Lightly holding the gas for revs on downshift

[rationull]

On my daily drive I need to downshift (manual trans) regularly for hills. I'm not practiced enough to double clutch quickly and smoothly, but I do try to match revs to avoid too much clutch slipping on the downshift. I've found the following strategy to work fairly well (i.e. allows smooth shifts) for downshifting a single gear at moderate speeds:

Just as the incline is starting, disengage the clutch with just enough pressure on the gas pedal that the revs rise to the where they should be. Shift down and re-engage the clutch.

Using this strategy, the clutch doesn't have to slow down or speed up the car at all. However, since there's a little pressure on the gas pedal there's more torque coming out of the engine than there would be with no throttle.

Really the only difference between this and a throttle blip to match revs is a little bit of timing. Is having a little bit of pressure on the throttle (and thus a little bit of added torque) while [quickly] working the clutch going to increase clutch wear? Should this practice be completely avoided in favor of more traditional rev matching?

 

[tomcat27]

double clutching a normal car? I'm not sure that I could do that without alot of practice. I also thought that was only for the big rigs that dont have syncros.
I dont see harm in what you are doing. do you always downshift (using the engine to slow the car) every time you slow down? I stopped doing that after putting 2 clutches in a car in 3 years. ok, my sister also trashed on the car... but hey! since then I haven't trashed a clutch in 20 years.
brakes are cheaper and easier to replace than a clutch.
I also used to shift my car without using the clutch - with careful throttling you can take the load off of the gears and put the car in neutral without clutching; then let the revs drop, apply light pressure on the shifter and the transmission will easily "snick" into gear when the gears match speed.

 

[aquariuscsm]

I always throw it in neutral and coast down hills.

 

[semaj281]

Double clutching isn't necessary on cars with synchros. Don't let Vin Diesel's stupid lines from the Fast and Furious make you think otherwise.
However I would be more in favor of traditional rev matching (i.e. clutch fully disengaged, then rev as you're moving the shifter) because your method might induce unnecessary clutch slippage during the process. I'm sure it's not a significant amount of wear, but it can add up.
And I also wouldn't downshift to slow the car down, unless I knew I'd have to accelerate again immediately. (i.e. slowing down to turn into a highway onramp and some offramps)
As for shifting without the clutch, it can be done with practice but I've been told not to do it on cars with helical cut gears, which is what most passenger cars have.

 

[semaj281]

Originally Posted By: aquariuscsm
I always throw it in neutral and coast down hills.

I've been told that with these computer controlled fuel injection engines, it would save you fuel to just leave it in gear and coast. That way once you let off the gas pedal, the injectors would actually stop spraying fuel, and the engine would be running on drivetrain energy. (the same type of energy used to push start a car)
But then again the difference in fuel savings isn't drastic, so do what's comfortable for you.

 

[TXMXTruck]

I fully agree with semaj281's comments regarding matching revs. Also, I've been driving manual transmission cars and pickups since 1977 and would never downshift to slow a vehicle.

My present 1998 5-speed full sized pickup has 243,000 miles on the original clutch. With my driving habits, the only time I have ever replaced a clutch has been due to something like a throwout bearing wearing out and making noise. I then do the clutch since most of the labor cost is already there....

 

[getnpsi]

Depending on the terrain and if I'm familiar, I actually downshift or at least neutral coast BEFORE the crest of the hill so I'm already slowing a bit, then gravity puts my speed back to where it was. The decel cut off happens quicker as well.

 

[labman]

Hope you stick to a more constant speed when there are people behind you and don't force them to slow down too. You could get rear ended by a cell phone user.

 

[JHZR2]

I always leave it in gear until about 1500 RPM if I know I am slowingstopping, to take advantage of injector shutoff. If I do need to downshift (I do not do this to get braking out of it), I do double clutch, as do I anytime that I am maintaining momentum, say, coasting in neutral for some reason. Makes engagement more smooth, and while the synchros will indeed make the shift happen, it is much happier to plop in gear when everything is roughly at the same speed.

 

[ls1mike]

Originally Posted By: labman
Hope you stick to a more constant speed when there are people behind you and don't force them to slow down too. You could get rear ended by a cell phone user.


Or say someone towing a 28 foot 5th wheel behind you who doesn't need to slow down. I hate it when people do that and they know I am behind them.

