Downshifting Wastes Gas?
04-11-2009, 10:50 PM
#21
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Tell your dad he's wrong.
Easy way to prove it.
1. Find a hill.
2. Drive to the top.
3. Turn the engine off but keep the steering wheel unlocked by putting the key to ON.
4. Have someone nudge the car to get it moving.
5. Downshift.
RPM's go up, no gas being used.
LOL
Momentum is a b****.
Easy way to prove it.
1. Find a hill.
2. Drive to the top.
3. Turn the engine off but keep the steering wheel unlocked by putting the key to ON.
4. Have someone nudge the car to get it moving.
5. Downshift.
RPM's go up, no gas being used.
LOL
Momentum is a b****.
04-11-2009, 11:20 PM
#22
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The car won't let itself detonate. It will do what it can to stay in stoichometeric ratios when you're in closed loop. It has a ping sensor to let the engine know when it's too lean and detonating. The ECU takes preventive measures to stop the detonation. I know for a fact that it retards timing. I'm pretty sure that it adds more fuel as well.
Look at the options... Idle, 800 RPM and braking, or 2,000 RPM and using the engine to slow down. Like you're using no gas because you've got your foot off the pedal? Do you really think the car is going to use any less gas then when it's in neutral?
No. You're using the same gas you would by depressing your clutch and applying the brakes or more.
You will waste gas at higher RPM. You save no gas by downshifting. You save your brakes at the expense of the clutch unless you rev match.
My OBD-II results show better gas milage when idling at 80 MPH then coasting in 5th.
Look at the options... Idle, 800 RPM and braking, or 2,000 RPM and using the engine to slow down. Like you're using no gas because you've got your foot off the pedal? Do you really think the car is going to use any less gas then when it's in neutral?
No. You're using the same gas you would by depressing your clutch and applying the brakes or more.
You will waste gas at higher RPM. You save no gas by downshifting. You save your brakes at the expense of the clutch unless you rev match.
My OBD-II results show better gas milage when idling at 80 MPH then coasting in 5th.
04-11-2009, 11:30 PM
#23
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Pretty simple in my opinion...
IF Fuel Injectors = Off while downshifting
THEN does not waste gas
ELSE downshifting wastes gas
Makes sense to me. Lower RPMs typically = more fuel efficient.
As its been pointed out, the fuel injectors do in fact shut off or nearly off during downshifting so it doesn't waste much gas, but as stated above... Downshifting to slow down isn't that great of an idea. Use your brakes.
On side note, your dad might have the right idea but from the wrong times. Back in his day, maybe the engine computer didn't shut off the injectors, so it did waste gas as it fed more to the engine to run at the higher RPMs. Still... wouldn't be a significant waste of gas as you're slowing down not accelerating... *shrug*
IF Fuel Injectors = Off while downshifting
THEN does not waste gas
ELSE downshifting wastes gas
Makes sense to me. Lower RPMs typically = more fuel efficient.
As its been pointed out, the fuel injectors do in fact shut off or nearly off during downshifting so it doesn't waste much gas, but as stated above... Downshifting to slow down isn't that great of an idea. Use your brakes.
On side note, your dad might have the right idea but from the wrong times. Back in his day, maybe the engine computer didn't shut off the injectors, so it did waste gas as it fed more to the engine to run at the higher RPMs. Still... wouldn't be a significant waste of gas as you're slowing down not accelerating... *shrug*
04-11-2009, 11:52 PM
#24
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QUOTE (njsharkracer @ Apr 11 2009, 06:55 PM)
I'll make this real simple. If your driving a manual tib and when you downshift and the engine is over 2k rpms with your foot off of the gas pedal, the engine will shut off the injectors. Therefore your not using any gas. Once you either give the gas pedal a push again or the engine drops below roughly 1800 rpms, the ecu will turn the injectors back on. If you drive a manual tib you'll feel the injectors turn off when you let off the gas over 2k rpms and feel them kick back in below 1800 rpms. I have a scangauge that reads this info and I can confirm it.
This is spot on. When you don't have your foot on the gas, the injectors shut off. No gas. When either it gets below that particular RPM or you give it some gas, the injectors start to flow again. DTN, sorry but you're wrong on this one. The car does NOT need to keep a stoich ratio to prevent detonation! The AF ratio simply goes to infinitely lean (all air, no gas). At this point, detonation is impossible because there is no gas in there. Simple as that. No gas, no detonation. Plus, the graphs posted clearly prove it.
You actually waste more gas by putting in the clutch, because the engine RPM drops below that specific RPM number, ithas to add gas to keep it from stalling. When momentum is already doing the work to keep it from stalling, it doesn't need to also add gas to prevent a stall.
Now probably where your dad got his idea, was from carburetor engines. With those, it's not possible to 'shut off' the carburetor as we can with injectors. In that case, it was a waste of gas to downshift as the RPMs went up and the engine automatically pulled more gas through. EFI changed that.
RedZ, you have a good idea here, but take it one step further. Do it once exactly as you said. Then do it again, but with the engine running. See what the difference is in stopping distance (or below a particular speed or RPM level). If it's a mile different, then it does add gas, and downshifting is pointless. If the difference is a few feet, then downshifting does save gas.
04-11-2009, 11:55 PM
#25
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Ok dtn, seriously the car does not maintain stoich when downshifting ok. There is no detonation during downshifting and no added fuel. the inj duty is very low almost like it's idling.
Unless you let the clutch off pretty fast there is not much extra wear on the clutch while downshifting. We aren't talking about going 80 mph and then putting the car in nuetral and your little computer isn't going to accurately tell you your estimated fuel mileage. Why the hell are you going 80mph and looking at the computer anyways.
When you use your brakes that is converting kinetic energy into friction energy into heat energy. It is more efficient to downshift gently and let the engine slow you down. There is no pinging. Even in an n/a stock tiburon you would here a ping.
I don't understand why everyone is against downshifting. Not wasting energy, being able to react faster, and it isn't much extra wear on the clutch unless you shift it nascar style.
Unless you let the clutch off pretty fast there is not much extra wear on the clutch while downshifting. We aren't talking about going 80 mph and then putting the car in nuetral and your little computer isn't going to accurately tell you your estimated fuel mileage. Why the hell are you going 80mph and looking at the computer anyways.
When you use your brakes that is converting kinetic energy into friction energy into heat energy. It is more efficient to downshift gently and let the engine slow you down. There is no pinging. Even in an n/a stock tiburon you would here a ping.
I don't understand why everyone is against downshifting. Not wasting energy, being able to react faster, and it isn't much extra wear on the clutch unless you shift it nascar style.
04-12-2009, 12:16 AM
#27
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^^
Are you serious!? coasting has nothing to do with what we are talking about and whether your car is on or off.
The info is here in the thread that the OP wanted. There is also some good information here I suppose so I am just going to close this thread. If another mod wants to re-open this feel free.
Closed
Are you serious!? coasting has nothing to do with what we are talking about and whether your car is on or off.
The info is here in the thread that the OP wanted. There is also some good information here I suppose so I am just going to close this thread. If another mod wants to re-open this feel free.
Closed