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My experience with the Rev-matching technique

Shehriyar Qureshi
·Apr 15, 2023·

Photo by the blowup on Unsplash

• Term breakdown
• The technique
• How it works

I was always interested in driving cars as a kid. This interest made it easy for me to learn to drive when I grew up and got the chance.

I don’t remember how I heard of this technique but was interested nevertheless.

Now, after some time practising, I’ve kinda got the hang of it.

If you don’t know what this technique is, here’s a quick rundown.

Term breakdown

Rev-matching, or if I try to interpret the expanded word, revolution matching, or if you look at the gauge cluster too much, RPM matching.

The technique

The term itself is generic. By itself, in the context of cars, it means matching the engine speed, which can refer to matching by either increasing or decreasing the engine speed.

It is, however, mostly referred to in the context of downshifting.

In the context of downshifting, it is the act of raising the engine speed (RPM) to match the clutch speed, which gets increased when the next shorter gear gets engaged.

How it works

Engineering Explained has a nice video of how it works

Taller gears have less torque and higher top speeds, shorter gears have more torque and lower top speeds.

For any given vehicle speed, any transmission gear will put your engine at a different RPM.

Jason Fenske - Engineering Explained

Fact in Math

For a given vehicle speed \(V\), engaged in gear \(G\), puts the engine at \(RPM_{G}\).

The relationship of engaging the next shorter or taller gear with the engine speed is as follows:

\$\$RPM_{G-1} > RPM_{G} > RPM_{G+1}\$\$

Note that we assume a constant vehicle speed for the above relation.

My personal experience

The technique itself seems simple i.e. we just have to increase the engine speed to match the increased clutch speed when downshifting. So what’s the right way to do this?

The collective answer is “It depends” But that doesn’t help us understand it. Let me go over my findings and try to explain why there is no definitive answer.

Modulation of the pedals

This has to be the biggest reason. Driving instructors commonly tell their students that the pedals are not an on-off switch, but are supposed to be modulated.

If they were an on-off switch, it would’ve been easier because the pedals would only have two states, and we would have an easier time knowing when to switch each pedal on or off.

But we don’t live in the computer world. We instead have to modulate the pedals. Each time we rev-match, we can change the timing of different portions of the technique and still get the same result.

The right way to rev match

We get the point now. When downshifting, we need to blip the throttle before letting out the clutch, so that the speed difference between the clutch and flywheel/pressure plate is minimal.

This is where we get different ways of blipping the throttle.

• blip the throttle, then let out the clutch

• blip the throttle as you’re letting out the clutch (TheTopher)

Essentially, this can boil down to:

• reduce the speed difference before slipping the clutch

• reduce the speed difference while slipping the clutch

Note that these both also happen at the same time. Even when we blip before slipping the clutch, there will still be a speed difference because we’re not 100% accurate, which will be corrected by the clutch slipping.

In conclusion of the above, there is no single right way to rev match. It is different for each car.

My personal experience

I started learning this technique on a Suzuki Mehran, which has a small inline 3-cylinder. I had a really hard time following the practices mentioned on the internet.

It all clicked when I started rev-matching on an inline 4-cylinder engine. That is when I realized that the blip timing depends on the engine as well.

Larger engines (4-cylinder) are slow to rev up, but also are able to hold their RPM before falling back down.

Whereas in Mehran, the small 3-cylinder engine was very quick to rev up, but the revs also fell very quickly.

Mehran Experience

Constraints

• The engine will not hold its RPMs for long before falling, very nimble engine

I realized it was way easier to start blipping as soon as I’m disengaging the clutch, so the revs don’t drop further.

Honda City Experience

This is the car where I was able to smoothly not just downshift, but also learn double-clutching and heel-toe downshifting.

Rev-matching is significantly easier on a 4-cylinder due to the fact that:

• when the clutch is pressed and I haven’t yet blipped the throttle, the bigger engine will hold its RPMs for a while, so I have more time between the clutch press and throttle blip since RPMs will not start falling instantly

• after blipping, the revs rise up after a delay due to the engine being heavier

• revs rise slowly

• revs hold their peak for longer

• revs fall slowly

This is because a 4-cylinder engine has more mass and inertia than a 3-cylinder, so it takes time for it to speed up and fall, but the revs are also able to stay at their peak before falling.

To test it yourself, pop the hood of a 4-cylinder, get yourself closer to the engine, and let someone very quickly jab the throttle to 0%-100%-0%. You’ll hear the intake sound first, followed by the engine revving after that. These both happen one after another.

Do the same on a 3-cylinder and you’ll notice that:

• intake sound and the revs rising overlap a lot more

• the duration is smaller

All of the above points make it very easy to not just rev-match, but double clutch as well. How? Because you’re able to overlap the steps while the engine is (slowly) revving up. When the engine reaches the peak RPM after the blip, you’ve already performed most of the double clutching and can match the final clutch release on the peak RPM.

In short, a 4-cylinder engine gives us more time to overlap the steps, while on a 3-cylinder, it needs to be done quickly and hence is harder to perform perfectly.