Jupiter ignition timing and Fuel

[Brian Cole]

Having been gently roused by Admin for not posting lately ( just shows how well my Jupiter is running) here is a question to think about over the coming Autumn evenings.
Like most Jupiters I would think, mine has quit a bit of slack in the distributor drive chain, even in the rod connecting the auto advance system. My question is, does this slack have the effect of retarding or advancing the ignition when the engine is under load? I would like to know this to compensate when setting the static timing.
I am also curious to know which grade of fuel most members are using. I normally use standard unleaded but is there any advantage in using a higher octane fuel in the Jupiter and if so what adjustments woul be required to the ignition /valve timing. I hope I'm not dragging up old material. I seem to remember someone saying that super grade will not have the dreaded additive and so could be topical. Brian Cole.

[Forumadmin]

Brian,
The slack will have no effect provided, when you do the static timing, you always turn the engine in the direction it normally goes. The slack in the vaccum advance might delay its onset when you stick your foot down, but on some dizies the vacuum pipe has a reservoir fitted to do precisely this anyway. But yes I suppose some grey matter could be exercised to ascertain what the advance curve should be (with modern petrols) and then figure out a way of obtaining it.

Fuel is pretty variable and so are Jowett engines. My engine is high compression so does complain if you do put the cheaper grades in, and needs a bit of retard on the micrometer if you do. The point of pink or pre-ignition will depend mainly on the compression ratio but also the fuel to air ratio, swirl of the mixture , temperature of the plug and cleanliness of the cylinder. Oh and head temperature. I think the more expensive grades will have higher calorific value and should thus give more power and mileage. As well as all that, the atmospheric conditions such as air pressure, humidity and temperature will also affect the mixture. On a damp cool morning at sea level with high pressure on the barometer, the car will go better.

The main thing to get right on a dizie is good bearings and no wear on the lobes so you get a consistent firing point on all four cylinders. But correct spark timing will also depend on balanced volumes and swirl patterns in the head, good valves and balanced and tuned inlet and exhaust tracts. Not something Jowett engines are renowned for.

[Keith Andrews]

Some basics of timing
higher compression... well dynamic compression ratio, cycinder pressure at a given rpms the lower the timing
So if a engine has been rebored it will want slightly les timing
higher the octane the more timing as slower burn, plus moern fuel mixes also tend to burn slower.
And the mixture ratio also changes the required advance.

intial timing, all that does ie set a stable idle and enough to fire up on but not enough advance to cook the starter armiture and solenoids....u can set it where ever u like , but keep under 10 degs.
Timing for when the car is moving, which is the important spec should be set at a a rpm that the cent curve is all in with no VA connected.... intial+ cent =total
To dial an engine timing in after a rebuilds, or running anything other than obsolete 65 octane British pool fuel.
intial timing, total timing , cruise timing @ 30mph and 60mph all need to be established individually, then the dizzy configured to those numbers.
Basically max pressure/ explosion needs to be at a given degree between 12 degs and 18 degs ATDC... in there will be a 'premuim' window of 1 to 2 degs.. the concept of simply increase decrease intial rather randomly just doesnt work.
Timing, hitting that premuim timing at a given load and rpm is like shooting a deer on the run the right lead time for that speed and allowing for wind and trajectory drop for a given cartridge and projectile weight...there is a 5" diam kill spot behind the shoulder...anywhere in there and the deer drops where it stands.....the further one gets outside that he more steps it will take before droping (car still runs good). but further out means anything from a couple hrs tracking it down, to days, to never finding the wourned animal at all.. car doesnt run.

why 12 to 18 degs ATDC? u have a short block infront of u, piston at TDC and u push down on it (the mixture explosion) it will not move... turn the piston slightly further down in the bore...still not move....bit further getting into the 12deg ATDC...hard to move but does....the further that piston is down the easier it moves...But there is a trade off, the lower the piston is when the max explosion takes place (over advanced) the less compression, the less intensity of explosion, less power...but sounds good, all noise no go.

Before any timing can be set up.. idle, cruise, total... the mixtures must be in the ball park....idle little lean mid 15s 02, same with cruise and power WoT low to mid 12s..these must be set with timing in ball park, then dial the timing specs in for the engine... Never use a vaccuum meter or 'ear' to establish specs... u can use it to tune the engine to those specs.