Spark Timing Considerations?
06-18-2019, 09:24 PM
#1
Racer
Pro Mechanic
Thread Starter
Spark Timing Considerations?
1981... L81... 350...8.2:1...2.87 rear end. Converted to early Q-jet and HEI.....I'm ready to dial in my new red, $40, ebay HEI distributor. I'm aware of the 36 degree max total advance, but are there other considerations to take into account such as gear ratio and compression ratio?. First off, the 8.2 compression ratio should be more tolerate of detonation (I use mid grade gas) and secondly cruise rpm is in the 1500-2000 rpm range. My question is: should total timing occur earlier, maybe in the 2000-2500 rpm range taking into consideration cruise rpm, and the compression ratio being tolerant of more timing at a lower rpm? I'm thinking something along the lines of 14 degrees initial and 22 degrees centrifugal all in at 2500 rpm. Vacuum advance (manifold source) should be responsive enough to prevent over advancing at low rpm.......Your thoughts?
06-18-2019, 10:33 PM
#2
Le Mans Master
The vac advance is going to have all of it’s advance in at those rpms. So as long as it doesn’t add more than about 14* for a total of 50* that should be fine.
however I would probably not have full mechanical advance in before 2500 and maybe more like 2800. Light the fire too early and there is a lot of negative work against the rising piston leading to lower performance.
You can experiment to see what gives you best overall performance. This may require limiting both mechanical and vacuum advance movement however as you increase your initial timing to avoid more than 36* all in and 50* at cruise. You might be able to get away with 52* at cruise. If it trailer hitches, or pings, it’s too much.
however I would probably not have full mechanical advance in before 2500 and maybe more like 2800. Light the fire too early and there is a lot of negative work against the rising piston leading to lower performance.
You can experiment to see what gives you best overall performance. This may require limiting both mechanical and vacuum advance movement however as you increase your initial timing to avoid more than 36* all in and 50* at cruise. You might be able to get away with 52* at cruise. If it trailer hitches, or pings, it’s too much.
Last edited by REELAV8R; 06-18-2019 at 10:36 PM.
06-19-2019, 09:57 AM
#3
Le Mans Master
The specs you suggest are pretty much in the zone. The springs can be tuned to stretch out the curve if you find you need to.
Cruise is not much of an issue. The load is so low you will not be in detonation range so the engine is very tolerant of the additional timing. It's when the load first comes on is when the centrifugal advance curve needs to be the concern. The vacuum advance will back off and you will be driving only on the centrifugal curve once you push the pedal down. Usually you want to reduce the centrifugal for better idle first. Springs can be as light as the fuel quality will allow at WOT but be sure the centrifugal advance does not begin advancing until you are off idle. Maybe 1200 RPM or so. Gotta play with it.
If you reduce the amount of advance possible in the centrifugal curve, the initial will increase because the top of the centrifugal curve needs to be at 36. The total sum of the initial timing plus all the available centrifugal will need to stay ideally at 36 degrees using those original wedge style heads.
The vacuum advance needs to be selected / limited to provide in the area of 12-16 degrees for a total sum of initial plus all the available centrifugal plus all the available vacuum advance not to exceed 52 degrees. This is just the icing on the cake for idle quality, lower operating temp, mileage and part throttle drivability.
Vacuum advance needs to be 100% fully in at idle when hooked up to a manifold vacuum source. Measure your vacuum at idle and make an effort to use a can that is fully advanced at least 2" lower than your manifold vacuum at idle. If your idle vacuum at idle is say 14", you want a can that is fully advanced at about 12" of vacuum or less. That way the RPMs will be steady and not have a tendency to "hunt" up and down.
Cruise is not much of an issue. The load is so low you will not be in detonation range so the engine is very tolerant of the additional timing. It's when the load first comes on is when the centrifugal advance curve needs to be the concern. The vacuum advance will back off and you will be driving only on the centrifugal curve once you push the pedal down. Usually you want to reduce the centrifugal for better idle first. Springs can be as light as the fuel quality will allow at WOT but be sure the centrifugal advance does not begin advancing until you are off idle. Maybe 1200 RPM or so. Gotta play with it.
If you reduce the amount of advance possible in the centrifugal curve, the initial will increase because the top of the centrifugal curve needs to be at 36. The total sum of the initial timing plus all the available centrifugal will need to stay ideally at 36 degrees using those original wedge style heads.
The vacuum advance needs to be selected / limited to provide in the area of 12-16 degrees for a total sum of initial plus all the available centrifugal plus all the available vacuum advance not to exceed 52 degrees. This is just the icing on the cake for idle quality, lower operating temp, mileage and part throttle drivability.
Vacuum advance needs to be 100% fully in at idle when hooked up to a manifold vacuum source. Measure your vacuum at idle and make an effort to use a can that is fully advanced at least 2" lower than your manifold vacuum at idle. If your idle vacuum at idle is say 14", you want a can that is fully advanced at about 12" of vacuum or less. That way the RPMs will be steady and not have a tendency to "hunt" up and down.
