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Old 03-24-2014, 02:32 PM   #11 (permalink)
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Originally Posted by johnlvs2run View Post
Sorry for my confusion. I've been coasting with the engine on to this point, and lose the 2 mph when going back to 5th gear. I've not used bump starting on trips yet due to the car not coasting very far, but will try it on the return trip tomorrow. I presumed that going back into gear shouldn't lose the 2 mph either way and even less so when the engine's already on. In any case, the previous answers should be helpful either way.

Okay, I will do that, thanks very much.

I learned on a stick shift long ago, and never had this trouble before.
That's what i figured, because you should not lose speed by bump starting...assuming your engine is OFF while you are bump starting.

There is no point in bump starting if the engine is already running

Bump starting allows you to turn the engine on again without using the starter/ignition...thus saving excessive wear on the starter.

The reason you feel friction with the engine on/pop clutch in 5th is because both engine AND transmission are spinning at different rates. When you pop the clutch, both are causing friction against each other, thus you lose too much speed.

If you bump start the car with the engine OFF, you are simply using the transmission to turn the engine over (instead of having the starter turn the engine over), thus there is less friction, thus you lose less speed.

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Old 03-24-2014, 05:47 PM   #12 (permalink)
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confusion

I see. If you're just going back into gear after a neutral engine-on coast, none of this stuff applies.

You just select the right gear for yor current road speed, blip the throttle to match the engine rpm to the trans rpm and engage the clutch.

I'm going to split this discussion into a separate thread to avoid adding to the confusion for future readers.
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Old 03-25-2014, 08:44 PM   #13 (permalink)
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If the engine's running when you go to re-engage a gear, you should lose no speed at all provided you rev-match as you re-engage the clutch.

That's what the "blip the throttle" part was about: bringing the engine speed up to where it should be for the current gear & road speed just before you engage the clutch.

I'll see if I can find a YouTube vid...
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Old 03-25-2014, 08:44 PM   #14 (permalink)
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PS: I split the thread.
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Old 03-25-2014, 11:03 PM   #15 (permalink)
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shifting from neutral to gear

I used engine off coasting during my entire 126 mile trip today, and couldn't tell the difference between shifting from neutral with bump starting or with the engine already on.

Bump starting up to 35 mph was very smooth and made no difference at all in the speed. I didn't try with the engine on at that speed. However, bump starting at 50 to 60 mph continued to have the same jerkiness as shifting from coasting with the engine already on.

I did manage quite a few shifts today with no loss of speed (engine on or off), which is quite encouraging. Jedi_sol's comment to shift slowly can be helpful to find out where the mistake is happening that's causing the loss. I will practice this more the next time.

Thank you for all the helpful suggestions!
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Old 03-27-2014, 06:38 AM   #16 (permalink)
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While no doubt people do get away with doing it, I'm not a fan of bump starting. The reason is the clutch pressure plate is designed for the engine to drive the car, not the car to drive the engine.

Google up a few images of clutch pressure plates and you will see that in most cases the connection between the clutch cover and the ring that clamps onto the clutch driven plate is made using straps of steel.

With the engine driving the car those straps are in tension - and very strong in that direction. Reverse the direction of drive and those straps are in compression and liable to buckling. Of course the torque required to bump start an engine is much less than the engine is capable of generating in the opposite (correct) direction.

There will always be some loss of speed with bump starting because you are transferring kinetic energy from the rolling car into the rotation/reciprocation of the engine parts. That the energy required to do the latter is small relative to the KE of the car means you can do it smoothly enough to be barely perceptible.

A minor point when rev matching is to match the engine revs to road speed - and hold there - before disengaging the clutch (depressing the clutch pedal). That way the gearbox shaft and gear speeds also rev match.

If you disengage the clutch before rev matching, the shaft and gear speeds aren't rev matched because they are disconnected from the engine. The inertia of the shafts and gears is low so it's not super critical but it is better to do it that way.

(That is why you de-clutch in neutral when double de-clutching. You don't have to come all the way off the clutch pedal when doing that, just enough to drag the driven plate with the pressure plate i.e. right at the clutch take up point.)

You will know when you are doing it right when there is very little resistance through the shift lever as you select the gear.

