Ignition timing & modern fuel

Hi all,

Edit: the car’s a 1969 2+2, triple SU carb, high compression engine, standard starter. The plugs look good. To me, at least.

When I set my ignition timing correct (with strobe, both idling and at 2500rpm), I always have the impression that the ignition can be set more advanced than that. When I do so, the car responds cripser. But, when I leave the ignition more advanced than it should be, the starter has a hard time starting the engine (the starter stalls), because the spark comes in too early.

I read (google [manchester XPAG tests]) that modern fuel burns slower, thus needing more advance. Could I (and you too) be suffering from this? Is it time to get a 123-TUNE distributor?

Thanks, Andrys

Andrys -

You might mention what engine, compression, carbs & starter you have – also what static advance you are having the starter issue with.

One possibility is that the starter is nearing the time for a rebuild.

I believe lower octane fuel requires less advance. At least that was the way it is described in the instructions for setting timing with the SNG Barrett ignition upgrade i’ve recently installed on my V12.

IIRC the recommendation is for 12 degrees advance with 97 octane gas, 8 degrees for 93.

you would need to recurve the distributor to get the best performance and easily start. having more total mechanical advance would allow more advance at 2500 while retaining the stock advance for starting.

I had this problem with a big block Ford engine from the 70’s. I ended up wiring in a delayed relay into the ignition circuit so that the engine would spin for a couple of seconds before the ignition would turn on.

Worked like a charm for years and really made life much easier on the starter. I stopped needing to replace starters every couple of years.

I am guessing they use a similar approach on the stop-start vehicles of today, but built into the ECU’s ignition timing control.

Hi Andrys If you look at typical web site pages on generic spark advance you’ll see that most authors recommend a total advance of 45 degrees for hemi engines (this is static advance plus mechanical advance). The hemi combustion chamber is big, spark plugs are off set to one side so it takes time for the flame front to traverse the chamber, and fully burn the fuel. Hence ignition is started much earlier than conventionally shaped chambers. The problem is that because of the large chamber and slow flame front hemi’s are prone to detonation. The solution is to run premium fuel and richer needles. Lean mixtures detonate easier. A number of years ago we did a fair amount of dyno work on both a stock, and a modified engine. There is significant power to be gained advancing ignition and increasing mixture strength. On the stock engine we saw 55 hp more at 45 degrees advance and using UO needles (Jaguar’s racing recommendation) at the higher end of rpm… All U (prefix) designated needles are essentially the same thickness in the idle portion, and slowly get richer - so you can use them on the street without a huge penalty- thou I’d recommend you use a UU needle which is half way between the UM needle (quite lean) and the UO. I run modified engines but at 9:1 compression. I set the idle timing at 20 degree btdc which will give me 39 degrees all in. Running on premium fuel here, described as 94 octane, with no ethanol, and UU needles, I have no problem with detonation. I’ve run the car in the US on ordinary premium - I think labled 92 with ethanol also with out issue, but the car really doesn’t like this fuel - won’t idle on it. The critical area for detonation is in the 2500 - 3500 rpm range while on partial throttle. Disconnecting the vacuum advance can help here. Starting with 20 degrees advance has not been an issue, but I use a geared starter.

Thanks for your responses chaps. I am happy to see that there’s actually science to back what I observed. I know there are a couple of websites about recurving the distributor. I’ll have a look at that.

Terry, an intriguing statement: 55hp more at 45 deg. Assuming 45 was “all in” (static & mech.), do you remember what the stock setting was (w/o vacuum) for total advance?
Over the phone, Bill Terry told me to limit advance to the mid 30’s. Now Bill has forgotten more than I will ever know about the XK. Maybe I misunderstood his meaning: (35 plus initial instead of 35 total?).

I am wondering how much performance I am giving away at 36 deg. all in at 3300rpm? 55hp?
The car does flatten out around 4000 rpm. (its a 4.2 stock config w/3xSU’s, opened up exhaust)

Hi David The 55 hp increase I spoke of was the result of both a timing increase to 45 degrees all in and going richer - specifically changing from UM (stock needle) to UO which is the needle Jaguar recommends for racing. To get 45 degrees out of a stock distributor you have to set your timing at idle at 25 degrees btdc (the stock distributor has 20 degrees mechanical advance) which was ok on the dyno, but not practicable for daily driving. I can only approximate the timing’s advance effect on hp because we discovered an internal problem in the distributor that was causing some minor issues in some of the middle tests, I would put the gain in going from 30 degrees to 45 at 24 hp - the balance of the 55 hp came from the richer needles. These tests were run with an engine dyno (not a road dyno) on a stock 9:1 rebuilt engine on triple S.U.s with individual K & N filters and a poor quality header. I don’t dispute Bill’s advise as there are variables of yours that affect this that he may not be aware of (nor am I). The obvious example is gasolene quality, the second is mixture strength. Running high advance and a lean needle is an invitation to detonation. I think if you ran high 30’s with an UU needle you’d be all right, you would see a significant power increase, and your mileage wouldn’t change much with normal driving…