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.

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

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.

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.

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.

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

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.

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.

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.

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) ;)

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.

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.

20 years, 180K miles, same clutch.

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

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.

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?

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

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.

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.

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 :)

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 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.

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.

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.

You mean you can't do that in the Yaris? oh come on!! :-P

Thanks for the info, when I get my SG2 I'll see what the engine is up to while downshifting and I'll work on throwing it into neutral when i coast rather then holding the clutch in with my luck the throwout bearing would go 30miles over the warranty.

Another point about riding on the clutch pedal rather then throwing it momentarily in neutral to coast.

I have no proof to back this up, but a buddy of mine (the kid with the mini) is the guy who taught me to drive a manual transmission car and he always stressed that you should never leave the clutch in because when you do that, the plates aren't touching, but they are still spinning which could cause them to generate more heat and warp because they are not designed to spin for long without being pressed together. What he said makes sense and I've driven a car with a warped clutch pad and it can be very difficult depending on the degree of warping that has occurred. He also stressed that you should engine brake whenever possible to save your brakes (he is a race car driver where brake wear and heat are a major concern) and like was mentioned here, it also saves fuel if your car has the fuel cutoff system in place when decelerating.

That makes a lot of sense too. The plates certainly CAN be touching, they just don't have pressure on them. The fricion disk can move freely between touching the flywheel or touching the pressure plate, there is nothing to keep it in the middle where it touches neither.

I just run along next to the car with the door open. When the car is going fast enough, jump in. Doubtless I will run myself over some day.

He he.. the difference in 1700 and 3200 lbs id pretty big..lol

Hell in my ninja 250 motorcycle I had a bad battery.. and you could push start it and drop the clutch to start it as the magneto woul d start making power to fire the ignition off... but even that was pretty hard work if it didn't pop off instantly...


When ever i bump start my car i do it in 4th or 5th gear as it prevents a jerk when i do it as the gear ratio is so high.. then i select the gear i need to drive in.. makes for smooth bump starting.. all it takes it that motor barly turning and it should fire up if its warm.

In my truck i do some nutral coasting and idling and some engine off coasting also depending on the situations.. like.. curves or other situations that may need good control of the car I idle it.. as its power stearing is very hard to turn when there is no motor running..

I once had an old, crappy Honda CM185 Twin Star with a weak electric starter and an idling problem. It would stall out at a stop light and I figured out that bump starting was much easier than kick starting it. I would just have to put it in second and rock it forward while dumping the clutch. The thing would fire up like it was meant to be started that way. I kinda wondered why they ever put two other starters on it. As cheesy as that thing looked, I really do miss that bike. It got more ugly stares at bike week than a morbidly obese wedding crasher wearing a speedo.:p

With the Tundra, push starting is a little more difficult but that's prolly due to the 4,000 pound difference between the two vehicles. So unless I already have the momentum, I'll have to rely on ol' sparky to fire up the beast. When I do bump start it, it seems to fire up right away, but it's not very smooth. The dormant engine gives a lot of resistance, even in fifth, so it does jerk a little. I'll have to practice a little, but I'd like some more reassurance that the cat isn't going to go up in flames. You know, especially with the manual specifically telling me not to push start it and all...

Just to illustrate my fear here...

Flaming cat:
http://www.worth1000.com/entries/274...4217vBda_w.jpg

The correct gear for you to choose would be the one that produces your rpm at idle or just a little higher for the velocity you are moving when you bump it.
I usually bumpstart in second when rolling down a driveway.

Catalytic converter damage just scared me away from EOC again...would anyone like to explain this more?

:-X

I think it's a bogus reason against EOC.

A modern engine will start burning fuel nearly instantly when started, even when cold. So the air-fuel mixture is burned before it gets to the cat. Even more true for a warm engine. Remember, there's no fuel being injected after engine cut-off so it's not pooling anywhere. Spark and fuel both begin when you spin it up by letting the clutch out.

I think the bogus caution likely dates back to the days of carburetors and points-plugs-condenser ignition setups, where miscellaneous starting issues would end up flooding the engine. If you THEN get the idea to roll it down the driveway and bump start it, all that flooded fuel gets dumped into the cat and would burn there as soon as the cat gets hot enough. Depending on exactly when and how the engine starts, of course. But the cat is designed to get mostly 'combusted' air-fuel mixture, not mostly raw fuel plus air. So the dumped fuel would be bad for the cat.

