Ignition Timing Education

[olddog]

I understand that there is a lag time between when the coil pumps out a voltage to the plug and when the air/fuel lights.

I also understand that the air/fuel burns and hopefully does not detonate all at once. Furthermore that the goal is to get enough fuel burned at the right time to maximize the pressure in the cylinder at the right time to get the most force to the crank taking into account the rod, piston, and crank angles.

But here is what I have never understood. Why is it that the timing is advanced up to somewhere up around 3000 rpm give or take, and then no matter how many rpms from that point on up, the timing stays the same?

For example say 20 deg at 2500 rpm WOT gives the most HP and then 35 deg at 3500 rpm WOT gives the most HP, why would 35 deg at 6000 rpm give the most HP? Intuitively it seems to me that 6000 rpm would require more advance like 45 or 50 deg maybe even 60 deg.

I hope someone can educate me on this.

[Gaz64]

I'd say up to about 3000 rpm, the first 10% of the burn time takes a fixed time period hence the need to advance the timing for increasing rpm.
After about 3000 rpm; the cam, cylinder head design, compression, intake, exhaust combo of the engine all start to work in harmony, the engine is breathing better, has higher volumetric efficiency, has higher cylinder pressure and turbulence, faster flame front across the chamber, so does not require any more advance.

Some engines like more than others and vice versa, some like fast curves, it depends on chamber design, CR, fuel, etc.

[JAM1775]

Yup...good explanation...Sting said the something similar ..Synchronicity!

[Jac Mac]

At Idle & any time the throttle is not wide open the cylinder can not get a 100% full of fuel & air, so the effective compression ratio is much lower than at higher rpm/full throttle so a certain amount of initial advance is required. Once you approach 100% cylinder filling then ignition timing can remain at a constant level. Supercharger, nitrous & turbo setups have high rpm ignition retard since they boost compression ratios by filling more than 100%-- some well thought out NA motors can also acheive slightly more than 100% fill @ certain RPM also.
How much total timing is a function of the combination of parts in the motor. For example if you & a mate build similar motors using all the same parts & his motor requires 38° total to produce the same power as yours does with say 32° total then you made a better job of getting all the clearances like squish etc right.

[olddog]

Ok the more dense the fuel air charge the faster it lights. This explains why the vacuum advance is important for part throttle operation. It makes sense that the more dense the charge the closer the molecules and the faster the flame can travel. It also explains why you need to back off the timing in a blown application.

However if this were the main factor, then why did the old mechanical advance operate on RPM alone, when at WOT, and the fuel injection engines pretty much do the same thing. Volumetric Efficiency increases with rpm to a point and then drops off again. Take a stock 5.0 for instance, it quits breathing around 3800 rpm and VE starts dropping like a rock. Then the theory should be that as the rpm goes up and VE increases the timing should be backing off. Then at above 3800 rpm as the VE drops off the timing should increase again.

I'm not trying to be argumentitive here. I would like to understand this.

[Jac Mac]

Originally Posted by olddog

However if this were the main factor, then why did the old mechanical advance operate on RPM alone, when at WOT, and the fuel injection engines pretty much do the same thing. Volumetric Efficiency increases with rpm to a point and then drops off again. Take a stock 5.0 for instance, it quits breathing around 3800 rpm and VE starts dropping like a rock. Then the theory should be that as the rpm goes up and VE increases the timing should be backing off. Then at above 3800 rpm as the VE drops off the timing should increase again.

I'm not trying to be argumentitive here. I would like to understand this.

Your spot on with this analysis-- the reason most if not all manufacturers dont bother to take advantage of it is that for most of the motoring public the cars will never be operated in a manner that requires it & the few that do are probably buying performance models with improved VE at upper RPM levels.

I know when I was building motors for a Restricted class of car here in NZ ( Holden 202ci 6 cyl breathing thru one (1) single barrel carb with a 1.25" dia venturi ) that I was able to make power by using the vac advance hooked to manifold vac so that when it was up around 6000rpm + it pulled in another 5° advance over the mechanical total. Worked fine until the techs banned the vac advance, which meant that we had to then play with the mech adv to achieve the same thing which was a lot harder to get consistant results.
Now obviously thats one situation where the carb creats the point of restriction that limits breathing , but in most cars with reasonable size carbs the point of restriction is usually @ the inlet valve / seat or throat ( evidenced by the fact that a bit of work in this area on most production cyl heads will show the most gains ), therefore the vac advance trick wont help much until the carb becomes the restriction.

