Bumpstarting - Worth it for half a mile or so? - Fuel Economy, Hypermiling, EcoModding News and Forum

Jakg · 03-14-2009, 06:41 AM

I'm trying to improve my MPG, and on the road I live on it's a quiet dead straight road for about half a mile - So far i've been practising coming round the bend at 50 MPH and then just turning the engine off, coasting the distance to my house and then starting it in 3rd at about 20 using the clutch.

I drive a 2002 Proton Wira 1.5 using the Mitsubishi 4G15 engine.

Is this bad for the car? Do the benefits outweigh the savings for a "normal" car?

Thanks.

lunarhighway · 03-14-2009, 08:20 AM

i've never seen any negative effects of occasional bump starting... i'm not sure how much it will save, that'll depend on a lot of factors... i find bumpstarting works best in 5th and 4th gear, i've even bumpstarted while i was coasting to a stop and the light turned green just before a was about to come to a standstill.

i think the forces experienced by the clutch is comparable or even lowever than when you shift gears or when you take of from standstill so i think think and abnormal wear wan resuly from a smooth bumpstart

MetroMPG · 03-14-2009, 09:56 AM

Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - Threads Tagged with bump starting

Of course you cause a small amount of wear to the clutch & drivetrain every time you bump start (there should be no "bump" if you're doing it well). Nobody can 100% guarantee the financial savings will outweigh the potential maintenance costs, or if you will even incur any maintenance costs.

We have several users who have been using the technique for hundreds of thousands of miles on the same vehicle, with no ill effects. (Yet.)

I do it.

FYI: Coasting experiment: engine off VS. engine idling - MetroMPG.com

skyl4rk · 03-14-2009, 10:59 AM

There are huge fuel efficiency benefits to engine off coasting. Bump start in a gear one or two higher than you would be driving at the speed you are bump starting.

hummingbird · 03-14-2009, 01:04 PM

I do it whenever I can avoid 100m of idling, without getting into trouble with traffic. My City is very forgiving about bump starts, and can take up from almost crawling in 3rd. If there is a bit of residual speed, I prefer 4th or 5th for bumping and then reengage using the technique so clearly explained by MetroMPG here.

Jakg · 03-14-2009, 03:47 PM

Cheers guys

whokilledthejams · 03-19-2009, 07:39 PM

I usually bump start after short coasts. I've yet to see ill effects.

As has already been pointed out, bump starting in a higher gear than you'd be driving in at that speed is ideal. It smooths it out a lot.

In my car, for example, if I'm coasting at 25-30mph, I can bump start in 5th pretty well. As soon as the engine fires up, disengage the clutch, and shift into the gear that you'd normally use at the speed you're traveling at.

jimepting · 03-22-2009, 12:12 AM

It is going to cause a bit of extra wear to the clutch mechanism, but many have good luck with bump starting. Skill helps lower the mechanical impact.

Perhaps the worst impact is on the synchronizers. When the engine is dead (cluster gear & individual gear stationary) and the third motion shaft (output shaft) is turning at several thousand RPMs, there is very high relative speed between the synchronizer/dog clutch parts, therefore there is a lot of synchronizer stress. Apparently modern synchronizers at up to the stress in many cases.

I have an AT, but I'd do some bump starting if I had a MT. I just wouldn't be doing it unless there was a clear and convincing fuel savings. That would include long coasts, and long downhills. It may make more sense to restart with the starter, since that is an "external" component and less expensive to replace. In addition, using the starter would allow for double clutching which would lower synchronizer wear considerably. I don't know of any actual data, but clearly a lot of extra shifts/clutch cycles is clearly outside the designers intent. JMHO

Christ · 03-22-2009, 12:24 AM

Just a note - if you're doing the work yourself, ease aside, the clutch is actually usually cheaper to replace than the starter, given new vs. new components.. rebuilt starters are often much cheaper, and can't be held as a comparison to new clutches.

The only problem is that many times, you have to perform all the steps to replace the starter even if you're changing the clutch, so better to have to replace the starter for the sake of ease. Then again, I drove one of my cars for 5 months with no starter installed... I used to push it and dump the clutch as soon as I got it rolling in 2nd gear to start it, no matter where I was. When I pulled the engine out of that car to put it in my next Civic, the clutch face still had the OEM wear indicator grooves in it, and the clutch was installed in 1991 (the car was a 1988 Honda Civic LX, the original owner "learned" how to drive stick in it.)

jimepting · 03-22-2009, 12:05 PM

I didn't mean to come down very hard on any of this. Clearly some folks have success with bump starting for the duration that they own the car. I think technique and practice may be the main factor.

Just a note - I don't think many folks do their own heavy work, such as clutch replacement, but that is just a guess. Yours is as good as mine ;-)