But that scenario is not likely in your modern computer-controlled car. If the fuel management and ignition are in bad enough shape that raw fuel continually gets to the cat, the car is going to be very tough to drive if you can keep it going at all. In that case, continued operation would likely ruin the cat converter with or without bump starting.

I just read this after a 250-mile drive to school when I did EOC 4-5 times and bump start...I made sure to shutoff while the tach was falling so hopefully the injectors were shut off even before I turned the key. Your post makes me feel better about it. Anyone else want to corroborate to make me feel even better? ;)

Wow, for the past week or so, I've been bumpstarting when I EOC but it was actually....bumpstarting. I didn't know it would work without a kind of a slowing down then a jerk on. I'll practice more once I get that kill switch installed (if only there was a relay diagram for my model). Reading this probably saved me a lot of drivetrain damage.

It obviously depends on how often you bump start. I did it compulsively, probably 50-100 times per commute.

I have also noticed that some cars (such as a Mira) aren't very bump-start friendly, for whatever reason. On other cars I have driven (e.g. early Suzuki Swift, carb model), the wear on the clutch would be imperceptible (as judged by loss of kinetic energy in the car). On a bump-start friendly car, it could probably be done with abandon.

I bump started for 2 years like this on the Mira and the clutch is on its last legs.

However, if I had restricted the bump starting to the largest hills (e.g. glides of maybe 20 seconds plus) instead of every opportunity to glide (even several seconds), I would have had the lion's share of the benefit and maybe the clutch would have lasted 10 years.

There is something to be said also for getting the clutch fully engaged before major acceleration, rather than a gentle easing out until 2000-2500rpm, and avoiding engine braking since the loss is in the form of kinetic energy, and brake pads are cheaper than clutches. As it is, a dedicated hypermiler will probably have brake pads lasting the life of the car, so you may as well use them for something.

I read , that bump mode isn't ideal for car catalyst.
Because sometime can unfired fuel mixture get to catalyst and damage him.
(I guess, when the car don't start at first bump)
I have catalyst and now for 1 month trying bump mode. But i'm afraid from braking my catalyst.

What do you think, how solve this problem?

You are at no more risk of damaging your catalytic converter with a bump start vs. a keyed start with any modern fuel injected vehicle. There is potential for damage with a carbed vehicle if the motor is allowed to turn over without the ignition on since the fuel flows regardless, potentially doing exactly what you suggest. Serious indiscretions there are usually followed by a large bang!

It is however worth noting that frequent engine off operation may allow the converter to go below it's optimal operating temperature, rendering it less effective during the cool times.

Thank you. Next question. I heard , that V-belt can be breaking. So its danger use bump mode. Is that correct?

If that is the case, it's probably best not to use the starter either. ;)

Best Gear?
 

Well I had posted a long reply saying that this method was not allways the best option in my Cobalt. However I just got in from testing the method as described here and it went pretty well.

So, as they say: Never-mind. ;)

Assume you car goes 70 MPH at 3000 RPM. At 7 MPH bump starting in the same gear is the same RPM as cranking the engine with the starter, about 300 RPM.

regards
Mech

jump (or bump) starting
 

Have to put in my thoughts.

There should be no bump involved. Do not "pop" the clutch. Think of the way you go up through the gears when accelerating. Unless you always give your passengers whiplash and chirp the tires when you shift up. Gentle is the word and use the same gear you would use when accelerating at the speed you are at. Having knowledge of what is actually happening when the tranny is in or out of gear and what happens when you push the clutch in helps. Oh and how the engine is involved in all that.

I have used this technique a lot over the years due to being too poor to waste gas, having a dead battery and no funds to replace it or just having to drive a POS the wouldn't idle well.

As far as reverse tourque harming the drive train, if done correctly it is the same as taking your foot off the gas when decelerating. In other words the clutch is designed for reverses in forces. That's why many clutch plates have springs in them.

As to EOC, I never thought about the pilot bushing (is it really a bearing now in cars/light trucks?) but you should never keep the clutch depressed for long periods of time. Shift to neutral and let the clutch out. Less wear on the through out bearing.

I have known and ridden with people who pushed the clutch in every time they let up on the gas. Why? They were told to by someone. Believed the car would stall otherwise, even at speed. Never was successful at converting any of these people.

So do your EOC right, enjoy but know you didn't invent it. Been around since there were cars with clutches.

And be gentle restarting it using the proper gear and don't "pop" the clutch.