For the specification of the Escort Cosworth drivetrain check out the transmission specs.
The standard drivetrain in the Escort Cosworth is good as long as you stay at the
standard power and don't misuse it every chance you get. In the category misuse
there's mainly one thing that will kill the gearbox and other drivetrain components
faster than anything else and that is standing starts on surfaces with good
traction (dry concrete and asphalt). Since there is no wheelsspin on such surfaces
the shock and torque when you let go off the clutch rather abruptly must be taken
up by the drivetrain. That takes it's toll after a while. Also changing gears at
high revs faster than the gearbox was designed to will shorten its lifespan. Such
shift-maneuvers will break and wear out the synchro-rings.
The MT75 gearbox is also very slow to change compared to many of the cars made
today. The MT75 gearbox is a very old design. It only utilizes a one-piece synchro
between each gear. This is most obvious when changing between 1st and 2nd gear due
to the large difference in speed between the two gears. Modern gearboxes usually
uses a two-piece synchro that greatly reduces the time required to change gear.
What breaks ?
The components that will let go first in the gearbox itself are the second and third
gear.
Braking a first gear is very very rare due to the low gearing and the lack of time
for engine torque to build up before having to change into second gear. Second and
third gears are the weak links due to them being on the secondary shaft and therefore
transfer of torque takes place from the primary to secondary shafts. The design of
helical gears means they force themselves apart as they turn creating thrust forces
against the end-bearings. With the and with increased
torque in a tuned engine this wears them down and allows the gears to move apart. As
this situation worsen less of the area of the gearteeth are in contact with the gear on the other
shaft and failure occurs. Fourth gear is a 1:1 ratio straight through the gearbox
and thus does not use any gears within the box. I have heard of one broken fifth gear
due to the same reasons as with second and third but braking fifth is rare due to
the smaller speed differential between the gears when changing from fourth to fifth.
Inputshafts for the MT75 gearbox. From left to right: Standard Escort Cosworth,
Quaife uprated, Quaife Big tooth, DOME.
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Two fifth gears for the MT75: From left to right: Quaife uprated, DOME.
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Diffs and driveshafts
The front and rear diffs will also break if they're used real hard. The rear
diff can be changed from the standard 7" to a 9" diff with housing and Limited Slip Diff from
Ford. That should be enough for almost anything.
In the front diff the casing (diff housing) is relatively thin and under increased load
it will flex. This flexing results in misalignment between the gears in the diff, this will sooner
or later result in failure. An uprated front diff casing can be bought from Quaife. This
cures the problem. This uprated diff-housing along with a Quaife ATB differential and
uprated driveshaft-joints creates a virtually indestructible unit. Many 500+ HP
cars use this setup with no problems.
Quaife ATB diffs: Autmatic Torque Biasing diffs. A type of torsen diff that distributes the
most torque to the wheel with the most grip. Said to take at least 600lbft torque. This is
due to the fact that
there are no planet gears which are the weak points in the original diff itself. Requires
rear diff casing to be machined. The machining is to remove enough of the
casing to allow the diff to fit inside. The price is £685 + VAT for the diff, £85 for new
bearings in the diff housing (recommended).
Normally the driveshafts and CV joints lasts for a while even though the front
right side driveshaft can pop out. This is normally due to engine mount wear or
excessive lowering.
There are two possible cures for that:
Don't lower your car so much.
Fit better engine mounts (Vibra Techincs Group A).
Some people also fit a longer driveshaft (available from Grahamgoode.com: GGR 722
Front o/s extended shaft), but that is not recomended because of this: A the
suspension uses it's complete travel the longer shaft puts excessive pressure on
the inner and outer CV-joints resulting in early failure.
The front prop-shaft usually fails after 100.000km somwhere. Or more correctly the bearings in
the crosses fail due to the heat from the exhaust downpipe that passes very close to the
propshaft. Remedy: Fit new crosses with lube-ports and inject new grease whenever you change the
motor oil.
