MG T-ABC

In the latest issue of the OCC Bulletin, Bjarne Bergengren of Helsingborg, Sweden suggest the need to advance the timing to 5 or even 10 degrees before TDC when using modern fuel. As I am running 92 octane in my TB, I'm considering advancing the timing. What are the advantages? What, if any, is the relationship between octane rating and timing advance. As US actane ratings for unleaded gasoline are running at 87, 89 and 92, respectively, what are the best advance timing settings for these octanes?

On Engine data, where can I find the specs for the XPAG; e.g., torque settings, standard bearing measurements, etc.? Happy 2000 everyone,

Skip (TB 0304)

In general, advance as much as you can get away with.
When you start to get pinging under load, back off. It all depends on carb settings, cam profile and timing, cylinder head shape, etc. But the idea is to get the maximum cylinder pressure to give the most oomph on the power stroke. Just keep an eye on temperature, and a ear open for excessive pinking, and you will have it. Aslo keep in mind, you cannot run as much advance on the cheap stuff as you can with the 92 octane. There is no difference in the actual fuel that burns between the 87 and the 92, it is just the additive package that changes. But even our cheap stuff in higher in octane than the mystery fuels that they used to run. Hence, the better advance curves that we can get away with. Also, if it looks like you can run a good advance, do check the total advance. You will probably want about 32-34 degrees total. Over 36, you are probably pinging up a storm.
Cheers...Andy

When you set the distributor timing staticly, as described in the workshop manuals, using a light with leads connected to the distributor and coil, you will end up with 5-6 degrees advance BTDC, when you check the engine timing with a strobe light. The distributor advance starts at low rpm.

Ths biggest thing about spark advance is the maximum advance allowed by the distributor action shaft and plate. The majority of Lucas distributors have a 15 degree (30 at engine) advance plate. The action shaft and advance plate are stamped with what advance maximum is possible with the plate. There was a factory bulletin , advising of the availability of a 17 degree (34 at engine) action shaft and advance plate. Calculations with the weight size, distributor case size, indicates that the maximum advance possible with the stock Lucas distributor is 19 degrees (38 at engine).

With the TF 1500 engine, the distributor action shaft and plate advance was reduced to 12.5 degrees (25 at engine). This was to ensure long life of the engine with the fuels available of the day. A 15 degree or 17 degree plate will give much better performance than the 12.5 degree plate.

John Seim

Hi Whitworth Head,

Well here are my 2 cents worth, plus some change. Am feeling negligent since I haven't contributed lately.

First of all, unless you are running high compression, you are wasting money buying premium fuel. XPAGs don't need the extra cost fuel injection cleaner additives, and the higher the octane the slower the burn rate, because the higher octane fuel blend is more stable under compression. The slower burn rate is why you can advance ignition timing a little further on high octane.

First, lets think about what ignition advance does. It starts combustion pressure on the piston before it is on the power stroke. This is negative force which robs power, and is audible as pinging when the extreme advance condition is approached. So I would argue that the greatest advance is not necessarily the best state of tune.

Timing at idle is not very important for performance or engine health, as long as the ignition can support a reasonably smooth idle. Timing at the fully advanced stage is important and will determine your engine's health and performance. Not enough advance is a common cause of overheating.

Static timing is fine for ordinary driving if your distributor is functioning properly. There are two ways to check your distributor. One is removing it and mounting it in an old fashion distributor bench top test rig. These are getting rare. The other is a timing light that incorporates a dial-in advance-retard degree function. This is the instrument I recommend.

Prepare by getting your crank pulley marked clearly and distinctly for top dead center and five degrees advance. You might want to convince yourself that your TDC mark is accurate by using a probe in #1 spark plug and put two temporary marks on your crank pulley, one for when upward motion of the piston stops and one when downward motion of the piston starts, half-way between these marks is TDC. A probe is a ridged pointer (the longer the more accurate) and some sort of measuring fixture. A real Rug Goldberg rig is what you'll likely build. And its time consuming but very worth while if you've never done this before the engine was assembled. Also remember that disturbing the carbon build-up on the piston crown can cause changes in measurement. So make a small clean spot by gentle scraping.

Set your timing using the static method at TDC (do this because you will first want to measure the maximum advance of your distributor). Warm up your XPAG and then connect your timing light. Now dial in 30, then 34 or 38 degrees advance on your timing light and adjust your distributor until you see your TDC mark at the pointer when your XPAG is running at about 2200 engine rpm (others may have better info on this spec since I'm writing without benefit of reference material). Hopefully you will have now determine which distributor advance mechanism you have. Then adjust your distributor statically again to 5 degrees advance. Now run your XPAG with timing light attached and dial in 30, 34 or 38 degrees (which ever you determined is your distributor) and run the engine around 2200 rpm. You should see your TDC mark line up with the pointer (note there will always be some wander due to point bounce or loose bushings). If you don't loosen your distributor and adjust it until you do then tighten it and double check. Then drop rpms to idle, dial in 5 degrees on the timing light and you should again see your TDC mark reappear at the pointer.

If it doesn't your distributor's advance mechanism is not operating to spec. This is a common problem, first because the distributor is not clean and/or well lubricated underneath the points plate, and second because I suspect replacement advance springs are not up to spec. But the important thing to note here is that your ignition timing at the fully advanced state is much more important because at this speed and above is where your engine is beginning to do its hard work, not at idle. And when it is working hard is when it is burning maximum fuel and generating maximum heat and stresses.

Lastly, what happens between these two distributor speeds is what is called the advance curve. You can plot this as a function of advance relative to engine rpm. The best shape of this curve (usually a couple of straight lines really) is determined by cam spec, altitude, carbs, and driving style - too deep a subject here, or for me without some study.

Cheers, Peter

Hi Whitworth Heads,

I was called to a great Alsatian dinner and sent me previous message in haste. Now I found an error in the paragraph starting:

Set your timing using the static method at TDC (do this because you will first want to measure the maximum advance of your distributor). Warm up your XPAG and then connect your timing light. Now dial in 30, then 34 or 38 degrees advance on your timing light and adjust your distributor until you see your TDC mark....

Remove "and adjust your distributor" in the last two lines above. I decided it best to first determine what your distributor does at max advance, but forgot to remove this statement.

Sorry for this error.

Cheers, Peter