justin00stang

Right, but if you're already on the edge fuel quality can change or carbon can build up, then you will start detonating.

Not always, depends a lot on the chamber and what you're doing with the car. On a turbo or SC car you can advance timing, it will keep on making power until it detonates, and if you detonate it enough you will break things. I think a lot of stock motor w/ blower failures are from tunes that are too agressive, one day they get some marginal gas and its all over.

An NA car is going to be a little more forgiving than blown but its still not a good idea to detonate one.

Adrenaline_Z

Good points.

I just think it's strange that an engine equipped with sensors (knock sensors in this scenario)
would scare people from going a touch more aggressive.

GueSS Who

I have a 15.1:1 compression 408 and really hogged out heads. I couldn't figue out why the car wouldn't make more power with more timing. It will not make more power with any more then 24 degrees. This has helped alot......this is why this site is good.


J-rod, sent you a PM

Lostpatrolman

iTrader: (3)

Doesnt real world driving stress the engine more and cause detonation sooner? Seems like a good idea to dial back a few degrees of timing if it barely made any difference.

J-Rod

If a motor doean't need ignition lead, then why add it? Too often folks seem to think that the more timing the better, and that isn't so.

If you ignite the chamber early you are only costing power. I'll agree that if you are making power thats one thing. Think about this for one second. If you can ignite a chamber's volume and have complete burn a X degree of spark lead, then its all burned. Why do you want to go to X+5 or X+10 degrees. You flame front is catching the piston on its way up the bore instead of at the top and on its way down.

But here is a real world example. As I cited above, we have a car with a high gear retard the car picks up mph and ET by taking out ignition timing under load in high gear...

Look at the dyno graph I posted on going from 22 to 28 degrees.

The problem I had with some of the skewed thinking I saw was that too much genI thinking was going into determining what is optimal timing for an LS motor. Do I run 36 or 38 degree of timing in my gen I motors, yes I do. Would I run it in a GenIII/GenIV, no way...

Just remember too much isn't always better... Its all about the combination.

Remember... Combo, combo, combo....

02sierraz71_5.3

iTrader: (2)

I just read this whole post and my head is swimming lots of great info thanks to all who contributed.
This question concerns my current setup where I did it *** back-wards the heads and cam before the full exhaust

Is it possible for a plugged up exhaust to cause detonation, that an otherwise freer flowing exhaust would eliminate?
Also the 5.3 motors have dished pistons the dish is an odd shape that is uneven across the piston sort of resembling a "t" shape would this shape cause hotspots with added compression?
what is the purpose of an iregularly shaped depression in cylinder heads?

Adrenaline_Z

Exactly, there is no point. The optimum spark lead has already been achieved, why mess it up.

This is not surprising. Optimum Timing is dynamic and engine load is dynamic.
On top of that, the engine comes in and out of resonance which changes
the charge density and VE.



My buddy's car is set to pull 2 degrees with this Crane ignition module.

For the timing experts, tell us what criteria warrants pulling timing to make
power. Why is this beneficial?

Adrenaline_Z

Not to the best of my knowledge. From my perspective, a "plugged exhaust"
would inhibit detonation because there is chance of residual exhaust gas to
remain in the chamber.

The fresh intake charge is displaced by the exhaust gas (which is inert).

I would think this would cool the chambers resulting in a lower chance of
detonation.

This is similar to how EGR operates in a light cruising situation.

Don't quote me, but I'm thinking a restricted exhaust will drop power by
reducing VE as opposed to promoting detonation.

justin00stang

The thunderbird supercoupe guys (m90 roots 3.8) have a very restrictive exhaust from the factory. They talk about needing to do exhaust before doing pulleys, because the stock exhaust is so restrictive.

I know the crown vic cars with the 4.6 will take a lot less timing than a mustang, partially because they have a very restrictive exhaust manifold that leaves a lot of heat in the chamber.

So yes, exhaust can have an effect on timing and the detonation threshold.

