I have a 3.8 with 9 to 1 compression and wondered what BTDC timing anyone else with this setup uses.
I am currently on 5 Deg BTDC and several people who are supposed to know there stuff have been surprised at this expecting it to be more like 10 Deg which is where I get pinking under load.
Thanks

 

You are correct. Do you have a 41063A distributor? (S Type)

EDIT: I read an article on this somewhere but can't find it. While it would appear counter intuitive it has to do with Jaguar's hemi head (combustion chamber) & relative piston design. Squish band & piston dome & relative onset of knock with the respective CR pistons.

 

Hi Gyln,

I know it is a rebuilt Lucas 22D6 but apart from that I will have to look.

 

In my Mk2 factory service manual, the static timing for a 3.8 with 9:1 c.r. is 5 deg advance with a paper air filter (and 10 deg with an oil bath, which is a bit surprising). If you set the timing using a xenon strobe light with the engine runninng at 550 rpm and the vacuum advance disconnected, the timing should be 8 to 9 deg btdc.

With a DMBZ.6A - 40665A distributor (standard on the 9:1 3.8 in Mk2) in a test rig, there should be a centifugal advance of 13 deg at speeds above 2,000 rpm. So on an installed engine with the vacuum advance disconnected, you should find a total advance of 5 + 13 = 18 deg at 2,000 rpm and above.
Edit: My mistake in the above! The degrees and rpm are distributor values on a test rig. As such, the strobe should give about 6 deg at 550 rpm and 31 deg at engine speeds above 3,500 rpm.

My inclination would be to disconnect the vac advance, have a 'reliable friend' hold the engine steady at around 2,500 rpm and check with a strobe. Those are the speeds where the timing should be appropriate.

Don't forget to re-connect the vacuum advance when you've finished!

The values above seem a little retarded for minimum advance best torque of a relatively low turbulence design like the XK. Edit: they are quite reasonable given the corrections. They are taken from the Mk2 factory service manual. Quite likely, the ignition advance of the XK (with a 9:1 c.r and standard domed pistons) is knock limited (with some fuels) and Jaguar may have gone further and allowed a good safety margin. If anyone has an E-type or XK150 service manual (or better still experience of setting the timing of a similar engine on a chassis dynamometer while carefully monitoring for knock), they may be able to provide alternative values. And, of course, any electronic control of spark timing will open up opportunities for more sophisticated settings and be an improvement on the compromises of a standard mechanical distributor.

 

Peter, is the manual talking about distributor degrees or engine degrees out on the dampener ?

 

Thanks Peter. So we now have S Type & Mk2. I wonder what car the OP's engine is out of. I was interested in what distributor the OP had as I have the full listing of Lucas distributors & their specs. It's huge. Have you any idea how many distributors Lucas made? Mind boggling.

The S Type manual is less informative so I've always referred to the Lucas publication.

I'm trying to get my head around the oilbath static setting on Mk2's.

I see from 1966 the Mk2 was supposed to have a 41064A distributor on 9 to 1 CR engines.

Lucas Distributor specs for those that are interested. Lot's of gaps but Jaguar well covered.

 

Jeff, That was a very polite nudge. Most internet users would have told me that I'm even more retarded than the spark timings that I've given! The manual doesn't actually specify engine or distributor degrees (or I can't find where it does), but I'm pretty sure that you are correct and they are distributor degrees. As such, the value at 550 rpm would be about 6 deg and at speeds above 3,500 rpm 31 deg. The last number sounds a lot more sensible to me for an XK combustion chamber.

Glynn, The manual lists 7 distributor variations for the Mk2. It's hard to say if the distributor, more specifically the spark timing part of it, is the most wonderful or horrible device on an engine. I guess Lucas had a massive database on spring, weight and vacuum capsule combinations to give an approximation to every timing curve?

 

Yes ~ I just restored my 41063A ~ had plenty of spares. Fitted a Pertronix Ignitor. Had it set up on one of these old beasts to spec and all behaves perfectly. Engine runs beautifully & zero knock thank heavens.

 

I remembered that when I had the distributor rebuilt he sent me a plotted curve for how he had set it up.
So going by this graph if I am reading it correctly which to be honest I very well may not be, my timing at 750 rpm should be 8 deg and 26 deg above 1800 rpm on the timing gun ??? or have I got it wrong.

 

Speaking as someone who forgot the first time round, I'll try to explain. The factor that we have to account for is that the distributor rotates at half of engine speed. Both the value on the advance axis and the speed axis have to be doubled. If the distributor set up plot shows 11 deg at 1,800 rpm and above and the engine static timing is 5 deg, then you should have an engine spark at:
5 + 2x11 = 27 deg at engine rpm of 2×1,800 = 3,600 rpm and above.

I suspect that I'm as clear as a Murray Walker F1 commentary.

All those numbers are of course with the vacuum advance disconnected.

 

Well said. Yes ~ the distributor rotates at camshaft speed. It is a 4 stroke engine. i.e half crankshaft speed.