Rev Matching vs. Double Clutching
08-31-2016, 05:22 PM
#16
Rennlist Member
It isn't a one thing or another.
You need to rev match when shifting so that the engine speed matches your wheel speed in gear.
Those of us who are old enough remember when Synchros were something that lasted about a year and so you had to learn to double clutch if you ever wanted to downshift. I had a 72 Alfa that gave up on its synchros quickly. But modern cars have these amazing transmissions that see to work for more than a year. Marvelous stuff. It amazes me that I can downshift without the double clutch. I do it to get into first while rolling but otherwise don't do it any more. Cool beans to do though
You need to rev match when shifting so that the engine speed matches your wheel speed in gear.
Those of us who are old enough remember when Synchros were something that lasted about a year and so you had to learn to double clutch if you ever wanted to downshift. I had a 72 Alfa that gave up on its synchros quickly. But modern cars have these amazing transmissions that see to work for more than a year. Marvelous stuff. It amazes me that I can downshift without the double clutch. I do it to get into first while rolling but otherwise don't do it any more. Cool beans to do though
"Rev Matching" has become a widely used term in recent years and I could never figure out why we needed the term since we already had "double clutching."
Watching a Bondurant video the other day, I finally figured out that they're different.
3 Options for Downshifting
1. No technique downshift
Actions: Push in clutch pedal. Shift to lower gear. Let out clutch.
- Good for changes when you're going slowly. For example, you've slowed down for traffic, the engine and trans are already at near idle and you're going just fast enough that you don't want to shift into first. You can put it in second and just move on. Simple. Takes essentially no practice or training. (When I buy a used car with a manual transmission, the first thing I want to see is how the previous owner drives the car & trans.)
- Bad for most instances in that increases wear on synchros and the clutch. Can also jerk the rear end loose when you need the traction in a high-performance corner.
2. Rev Matching.
Action: Push in clutch pedal. Shift to lower gear. Blip the throttle to increase the engine rpms. Let out clutch.
- Good for high performance driving. Slightly faster than double clutching. Goal is to get the engine close the rpm's that it will be running when you let out the clutch. Reduces clutch wear. Makes a smoother transition to keep you from losing traction in the rear when you engage the clutch.
- Bad in that it has higher wear on syncros (because you have already shoved the stick into the lower gear before blipping the throttle).
3. Double Clutching
Actions: Push in clutch. Shift into neutral. Let out clutch. Blip the throttle to increase the engine and transmission to close to what it will be in the lower gear. Push in clutch. Shift to lower gear. Let out clutch.
- Good in that it reduces wear on both the syncros and the clutch. Makes a smoother engagement (like rev matching) to maintain traction. Will save you money over time with less servicing and wear on the transmission and clutch. There are older transmissions that require this. But all transmissions benefit from it.
- Bad - takes a little more practice to do it quickly and well. Takes a fraction of a second longer to do it than rev matching--realistically, only relevant to professional racers.
Watching a Bondurant video the other day, I finally figured out that they're different.
3 Options for Downshifting
1. No technique downshift
Actions: Push in clutch pedal. Shift to lower gear. Let out clutch.
- Good for changes when you're going slowly. For example, you've slowed down for traffic, the engine and trans are already at near idle and you're going just fast enough that you don't want to shift into first. You can put it in second and just move on. Simple. Takes essentially no practice or training. (When I buy a used car with a manual transmission, the first thing I want to see is how the previous owner drives the car & trans.)
- Bad for most instances in that increases wear on synchros and the clutch. Can also jerk the rear end loose when you need the traction in a high-performance corner.
2. Rev Matching.
Action: Push in clutch pedal. Shift to lower gear. Blip the throttle to increase the engine rpms. Let out clutch.
- Good for high performance driving. Slightly faster than double clutching. Goal is to get the engine close the rpm's that it will be running when you let out the clutch. Reduces clutch wear. Makes a smoother transition to keep you from losing traction in the rear when you engage the clutch.
- Bad in that it has higher wear on syncros (because you have already shoved the stick into the lower gear before blipping the throttle).
3. Double Clutching
Actions: Push in clutch. Shift into neutral. Let out clutch. Blip the throttle to increase the engine and transmission to close to what it will be in the lower gear. Push in clutch. Shift to lower gear. Let out clutch.
- Good in that it reduces wear on both the syncros and the clutch. Makes a smoother engagement (like rev matching) to maintain traction. Will save you money over time with less servicing and wear on the transmission and clutch. There are older transmissions that require this. But all transmissions benefit from it.
- Bad - takes a little more practice to do it quickly and well. Takes a fraction of a second longer to do it than rev matching--realistically, only relevant to professional racers.
08-31-2016, 05:29 PM
#17
Rennlist Member
It isn't a one thing or another.
You need to rev match when shifting so that the engine speed matches your wheel speed in gear.
Those of us who are old enough remember when Synchros were something that lasted about a year and so you had to learn to double clutch if you ever wanted to downshift. I had a 72 Alfa that gave up on its synchros quickly. But modern cars have these amazing transmissions that see to work for more than a year. Marvelous stuff. It amazes me that I can downshift without the double clutch. I do it to get into first while rolling but otherwise don't do it any more. Cool beans to do though
You need to rev match when shifting so that the engine speed matches your wheel speed in gear.
