figuring out engine timing

[fiznat]

Okay, I've been thinking about this alot, and I think I've got it all set in my head right now. I just wanted to verify this with you people who actually know what you're talking about to make sure I'm right:

Things I think I know:

1. Stock timing always happens some time before TDC (BTDC), and is advanced as engine speed increases to account for the time it takes for combustion to spread --- ideally getting the highest pressures at TDC.

2. Retard can be introduced to avoid detonation in 2 ways as I understand it:

A. retard can effectivly LOWER the compression ratio. This had me confused because I didnt know why compression would go down if the spark happened closer to TDC... it seemed to make sense that the closer the spark happens to TDC the higher the compression, but this doesnt account for the time it takes for "full combustion" I guess. (what is this delay called? fuel burn time?) Therefore, retarded timing effectivly LOWERS CR's because the maximum pressure applied to the piston by the fuel happens some time AFTER TDC. This is useful to avoid detonation caused by either too lean a mixture or too high CRs, and also to increase emissions (at the cost of power).

B. I am not sure this is right, but I am guessing that the extra fuel + air in a boosted cylinder burns faster than NA air + fuel. This would mean that the regular amount of advance provided by the ECU (or whatever) is TOO MUCH, and the maximum fuel compression would occur BEFORE TDC. This would cause knock. In this scenario, retard isnt introduced to lower CRs, but to get that fuel burning at it's maximum right back at TDC.

3. Retarding the timing results in an increase in EGTs. This isnt because of higher compression (like I had thought), but because the fuel starts to burn later, and therefore has JUST finished burning (or is even still burning) by the time the exhaust valve opens.

Okay guys, lemmie know if I'm right, wrong, or a total moron...

[WDRacing]

Hmm, how to explain timing. You've got the basic gist of it.

Think about the Air/Fuel (AF) mixture being pushed up towards the spark and being compressed and mixing as it does so. Now if ignite the spark at TDC, the piston will get power to travel down and repeat the cycle. If you ignite the spark prior to reaching TDC, then it is being compressed as well as exploding. If the explosion comes on to early, then the piston will try to travel down before it is actually supposed to. Its actually preignition or detonation. The key is to find the exact point in the combustion process where the spark should be ignited to neither rob the rotating assembly of power, but you don't want it to become paracitic either. By paracitic I mean having the crank pull the piston down rather then it being forced.

Timing gets advance because the pistons are traveling up to fats for the actual combustion to take place in time to drive the piston back down. So it gets ignited earlier, but only early enough to where the combustion is just becoming complete as the piston reaches TDC, or close to.

As far as boosted air burning faster then if the engine was NA, I don't think that is correct. There are different amounts of AF in the cylinder, which is why the timing should need to be changed. The air is hotter in most cases then a NA motor.

Thats how it all makes sense in my head bro, I might have just completely confused you and everyone who will read this. But I had to give it a try.

The simplest way to change your timing under boost, buy and MSD boost timing retard box, or a super coil that allows you to set each rpms max timing curve. Start off conservative, use a knock meter and add timing slowly. Be careful though, you can blow a motor just as quick as adding to much boost.

Hope that helped some...lol.

WD

[I am Technoman]

Good job WD. Ignition timing is hard to explain. On a boosted motor wile under boost we all know compression is greatly increased and like WD said temps are higher and when you have all this compression with higher air temps a boosted motor can suffer from “pre detonation” “firing without the aid of the spark” which we have all heard the pinging sound that is the resistance between the piston and the combustion itself causing the piston to go side to side. I do believe a cylinder that is forced will burn A/F mixture faster because of the extra and I do mean extra oxygen compressed inside the cylinder. We use Higher octane to burn slower because if this.

On a N/A motor on the exhaust stroke the exhaust valve opens and the A/F mixture is pulled in from vacuum of the piston going down and the pressure of the out side air 14.7 at sea level I think! When this happens on a boosted cylinder the intake manifold is pressurized by the turbo and when the valve opens the cylinder is forced with fuel and air. This is the reason 3 or 5 angle valve jobs are better because of the seal and the valve is less likely to leak. So what I’m trying to say is on a boosted motor when the piston is all the way down it has X amount of pressure coming from the turbo during boost and now the piston has to travel back up causing massive amounts of compression and heat making the cylinder more likely to detonate so we confiscate by running a rich fuel mixture which cools cylinder temps and the turbine.

[fiznat]

As far as boosted air burning faster then if the engine was NA, I don't think that is correct. There are different amounts of AF in the cylinder, which is why the timing should need to be changed. The air is hotter in most cases then a NA motor.

I think you're mostly right. The higher pressure air (much more combustable oxygen avail.) and hotter temperatures make the mixture much more "ready" to explode than a regular NA mixture at normal pressure. This increases the danger of preignition, like you mentioned, but I think the mixture also burns faster. I got this from the SDS EFI website:

Quote »http://www.sdsefi.com/meltdown.htm "]Most naturally aspirated engines require between 30 and 38 degress of ignition advance to achieve PCP at the correct crank pin position to make maximum power. By compressing the mixture through turbocharging, the rate of flame front progression increases and slightly less ignition advance is required to achieve PCP at the correct moment.[/quote]

They pretty much call the "burn time" or "mix time" or whatever here "PCP," but the idea is the same.

Thanks alot guys, I think we're pretty much all right. It's a little funky to think about, but I think we did a good job of explaining it .

...Although I'm still unsure about my section A stuff. I suppose it's theoretically possible, but I think ignition that late would pretty much melt exhaust valves and energize turbos to the point where even the wastegate couldnt keep up. Also at that point you're pretty much undoing some of the extra compression you added with the turbo, so you could acheive the same thing (without the hot EGTs) by simply turning down the boost.

Cool man, glad I finally got my head wrapped around this stuff!

[I am Technoman]

Yea it’s really hard to think out something that works in milliseconds. Gives me a headach!