Question about engine timing

"Jake - Has Bad Luck So You Don't Have To" (murdersofa) 12/13/2014 at 10:07 • Filed to: None

1

16

My car has about 40 degrees of timing advance when cruising on the highway when running on mid-grade. When running on regular fuel, I have 40 degrees of timing advance - 13 degrees of knock retard (yes, my engine knocks like a door-to-door salesman). I really don't understand how igniting the fuel 40 degrees before top-dead-center is in any way efficient, or why the 13 degrees of knock retard is really a bad thing. All I know is the less knock I have (from higher-grade fuel) the more powerful asplosions come from my 3.8 liters of fury mild annoyance. Furthermore, watching videos of people with Torque overlays on their video, their cars rarely stray from 0 degrees of timing, occasionally wavering up to 3 degrees either way.

Can anyone Explain Like I'm Five what the deal is with timing in modern engines?

E. Julius > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 10:33

3

I'm not an engineer, but I've read a decent amount about this, and I'll do my best to explain what I do know until somebody more knowledgeable corrects me.

Ignition occurs before TDC because it takes time for the flame front to propagate through the fuel air mixture, as well as for maximum gas pressure (which is what acts on the pistons to move them through cylinder) to build. Igniting at TDC would be substantially less efficient, because the piston will have already traveled further down the cylinder when combustion is complete and the maximum gas pressure has been generated.

I own dead car brands only > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 10:41

1

pop over to PontiacBonnevilleclub and ask this question, it doesn't sound right to me, you would want 0-3 degrees of KR at most. I'm not sure about timing though

Little Black Coupe Turned Silver > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 10:43

0

I don't really understand it either, but I believe it's more about the time the valves are open, and not when the actual firing it happening. The longer amount the intake and exhaust valves are open are needed at higher RPMs, to help get the fresh air in and exhaust out faster.

I question that reading of 40 degrees of timing on your car however... I know my car runs at 27 degrees, and that's because it's been tuned it to be that high, since I run E85 and benefit from it being increased. From what I understand normal gas cars run much lower than that.

E. Julius > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 10:44

1

The reason timing is retarded for knock is as follows:

Lower octane fuel is easier to ignite, which means the flame front will propagate through the fuel air mixture more quickly than a mixture with higher octane fuel. If it expands fast enough, it will actually be resisting the piston as it travel up the cylinder toward TDC, causing knock. The timing is retarded so that the piston is closer to TDC when ignition occurs, thus mitigating this problem.

There are other reasons knock can occur, such as hot spots caused by carbon buildup, compression heating of the fuel air mixture (which is why engines with high compression ratios need higher octane fuel), or issues with the fuel delivery system. Since modern computer controlled engines can change things like timing and fuel injection very precisely and on the fly, they are able to modulate these parameters in order to avoid knock, which is what your engine is doing.

E. Julius > Little Black Coupe Turned Silver 12/13/2014 at 10:54

0

While valve timing/lift does play a large role in the combustion process for the exact reasons you stated, ignition timing is also a major factor that determines whether an engine knocks or not. This is because many types of knock are due to ignition/combustion not occurring at the right time relative to the piston's position with the cylinder.

Demon-Xanth knows how to operate a street. > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 11:26

1

what car is this? Those numbers sound way way off. At 40 degrees you're just about igniting with the intake open.

E. Julius > Demon-Xanth knows how to operate a street. 12/13/2014 at 12:00

0

Intake valve close usually occurs between 40° and 60° after BDC, which would be 80° before Jake's cited ignition timing.

AMGtech - now with more recalls! > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 12:36

1

Timing is advanced because, to put it as simply as I can, fuel always burns at [approximately] the same rate but engine speed increases. So in order to have as much of the combustion process as possible happen while the piston is moving downward, the fuel needs to be ignited sooner.

Look at the game of baseball. If the pitcher pitches the ball at 70mph the batter will need to swing later than if the ball were pitched at 100mph, otherwise he would swing too early, or inversely he would swing to late.

Knock retard is used to account for variables in the ignition process. But yours sounds too high to me. I suggest a carbon cleaning process, such as GM Top End Cleaner, or BG fuel injection service, then replace your spark plugs. Also, hot air intakes can increase knock retard.

tromoly > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 12:48

0

40 degrees advance? Are you running Methanol? That doesn't make any sense on a Gasoline engine.