Visceral

iTrader: (1)

I think all of these examples come dow to load, and the effect of load on optimal timing. More load, less timing. That certainly should be accounted if you swap an engine from a Crown Vic to a Fox Coupe, and certainly would be nice to have optimized when you change gears (and load) (which requires some expensive electronics). I think the Digital-7 is like $800

Adrenaline_Z

Yes, that would certainly be a problem. It would heat up the intake charge.

I wondering though...how much of the heat retention is a problem, as opposed
to pumping losses?

It would have to be a highly clogged converter, or poor tuning to cause
detonation I would think? Simply because, the chamber is rarely scavenged
fully before the next intake cycle on the average street/strip motor.

Residuals are always present outside of a small tuned RPM window. Know
what I mean?

02sierraz71_5.3

iTrader: (2)

I bumped my compression up from 9.1:1 stock to 10.5:1 on a 5.3 truck motor I was having problems tuning out kr, now that Ive gotten rid of the stock y pipe and cats. I can run more timing without knock. The long tubes are next should have no problems then.

Nine Ball

iTrader: (38)

TTT for new forum

2000SSNB

iTrader: (4)

This is an informative discussion. One factor I have not seen in this discussion is the amount of time it takes for: the moment the spark is fired
electrically and the flame front to fully burn the gases in the compressed space unitl max pressure builds. This time is probably relatively constant,
the rpm is not, so different rpm translates into different ignition timing
needed. If one takes 14 deg as the optimum pressure point ADTC,
for each rpm there should be different igntion timing, That is why
there are entire maps in the PCM.

So, what is the time it takes from the instant the coil fires and the flame front burns the mixture to the point of max pressure?

Gert

Bink

Good question...I don't know?
Nanoseconds?

MadBill

Quote< This time is probably relatively constant,
the rpm is not, so different rpm translates into different ignition timing
needed. If one takes 14 deg as the optimum pressure point ADTC,
for each rpm there should be different igntion timing, That is why
there are entire maps in the PCM. > End quote
In fact, this is true only at lower RPM, which is why most WOT spark tables show increasing advance only up to the mid RPM range. Thereafter, increasing mixture turbulence speeds the burn rate as much as RPM shortens the available time, leading to a fairly constant (or even reduced) spark advance requirement.
BTW, although that 14 degree ATDC peak pressure point is quoted here and elsewhere, it is an approximation for most engines, not a set-in-stone absolute number for all. During my years at GM, the concept was 'invented' twice, millions spent on controlling SA to some magic value between 14 -19 ATDC and abandoned as the exceptions piled up to the point that every engine had to be completely mapped anyway. History being what it is (ignored generally), I believe it has been recently patented despite decades of "prior art" (a legal term for 'nothing new here.')
Therefore, the answer to you last question is; "The time it takes the engine to rotate (SA + ~15 degrees) at the RPM in question."

04Yukonxl

We had another example of this on the dyno the other day. We had a car with a new cam put down 362 rwhp with 26 degrees of timing over the entire rpm range. We dropped the timing to 22 degrees from 3000 up and picked up 20 rwhp. People were amazed at how much power we picked up by retarding the ignition. We need more time to optimize the combo but the theory here has been confirmed.

04Yukonxl

I see a lot of people setting their advance up to where it starts to knock and then back it off, thinking they need to run as much advance a possible.

I have a theory that I'd like to confirm related to rpm and advance. The engine gets more efficient at higher rpms due to higher velocity. This allows for better ignition conditions which should require less spark advance. As rpm goes up, the spark should be retarded. Comments?

J-Rod

Faster engine speed means there is less time for the pre-flame reactions in the end gases to occur, thus reducing the tendency to knock. On modern engines like ours with an engine management system, the ignition timing may be advanced with engine speed and load, to obtain optimum efficiency at incipient knock.

The tendency to knock increases as spark advance is increased,(as a theoretical example) 2 degrees BTDC requires 91 octane, whereas 14 degrees BTDC might require as much as 96 octane.

If you advance the spark, the flame front starts earlier (the piston is farther down in the hole , and still on its way up), and the end gases start forming earlier in the cycle, providing more time for the autoigniting species to form before the piston reaches the optimum position for power delivery (remember the MBT and the LPP we discussed before), as determined by the normal flame front propagation (keep in mind this has to do with quench, squish, turbluence, and chamber design we all touched on before) . It becomes a race between the flame front and decomposition of the increasingly-squashed end gases.

