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Timing

7K views 64 replies 35 participants last post by  charbilly2001 
#1 ·
I have a question for all the people that give answers here, just for my own edification, nothing bad. And, I will step back on this one, letting the rest of you come up with all the ideas you have, for a few days. I'll be back to read the responses, then outline why it is needed.

The question, why do engines need timing advance curves to run efficiently?
 
#2 ·
Flame Bait :secret:
 
#6 ·
Simply, it takes time for the air and fuel to get into the cylinder.
That is part of it but more so it takes time for the fuel air mixture to burn.
 
#7 ·
because of the different rate of burn at different rpms.. well, not exactly the different rate of burn, but rather the different amount of lead (in degrees) that it needs to complete the burn at the optimum time to get maximum use out of the energy released..
 
#8 ·
Perhaps a moderator will move this thread to "Engine".



Turbulence increases flame speed. Turbulence is lessened at low throttle opening. Turbulence increases with throttle opening and with increased RPM due to velocity of incoming air stream, and piston speed.

Mixture strength--fuel rich or lean--changes flame speed. Lean mixtures burn slow. Might be that over-rich mixtures burn slow, too. I've kind of forgotten.

A slow-burning fuel/air charge needs more advance to get the mixture burnt 'n' pressurizing the chamber at a point when the piston is in a position to accept the pressure and turn the crankshaft.

Emissions control--depending on the manufacturer's strategy for reducing tailpipe emissions, timing can be changed to effect the chemical reactions thereby achieving emissions reductions.

Detonation--need to assure that ignition timing is not over-advanced under conditions that could induce detonation (although GM claimed that PART-THROTTLE "ping" was "The Sound of Economy".)

Timing is varied based on engine speed, engine load, and considers projected mixture strength (assuming "stock" carb tuning) and even altitude or emissions regulations of the region the vehicle is sold in, to optimize goals set by Engineering, within limits of cost/complexity/serviceability. Engineering goals can include power, mileage, emissions reduction, NVH, and other things I can't even contemplate.

How do I score, Professor?
 
#10 ·
What Shurkey said would have been my answer

"Timing is varied based on engine speed, engine load, and considers projected mixture strength"

All about Load .......... thats why race cars can use locked or quick advance no vacuum advance ........ they get into 100% load fast

now school us Dave
 
#12 ·
Timing curves are for grocery getters and lawnmowers. Locked timing is for performance cars :laugh:
 
#14 ·
Keep going, a small few of you are fairly well on target. Not fully correct, but, getting this subject opened up, and continuing education about it, is always a benefit.

I would prefer tech answers, NOT the usual flames and harassment.

More, please.
 
#15 ·
My guess is because the engine firing is set by the distributor and timing and related gears.

The time available for ignition charging and firing from the primary to the secondary circuit becomes less and less as rpms increase.

If what I read on the internet is true, the speed of combustion, etc, is about the same no matter what the engine rpm.

So within that window, you have to move the whole fire the mixture event earlier so it has time to burn.

If you fired it earlier at low rpms, its so early that its too early and would kick back.
 
#16 ·
It's about the fuels burn rate. Lean mixtures are harder to ignite and complete combustion process, like at idle and cruise, they need more time to fully ignite/burn. The fire has to be lit early so the combustion process has enough "time" to fully burn the intake fuel charge.

When moving to a rich mixture, like WOT or a choked position (initial start-up), that mixture burns/ignites easily and fast, so the ignition timing needs to come in later. If applied to soon you will loose power and efficiency.

You can prove this by running an AFR meter. Change nothing but the timing at a given/constant rpm and watch the efficiency change.

If a "street engine" has a locked timing scenario it will loose power, efficiency, and fuel mileage. It needs the timing "curved" to match the load/fuel applied to keep the AFR in the "sweet" spot.
 
#20 ·
for optimum burn the spark has to happen always at the same place , so as the piston speed increases in rpm you need to adjust > anticipate , to fire at that same place
 
#21 ·
Ideal timing advance is a combination of burn rate of the fuel used (octane), air-fuel mixture ratio, engine rpm, load, combustion chamber temperature, and combustion chamber design. Other factors can play in too like oil in the mixture, friction, etc. Did I leave anything out?
 
#22 ·
There are all kinds of variables between different engines but the one constant answer is charge density vs load . A throttled engine at a constant speed equals low charge density or slower burn time. More throttle and.more load equals denser charge and less time needed for combustion. So we need a system to ignite the mixture at the right time taking into account the right time is constantly changing based on these 2parameters
 
#23 ·
There is a fair amount more to it, in the physical sense, will get into it a bit later, as my place froze all the hot water pipes last night, and I am now forced to become Dave, the Plumber. Think rod angles, crankshaft rotation degree position efficiency, pressure expansion rates, and 'push', that sort of stuff.

You guys are on the right track, good, keep going.
 
#35 ·
There is a fair amount more to it, in the physical sense, will get into it a bit later, as my place froze all the hot water pipes last night, and I am now forced to become Dave, the Plumber. Think rod angles, crankshaft rotation degree position efficiency, pressure expansion rates, and 'push', that sort of stuff.

You guys are on the right track, good, keep going.
LPP, Dave.
 
#27 ·
It still comes down to the amount of time it takes for a fuel air mixture to burn. Yes it is that simple.
 
#28 ·
Which is why running race fuel in an engine that doesn't need it actually costs power.
 
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#30 ·
Bottom line is that advance is needed to give combustion a head-start for pressure to be applied at the appropriate time on the piston. But my question is, without using a dyno, how would one determine what is appropriate in terms of timing? Drag strip is helpful for wot but what about all that part-throttle stuff?
 
#31 ·
I apologize for not being here the last day or so, still going through a lot of plumbing issues that I really didn't want to get into in the first place. Just like the old adage says, the more you fix, the more you find wrong. Looks like it might take me through the weekend to get this mess fixed.

PLEASE, keep this topic going, I like that so many are interested in opening up the learning, and open discussion processes, and hope to get back to it when I finally dry off from my water works project.

Thanks, everybody.
 
#32 ·
Timing is advanced with increase in RPM. As RPM increases combustion time decreases. To maintain efficient combustion, timing must advance in some correlation dependent on carb, manifold and cylinder head flow and turbulence, valve size and relation to cylinder bore wall, not to mention cam lift, duration, overlap, etc.
And other stuff.


Am I close at all?
 
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