Robert Bowen
Robert Bowen Editor
1/4/18 1:15 p.m.

Manual transmissions are crude, simple devices. Most of the internal parts are big, heavy and made from steel alloy, which makes them tough. The basic technology—helical gears and sleeve synchronizers—has been around for more than 100 years. This is good, because we enthusiasts tend to ignore our gearboxes until something goes wrong—like when nasty noises appear or the shifting becomes …

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jdoc90 New Reader
1/5/18 7:51 p.m.

Of course none of this will help an alfa romeo 5 speed transmission 2nd gear downshifts .The grunch was added to the dna and cannot be overcome even by stem cell transplant .lol  So the 4% of Americans who can use a manual transmission can now keep the 1% of cars still running with one going longer until they are outlawed and replaced by a toaster with wheels ! viva la manuale !

mad_machine GRM+ Memberand MegaDork
1/5/18 9:45 p.m.

Both of my manual equipped cars come with fragile transmissions. Both are flawed from the factory simply because they were designed for lower power levels than the engines they are bolted to. The Fiat Box is not a bad one, but it's biggest flaw is that it uses springs to hold it in gear instead of springs to push it out like most modern boxes.

The saab 900 box was never designed to hold the 185hp the turbo engine puts out. Combine that with the heat of the turbo next to it and the temeratures of the engine oil that uses the side of the box as the sump, and you are looking at a transmission doomed to an early death.

Empathy will go a long way. When I was much younger I destroyed a rebuilt fiat box in the course of a summer by doing reverse to forward burnouts. Reversing the power through the gears quickly shredded that transmission. Older and hopefully wiser now, and with many years driving big commercial trucks with manuals, I have no urge to powershift or rush a shift.

Vigo UltimaDork
1/5/18 11:03 p.m.

I disagree with the statement that rev matching without double clutching will do anything to ease the job of the synchros. IF the clutch is working properly, there will be almost no power transferred while the clutch pedal is pressed, so the speed difference between the mating gears will not be substantially reduced. If you have an unsynchronized reverse gear, it's pretty easy to tell whether your clutch imparts any rpm to the input shaft and all of its meshed gears by simply slowly shifting into reverse while the clutch pedal is pressed. Most (maybe all?) unsynchronized reverse transmissions use an idler gear that slides back and forth to engage. If the gears being meshed by the sliding idler gear are not at a stop, the 'clash/grinding' of the gears can be heard or at least felt through the shifter. If your reverse idler doesnt 'clash' trying to engage reverse while revving the engine with the clutch pressed, that means that all those gears are stopped and that by extension revving the engine while the clutch pedal is pressed is not going to do anything for your synchros because it doesnt transfer enough torque to overcome the friction of all the parts and actually impart rpm to the input shaft.

What rev-matching without double-clutching will do for a racer is reduce the shockload to the drive tires when you release the clutch. If you downshift the trans without rev-matching, you are asking the tires' contact patch to transfer energy from the movement of the car to spin up every component upstream of the synchro you just engaged by probably thousands of rpm. The input shaft, all of the gearsets, the clutch disc, pressure plate, flywheel, crankshaft, rods, pistons, timing parts, cams, everything on the belt drive, yadda yadda yadda. Spin that 100+ lbs of crap up by a couple thousand rpm in a fraction of a second. That's a pretty huge load, sometimes enough to break the tires' traction even while coasting in a straight line. If you happen to be already using 90% of your available traction turning around a racetrack, that additional load on the tire will cause you to break traction and depending on which wheels are driven you'll either oversteer or understeer.    


 Cliffnotes: rev-matched 'single-clutched' downshifts are less likely to cause you to lose control while turning hard on a racetrack. Rev-matched 'double-clutched' downshifts do that AND reduce wear on your synchros both on the track and on the street.


mad_machine GRM+ Memberand MegaDork
1/6/18 7:50 a.m.

In reply to Vigo :

rev matching also reduces the "reverse" load on the clutch, transmission, and differential as those units now have to deal with torque coming FROM the drive wheels back to the engine rather than the other way around. If you can bring the revs up to where they would be, it is a softer transition to the entire drivetrain

Knurled. GRM+ Memberand MegaDork
1/6/18 9:29 a.m.

In reply to mad_machine :

Depends on if your transmission has weak synchros or weak mechanicals.


Of course the real fix is to not use the clutch at all, no synchros = no problem!

jerel77494 New Reader
12/7/21 3:05 p.m.

Check the online forums, too.  Sometimes you'll find info that can help.  I replaced the gearbox oil in my NB Miata with the "forever" oil Ford uses in their Mustangs and it shifts even smoother now.  Plus it's a synthetic, so the box runs cooler.

KozyB GRM+ Memberand New Reader
9/9/23 3:19 p.m.

Another vote for double-clutching with rev matching!  Practice it, make it a habit.  Your transmission will thank you, and your work-a-day drives become opportunities to be smoother and more one with your car. 

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
9/10/23 3:11 p.m.

