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Leaded/unleaded after fitting Megajolt
David Jenkins - 5/7/07 at 07:22 PM

I think I know the answer to this, but it's quiet this evening (the wife's watching The Bill) so I thought I'd air this thing in public (oooer).

I have a good-old x-flow that has been rebuilt with an unleaded head. When it had a good old-fashioned mechanical dizzy I found that I had to run super-unleaded 'cos when I used standard unleaded the engine used to run on for quite a while after turning off the ignition - very worrying and it made me nervous about what I could be doing to the engine. I couldn't hear any pinking, but I couldn't hear much anyway when on the move.

Now the dizzy's been replaced with MJ, and up until recently I've been running on super-unleaded, just in case. A week or so back I finally put in the 'dual map' switch and installed a second map that was around 5 degrees back from the original (no science, just guessing).

The car now ran sweetly on the standard unleaded using this second map, so I was happy. I put my 'knock detector' into operation and there was no sign of distress. Also the car seemed to be pulling as well as ever, and generally All Was Well.

Then I switched to the original map whilst tanked up with std unleaded - and the car still ran well, with no sign of pinking and no running on when the ignition was turned off.

I can offer 2 guesses for this behaviour...

1. MJ somehow works better automagically so the engine doesn't suffer pre-ignition. (OK - maybe not).

2. My super-unleaded map is not sufficiently 'ambitious' and could stand some further advance settings.

Any opinions/comments?

David

scottc - 5/7/07 at 07:26 PM

I think I'm gonna go with option 2.

You should be able to run a bit more advance with super unleaded over your bog standard unleaded.

incidentally what spark plug gap you using?

David Jenkins - 5/7/07 at 07:28 PM

quote:
Originally posted by scottc
incidentally what spark plug gap you using?


0.8mm at the moment, but I'm considering going to 1.0mm next time they come out.

Doofus - 5/7/07 at 08:29 PM

Option3 - the old ignition would run-on due to the alternator light keeping the ignition going after the ignition was off.
A bit of a long shot I know.
Was the mech dizzy points or electronic?

Paul.

scottc - 5/7/07 at 08:45 PM

David are you using EDIS?

The Focus runs a 1.3 or 1.4 mm gap, not sure what compression ratio the X-Flow has compared to the focus (or any other zetec), but I know of a bloke running a Porsche 924 running a 1.4mm gap, said Idle was much smoother and it made a noticable improvement.

David Jenkins - 5/7/07 at 09:08 PM

quote:
Originally posted by scottc
David are you using EDIS?

The Focus runs a 1.3 or 1.4 mm gap, not sure what compression ratio the X-Flow has compared to the focus (or any other zetec), but I know of a bloke running a Porsche 924 running a 1.4mm gap, said Idle was much smoother and it made a noticable improvement.


Scottc: Yep - the MJ just manages the timing - it uses the EDIS to do most of the hard work.

The standard x-flow compression is 9.0 : 1

Doofus: I started off with mechanical, changed to Valencia electronic, but had the same effect with both. If there was a problem in that area, I would have expected the same result no matter which fuel was used.

UncleFista - 5/7/07 at 09:11 PM

quote:
Originally posted by scottc
David are you using EDIS?

The Focus runs a 1.3 or 1.4 mm gap, not sure what compression ratio the X-Flow has compared to the focus (or any other zetec), but I know of a bloke running a Porsche 924 running a 1.4mm gap, said Idle was much smoother and it made a noticable improvement.


Ford started recommending 1.0mm gaps not long after launching the Zetec. The standard gap turned out to be too big, causing the plug leads to break down.

Plug leads are a PITA with Mondeos, they're not long lived and aftermarket ones don't last as long as Ford (£££) items.

lotustwincam - 5/7/07 at 09:42 PM

I stand to be corrected, but surely the ignition timing has absolutely nothing to do with the engine running on after it is switched off?

Ford's cure for the original problem was to fit an anti-dieseling valve in the inlet manifold. This opened to atmosphere when the ignition was turned off. The problem was most evident when using low octane fuel that would ignite under compression alone. The running on was a very rough idle and couldn't be compared to a normal idle.

So, having said all that, I can't explain the running on, but would think that your MAP is a litttle bit conservative if you're not getting any pinking with normal unleaded.

Out of curiousity, what is your max advance and at what RPM is it at?

I'm about to go MJ, with a 1660 Xflow and a CR of approx 11.25:1


Drew

David Jenkins - 6/7/07 at 08:39 AM

quote:
Originally posted by lotustwincam
I stand to be corrected, but surely the ignition timing has absolutely nothing to do with the engine running on after it is switched off?


My current know-nothing theory is that something was getting too hot in the over-advanced state (e.g. a plug end) and that was keeping it running.
I do know that a change of fuel stopped that immediately.

quote:

Out of curiousity, what is your max advance and at what RPM is it at?

I'm about to go MJ, with a 1660 Xflow and a CR of approx 11.25:1


Drew


Currently it's 41 degrees at 5000 rpm for the super unleaded, 36 degrees for standard.

Your CR is rather higher than mine!

cheers,
David

martyn_16v - 6/7/07 at 08:59 AM

quote:
Originally posted by David Jenkins
1. MJ somehow works better automagically so the engine doesn't suffer pre-ignition. (OK - maybe not).


Maybe, at least to some small degree. A worn out old mech advance dizzy will suffer from a fair amount of timing scatter. So while it was originaly specced to run at X advance at Y rpm and Z load, it could actually have been a few degrees off. Add to that the use of only a couple of mechanisms (base timing, rpm advance and vac advance) to provide changing timing over a varying range of operating conditions and even a perfectly functioning mech dizzy can only approximate the ideal spark map for an engine. The more flexible digital map of the MJ can be set much closer to ideal, allowing you to run closer to the limits of pinking all over the place, instead of having to back off the timing for the whole range just because of one bad spot on the 'map' from the dizzy.