I want a '78 advance (thanks Ray!)

[81xsproject]

So ever since Ray said that the 78 timing advance had the most kick I have been looking for one. I need a mech adv for my MSD ign conversion and haven't seen one on the bay since I started looking. Anyone got one they could part with?

[TopCatGr58]

Hey Jessie,

A review of the ignition module thread, where Jerry and others and I discussed the use of 81 modules for XJ's, shows that for the early XS11's, the static timing was 10 BTDC, and then the cent. adv. provides a max. of 26 additional degrees for a total/max of 36 degrees, which is also the same max that the XJ's TCI provides at 4750 rpm or above.

Crazcnuk who races cars, posted about the max adv. they would program into their race engines distributors was about 32 degrees maxing at 6k rpm.

I'm no race engine developer, but I think there is a MAX timing advance amount for the under WOT/Full throttle power curve. This is also why Yamaha provided the Vacuum advance/retard mechanism, to allow the advance timing to return to the LOWER more powerful timing advance for heavy load vs. the higher advance amounts for low load lean fuel cruising rpms.

I remember Ray's post about his seat of the pants feeling for the more advanced timing, but since you'll be using the MSD system with the Cent. Adv. but not the vacuum advance, then I don't think you would necesarily need to restrict yourself to the 78 unit, but could use the 79-80 ones as well. And then either just adjust the plate for a little more static timing before the cent. kicks in, or you should be able to do some fancy programming with that assortment of timing chips I saw in the photo of your system.

I'll do a NET search to see if I can find any specs on max ignition timing values for given rpms, but as we've found out with what Yamaha did with the XS11, they may have already figured that out also...at the 36 degree limit!?!?
T.C.

[81xsproject]

Well TC, I don't know if I am going to be able to rid my self of the vacuum retard. I am currently planning on having it. So it will be a full 78 curve.