Ignition setting using unleaded?

[ChrisC]

Folks,

Question for those people who have had their car tuned on a dyno. What was found to be the best ignition setting BTDC? The manual says 10 degrees but that assumes the use of 5 star petrol.

Unleaded is not so good so logically the timing needs to compensate.

Thoughts please as I am a little out of my depth on this.

If it makes any difference I am running an electronic distributor (not a 123) and have performance cams etc...

Cheers

[kingzetts]

Chris,
With a fixed advance curve (I assume your dizzy is not one of the programmable variety?) you're really asking the wrong question about timing. You need to select an idle setting which gives you as much advance as possible without pre-ignition at the critical point in the rev/load map - usually medium revs (3500 or so) at full throttle I believe on an XK.

To illustrate, I used to have a JAG 123 dizzy on my nominally standard 8:1 3800 engine running regular 95 RON unleaded. Starting from the "standard" 10 degrees BTDC idle setting, it ended up set at only 2 degrees BTDC after CMC tuned it for me. They tuned it by getting me to drive it along the road near their premises and pulling full throttle in 4th - you could hear the pinking clearly - and then backing off the idle setting progressively until the pinking went away.

Now running Megajolt, idle timing is no longer directly linked to advance at other load points. So mine now has an idle setting of 2 degees at idle with vacuum disconnected, but 12 degrees at idle under full vacuum.

Seeing as you have non-standard cams and possibly other changes, I'd start with 2 degrees and see if you are free of pre-ignition under load, then gradually advance the idle setting and repeat the road test until you just get pinking, then back off a degree or two. Much easier to do on a dyno, of course.

[ChrisC]

A fair point - thanks...

now I just have to wait until summer to drive the car but saying that the engine is not run in yet so putting it under that kind of load isnt a good idea

Maybe I will simply back it off a little more to say 5 degrees and wait until the engine is run in to fine tune it.

Cheers

[AussieEtype]

Surely whether petrol is unleaded or not is irrelevant - what is important is the octane rating of the fuel. 98 Octane (unleaded) has about the same rating of the old 4 Star Leaded so ignition settings would be the same.

I assume in the UK there is no 100 Octane Unleaded but I assume you do have 98, 95, 91 Octane Unleaded and some variants for other fuels with Ethanol (yuk) in it - these normally rate about 1 octane point higher than their 100% petrol equivalent.

Garry

[ChrisC]

Perhaps a slightly mis worded title...

UK Unleaded is 95 Ron so about a 3 star. Super Unleaded is higher and might be 98 (so 4 star). But as John suggests - my dizzy is mapped so a Dyno is the only real way of setting the ignition but all i was after was an initial setting given we dont have 5 star in the UK.

[Woolfi]

Be careful to believe, that you can detect detonation by hearing the "pinks".
High-speed detonation is much more dangerous for the motor, than typical pinking. High-speed detonation can not be heard so easy. If you set your ignition, that there is no pinking, still the motor can have high-speed detonation.
Check in google for detonation , high-speed pinking , pre-ignition.

Regards Wolfgang Gatza