Rev Matching

[wheelhaus]

Originally Posted by RL_BRZ

wheelhaus >> thanks for sharing your info... How will I know how much RPM should I blip the accelerator pedal while downshifting per gear? It's their a table for correct RPM per gear while downshifting?

Actually, yes. See below. This graph is not from an 86, but from a 2007 STi and is shown in KPH, the STi certainly doesn't have a 325mph top speed... The 86 is geared shorter, so all the lines will be farther to the left, but this will work well to express the concept.

Anyways, there are two variables, road speed and engine rpm. The relationship between them is the transmission ratios. Look at the graph and compare the vertical relationship at any given speed. This will give you an idea how far the RPM need to jump to bridge the gap from a higher to lower gear.

Lets say you're in this STi, traveling at 50kph (31mph). If you're in 1st gear, the engine rpm will be 5800rpm, 2nd would be 3600rpm, 3rd 2500rpm, and 4th only 1800rpm. Notice how the gap is very narrow at lower rpm, and much greater at high rpm, all at the same road speed. Now move on to 100kph (62mph) and the relationship is the same, larger gaps at high rpm. As you practice, you'll learn this relationship.

Learning a given car is a practice of three observations in particular, ( @serialk11r touched on this). First two, engine response and clutch feel. Since the 86 is NA, it has rather quick response at all rpm, this is good, as it provides a consistent "feel" when rev matching at different RPM. Third, you must learn the gaps between each gear for a particular car. Since the 86 is designed as a sports car, the gears are pretty evenly spaced until you get to 5th and 6th. Taller gears are closer for two reasons; 1- resistance (wind and mechanical), and 2- reduced torque in higher gears. as you increase the ratios, the torque to the driven wheels is exchanged for speed. So, 5th gear is a closer gap as you need to keep the engine within the powerband to optimize the peak of it's power curve when coming from 4th. From 4th at redline, it's only an 1100rpm shift...

For comparison, many consumer cars (take a generic 5speed for example) have uneven gaps throughout the spread, for instance a car may have a REALLY short 1st gear, and a REALLY tall 5th gear, but 2-4 could be a close spread across the most common speeds the car would see. Short 1st would be for easy starts, tall 5th would be for highway cruising. The STi is the same way, you can see 5th gear is much closer, but 6th is a taller gap because it's almost useless on a race track and instead left for highway cruising and MPG. (A race car tranny would be different, where 6th would be geared for peak powerband for the longest straight on a given track).

The point is that as you practice, you'll begin to "feel" the required rpm jump necessary to grab the next lower gear. This relies mostly on your RPM before the downshift. If you're at 5000rpm, you may need to jump 1800rpm. If you're at 1500rpm, you may only need to jump 400rpm. If you were to visualize this graph over your tachometer, the lines would constantly move with the tach needle, but this visualization may help you learn, the lines grow apart at high RPM, and contract at low RPM. With practice, you'll learn to just go by feel and the sound of the engine.


edit- A trick I've used on previous cars is to shift between two gears and maintain a constant speed. For instance, cruising 50mph down a road, shift between 3rd and 4th repeatedly, and 4th and 5th repeatedly without changing speed. Doing this between different gears at different cruising speeds helps you learn the gaps. Upshifting between two gears will net the same gap as downshifting the same two gears. So even if you're only accelerating to get on a freeway, watch where the RPM leaves one gear and catches the next. At that RPM, you'd need to rev the same amount to downshift to the previous gear again.

[wheelhaus]

Originally Posted by vascopajama

Dave
2011 Kawasaki Z1000

Hi Dave.
I too ride bikes Z9s. Z1000s, now a Suzuki GSX. The thought of changing down without matching scares me as it should anybody that rides. It amazes me that you see riders around that dont match. They've obviously worked out a way of getting all their ham fisted changing over before the lean in in the wet, Otherwise they'd be all but gone!
And of course the same goes for cars if your into driving well that is. You can forget about going fast if you can't match your revs.
I've often wondered if your saving your synchros by 'double declutching' I often do it in older cars if I think the synchros are on the way out.
Do you think changing is easier (goes into gear easier) if you match but don't double declutch? I guess no as the engine isn't conected to the gear box when the cog swap is going on.

For bikes, they use a sequential dog box, so you're always in a gear.. The back-torque-limiting designs found on some sport bikes means you can bang down the gears and not upset the bike. It's great for race bikes, but pretty lazy for street bikes... Even still, I want one, lol... Rev matching on a bike with a normal clutch is crucial! It's pretty dangerous unless you're just cruising lazy. Riding at full lean is dangerous as it is, throw in a botched downshift and you're served up with a guaranteed lowside.

I 'double declutch' because I enjoy it, it makes the whole downshift feel smoother when the stick just slips right into the gear without a bump. It really does feel easier and smoother. It does reduce wear on the synchros, as much as it's debated, I believe it's worth it.

[SupraLove]

Quick Questions:
1. It isn't really necessary, but it preserves your synchros, which are hard to replace.
2. Yes, it is.
3. Only single-clutch when accelerating. Double clutch is only for downshift.

The second is correct.

B) Clutch in, shift down and simultaneously rev engine to desired RPM, release gas, clutch out, then get back on gas to accelerate

You are very welcome! :happy0180:

[wheelhaus]

Originally Posted by SupraLove

...
B) Clutch in, shift down and simultaneously rev engine to desired RPM, release gas, clutch out, then get back on gas to accelerate

Technically that works, and I used to do it that way, but it's technically incorrect because you're revving the engine before the clutch is re-engaged, you're using clutch slip to speed up the input shaft instead of using the engine rev to do it for you. I also had the same misconception while I was learning.

Proper double 'de'clutch downshift (3 steps)
1- clutch in - neutral - clutch out (this re-engages the input shaft to the engine while no gears are engaged)

2- rev match (this revs the input shaft with the engine)

3- clutch in - shift down - clutch out (here the stick slips right into gear, the input shaft is already at engine RPM)

aaaand accelerate (or repeat).

You blip the gas once. You punch the clutch twice. The stick movement is normal for a downshift, but there's a pause while in neutral.