"Do not downshift into first when ... moving faster than 15 mph"

[shogun32]

My brother wasn't to happy when I had to replace the clutch in his mustang.
The clutch literally exploded.

you were lucky. Had the clutch held you'd have grenaded the engine from overspeeding the rotating assembly. Fuel shut-off wouldn't have saved it since mechanical drive from the wheels would have overwhelmed it.

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[RalphK]

Never got to the point of letting the clutch out, it exploded before I had a chance to let it out. Held the clutch in and coasted to a stop. It was a very loud bang when the clutch disintegrated.

 

[Zooks527]

Down shift into first either just before you stop or stopped. Anything else beats the snot out of the synchros. Leave it in second. Plenty of torque to get you up to speed again when your are already moving.

That is how I shift. That wasn't the question being asked.

That being said, a downshift into first puts no more stress on the syncros than an equivalent downshift in any other gear. The only thing the synchros effect is the rotational speed of the input shaft, the change in inertia of which is a function solely of the change in rpm from one gear to the next. There's far more force transmitted through the cones in the synchros doing a skip gear shift (up or down) than from shifting into first at 16 mph.

 

[ice445]

That is how I shift. That wasn't the question being asked.

That being said, a downshift into first puts no more stress on the syncros than an equivalent downshift in any other gear. The only thing the synchros effect is the rotational speed of the input shaft, the change in inertia of which is a function solely of the change in rpm from one gear to the next. There's far more force transmitted through the cones in the synchros doing a skip gear shift (up or down) than from shifting into first at 16 mph.

The way I understood it is that the synchro isn't the problem, it's that the difference in engine speed versus driveline speed is very high in 1st, and the transition is very abrupt because of that, creating excess shock load or clutch wear. But I could be wrong.

 

[Vlad Soare]

I can tell you what happens when you downshift into first at excessive speed. When I was 15 I wondered what would happen if I downshifted into first at about 50mph. My brother wasn't to happy when I had to replace the clutch in his mustang.
The clutch literally exploded. Oops

That can happen in any gear. Shift into second at 100 mph, or into third at 140 mph, and your clutch will explode as well.
The question wasn't about excessive speeds. 25 mph, for instance, is by no means excessive for first (not with the D4, anyway) and is extremely unlikely to blow up the clutch.

The way I understood it is that the synchro isn't the problem, it's that the difference in engine speed versus driveline speed is very high in 1st, and the transition is very abrupt because of that, creating excess shock load or clutch wear. But I could be wrong.

I can see why that difference may put more stress on the synchros. But on the clutch, as long as the auto rev match works fine (or you match the revs yourself), there should be no ill effect at all.
Now, if you don't match the revs and dump the clutch like an ape, then of course you'll stress the hell out of the clutch and driveline, that's for sure. But we're not talking about that.

Some people seem to think that a six-speed gearbox has five actual gears, plus an extra aid for driving off. Personally I don't see it like that. I see a six-speed as a six-speed. There's nothing magical about the first. It's just a gear like any other.

 

[shogun32]

There's nothing magical about the first. It's just a gear like any other.

true, but with 3.73 gears 2nd is just fine below 10mph. No, it won't be leave-black-marks fast but perfectly useable. Yes I do consider first as a 'throwaway' gear only useful for starting from a stop and zooming to 50'ish MPH. Then again 6th on the D4 is a useless appendage as well. Every time I drive a pre-18' car I get pissed anew at Ford.

 

[Robottrainer]

That is how I shift. That wasn't the question being asked.

That being said, a downshift into first puts no more stress on the syncros than an equivalent downshift in any other gear. The only thing the synchros effect is the rotational speed of the input shaft, the change in inertia of which is a function solely of the change in rpm from one gear to the next. There's far more force transmitted through the cones in the synchros doing a skip gear shift (up or down) than from shifting into first at 16 mph.

Yes he did ask about what harm it has on the clutch. I'm saying its hard on the synchros. You are right in a sense however if there is large ratio difference between first and second, the blocker rings have a harder time dealing with the rpm difference of the gears/countershaft.

https://www.carthrottle.com/post/this-is-why-its-difficult-to-downshift-into-first-gear/

 

[Sivi70980]

true, but with 3.73 gears 2nd is just fine below 10mph. No, it won't be leave-black-marks fast but perfectly useable. Yes I do consider first as a 'throwaway' gear only useful for starting from a stop and zooming to 50'ish MPH. Then again 6th on the D4 is a useless appendage as well. Every time I drive a pre-18' car I get pissed anew at Ford.

So you have a 4 speed with a granny gear and an overdrive you almost never use? Meant to be light hearted of course.