 

[The_Jeff]

OP never mentioned if it was downshifting for UPhills. His procedure is good for saving clutch but does nothing for synchros. Really though since 2nd and maybe 1st and 3rd are the first synchros to wear out I would not worry about wearing 4th.

My stupid modern cars have increased idles when rolling as an emissions dashpot, which makes snicking into other gears easier. I still double clutch (involving a blip in neutral) not only because it's fun but smoother going into 2nd in my high mileage stuff.

 

[rationull]

Sorry I wasn't clear but I was talking about downshifting to climb, not to descend. I don't generally downshift to slow down, although I will downshift for steep hills occasionally depending on traffic and weather conditions.

I stay in gear for downhills to use deceleration fuel cutoff as others have alluded to here. Plus if I shifted into neutral for the grade on my commute and didn't ride the brakes I'd probably be going 100+ mph by the bottom which wouldn't be all that fun

I'm not too worried about my 4th gear synchros, mainly the clutch. And I'm not even really "worried" about the clutch, but it occurred to me the other day that even though my strategy doesn't cause the clutch to change the car's speed very much it might wear it a bit due to having a little bit of throttle while shifting.

I can rev match semi-decently without holding the throttle on shifts but with a DBW throttle it can take some practice to give the pedal the right amount of "blip".

 

[PeteTheFarmer]

My guess is that you'd get less wear on the clutch disk with your method than by not increasing the revs at all, since if you do it right the difference in speed between the flywheel and the clutch disk should be reduced. The extra engine torque that you mention would concern me more with its possible effect on the rest of the drivetrain than the clutch disk.

 

[rationull]

Originally Posted By: PeteTheFarmer
The extra engine torque that you mention would concern me more with its possible effect on the rest of the drivetrain than the clutch disk.


The rest of the drivetrain would already be spinning at approximately road speed anyway though, wouldn't it? Since the engine torque isn't enough to change the vehicle's speed then it should have a negligible affect on the rest of the drivetrain, by my thinking.

 

[PeteTheFarmer]

If it's not enough to cause acceleration, than I concur it would be negligible. If you had a more severe downshift where you would need to hold it at say 3K rpm to get a good rev match it might be a problem.

 

[rationull]

The scenario I'm talking about calls for an RPM change from about 3000 RPM to about 3500 RPM (rounded up) from 5th to 4th gear. Just barely holding 3500 RPM or so doesn't provide enough torque to accelerate in my 1.8l Honda engine

 

[PeteTheFarmer]

So the RPM difference is 500. Since you're not letting the engine drop to idle, the small amount of throttle you need for the increase shouldn't hurt anything. Some might question why you're even bothering to rev match with such a small difference, but its good practice, and becomes a habit after some time.

 

[willix]

RPM matching? Having comfort with the machine is of utmost importance. Its drive line fung shway. To downshift without upsetting the apple cart. It similar to riding a bicycle, once learned. You just spike the throttle find the sweet spot, and it drops in gear without force and clutch engagement is uneventful except for smoother increase in rpm to power up, power down. This proccess happens inside 1 second of time.

 

[JohnBrowning]

As long as you are not well past the maxmium listed speed for each gear by the OEM you should be fine. I know how to double clutch and have a commercial drivers liscense but never double clutch my daily driver! WHy would I TOyota but synchros in the transmission for everything but reverse!

You shouldnot need to blip the throttle at all for a down shift!

 

[JohnBrowning]

Oh an just so you know Eaton wants all the old timers included to use a 1/4 inch of clutch. If they could get the dinosaurs still driving tractor trailers to use 1/4 of clutch the transmissions would last 1 million miles as well. Even though it feels smooth the input shaft takes a beating. Eaton was one of the sponsors of the truck driving school I went to. We had on top of everything else a $50,000.00 shift simulator that simulated all the various transmissions in use you had to select engine and transmission combo at each simulation run. It was not forgiving at all lots of grinding going on if you did not get it right.

We also drove trucks that where riged to simulat driving on ice and jack knife simulator truck as well.....

In fact it will not be long before only owner operators will have full on manualy shifted transmissions becasue auto-shifters are getting so reliable....A computer can shift the truck better then a person and gurantee that 1/4 of clutch is used. Oncethe pressure to drive down fleet warranty costs and fuel use hits even harder you will see more and more of them. RIght now USA has all auto shifts I think.