High octane fuels produce end gases that take longer to autoignite, so the good flame front reaches and consumes them properly. (Remeber this, higher octane isn't more power. Its preventing the stuff from lighting off to early).

The ignition advance map is partly determined by the fuel the engine is intended to use. The timing of the spark is advanced sufficiently to ensure that the fuel/air mixture burns in such a way that maximum pressure of the burning charge is about ~15-20 degree after TDC. (But again, this goes back to LPP, and MBT dicussed above)

Knock will occur before this point, usually in the late compression/early power stroke period. The engine management system uses ignition timing as one of the major variables that is adjusted if knock is detected. If very low octane fuels are used ( several octane numbers below the vehicle's requirement at optimal settings ), both performance and fuel economy will decrease.

Gasoline, a hydrocarbon-based fuel, needs to be atomized and emulsified (broken down into small particles and mixed with air) to burn. It will not burn by itself in liquid form. When atomized, gasoline has a laminar burning velocity of approximately 0.5 meter/second (m/s) or 1.64 feet/second. As a comparison, acetylene mixed with air burns at a rate of 1.58 m/s or 5.18 feet/second. The slow laminar burning speed of gasoline poses an interesting problem when used as a fuel for an internal combustion engine.

Since this is best represented using metric measurements, ignore the dimensions but accept the concept. Given a cylinder with a 100 mm diameter and an ideal central location for ignition, the time for a gasoline-fueled flame to travel this distance is 100 milliseconds. The problem is that when an engine of this dimension is running at 3000 rpm, there is only a window of 10 milliseconds for the combustion event to take place. Obviously another force must be at work because we all know that a gasoline engine can operate at speeds substantially higher than 3000 rpm. The key is to increase the burn velocity.

It has been established that the flame in an engine travels across the bore at a rate of 10-25 m/s. This is substantially faster than the velocity stated earlier, but it is the reason why gasoline can be used as a motor fuel.

To increase burn velocity, turbulence needs to be introduced to the combustion event. In an engine, this is accomplished by the induction and compression process along with the design of the combustion chamber. Here is where the LS motors shine compared to a Gen I SBC, we've discovered how important the chamber is, and with a better chamber design we come out with more power, and the need for less spark lead is achieved, while still making more and better power

During pre-mixed combustion, the effect of the turbulence is to break up or wrinkle the flame front, creating burnt gases in the unburned region, and vice versa. This effectively increases the flame front area and speeds up combustion. Though diffusion is usually associated with a compression ignition engine, (better known as the diesel), it can also occur in a spark ignition engine when stratified charged. The fuel would be injected in a fine spray and the turbulent air motion would sweep away the vaporized fuel and combustion products from the fuel droplets, speeding up the burn velocity.

Some good long articles/FAQ's on this.

http://blizzard.rwic.und.edu/~nordli.../gasoline.html
http://www.highperformancepontiac.co.../0209hpp_fire/

MadBill

Excellent description, J-Rod!
I suspect a lot of the desire to advance spark to the border of detonation regardless of other factors (e.g. power) results from typical OEM engine design and calibration. When designing for a specific minimum fuel octane, it has been found by them that the best power and economy are achieved with slightly retarded ignition timing at a CR which is slightly too high to run knock-free at MBT. This means there will always be power to be gained by adding spark and either reducing the built-in 'insurance factor' or increasing octane. (typical approach of aftermarket chips/tunes)
If your particular combination's octane requirement is several full points below that of your selected fuel, you can advance well beyond MBT and lose significant power without a whisper of detonation.
Similar to 04Y's experience (but too many years before!) we started tweaking on my buddy's new 1970 GTO. With a tank full of Sunoco 260 (~106 Research octane) and a stop watch, we cranked the timing up 2 degrees at a time, waiting for knock to signal the end of our 'power gains'. At some point it dawned on us that we were at 46 degrees and had lost over a second and a half in our zero to 100 MPH runs. Turns out the old goat liked no more than ~ 37 degrees, and needed no more than 91 octane to use it...