Jake - Has Bad Luck So You Don't Have To > E. Julius 12/13/2014 at 14:49

0

That all makes sense, but then how are other cars igniting right at tdc? Ludicrously high octane fuel?

E. Julius > Jake - Has Bad Luck So You Don't Have To 12/13/2014 at 14:59

0

I have far from an exhaustive knowledge on the subject, but as far as I know a properly tuned automotive engine shouldn't be igniting at TDC under most conditions. Maybe at a low RPM idle or something on small displacement engines, where the flame front doesn't have to travel as far and there's not as much fuel to burn up. Under the conditions you specified in your post though (cruising at highway speeds), I don't think 0° ignition would be remotely desirable for most cars. Do you know of any specific cars that do that? I know the theory but not any examples.

briannutter1 > Jake - Has Bad Luck So You Don't Have To 12/14/2014 at 17:31

1

I'll be really general here because I don't want to look up the specs for your 3800. On a stock engine, the "mechanical" timing curves are around 5 degrees btdc or so at idle (almost no cylinder pressure) and rises to a point around 3500 (that may be 28 degrees or so) and flattens out after that. Vacuum advance advance is stacked on top of this. Perhaps another 15 degrees. This means (at part throttle) the idle would be 20 total btdc. The 28+15 would result in a part throttle mid rpm number of 43....(still very little cylinder pressure). At wot when there is no vacuum signal and the total timing regresses to the "mechanical" setting of 28 degrees or so because that's when cylinder pressure is at it's highest and detonation the most likely to occur. Whether the engine is older with a distributor and mechanical and vacuum advance, or a ECU controlled spark timing (using map sensor data); the engine itself still demands much the same thing based on physics.

As others have noted, the flame front takes a long time to spread from the plug and the manufacturers are usually timing peak pressure to occur at 15 to 25 atdc for maximum leverage on the crank. So if you're pinging at part throttle, you can retard the timing (all three numbers would change the same amount). You can go to a colder spark plug too. if the engine combo is stock, you likely have a lot carbon buildup (which glows and ignites air/fuel at the outer regions of the cylinder and the two flame fronts collide causing detonation). Let me know about your engine combination and what control you have over the computer or distributor, and I can give you more details.

Jake - Has Bad Luck So You Don't Have To > briannutter1 12/14/2014 at 18:50

0

Engine combination? Not sure what that means, but the engine is 3.8l V6, 9.4:1 compression ratio, 228,000 miles, 3.4in stroke/3.8in bore, for what it's worth.

I've got access (sporadically through a friend) to tuning software that gives me complete control over the PCM. As it stands I have 0 knock retard when running on premium (92 octane), and 3-4 degrees when on mid-grade (89 octane), and up to 12 degrees on regular (81 octane).

briannutter1 > Jake - Has Bad Luck So You Don't Have To 12/14/2014 at 18:55

1

ok, the reason I mentioned combination was in case your engine was modified. Is it supercharged? Is it detonating at WOT or is it ratting at part throttle heavy load. Any specific RPM it sets in? With that many miles, it's going to have a lot of carbon buildup and I think the cold plugs and a carbon treatment (or treatments) would help if you have the stock tune in it and are still having problems. A colder thermostat will help.

Jake - Has Bad Luck So You Don't Have To > briannutter1 12/14/2014 at 18:58

0

As far as mods the car just has a moderate tune that wrings out a few more HP on higher-octane fuel (the reason for the knock on regular fuel, likely), might-as-well-not-exist high-flow warm air intake, high-flow exhaust, and that's it.

Detonation is mostly a non-issue now with higher-octane fuels.

Timing advance seems to be the same on a cold engine as a warm engine, and I see 40 degrees at 1700rpm @ 60mph cruising on the highway (3% throttle average).

I have recently done Seafoam through the vacuum lines and some through the fuel

No idea what condition the spark plugs are in.

briannutter1 > Jake - Has Bad Luck So You Don't Have To 12/14/2014 at 19:03

1

are you able to log knock so you see what load kpa/rpm it's happening so you can lower the timing in those specific cells? You can do it by ear, but it's hard to see what kpa it begins. Personally, for the % of gas price between 87/89 and 91/93; I've always sucked up the few pennies per tank in exchange for power. Plugs will help and the colder thermostat means the water in the heads is cooler (as opposed to iat) and this is what will lower the auto ignition.