Those of us who are old enough remember when Synchros were something that lasted about a year and so you had to learn to double clutch if you ever wanted to downshift. I had a 72 Alfa that gave up on its synchros quickly. But modern cars have these amazing transmissions that see to work for more than a year. Marvelous stuff. It amazes me that I can downshift without the double clutch. I do it to get into first while rolling but otherwise don't do it any more. Cool beans to do though
08-31-2016, 06:31 PM
#18
Rennlist Member
FYI, Bondurant hammered us on heel/toe (rev matching) for the first two days of their 4 day Grand Prix school. The entire program is focused on car control through weight management and being able to rev match while downshifting is a key component of this. So I'm fairly certain it has nothing to do with the transmission, at least as far as their program is concerned.
08-31-2016, 06:56 PM
#19
I had a 73 fiat with the same issues... for most.. this is where i developed my technique for making synchros last . It wasnt double clutching, it was dragging the clutch through the shift and bliping through neutral to spin up the lay shaft... if you havent tried it, give it a shot .. it does work and work well.
08-31-2016, 07:35 PM
#20
Rennlist Member
try it sometime. it works for your skip shifts too.
Mark
FYI, Bondurant hammered us on heel/toe (rev matching) for the first two days of their 4 day Grand Prix school. The entire program is focused on car control through weight management and being able to rev match while downshifting is a key component of this. So I'm fairly certain it has nothing to do with the transmission, at least as far as their program is concerned.
08-31-2016, 07:56 PM
#21
I guess in theory, this might be true, but in practice i can show you 10 years of racing and the part numbers can still be seen on my clutch discs when pulled off the car! so, very mimimal wear. And if you understand the force it takes to spin up just the layshaft and gears, you can see its VERY minimal.
try it sometime. it works for your skip shifts too.
try it sometime. it works for your skip shifts too.
For example, my clutch setup is very sensitive to slippage. It is not good for it. I use a 212mm four puck no spring clutch disc that is almost an on/off switch. The difference between significant grab and no grab is not very much and would be VERY hard to modulate as would be required for your technique to work..if it even could work...with my setup.
08-31-2016, 08:02 PM
#22
Rennlist Member
One car and one person does not mean it will work for every car and every person......
For example, my clutch setup is very sensitive to slippage. It is not good for it. I use a 212mm four puck no spring clutch disc that is almost an on/off switch. The difference between significant grab and no grab is not very much and would be VERY hard to modulate as would be required for your technique to work..if it even could work...with my setup.
For example, my clutch setup is very sensitive to slippage. It is not good for it. I use a 212mm four puck no spring clutch disc that is almost an on/off switch. The difference between significant grab and no grab is not very much and would be VERY hard to modulate as would be required for your technique to work..if it even could work...with my setup.
but, on any street car, even the ones with the on or off type of engagement, it works well.
08-31-2016, 08:14 PM
#23
Perfect Angel
Rennlist Member
Rennlist Member
2. Rev Matching.
Action: Push in clutch pedal. Shift to lower gear. Blip the throttle to increase the engine rpms. Let out clutch.
- Good for high performance driving. Slightly faster than double clutching. Goal is to get the engine close the rpm's that it will be running when you let out the clutch. Reduces clutch wear. Makes a smoother transition to keep you from losing traction in the rear when you engage the clutch.
- Bad in that it has higher wear on syncros (because you have already shoved the stick into the lower gear before blipping the throttle).
08-31-2016, 09:18 PM
#24
Rennlist Member
I don't feel like this is quite right. It's hard to gauge since at this point in my illustrious driving career the heel-toe blip downshift is automatic. But I think that I clutch/blip/shift to lower gear, clutch out vs clutch/shift to lower gear/blip, clutch out. If done at the right time it's almost all simultaneous so maybe it makes no difference.
FWIW my sequence is:
1.Brake
2.Clutch in/Blip
3.Lower gear
4.Clutch out/Gas
1-1.5 sec total?
Doesn't everyone do that?
08-31-2016, 09:28 PM
#25
Rennlist Member
08-31-2016, 09:54 PM
#26
Racer
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I'd say I start my blip as the lever passes through N. Closer to the incoming gear than the outgoing. On the street doing lazy h/t the blip is definitely toward the latter half of the motion.
08-31-2016, 10:00 PM
#27
Rennlist Member
Yeah. It makes no real difference since on track it's not a disjointed motion. Symantics.
I'd say I start my blip as the lever passes through N. Closer to the incoming gear than the outgoing. On the street doing lazy h/t the blip is definitely toward the latter half of the motion.
I'd say I start my blip as the lever passes through N. Closer to the incoming gear than the outgoing. On the street doing lazy h/t the blip is definitely toward the latter half of the motion.
I didn't have to rebuild any trannys, except the one on the 911 after 8 years of heavy track duties.......
08-31-2016, 11:37 PM
#29
Rennlist Member
For me blip timing varies car to car, partly due to differences in throttle sensitivity and flywheel spin up and spin down
08-31-2016, 11:47 PM
#30
Rennlist Member
This is so tru. I have two 996 cups cars and one has a Sachs OEM clutch and the other has a Tilton clutch which has a larger disc. The blip timing required are very different between the two.