How to smoothly bump start / clutch start an engine

[trebuchet03]

Originally Posted by PaleMelanesian

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.

Can you substantiate that claim?

Technically, yes - it does. About ~5 seconds worth of extra fuel for a v6, more for a 8 and less for a 4 cylinder (this according to the Florida section of ASME)

[bennelson]

I was watching Doax while he was driving us over to look at the Metro the other day.

If the passenger wasn't paying attention, you wouldn't even know he was restarting the engine with the clutch. The only thing that really gave it away was him reaching for the key to kill the engine, then turn it back to the run position.

Give him a kill switch on the steering wheel and you wouldn't even know anything was happening (nice, quiet engine that kills fast is great too).

I was trying some pulse-kill-the-engine-glide-bump restart the engine driving on the way back from picking up the truck cap. After seeing someone else do it, I did a much better job with the clutch engine restarts.

It doesn't take much, just let the engine barely taste a little of the spinning wheels, and it starts right up.

[diesel_john]

at 21 to 1 my diesel does not bump start well even in 5th. i have elected to just let it idle and pop into neutral, and of course match rpm's clutch out in neutral before going back to 5th, to save the synchronizers.

at idle it uses about a pint of fuel an hour.

i need a compression release, prob is the valves only clear the pistons by 0.050" on this engine, could invent a release in the glow plug, i guess.

[elhigh]

Originally Posted by NoCO2

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.

20 years, 180K miles, same clutch.

Teaching the kids to drive is hell on it, though.

[boxchain]

I have never replaced a clutch. I'm worried about the bearings that have spent the last 5-60s draining their oil, which is why I bring it up to idle or pause before loading the engine.

I often don't use the clutch to shift, esp in the higher gears, but that's another topic.

[Twerp]

Okay, so bump starting might not be that bad for the clutch, but what about the catalytic converter. The owner's manuals for both of my vehicles say that the cat might be damaged if I push start my vehicle. Is that the same as bump starting or are they talking about something different?

[homeworkhome53]

I seldom bump start over 20 mph and usually less and always in 5th if coasting to a stop. I will briefly bump just before applying the breaks the car is an easy starter, very smooth. The car will momentarily rev to 1500 rpms then settle down to idle. The wear was a concern but the car is designed to pull the car from a dead stop in 1st at start up. I think the low speed bump is about equivalent, but I am no engineer, but you are trying to get all that weight moving (in a 1st gear start) as opposed to over coming compression and a much smaller weigh in the spinning mass of the engine. I'm sure one of you engineer types could come up with the forces involved, 1st gear start vs low speed bump start in 5th. How much load the clutch carry for each. The only difference I see is the change in direction the load is applied which may be designed into the clutch, it may be designed for 1st gear starts and not for bump starting, again no engineer here.

homeworkhome53

[Blue07CivicEX]

I had actually had a question about this and was glad to see the topic brought up. This is slightly off topic but, engine breaking, what are the advantages and disadvantages? My dad taught me to do it when I was re-learning to drive standard a couple years ago (I'm only 22) and I have been doing it ever since assuming it was good for something but it seems revving the engine as you slow down would use more fuel even if you're not on the throttle. Does this method only save the breaks? If so I think I'd rather save the gas and replace the breaks earlier.

On another note, if I'm doing P&G without shutting off the car just pulsing then holding the clutch in until it slows down, then speed matching, would I be better off to pop the car out of gear then just a quick clutch to put it back in or will holding the clutch in for the period of the glide be safe on the clutch/tranny?

Also, I'm an Engineer and have learned more in a few days on here then 50% of my classes in college.

[JJW]

Originally Posted by Blue07CivicEX

I have been doing it ever since assuming it was good for something but it seems revving the engine as you slow down would use more fuel even if you're not on the throttle.

It depends on the vehicle, but some have something called DFCO, decelleration fuel cut off. When the computer senses the car is decellerating and in gear, it will shut off the injectors. That means you are burning no fuel while in that state, however, you do get the resistance of spinning the engine. Apparently some cars even play with the valves in this state to reduce engine braking loses. From what I've read, the reason it exists is to prevent unburnt gasoline from reaching the catalytic converter, however hypermilers can use it to their advantage by deciding when to use DFCO and went to coast out of gear.

Originally Posted by Blue07CivicEX

would I be better off to pop the car out of gear then just a quick clutch to put it back in or will holding the clutch in for the period of the glide be safe on the clutch/tranny?

Thats a bit of a point of debate, but I do know that while your foot is on the clutch, the throw-out bearing is engaged and spinning. That bearing is intended to hold the clutch face away from the flywheel/pressure plate in the moments while you shift, not for extended sessions. Personally, I prefer to shift to neutral and disengage the clutch (release the pedal) so that while I may be spinning the input shaft of the transmission, I am at least using bearing surfaces designed to be used full time and which are most likely bathed in lubricating fluid, unlike many throw-out bearing designs which are simply sealed bearings since their duty cycle is typically low. Some cars seem to be fine with holding in the clutch, some seem to eat throw-out bearings. I do know I prefer not to have to replace them, which involves pulling the tranny most times.

As for the original topic of bump starting, can anyone comment on what Twerp said? My manual also advises against "severe catalytic converter damage" if you start the engine by engaging the clutch. Is the danger the combustion of accumulated fuel vapor ahead of the cat? I've done this for years only to now hear that it could be hazardous to my vehicle (of course, I started doing it on a 69 VW which was probably a tad more tolerant of abuse)

EDIT: By the way, one of my favorite "stupid Metro tricks" was that I could push start the car by opening the door, kicking off like a skate board until it moved, and then smoothly letting out the clutch

[ttoyoda]

Okay, so bump starting might not be that bad for the clutch, but what about the catalytic converter. The owner's manuals for both of my vehicles say that the cat might be damaged if I push start my vehicle. Is that the same as bump starting or are they talking about something different?

If the ignition is on and the engine starts right away that is fine. If some defect creates a condition where fuel is sprayed into the engine with no spark to ignite it, that fuel will ignite in the cat if the cat is hot. Do that long enough and the cat housing will melt.

I'm sure one of you engineer types could come up with the forces involved, 1st gear start vs low speed bump start in 5th. How much load the clutch carry for each.

I don't think the clutch is the issue. There is a thrust bearing, often at the center crankshaft journal. This bearing resists the push of the throwout bearing when the clutch pedal is pushed in. When the engine is stopped, there is no oil pressure (obviously). The thrust bearing is getting no lubrication. Every time the engine is restarted, that thrust bearing wears a little (is is just a plain bearing shell, no balls or rollers). This is also true on cars that make you press in the clutch to start them. But usually cars are not started and stopped that often over the life of the car. And you don't want to have to go in there to change that thrust bearing. That said, I do bump start by car sometimes just for the practice.

would I be better off to pop the car out of gear then just a quick clutch to put it back in or will holding the clutch in for the period of the glide be safe on the clutch/tranny?

If you keep your foot on the clutch pedal a lot you will wear out the clutch throwout bearing. If you are in neutral, (clutch pedal out,) a lot with the engine and transmission turning at different speeds, you will eventually wear out your pilot bearing, but that is not too common.

it seems revving the engine as you slow down would use more fuel even if you're not on the throttle.

No, because the throttle body or carb butterfly valve is shut. So the engine is not getting air, and not using extra fuel. (unless you are driving a car with a carb from the 1960s). This is a case where the engine speed is high only because the momentum of the car is spinning the engine. Not because the engine is using fuel. Another advantage of slowing the car with engine drag is it encourages smoother driving, which is less wear on all mechanical parts.