How strong is each gear on the MT75 4x4? I know 3rd gear starts giving problems around 350 bhp, and sometimes 2nd gear also gives problems. But how strong are 4th and 5th gear? And would an MT75 with semi-straight cut 1st, 2nd and 3rd gear and standard 4th and 5th be OK for a 400 or maybe 450 bhp car? As the downside of having all gears semi-straight cut is the annoying noise on long trips on the motorway. And would it be possible to mix with standard gears and semi-straight cut?

 

I do not think this will help much but my car did 30,000 miles on the std box at 330 bhp then as soon as I went to 420 on low boost but with loads of low down torque due to the twin scroll borg warner the box failed under very normal use...rattling third and second in under 100 miles.

Best speak to Bara at bara gearboxes

 

So why would it not help changing 1st, 2nd and 3rd gear to semi-straight cut, as you say you had problems with 2nd and 3rd?

 

Today I had a look on how the MT75 is build up and 4th gear is direct link between the input shaft and main shaft, so not using any gears. So I guess that means it shouldn't break either and 4th gear should be quiet on a semi-straight cut gearbox, because no gears are used in that situation.

I also looked at the Oppliger semi-straight cut gear set and it looks like it's doesn't include the 5th gear cog for the main shaft. So if it's meant to use the original 5th gear that should be quiet too, but in that situation it's still using the new gears on the input shaft and the layshaft which might be noisy.

 

Has anyone done this to a standard box and is there a real benifit?

 

As far as I'm aware Marc is 2nd and 3rd that takes all the abuse so theoretically I see no reason why what you are planning to be an issue, however that being said even uprated gears are not all that and its the torque that kills them, my frien kept destroying boxes (rwd mt75) so got a new box bigger stronger bearings and uprated gears (not cheap) 1st outing 7th lap at brands hatch it stripped the gears (running 430bhp 530Nm). Now he's going T5 and after myself also just doing another box im also contemplating T5 or possibly 350z box.

 

 

and both times this has happened it wasnt during shifting either.

 

Ha /\ ive got a box with a 3rd gear that looks like that too

 

THAt looks expensive!
just to add this is an older thread I have bumped

 

I am in exactly the same boat as you.

My engine and box is coming out March/April and the box wont be going back in, unless I do something to it

 

The other issue is being so far south of the country any shows I attend will mean a 2-3 hour trip at motorway speeds so really don’t want a noisey 5th gear

 

My oppliger box is semi straight cut and I can drive with the stereo on, you can hear the box but it is a nice sound, still acceptable though.

You are welcome to come and try mine. I am near Guildford.

 

Thankyou for the kind offer, Guildford is still a fair trek for me!.
Have emailed Pete regarding the straight cut 2nd and 3rd gear set but led to believe these kits are no longer produced so may have to look for used

 

"thinking outside the box!"

get some of those noise cancelling headphones for planes and the constant whining of the gears will be picked up by the mike and you could feed the headphone signal into your car in anti phase!

 

I would t say 5th gear on my box is any quieter than any of the others.

 

Don’t fancy driving with head phones it’s a road car so that would kill the fun for me having to do that.
Pete still does the straight cut gear set in 2nd and 3rd for £1800 all in, this is probably the route I will take

 

It's been quite a while since I started this topic. In the meantime I found out it's not only 2nd and 3rd gear breaking, but also the gears from the input shaft to the layshaft can break. So if you have a stronger 2nd and 3rd gear, you still run the risk of breaking the gearbox.

 

Think I read still no launches as 1st gear also a risk.
Thats not a concern to me it was mainly firing the car out of tight bends then flat out in 2nd and 3rd.
A used fixit box would have been a consideration but probably £500 off budget as looking to keep to 2k.

 

if spending 1800 on gears that could still break the box i would spend the extra and get a proper box for it

 

Is agree, if you spend £2k and it breaks you have lost it all but you can pick up,second had fixits etc sometimes for £2500 / £3000 although most are full straight cut.

I went through the whole process and ended up over £5k to do the oppliger close ratio long box with semi straight cut. I find the lower gears really suit the cossie engine as the standard is quite overheated for mega top speed.

 

The issue is defining a proper box as the others have had failures also, nothing seems 100% fit and forget.
Main issue is the noise in 5th for me personally, it would be nice to have slightly different ratios but that would come in at over 4K looking at new prices?

 

Out of interest see a new sqs gearkit is 1700 euros less vat, does anyone know the cost of building a supplied gearkit in a gearbox?m and had anyone used one of these kits of late?