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M46 power handling

I

igy569

New member
Joined
Jun 25, 2007
Location
Uxbridge, MA
This may be a stupid question, but.... how much power can I put through my M46 before it is likely to grenade on me?
As of right now, the tranny is fine. Quiet and smooth. This is my daily driver, and I tend to be pretty gentle with it. I just like to have that power on tap when I want it. I am doing a custom intercooler, and a blow off valve with a boost controller. I plan to buy the pillar pod, so I can install a proper boost guage. I will probably put in an oil pressure as well. Have not decided on the third one yet.
ANY input is greatly appreciated.

Owen in Mass.
 
The M46 is rated to handle up to 300 HP. The weak point is more the overdrive. 2 exists P and J type, one is more resistant than the other one, but cannot remind wich one.
 
Yeah.. Twist force kills the transmissions. I killed my M47 with about 240ft/lbs to the pavement from a moderately modified B230FT
 
My B21ET currently puts out 400 Nm at the wheels, with an M46.

The problem is your left foot, if you're gentle with up and downshifts the gearbox will last, also good oil is crucial.
 
P-type OD's are stronger - they have a thicker mainshaft. Unfortunately, they only come on the back of alloy-cased M46's, which aren't as durable as the earlier iron-cased variety. The best M46 solution would be to have a machine shop modify the iron-case M46 to accept the P-type OD.

I just run an iron-case with J-type OD. It has no complaints in my basically stock 745T pushing 13psi.

It seems the median torque required to break an M46 is around 225 ft-lbs, but the standard deviation is large; some have broken them with an NA B21, others have put ~300HP to the ground repeatedly without issues.
 
oeb said:
The problem is your left foot, if you're gentle with up and downshifts the gearbox will last, also good oil is crucial.
Click to expand...

Bingo. This is a universal truth of most any stock-ish manual trans. Another thing that will help is keep a weaker link in the chain if you are really making power, don't put enough tire on it to always hook up, let it spin instead of breaking, or let the clutch slip some, I prefer the tire undersizing/stickiness though.
 
11110000 said:
P-type OD's are stronger - they have a thicker mainshaft. Unfortunately, they only come on the back of alloy-cased M46's, which aren't as durable as the earlier iron-cased variety. The best M46 solution would be to have a machine shop modify the iron-case M46 to accept the P-type OD.
Click to expand...

A few corrections here...

No machine work is needed to the iron case to use it with a P-Type overdrive. You just fit the guts of the aluminum case box in the iron case. The gears are not any bigger or smaller in any of the boxes unless you replace with a Quaife set or some such. Any thoughts about 'case wiggle' or 'flex' are BS. The internals fail cuz they are too damned small for preformance use.

I've had M-46 boxes fail with stock power enough times that I gave up on it for any manner of performance use.

The box with the J-Type overdrive WILL break the mainshaft the first time you get grip+250lb/ft to the ground every time.

I'm a savage and the P-Type overdrive never failed me. Can't say the same for any gearset I've fitted in an M-46 case including the pair of Quaife close ratio gearsets I broke badly.

Some of the fellers on the board have tall tales of M-46 boxes holding up fine to all manner of savagery. Methinks that many of them are full of something akin to what the dairy guys spread on my field to make things soooo aromatic and grow quickly round these parts.

T-5 FTW.
 
i would say that the m46 is good for about 220 whp once you go over that its just a matter of time.. I am not saying you cant run more than 220 wheel i ran 330 whp threw mine and around 350tq but i also replaced them about every 3 months.. Never had an overdrive fail on me but i never engaged it when i was in the boost...
 
my od failed years ago, and i beat my m46 w/malice every time i got in it, but it did fail after all and i have little traction, now i fear for the m47
 
I think my M47 ate 3rd gear at roughly 220-ish WHP levels. Not a botched shift, smooth shift into 3rd, back on the gas, and only after the boost hit ~18 psi did it come apart.

I have noting to really back up the idea, but I suspect M47's and M46's probably handle very similar amounts of torque. And the amount they can safely handle goes down bit by bit with every mile that the trans wears.
 
This is all great info guys. I believe this has answered my question tho. I am not planning huge power. I am just going to tweak it a bit. So it looks like the M46 should be OK for now. I am planning to leave it all stock, just up the boost, and improve the intake/exhaust.
This forum is awsome. Lets keep the bricks going!!
 
And like many have said, they are cheap used, so are the slushboxes. And not hard to swap out. I'd get another car before I went through the trouble of a t5 swap unless somebody really makes it boltin personally, and even then I'd prefer it had a v8 attached to it, lol..
 
igy569 said:
This is all great info guys. I believe this has answered my question tho. I am not planning huge power. I am just going to tweak it a bit. So it looks like the M46 should be OK for now. I am planning to leave it all stock, just up the boost, and improve the intake/exhaust.
This forum is awsome. Lets keep the bricks going!!
Click to expand...

If you're not planning on gettin buck nasty I'd swap in a 47 if I were you.. much lighter and less complimufuggincated.
 
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