Most electronic ignition systems available now also retard ignition timing by about 4° at high RPM and the 'original' Morse type timing chains also tended to do the same to the cam timing at high rpm as centrifugal force made them ride higher on the sprocket on the tension side ( as long as there were no torsional effects making it snake ) which would retard both cam/ignition timing@ high rpm.

That should get you thinking-- gotta go watch the last Aussie V8 race right now.

[olddog]

Thanks Jac.

One of the several things that got me thinking about this was another club cobra members dyno chart. The VE was still running flat, but the torque was falling off, as the rpms went up. When the VE stated to drop the torque dropped much faster. I have since seen several other dyno charts that showed similar results.

It seemed to me that if the VE is running flat and the toque is dropping then the most likely reason is that the timing is not right.

The Ford ECU timing tables does add a few more degress of timing from 3500 up to about 4500 or so for the 5.0 engine. This is another thing that has got me thinking. When you are no longer limitted by centrifugal weights, springs, and vacuum, and actually have a computer to work with, you are now free to do pretty much anything you want. You can make the timing curve go up or down in any shape you want. The issue is you have to understand what the engine needs, or you are going to waste tons of dyno time trying to figure this all out.

A third point, I built a spread sheet to calculate the time it takes for the crank to move from the point votage goes to the plug until the crank gets to TDC. Here is what I found:
At 6^ timing and 600 rpm it takes 1.67 miliseconds for the crank to go to TDC.
At 35^ timing and 3500 rpm it takes 1.67 miliseconds for the crank to go to TDC.

So it looks like the mechanical (or EFI WOT) timing is based on a fixed lag time for the fuel to burn. However if you carry this to the next logical step then you need:
At 60^ timing and 6000 rpm it takes 1.67 miliseconds for the crank to go to TDC.

But this is not the case, as most engines will stay at 35^ at 6000 rpm. If you calculate where the crank is at after 1.67 miliseconds it is at 25^ after top dead center. So either the burn rate changes dramatically or a lot of Hp is lost due to less than ideal timing. At this point I suspect a little of both is going on.

[Jac Mac]

It would be fair to assume that most road going naturally aspirated motors produce maximum torque within a 2500 / 4000 rev range so your 35° advance is in for most of that- now another critical factor here is rod ratio & how long in terms of crank degrees the piston is parked around TDC. Apart from all the mechanical reasons like less piston skirt thrust loads etc , this time at TDC allows the flame front to burn thru the fuel mix & create more pressure at the point where the piston/rod assy is getting the maximum leverage on the crankshaft throw. In a short rod combo the piston would be further away down the bore to the point where it is virtually running away from the pressure ( an over exageration, but should help explain it ). Low compression ratios and lots of ignition advance can be quite scary because if there is not proper mixing of the fuel/air you will get lean and rich areas in the fuel mix and this can /does lead to preignition & detonation given the right circumstance's even at relatively low RPM-- once this happens your on a downward spiral to trouble as the temps will spike in that cyl and increase these problems.
Main thing is to be aware of what might/can happen & back off timing & throttle at the first hint of problems-- most of which you only learn by doing it wrong---once!
Need to add a bit more to this--you must address what the engine wants/needs at all times and in regard to acceleration even get a step in front to what its about to need, dynos can only do so much and you have to be aware continuously of the what effect airflow can have at speed,particularly around carbs and ram tubes etc. I have seen motors that produced great dyno numbers only to lose 25/30% of that on installation in the car due to carb air supply alone. while you might have been acheiving close to 100% VE on the dyno, you wont get that in the car if your drawing hot air off the exhaust/radiator or even off the exhaust of a competitors car if your drawing it from up front. Again this is only one facet of the engine package & the guy that wins has all or more of his ducks in a row with everything on the car- handling, brakes, etc mean as much if not more than outright HP. Its not easy to drive for any length of time in a high HP car if the thing wont stop & turn when you want to.