So, why does the idle speed increase when the timing is advanced by rotating the distributor?
My feeble brain says that if the engine turns faster with the same throttle setting that it is running more efficiently. What is the downside that I'm not seeing?
Will the engine overheat due to low flow of cooling air at idle with an advanced timing setting?
Musings about Idle Timing
- vwlover77
- IAC Addict!
- Location: North Canton, Ohio
- Status: Offline
Musings about Idle Timing
Don
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78 Westy
71 Super Beetle Convertible Autostick
"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen
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78 Westy
71 Super Beetle Convertible Autostick
"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen
- Amskeptic
- IAC "Help Desk"
- Status: Offline
Re: Musings about Idle Timing
A) Advancing the timing at idle speeds up the engine because it increases the "springiness" of the compression/combustion pulse against the piston just before TDC. This seems more efficient to have a faster idle speed for a given throttle position, but it is cancelled out by increased friction, and the pumping losses against the closed throttle plate. Believe it or not, the most efficient idle would be wide open throttle at 4* BTDC where you set the idle speed by leaning out the mixture.vwlover77 wrote: A) why does the idle speed increase when the timing
is advanced by rotating the distributor?
B) My feeble brain says that if the engine turns faster with the same throttle setting that it is running more efficiently. What is the downside that I'm not seeing?
C) Will the engine overheat due to low flow of cooling air at idle with an advanced timing setting?
B) VW purposely made the idle timing less efficient (particularly with the air-injection buses at 10*ATDC) so they could cram more air through (that's the move towards the WOT above which is more efficient) and then lean it out a bit. You are correct that the engine would spin faster at a given throttle position with more advanced timing. But actual efficiency is a tricky little deal with many rich and subtle variables.
C) Note that the actual quantity of air allowed into the cylinders at idle is very small, just a wisp of fuel and air. Whether or not the spark is advanced or retarded has very little effect on cooling system capacity. If you had the timing so advanced that the combustion was actually expending itself on trying to run the engine backwards, you would have serious pre-ignition heat on the piston crowns, but it would be extremely localized overheating, nothing that the cooling system could address.
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles
- vwlover77
- IAC Addict!
- Location: North Canton, Ohio
- Status: Offline
- Amskeptic
- IAC "Help Desk"
- Status: Offline
Diesel engines actually do the WOT method, with fuel injection quantity as the only speed control. Half the noise of a diesel is the unfettered intake. There is no engine vacuum with a diesel. That's why they are more efficient. They have to have a vacuum pump for brake boosters and vacuum operated climate controls.
Colin
Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles