This may seem like a silly question to some, but hear me out on this and tell me what you think..

If a small amount of fuel burns faster than a large amount of fuel, wouldn't a leaner mixture mean a slightly more advanced ignition?

If this is true, adding fuel to tune out knock may actually be retarding the ignition time slightly as well, right?

I've always wondered if it was the increased volume of fuel that attenuated knock, or if it was the resulting slower burn of the fuel that nudged the timing back just enough to suppress it.

 

fuel adjustments does impact knock/timing. it also depends on what fuel your using, if your using a lower octane which is faster burning than a higher octane then you want to retard the timing.

you control the timing, with a reflash

The timing will not go higher than what you set it to with changing the fuel, it will retard the timing if knock is present from the mixture being 2 lean

adding fuel just slows down the burn rate but the same timing is there.

timing is a measurement when the fuel starts to burn, adding fuel to surpress knock is giving the flame front more fuel to burn because the fuel burnt 2 fast before reaching TDC from the previous log/tune

 

adding fuel, or removing fuel only changes the density of the charge. Too much fuel will slow the burn down, as well as being too lean can actually burn slower. The ignition timing is beginning the burn at the given point in the tables. What changes is WHERE the peak torque on the crank occurs. If you add fuel and the mixture is too rich, you've effectively slowed down the point of peak torque (or possibly moved it closer to your ideal point after tdc). AFR does affect where timing SHOULD be, but it doesn't change or move the ignition event. Only the point of maximum torque.

 

That's what I meant. Thank you. I know that the beginning of the ignition event is determined by the map, but wasn't sure if the burn activity would be slowed by adding more fuel.

 

Which is why some form of lean spool is preferable to none. I think alot of the individuals that are naysayers to disabling it completely only to reinstitute, dont fully understand that point. Thanks for clarifying that 9sec9, I "knew" the concept but didnt have the tech behind it.

 

I'm sure I'll be shot at sunrise for expressing my opinion, but that's the philosophy that's worked for me. The most ideal way to 'tune' a car would be by peak load at a given crank angle. If we could only measure the load in comparison to crank angle, then set timing to 'MAKE' peak torque occur at a given of 14 degrees or 15 degrees after tdc, we'd all have it made. The goal is getting to that point, but not knowing how our vairables, (AFR, Ign timing, packing of the cylinders, backpressure, etc) are affecting that peak point. It would be so much easier to see measurement of when peak occured (ABSOLUTELY) and make our adjustments accordingly.

 

Good point man

 

why would it be ideal to make peak torque occur at 14 or 15 ATDC?

 

Imagine the Tai Chi old man in your mind. If he only weighs 100 lbs and a 200 lb man charges him (in other words a piston is being pushed by a rod up a cylinder at break neck speed). Now, the old man can apply all 100 lbs with a palm thrust into the 200 lb man that's charging and what is going to happen? Probably, the old man breaks his arms because the force was applied TOO SOON and something had to give, HIS ARMS (or the piston and rods). Now, imagine DELAYING the old man's force until just the right time (NOTICE THE WORD TIME) and just as the 200 lb man get's to the old man, instead of pushing AGAINST the 200 lb'er, the old man uses all 100 lbs to begin his push a little later and his full pressure is on the BACK SIDE of the bigger man. This uses the inertia created by the moving object to the old man's advantage and puts maximum thrust on the bigger man (in this case the piston, as it passes). This maximum thrust at 13 - 15 degrees (considered by some to be ideal) is the goal. Now, if the fuel burns too fast and you have too much timing advance, it's possible that you're going to be like the old man and apply your force on the piston too soon. BROKEN RODS. By waiting a little longer (retarding the timing from WHERE IT WAS) would avoid that, but still not necessarily maximize all of your parts, it would only AVOID something bad from happening. It could also give you silence as far as knock, but still not maximize what you're after. By increaseing the burn rate, or slowing the burn rate will have a great impact on how far past tdc the peak pressure will occur.
I'm not trying to be goofy, just sometimes it's easier to think of it as something more visual.

 

Well, actually, peak cylinder pressure creating maximum torque on the crankshaft. The 14-15* is because that is the angle that will produce the greatest moment of force along the crankshaft. That angle is a function of many things, like rod length, stroke, etc. There is a specific angle that will yield the most force actually being used to rotate the crankshaft, rather than pusing along the axis of the rod, for example.

Eric

 

Remember, just because the burn begins at 20 degrees BTDC, doesn't mean that your peak pressure will be the same as some one else's at 20 degrees. If you have 10.6 to 1 AFR (a little rich) and he has 12.5 to 1 AFR, their burn will occur faster and maybe hit peak pressure at 14 degrees AFTER tdc. Your's on the other hand, because of the slower burn, may hit it at 25 degrees after tdc and be too late for your maximum torque. That's why ALL of the tuning is required, not just a timing map, or a fuel map.

 

9sec9,

I like your analogy. One I always heard is even easier to understand:

It's like pushing someone on a swing. If you try to push to soon, you get your teeth knocked in (too much ignition advance). If you push too late, you barely get a push at all (too little ignition advance). But, if you push just at the right time, then all of your power goes into pushing the person (perfect timing advance).


Eric

 

This is correct. All of my statements are based on the same rods, stroke in our Evo motors. A stroker would obviously change things some.

 

LOL that crazy I use the same analogy but with a nail and hammer

 

Stoker motors have HIGHER PISTON SPEEDS and because of that, they will travel further in the same length of time (BURN TIME) and that will cause the peak pressure to move to a different point in crankshaft degrees ATDC. Remember, the burn rate affects when the peak pressure occurs, so if the piston speeds up, and the burn rate stays the same, the peak pressure will occur LATER in the cycle, provided that the timing is left unchanged.