Ignition timing - technical

Long time no see !

How's the S2000 ?

Nope :)

Because before we had ignition maps and computers there was no connection between timing and throttle plate position. (other than a slow acting advance retard vacuum diaphram that only worked on "cruise" position due to a drilling in the carb body next to the throttle plate - not manifold vacuum)

So the real "correct" timing at idle, would have been far too advanced (and cause detonation, power LOSS) at WOT conditions at lower rpms So the "BASE" timing, typically a few degrees btdc was OK for a)starting b)idle, because here fuel efficiency is unimportant and so long as it ticks over smooth and starts its ok! c)correct for full throttle at low rpm, as in top gear in town say.

Ideally you need it advanced more at idle, because there is little charge in the cylinder, and flame speed is slow. It then needs to go back to the "retarded" position as you put your foot on the pedal, because flame speed increases due to the more dense charge and the effective compression ratio increase.

But a mechanical distributor cant do this, so it was a compromise with your throttle closed/idle.

All it can do is increase the advance with centrifugal weights and springs to compensate for the fact that the engine is turning faster and the flame speed stays the same.

A perfect (impossible) head would need zero advance at any speed, and would fire at TDC at all throttle openings and rpms... The closest you can get is say something like a TSCC suzuki gsxr head. Because it uses only 28 degrees or less MAXIMUM advance at 11000 rpm! Thats why they make so much BHP per litre.

Yes- you get dreadful HC's if you have a too far advanced spark at idle because burn speed is slow.

Don't forget that with EM controlling sparks and fuelling, you can maintain idle speed against a load by having say the base timing at 10 degrees at idle speed, say 900rpm, but if it drops then the map tells the ECU to advance the timing below target idle speed- and if idle speed is above target but with the throttle shut (as indicated by the TPS) it should retard the timing. This method is far better than using an air bypass valve alone- which you would use mainly at cold start.

Tim..

Good god hows that for a memory? S2000 is great thanks and i can't think of anything else i would have thats currently on the market, she's done

Gary

Nom wrote:

Thanks Tim, Thats where the conversaion started from, you advance the timing to pick up the idle speed, should it drop ie at 900rpm say 10DBTC if it drops to 850 advance to 12BTDC etc. We then sat and thought that if it runs faster with more advance, it must be burning more efficent and therefore why wouldn't we have it at that timing in the first place? We wondered if high Nox was the result, but somethings not stacking up here. More advance must mean more power cos the speed goes up, you say you get high HC because of poor burn but that would mean you were burning less of the fuel that before, so the speed should go down.

Fixed amount of air, fixed amount of fuel, just when you light it changes.

strange.

gary

Tim (Remove NOSPAM. Registry corupted, reformated HD and lost alot of stuff :( wrote:

As you advance the timing at idle you will get little change in hydrocarbons, as most (at least 95% plus) of the mixture is burned regardless. The Oxides of nitrogen may rise though if its advanced far enough to cause "high" cylinder pressures.. The extra power comes from igniting the fuel/ait at a "better" position for power so the small charge can be used more efficiently, with less wasted as heat and "pressure" in the exhaust. It gets burned an finishes burning before the exhaust valve opens. Which is why you will notice it gets quieter too when you advance it - at least without exhaust silencing...

Burgerman for someone from Grimsby (where I did a lot of my growing up) you aren't half knowledgeable.

Have they put a gatso between every lamp post on the Laceby bypass yet or even installed armco down the centre to stop more people wrapping their XR3i round said lighting implements?

The M180 is fantastic. I remember going to Scarborough from near Sheffield very quickly one day down that road. Little traffic too.

Thanks! It was hard earned...

No, not yet!

And the M180 is still deserted and camera free too! (Should you want to speed test anything serious!) Personally I did my speed testing at Bruntingthorpe, (PB), and at the strip...

Tell me about it!! I'm working on a system to provide a lot of advance at idle rpm on my modified engine (285 degree cam), just to get it idling level. I've had a smooth tickover simply by swapping the vacuum feed from the carb take off port, to one in the side of the manifold. .... But it won't start when cold. So next step is to utilise an electro-valve to bleed off the excessive vaccum via a switch on the dash until a bit of heat is in the engine, then switch the vacuum back on .... possibly control it via the switch on the choke control lever.

I suppose theres one good thing about a high overlap camshaft ... it promotes exhaust gas reburning ... less nastys out the rear!!

-- (Scum Mail Bouncer In use). (Remove the two "n" from email address to reply directly).

Regards..... Steve.

Hello FEo2.

FW> Tell me about it!! FW> I'm working on a system to provide a lot of advance at idle rpm on my FW> modified engine (285 degree cam), just to get it idling level. FW> I've had a smooth tickover simply by swapping the vacuum feed from FW> the carb take off port, to one in the side of the manifold.

Didn't the engine lose a lot of low revs torque by doing that? I know my Niva did when I had tried it. Ended up with restricting centrifugal mechanism travel to get about 18 degrees of advance at idle keeping the same total amount at high revs. The engine (284/292 cam) could use more but the starter motor wouldn't crank it 4 times out of 5.

FW> .... But it won't start when cold.

Excessive advance jams the starter motor?

Sergey

... If cows have horns, why do they moo instead of honk?

Thants what we thought, so if thats the case why wouldn't we run with the max advance for the amount of fuel/air in the cylinder and reduce the amount of fuel/air that can get in to set the tick over speed?

I think basically what i'm saying is if you can increase the idle timing to recover a dropping tick over, it can't be advanced enough in the first place.

Gary

Burgerman wrote:

One option i've used on my rally cars in the past is to have the ability to cut the ingnition while cranking, so you crank the engine over on the starter and when it's spinning you throw a switch to activate the ingition. That way you over come your advance problems by using the flywheel.

might be worth a try.

Gary

FEo2 Welder wrote:

You can with electronic computer controlled systems. But with "old style" distributors it would have been too advanced to start up easily, and it would "pink" as soon as you opened the throttle because now there would be too much advance with the throttle open.

You see the burn speed increases with the charge volume...

One reason it might not be a good idea is that if you are using a catatonic diverter then it will mean much cooler and lower quantity of exhaust gas at the same idle speed, so the catatonic diverter will not function properly in town... (They dont much at the best of times!)

Agreed.

Modern electronic ignition (and fuel injection for that matter) is approximately 1,000,000 times better than the stupid kludges it has replaced.