In reply to KozyB :

Like everything, there is a lot of gray area.  The ratio and therefore speed change makes a big difference.  5-4 downshift in a Miata trans at highway speeds?  You're not going to affect much by allowing the synchro to speed up the geartrain.  Trying to downshift most transmissions to 1st gear at anything but walking speed?  Give it a double declutch to rev up the geartrain.  Something in the middle?  Big fat load of "it depends"... the larger the ratio gap, the heavier the transmissions' rotating assembly, the more the transmission will appreciate it.


This is one really nice thing about transverse units, the shifted gears are usually on the input shaft, so a shift only has to change the speed of the input shaft and the clutch disk.  Rear drive transmissions usually have the shifted gears on the output shaft, so a shift has to change the speed of the (rather heavy) countershaft, and all of the driven gears (which are slaved to the countershaft), and the input shaft, and the clutch disk... basically ALL of the rotating mass that is not the output shaft.


It would be amazing if someone made a RWD transmission that was "backwards".  I am 95% sure that Honda S2000 transmissions are.

weedburner Reader
9/10/23 6:35 p.m.

There is a seventh step to maximizing a manual transmission's potential that most seem not to be aware of- optimizing the clutch's engagement rate. Excessive clutch torque capacity makes it possible for sudden engagement of the clutch to send huge spikes of energy back and forth between the engine/transmission/drivetrain/tires. Those spikes occur not only during launch, but can also occur after the shifts. Sure an accomplished driver can minimize the effects manually, but the more excess clutch torque capacity there is, the narrower the driver's margin for error will be. If you have a weak transmission, it may not take much of a driver error to break it.

Here's a link to one of my webpages with background info on how the clutch can greatly intensify the energy spikes sent thru the transmission. The page is aimed at drag racing, but the same general forces have the potential to wreak havoc on your transmission and tire contact patches any time the clutch is engaging...

In an ideal world, the proper way to minimize the problem would be to install a clutch that is ideally suited to the application. The problem with that solution is that the steps between different levels of clutch capacity are fairly large, usually forcing you to choose a clutch with too much torque capacity just to make sure you have enough. Fortunately if you must choose a clutch that has excess torque capacity, there are external devices that can make an overkill clutch a better match to it's application.

Many oem's actually use a "clutch delay valve", basically a one-way restriction to slow clutch engagement for the purpose of protecting the drivetrain. There's also lots of u-tube videos out there dedicated to removing those clutch delay valves. You can get away with small amounts of simple in-line fluid restriction without much trouble, but when you restrict fluid return from the slave to the master in quick successive strokes like racer's do, the master cylinder's internal return spring causes additional fluid from the reservoir to be drawn past a collapsing cup style piston seal. This is not a problem during casual driving, as the additional fluid drawn past the seal has time to return to the reservoir before the next shift. But when clutch pedal cycles come in quick succession, that excess fluid does not have time to return thru that small compensation hole which in-turn causes an increasing pump-up effect that messes with driver coordination. In the end the driver thinks he missed a shift, but what actually happened was the pump-up effect caused over-stroking of the slave, which in turn produced an rpm flare caused a delay in the clutch grabbing after the shift.

Here are some examples of aftermarket flow restriction valves that function much like the factory delay valves, except these aftermarket versions are adjustable...
...Tilton Flow Control Valve is a flow restriction device that allows you to adjust your clutch's engagement rate. You can read about it as well as the reasons that you might want one here...
...Clutch Masters Flow Control Valve does basically the same thing as the Tilton, you can read about it here...
...Magnus Launch Control Device eliminates the pump-up effect by adding a solenoid bypass parallel to the in-line fluid restriction, making the restriction path active only during launch. You can read about it here...

While flow restriction devices are effective for reducing impacts to the transmission, a common problem with all the flow restriction devices is that they all restrict fluid return flow during the entire pedal release cycle- including the deadband area before the clutch actually begins to engage. This can be a big deal if you are a drag racer, as delayed travel thru the clutch pedal's deadband area during launch also adds to your car's reaction time. The typical "fix" for the resulting slow reaction time is to pre-load the clutch against a hand brake prior to launch, which serves to miminimize the clutch pedal's deadband travel. The problem with that in a drag race scenario is that that pre-loading the clutch causes it to heat up. If the opposing racer knows you are pre-loading the clutch prior to launch, they will likely delay their staging to put additional heat in your clutch, which will likely cause your pre-loaded clutch to pull thru the beams pre-maturely resulting in a red-lite.

The ClutchTamer is a clutch engagement control device that is unique in that it controls pedal return rate instead of fluid return rate. It does this by acting directly against the pedal itself, which completely eliminates the problem of drawing additional fluid past the master cylinder's piston seal. Also because the 'tamer does not become active until after the pedal has passed thru it's deadband area, reaction time of the car does not get delayed. The need to pre-load the clutch to improve reaction time is completely eliminated. You can read about my ClutchTamer device here...

In addition to the ClutchTamer, I also make the Hitmaster 2-stage clutch control device for drag racers. My ClutchTamer device I linked above is a better fit for the typical Grassroots guy.


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