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-   -   How to smoothly bump start / clutch start an engine (https://ecomodder.com/forum/showthread.php/how-smoothly-bump-start-clutch-start-engine-1141.html)

MetroMPG 02-22-2008 08:33 AM

How to smoothly bump start / clutch start an engine
 
This question comes up enough that it's worth a dedicated thread.

The goal is to re-start the engine at the end of an engine-off coast, while the vehicle is still in motion. You want to impart as little shock/wear to the drivetrain / passengers as possible.

Don't ...
  • ... don't simply dump the clutch in the gear you plan to use. Not smooth! It's hard on the drive train & potentially dangerous depending on the circumstances (sudden engine braking of the drive wheels).
  • ... as with other advanced techniques, DO NOT try to learn this in traffic! Learn on a deserted road or a parking lot where there's no traffic.
Do this instead:
  • 1) select a higher gear than you would otherwise be in for the vehicle's current road speed. (EG. I'll typically select 5th to clutch start at speeds where I'd normally be in 3rd, 4th where I'd normally be in 2nd, etc.)
  • 2) let the clutch out part way, smoothly & relatively quickly, only as far as needed to spin up the motor
  • 3) immediately depress the clutch as soon as the engine spins up
  • 4) select the appropriate gear for your current road speed
  • 5) "rev match" before releasing the clutch again - that means, blip the throttle to bring the engine speed up to match the transmission speed before smoothly re-engaging the clutch.
Note on (1): you obviously can't select a higher gear if you're already traveling at speeds where you would return to top gear anyway. But you should still do the "partial" clutch engagement and immediate dis-engagement to restart the motor, and the rev-match to re-engage the clutch.

Note on the first "don't" - one instance where you might simply dump the clutch in the gear appropriate for your road speed is if you need the engine on NOW (maybe for safety reasons), and the extra 1.5 seconds to switch gears & rev match would take too long.

With practice using this technique, "bump" starting should become a misnomer. Clutch starting the engine in this way should be nearly imperceptible to a passenger.

Gone4 02-22-2008 12:35 PM

Thanks for posting this. I for one was not sure on the specifics. But I haven't gotten to try this yet either.

NoCO2 02-22-2008 12:46 PM

Great post, I always wondered how people do this without wreaking havoc on the engine and drive train components. How does this method effect clutch wear though? It seems like using it to spin the engine while it's only half way engaged would wear the clutch much faster then normal. That then would bring into account the replacement price for a clutch into your cars operating cost.

MetroMPG 02-22-2008 12:57 PM

Clarification: I wouldn't say it's only half-way engaged and slipping - just that you don't need to completely dump the pedal to spin up the motor, as the clutch is fully engaged well before the pedal is fully released.

Does clutch starting cause more clutch wear? Definitely. Compared to what, though?

I doubt it's as much wear as would be experienced by people who commonly use their transmissions for engine braking, and slip their clutches each time they shift down to do so.

EDIT: the choice of a higher gear for clutch starting also minimizes wear, since the input shaft is spinning slower when the clutch is engaged.

NoCO2 02-22-2008 01:15 PM

It would be interesting to hear how often people who use this method of bump starting have had to replace their clutches in the past.

::EDIT::
You bring up a good point about the gear braking. My best friend has a '02 Mini Cooper and he has nearly 70k on it so far and just recently replaced his clutch a month or so ago for the first time. He autocrosses (races) this car every weekend so if it took 70k for him to need a replacement after racing conditions then I guess the point is null.

basjoos 02-22-2008 02:18 PM

I've been bump starting for years and have 465,000 miles on my original clutch.

Ryland 02-22-2008 03:04 PM

Simaler to Frank Lee, I let it out in a simaler manner as I do when coasting down the driveway in the morning, and it's alwas smooth enough that the only thing a passenger notices is that the oil and battery light on the dash went out, I've heard of people who's passengers freak out, and if that is the case then they need to revisit what they are doing, or let someone else drive.

MetroMPG 02-22-2008 03:07 PM

It's going to be different from car to car as well. Everyone here has probably driven enough different vehicles to know some are more forgiving than others.

boxchain 02-23-2008 12:13 PM

Good writeup. I'd like to add that I was able to clutch start in 4th at ~5mph yesterday.

I wonder how much energy (in lost momentum) this is using vs. electrical energy that the starter would use, and if there is a significant difference. Prob not much of a difference.

PaleMelanesian 02-24-2008 12:07 AM

I think (don't know for sure) that some fuel-injected cars will add extra fuel when you crank the starter. Bumping would eliminate that.


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