SL-6 Timing is What???

[mydart270]

i have question that's bugging me. Right now my timing is set at 5 deg BTDC in my SL-6. Lots of people say "bump it up to at least 10 - 12 deg BTDC. Wow....hard starting I assume and engine runs hotter I think.

can someone pls tell me why this recommendation exists? My car is a 1968 Dodge Dart 270, SL-6, A/T., no A/C. Very nice and runs like a top except for kick down inconsistent, carb adjustment I hope.

Many thx...great forum, great people, great experience.

alan

 

[Badvert65]

If it is running well, I would leave it alone. Sometimes you just have to find your engine's sweet spot.
That being said, you should verify that the balancer hasn't slipped (not uncommon on older engines). Just verify that the balancer mark is at zero when the #1 piston is at Top Dead Center. If the balancer has slipped, you won't really know where the timing is set.

 

[Mattax]

per '68 Plymouth FSM, 5* BTC at 650 rpm is correct spec. It should start advancing above that so its between 5 and 10* at 950 rpm. Seems like yours is working fine, so wouldn't worry about it too much.
Generally retarding the timing can make it run hotter. Later ignition results in more of the burn occuring when the piston is further down the cylinder. That heat then goes into the cylinder walls and exhaust port.
CAP equiped vehicles purposely used less advance at idle. '68 Plymouth FSM for a 225 spec's 0* @ 700 rpm.

People advance timing when hot rodding and using a cam that doesn't burn efficiently at idle. (Advancing helps because of the slower burn.) They also advance when trying to overcome the poorer performance of the later smogged engines, or because someone suggested they do it. (guilty as charged - I had several 70s /6 when younger). Finally, when running on the drag strip, engines temperatures are usually a bit cold - sometimes a bit more advance will make the make the car quicker in that situation. Factory spent considerable time and effort developing and testing the timing curves for max efficiency and power. Sometimes Dodge and Plymouth came up with slightly different specs but they are usually similar. Non-CAP curve is going to be pretty good starting point even if you want to experiment.

 

[MCPObr44]

per '68 Plymouth FSM, 5* BTC at 650 rpm is correct spec. It should start advancing above that so its between 5 and 10* at 950 rpm. Seems like yours is working fine, so wouldn't worry about it too much.
Generally retarding the timing can make it run hotter. Later ignition results in more of the burn occuring when the piston is further down the cylinder. That heat then goes into the cylinder walls and exhaust port.
CAP equiped vehicles purposely used less advance at idle. '68 Plymouth FSM for a 225 spec's 0* @ 700 rpm.

People advance timing when hot rodding and using a cam that doesn't burn efficiently at idle. (Advancing helps because of the slower burn.) They also advance when trying to overcome the poorer performance of the later smogged engines, or because someone suggested they do it. (guilty as charged - I had several 70s /6 when younger). Finally, when running on the drag strip, engines temperatures are usually a bit cold - sometimes a bit more advance will make the make the car quicker in that situation. Factory spent considerable time and effort developing and testing the timing curves for max efficiency and power. Sometimes Dodge and Plymouth came up with slightly different specs but they are usually similar. Non-CAP curve is going to be pretty good starting point even if you want to experiment.

What is the cutoff year for pre and post smog vehicles with slants?

 

[Mattax]

What is the cutoff year for pre and post smog vehicles with slants?

The first big changes to reduce smog related emissions control was called CAP. Based on the MTSC pamphlets and shop manuals I've looked at, I think the first model year for CAP was '67, and just for California sales. Then in '68 it was implemented across some or more of the US and Canada. By '69, all US and Canada.

The big changes with CAP involved idle timing, fuel mix and engine speed. Timing on decelleration was also an early concern. Early 70's was when it started to effect performance and economy, especially with things like delaying the timing advance. Early changes not very noticible, although even Chrysler notes to their techs they will see lower vacuum at idle in the last pages of this booklet. 1967 Chrysler Imperial CAP System from the Master Technicians Service Conference Session 241

This graph of fuel mix vs combustion products shows the effect of running making the CAP equipped cars run leaner at idle. Most efficient idle AFR is generally around 13 - 13.5 but for CAP it needed to be 14.2:1. Initially the big focus was reducing CO. Ironically the early anti-smog setups made the NOx worse if you discount the effect of running leaner and with less timing. Later this was addressed because of course it was a problem too.