relationship between vacuum and dwell
Post Fiend
Joined: Jun 2002
Posts: 13,873
Likes: 3
From: Atlanta GA
relationship between vacuum and dwell
Dwell is the amount of time the points stay closed and a small dwell means the points stay closed longer, correct? The lower the number the more retarded the advance, correct?
So how come setting dwell to 15% produced the most vacuum (18) on my truck, but, lousy driving. Setting to factory specs (27%) produced lousy highway driving/shaking and low vacuum. A setting of about 20-22% dwell produces okay vacuum (15) and okay driving.
Should I set to the best dwell for vacuum and work backwards from there? I know the motor was recently rebuilt because there was no sludge on the heads when I took the valve covers off. I am just trying to figure out how to tune (being my first points vehicle) the engine.
FWIW: Seems to like 93 octane gas.
So how come setting dwell to 15% produced the most vacuum (18) on my truck, but, lousy driving. Setting to factory specs (27%) produced lousy highway driving/shaking and low vacuum. A setting of about 20-22% dwell produces okay vacuum (15) and okay driving.
Should I set to the best dwell for vacuum and work backwards from there? I know the motor was recently rebuilt because there was no sludge on the heads when I took the valve covers off. I am just trying to figure out how to tune (being my first points vehicle) the engine.
FWIW: Seems to like 93 octane gas.
Posting Guru
Joined: Aug 2002
Posts: 1,214
Likes: 2
From: Minneapolis, MN
relationship between vacuum and dwell
John, you are correct that dwell is the time that the points stay closed. It's measured in degrees° (alt+0176). One thing to remember is that when you change the dwell you also change the timing. Small dwell would mean the point gap increases and so increase timing advance. If the breaker plate is worn, when the vacuum advance moves it can also change the dwell. If the dizzy bushings are worn, the shaft can move around changing the gap/dwell. Watch what it does through the rpm range. Short dwell might not give the coil enough build up time at higher rpm's and long dwell the points will just barely open. Set the dwell for the most stable reading and adjust the timing after for best running.
Just my $.02
Greg
'77 F-250 Camper Special 400ci driver
'76 F-250 Supercab 360ci current project
'71 Mach I 429CJ in storage
'79 F-150 for parts
huct on foniks wurkt fer me
Just my $.02
Greg
'77 F-250 Camper Special 400ci driver
'76 F-250 Supercab 360ci current project
'71 Mach I 429CJ in storage
'79 F-150 for parts
huct on foniks wurkt fer me
Elder User
Joined: May 2002
Posts: 799
Likes: 0
relationship between vacuum and dwell
You're correct: dwell refers to the closed portion of the points operation.
Interesting observations.
Here's a thought: Adjusting things for best idle may not be best for normal driving since the conditions are different. As long as your plug wires cap and rotor are good. As long as and your spark gap is right and you're sure your timing marks haven't slipped. As long as all of those are solid, you can be free to experiment and get a feel for what's best regarding dwell.
It sounds like you've found the best settings. It's hard to knock carefully done experiments. Theory is great, but the final proof is always in good experimental practice.
--Matt
Interesting observations.
Here's a thought: Adjusting things for best idle may not be best for normal driving since the conditions are different. As long as your plug wires cap and rotor are good. As long as and your spark gap is right and you're sure your timing marks haven't slipped. As long as all of those are solid, you can be free to experiment and get a feel for what's best regarding dwell.
It sounds like you've found the best settings. It's hard to knock carefully done experiments. Theory is great, but the final proof is always in good experimental practice.
--Matt
Thread
Thread Starter
Forum
Replies
Last Post
