbo 4-cylinder it needs to be a little more in the powerband :)
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yeah, but my lt1/t56 has a tall rear end(3.31? i thought), if it's shortened a bit, would it be in a redblock's power band?
 
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