If you get really good at it, it is possible to select the gear without depressing the clutch pedal at all. Don't practice that on a car you care about though.
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Old 04-03-2014, 12:49 PM   #17 (permalink)
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jumpy RPMs

This past trip I watched the RPM dial to match 2000 to 2250 RPMs with shifting back into gear, which was not as intuitive as I thought it should be. Just a slight pressure too much caused the RPMs to jump to 3000, and a slight pressure less caused them to go down to 1000. I had to watch and consciously hold the pedal in exactly the same place, so the RPMs would stabilize and then the shifting would be smooth.

Thus the issue is not the clutch. It's the extremely jumpy gas pedal & throttle.

I looked and am pretty sure the throttle is operated by a cable, not drive by wire, but it seems to have the same type of issues. Is there a relatively simple way to even out the pressure and throttle response so they are more linear?
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Old 04-03-2014, 02:44 PM   #18 (permalink)
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Quote:
Originally Posted by johnlvs2run View Post
Is there a relatively simple way to even out the pressure and throttle response so they are more linear?
Honestly, its all about TIMING and practice practice practice.

the reason your car either jerks or lags after shifting back into gear is because the engine/tranny speeds are simply not matching perfectly when you release the clutch.

This means you need to know what the rpm should be at any speed, at any gear...which is a bit daunting

when you get good at it, no need to hold the clutch or rpms, you should be able to just clutch, blip, shift in one smooth motion and no jerkiness

Too bad you're not in so. cal, I can show you how
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Old 04-04-2014, 03:15 AM   #19 (permalink)
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I realize what you are doing wrong , holding the throttle at a steady 2000 rpm is not rev matching. That is the reason that car seems to JERK whenever you release the clutch. This is because at whatever speed you are traveling, 2000 rpm is either too high or too low. it does not exactly match the revs you need to be at that given speed that gear you select.

For example let's say you are traveling 60 miles per hour on the freeway.

In 5th gear their revs are at 2500rpm

In 4th gear the rev are at 3000rpm

In 3rd gear their ribs are at 4000rpm

In 2nd gear the revs are at 5000rpm.

Then if you are coasting in neutral let's say for example you want to shift into 3rd gear from neutral while driving 60 miles per hour you will need to Blip the throttle to 4000 rpm.

This is called "rev matching." You blip the throttle so that the engine rpm matches the transmission speed.

One way to practice this is every time you downshift, keep an eye on how far the revolutions increase between each gear.

Now go back to our example if you are driving 60 miles per hour in 5th gear then you down shift into 4th gear revolutions jump by 500 rpm then if you downshift to 3rd gear revolutions jump another 1000 rpm. Therefore if you are in 5th gear and you want to down shift to 3rd gear you need to blip the throttle so that the RPM's jump up 1500 rpm, then release the clutch.

Eventually when you get better you should be able to down shift through all of the gears and the car will not jerk. You will also be able to determine what the rpms need to be at for any speed and any gear.

Therefore as I mentioned in my previous post shifting smoothly is just something you will need to practice because you will need to get familiar with your transmission and engine and know how many RPM's jump between any given year at any given speed. trust me it just takes a lot of practice. Yes you can smooth things out by letting off the clutch a lot slower but then all you are really achieving is just extra wear on the clutch.

Try to find some YouTube videos on rev matching and how to downshift.

On my del sol that I owned for 10years, I knew the gear ratios like the back of my hand, I could drive it smoother than than an automatic transmission. Now I have a new car and It's annoying because I have to learn the new grear ratios all over again.
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Old 04-04-2014, 05:38 AM   #20 (permalink)
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Yea this sounds like you just haven't learned to rev match yet. With your car parked, play with the throttle a bit. Aim for 1500rpm, 2000, 2500, etc. and try to get a feel for how much of a jab you give the throttle to achieve that speed from idle. Then hold the engine at a higher rpm say 2000 and then try to hit 2500, etc. When you drive the car around daily, when you slow down always perform downshifts, and you'll learn very quickly. The point is that the clutch should not be doing any work on a downshift.

Once you know how to do this, you should be able to execute shifts as smoothly as an automatic transmission.

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