Clutches
The standard clutch can't handle much more than 330hp and the torque that comes with
that. It might even slip at lower poweroutputs.
This is how it should be. It is a built-in safety margin by Ford that basically
makes the clutch the weakest link in the drive train. Breaking a gearbox whilst
using the standard clutch should not happen. Even at 330BHP with the correct uprated
clutch, few gearbox problems should result from spirited driving. Obviously
continued hard launches will drastically shorten the life of the clutch. Problems
usually occur when paddle-type clutches are used in conjunction with the
standard gearbox as this effectively moves the weakest link in the drive train from
the clutch to the gearbox itself.
Replacement clutches are readily available in different specs from upgraded organic
plates with standard Motorcraft covers, to 4 and 6-paddle sintered bronze ones with
really agressive covers. The latter will not be much of a joy to drive in the city
and when in heavy traffic as they engage very abrubtly. It's either in or it's
out, there's no slipping. There are also multi-plated clutches but those aren't really
needed. And they're expensive.
Possible gearbox upgrades
- Pete Doughty's DPE boxes: There was a lot of problems
at first with these, but they are all fixed now with one car with 552hp having no
problems even after a lot of full throttle launches.
You must deliver your standard gearbox to him and he does either a second and 3rd
gear conversion or the second gear, 3rd gear and main shaft. The gearbox will be
very noisy in second and third with the first option and in all gears except 4th
with the second option due to the mentioned gears being straight cut.
The prices are £1800 + vat for 2nd & 3rd. £2200 + vat for 2nd, 3rd & main shaft.
Pete Doughty, Nothingham, England. Phone: 01159 262073
- DOME gearkit: This is a helically cut synchro gear kit produced by a Swedish
company that also produces the gears for the McLaren F1 cars. Very
strong (they have never had a gear brake) and uses the standard ratios.
There are a number of different so called "Swedish" gearkits which are not
all the same. Sweden has a number of engineering companies producing gear
kits with varying degrees of quality.
- Quaife Uprated gearkit: This is quite like the DOME kit,but the gears are
only slightly bigger than the standard gears. They are however heat treated
and shot peened to increase strength.
- Quaife Big Tooth uprated gearkit: As above but with bigger teeth (stronger and noisier).
- R&D gearkit - has straight cut 1st, 2nd, + 3rd gears (all standard ratios). Cost
is around £2200
- R&D 6 Speed Straight cut Dog engagement gearbox: This gearbox uses straight cut big
tooth gears to dramatically increase strength. The design of straight cut gears actually
forces them together so unlike helically cut gears. The engagement is by dog ring and not
synchro, this allows a phenomenally fast change, basically only limited by how fast your
hand can move the lever, you also don't need to use the clutch when gearing up,
thus increasing the speed of the change even more. There is an excellent choice of
ratios. Modificationsneeded to fit the box are: Clutch actuation must be hydraulic
and by a center push bearing and the hole where the gearlever comes into the car needs
to be made slightly bigger. The cost for the complete built gearbox is £4,750 + VAT. It
is very loud due to the straight cut gears, but very strong.
- Fixit gearkit - from sweden.
- Quaife group A gearkit costs around £5000. It has different ratios (for rally), so limit's
topspeed to something like 130-140 mph.
- Quaife group A gearbox, basically an MT75 with their gearkit already fitted, costs £7000.
- Quaife 6spd sequential gearbox - This is the ultimate gearbox upgrade from a
performance standpoint of view. It is very very strong, doesn't need to be clutched
and ratios can be had exactly as you want them.
As with everything really serious (solid) there are downsides:
1. The cost: £10.000+.
2. Driving it in the city and traffic can be a pain.
3. The dog-rings wears a lot faster than the synchro rings on a normal
synchro-box. They may last 10.000km in road use, then it's out with the
box and change the dog-rings.
4. Requires modifications to the tunnel, front diff and propshaft. Also requires
modification to the clutch actuation.
Quaife 6 speed sequential gearbox.
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Quaife ATB diff.
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