Warning: Bad to bump start and skip gears - Page 3 - Fuelly Forums

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Old 05-19-2006, 09:53 AM   #21
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Also, I gave a shout to a fuel engineer...

Question:
I would like to know some information that came up on another website I have found and have generally agreed with. In one of their threads, they stated that bump starting with manual transmissions would improve the fuel economy. (Engine off = unlimited fuel econ.) To do this they would turn off the engine while in neutral and then selecting a high gear to restart the engine by ?bumping? the clutch to restart the car without the starter) I do agree with this fully, and I do practice this technique while rolling down large hills. What I was wondering is the negative effects of practicing this technique on a vehicle? Examples have come up such as: CV Joints, loss of oil pressure, and resulting damage on restarting the engine from lack of oil.

Answer:
"Squirt and coast" is a recognised way to improve economy, and makes clear theoretical sense. There are really three potential problems:

1) While coasting, you have less control of the vehicle - no engine braking down hills, no ability to accelerate suddenly if required, no power steering, and (after a few applications) no servo brakes either. Depending on circumstances, this might or might not be dangerous. Cars that deliberately knock out the engine, like the Prius, use electric assistance for brakes and steering instead

2) Effect of multiple engine restarts. Obviously this is not ideal, due to lack of oil flow etc, but is probably OK as long as the engine is warm (low oil viscosity)

3) Effect of starting engine by "bump starting" not the starter motor. This seems a bit more risky, but is probably all right as long as you feed the clutch in slowly and are in a high gear (so the revs are not too high)

Overall, I suspect it is probably OK, but am not a mechanical expert so can't really be sure. The safety aspect bothers me, though.

Also, don't forget that modern cars completely cut off the fuel on overrun (eg rolling down hills with your foot off the accelerator pedal) anyway, so the added benefit of selecting neutral may not be that much.
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Old 05-19-2006, 10:06 AM   #22
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BUMP AND GRIND.... been doing it for years.
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Old 05-19-2006, 10:10 AM   #23
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The loss of power brakes could be overcome relatively easily with a small vacuum pump fitted to a vacuum reservoir (which probably already exists). Turbocharged cars have these, and I'm willing to bet you could source the parts on the cheap. The control system can't be terribly complicated; just a vacuum switch and maybe a relay.
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Old 05-19-2006, 11:17 AM   #24
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Quote:
Originally Posted by brick
Turbocharged cars have these, and I'm willing to bet you could source the parts on the cheap.
diesels also use electric vac pumps. as do most EV conversions.

if this is a serious concern of someone who wants to coast ICE off, the parts are readily available in scrap yards or purchased new for doing a retrofit - "Gast" is a commonly used EV pump brand.
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Old 05-19-2006, 11:34 AM   #25
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Originally Posted by MetroMPG
"Gast" is a commonly used EV pump brand.
We have a Gast Medical Grade Vacuum Pump here at work.
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Old 05-19-2006, 11:41 AM   #26
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My brakes stay good on ICE off coast for qiute sometime. ABS stays active. I dont think I need to add a back up vac. system. Ive tested the cars stoping with no vac and I have no trouble laying into the pedal to bring them to a stop.
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Old 05-19-2006, 11:58 AM   #27
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Quote:
Originally Posted by psyshack
My brakes stay good on ICE off coast for qiute sometime. ABS stays active. I dont think I need to add a back up vac. system. Ive tested the cars stoping with no vac and I have no trouble laying into the pedal to bring them to a stop.
I think the fortunate thing about having a small car is that the brake booster is optional. On my honda n600 there are no parts available for rebuilding a brake booster any more, so people are just starting to remove them. Luckily the car only weighs 1200lbs
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Old 05-19-2006, 12:34 PM   #28
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Quote:
Originally Posted by tomauto
Well, when I bump start, I never put the clutch out all the way.
When restarting...

Check that ignition is on >
Push clutch in >
Select gear (4th or 5th) >
Slowly ease out clutch until it engages and starts vehicle >
(this is usually about 1/2 way)
Push clutch in again and select highest gear >
Climb hill >

Speeds for starting [bump in this gear]

30 or less (coming to stop) - [3rd]
30 or more (around town) - [4th]
Highway (any speed) - [5th]

Also, when getting back into gear on the highway, try and rev match as best you can.
In my Saturn, I use 4th gear down to about 20 for bump starting and 3rd for speeds below 20.
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Old 05-19-2006, 06:02 PM   #29
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I use 2,3,4,5 -- 6 if there is one. I dont like to use 1,R. Im not awful smooth about it. Some of them come off perfect some are real bangers.

Theres so much going on with a bump it isnt funny. Speed, the right gear. Did the engine shut down in your favor when you key'ed off or hit a kill switch. is there a valve in the wrong place for the ecu to think thru ?? are your points open or closed ?? is the condensor in the right state of charge or discharge. On and on and on.

Ive never hurt a tranny be it MT or AT and Ive beat more than my share HARD. From big block American Iron to the worst the brits and the usa have ever made. I can share with you what not to do to a tranny but im sure 99% of the folks here arent doing those things to one.

Im a fluids maint. manic! i change oils and lubes and use what I think is the best for the money. That one action and a bit of thinking will do more for the life of a driveline than rev matching, bumping or just flat out parking a car.

I have my 1st two wheel transport still. My 1968 Honda 50 Mini Trail. It still runs. its never been rebuilt. the cam chain is so slack it rattles in the engine. It burns oil and the piston slap is only over shadowed by the cam chain. its a man/auto. just stomp on the shift lever and it still bangs a gear. the rest of the rig is worn out! The tranny is soild!

so dont worry about your trannys. just maint. them and go!!!

psy
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Old 05-20-2006, 01:41 AM   #30
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Quote:
I can share with you what not to do to a tranny but im sure 99% of the folks here arent doing those things to one.
i want to know!!!

as for tranny fluid, which are your favs psy?
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