I'm with @Vlad Soare on this one. I use all 6 gears every drive and I'm eventually going to have 4.09 gears and I'll still use all 6 gears all the time. Maybe it's us being too literal. I will say in traffic I'm in 2nd till around 8 or 10mph when I'll clutch and if slows more like down to 5ish, go into 1st. Basically like all of us, use appropriate gear as to not bog the car down under like 1200-1500rpm. I have zero issues being in 6th at 50mph. Do I floor it in 6th at 50mph? Of course not. But cruising in traffic varying from 50 to 75, 6th gear is where it stays. Obviously down into 5th or lower if I need to accelerate faster or happen to cross a Mexican border.

 

[Zooks527]

Yes he did ask about what harm it has on the clutch. I'm saying its hard on the synchros. You are right in a sense however if there is large ratio difference between first and second, the blocker rings have a harder time dealing with the rpm difference of the gears/countershaft.

https://www.carthrottle.com/post/this-is-why-its-difficult-to-downshift-into-first-gear/

But it's not harder on the synchros, at least in the Mustang.

One thing everyone gripes about is how damn close the ratios are in the 2018+ MT86. Looking at the difference between ratios, you get:

1 - 2: 1.54​

2 - 3: 1.47​

3 - 4: 1.42​

4 - 5: 1.23​

5 -6: 1.31​

​

There's only a 5% difference in the rotational kinetic energy required to execute a shift from 2nd to 1st at 16 mph than there is to execute one from 3rd to 2nd at 24 mph. As change in angular momentum of the input shaft plus clutch head is a function of the square of the change in angular velocity (RPM), any downshift that imposes an RPM change of more than 800 rpm will put more wear on the synchros than the Owner's Manual's dreaded "2nd to 1st downshift at 16 mph".

An examples of this would be 4th to 3rd at 44 mph (2000 rpm to 2850 rpm). No one is going to try to argue that such a shift is harmful to the synchros, yet it's requiring the 3rd gear synchro to supply more energy to the input shaft than the shift that the manual warns about.

 

[Grintch]

The manual also probably states don't drink engine coolant too. Are you going to question that as well? I'm not one for arguing against a manufacturer's suggestion written in a technical document.

So you agree with shifting to 6th at 50 mph?

Why does the OP want to shift to first? At the track or even the autocross I can't imagine the shift would be worthwhile, you would lose more time downshifting shifting then almost immediately upshifting again than you would gain from having the extra torque multiplication for a few seconds.

I wouldn't worry about the damage warnings, it's about like the ones that warn about using the "most track ready Mustang ever" at the track. Or about using the built in track functions like the line lock.

 

[Zelek]

So you agree with shifting to 6th at 50 mph?

Why does the OP want to shift to first? At the track or even the autocross I can't imagine the shift would be worthwhile, you would lose more time downshifting shifting then almost immediately upshifting again than you would gain from having the extra torque multiplication for a few seconds.

I wouldn't worry about the damage warnings, it's about like the ones that warn about using the "most track ready Mustang ever" at the track. Or about using the built in track functions like the line lock.

I've shifted into 6th at 50 mph on a relatively flat area with a lower speed limit. 3.73 gears make it not as bad. As long as you don't lug the engine up a big hill, there's no harm. Is it common? No.

 

[shogun32]

I'll be driving along at 70mph for 5 minutes before I remember, oh yeah 6th gear. And that's with 3.73 gears and D4. Like on the Camaro it's nothing but a fuel efficiency dodge. It's serves no other purpose.

 

[Zooks527]

I don't know when I would want to do this but, for sake of argument, 15 mph is 2200 rpm in my car (2019 GT MT PP1). How would downshifting into first at 16 mph (2350 rpm) from second (1525 rpm) do any damage to the clutch?

Why does the OP want to shift to first? /QUOTE]

Is there some rule in this forum that you have to ignore what someone says before quoting him, or do people just find it easier to make stuff up?

The question is why would it do any harm? Not the usual weasel "may harm", "might harm", "could harm" but an explicit "will damage the clutch".

 

[shogun32]

The question is why would it do any harm? Not the usual weasel "may harm", "might harm", "could harm" but an explicit "will damage the clutch".

every time you use the clutch you are "harming" it if you want to get pedantic - a friction lining is spinning at dissimilar speeds to a metal plate and between the 2 of them they are dragging the 2 sides to the same speed and the process generates friction, heat and wears away at the abrasive material on your clutch disk. But 'NO' it will not cause harm if you're using your brain and not just stepping off the pedal.

Happy?

Clutch like brakes are a sacrificial wear item. If you abuse them you'll be replacing them more often than if not.

 

[Zooks527]

But 'NO' it will not cause harm if you're using your brain and not just stepping off the pedal.

Which goes right back to the original question, which everyone is avoiding in order to fire off a snarky comment. Since no one can come up with any cogent reason why it would harm the clutch, why is there an explicit "will damage" warning in the manual.

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