Vacuum advance tuning to maximize cruise MPG?
- Thread starter mderoy340
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I edited post 22, to maybe help explain my thinking. My thinking may be getting fuzzy in my middle tears. And Im not too proud to change it. So the new part starts at "consider how it works".
46* @idle, 60* @ 2200. That cant be good. I consider that messed up.
The object of this exercise is not to force the engine to accept huge amounts of timing.
The object is to give it only as much as it asks for.
If you start the fire too early,Then 2 things can happen; 1) The peak pressure will occur too early, at a place where the rod cannot deliver it efficiently to the crank, and 2) the pressure may be too high before TDC and it may actually try to force the piston back down, the way it came. This is sometimes called negative work. This was evident when you had to increase the idle speed when you over-advanced it to 46*idle timing There may not be enough pressure at small throttle openings to cause the dreaded rattle, but none-the-less, it may be too much, and will eventually lead to bearing damage.Thats why I called it messed up.
-Once you get the dial-back installed, you will be able to prove this quite easily. I suggest you set the base timing with the device set to the middle of its range. Then you will be able to retard it as well, from the front seat as you are driving. If the MPH goes up when you retard it, you will know its too far advanced. If it changes very little, it may be advanced way to far.
Yeah so, Ima gonna be gone for a few days, So Id like to say Best of luck to you and may all your efforts pay off.
46* @idle, 60* @ 2200. That cant be good. I consider that messed up.
The object of this exercise is not to force the engine to accept huge amounts of timing.
The object is to give it only as much as it asks for.
If you start the fire too early,Then 2 things can happen; 1) The peak pressure will occur too early, at a place where the rod cannot deliver it efficiently to the crank, and 2) the pressure may be too high before TDC and it may actually try to force the piston back down, the way it came. This is sometimes called negative work. This was evident when you had to increase the idle speed when you over-advanced it to 46*idle timing There may not be enough pressure at small throttle openings to cause the dreaded rattle, but none-the-less, it may be too much, and will eventually lead to bearing damage.Thats why I called it messed up.
-Once you get the dial-back installed, you will be able to prove this quite easily. I suggest you set the base timing with the device set to the middle of its range. Then you will be able to retard it as well, from the front seat as you are driving. If the MPH goes up when you retard it, you will know its too far advanced. If it changes very little, it may be advanced way to far.
Yeah so, Ima gonna be gone for a few days, So Id like to say Best of luck to you and may all your efforts pay off.
mderoy340
Well-Known Member
I took AJ/FormS advice and made some changes. The adjustable timing box I got does not work so I'm doing this old school now.
It was 80+ temperature today so good for a normal tune today.
I started with setting initial timing with a vacuum gauge at idle and ended up with 24* 16" vac. Instant start even after heat soak. 13.1 AFR in neutral.
11* mechanical advance all in by 2300, Total mechanical = 35*
Drove the car without vacuum advance 13.9-14.3 AFR cruise, 12.3 WOT and had no ping. Bumped up total mechanical to 38* and had slight ping. I put it back to 35* total.
I decided to work with ported vacuum; lack of engine brake decelerating with full manifold vacuum and manual brakes is annoying. I had an 8.5R vacuum can and ground down the mechanical stop on the lever. I adjusted the can to start moving at 8" VAC and is all in at 15". I drilled a hole in the distributor housing and tapped it for a 4-40 screw. I'm using this as an adjustable mechanical stop for the vacuum advance plate. The screw does not interfere with the cap and is easy to reach with the distributor installed. See pic.
Total timing with vacuum advance for now is set at 45* Drove the car in cruise mode; 45-70 mph and once I'm at steady speed I'm at 18-19" Vacuum, 13.9 14.3 AFR. No ping with hard acceleration.
Tomorrow I'll slowing adjust the vacuum advance max limit and see how the car reacts.
It was 80+ temperature today so good for a normal tune today.
I started with setting initial timing with a vacuum gauge at idle and ended up with 24* 16" vac. Instant start even after heat soak. 13.1 AFR in neutral.
11* mechanical advance all in by 2300, Total mechanical = 35*
Drove the car without vacuum advance 13.9-14.3 AFR cruise, 12.3 WOT and had no ping. Bumped up total mechanical to 38* and had slight ping. I put it back to 35* total.
I decided to work with ported vacuum; lack of engine brake decelerating with full manifold vacuum and manual brakes is annoying. I had an 8.5R vacuum can and ground down the mechanical stop on the lever. I adjusted the can to start moving at 8" VAC and is all in at 15". I drilled a hole in the distributor housing and tapped it for a 4-40 screw. I'm using this as an adjustable mechanical stop for the vacuum advance plate. The screw does not interfere with the cap and is easy to reach with the distributor installed. See pic.
Total timing with vacuum advance for now is set at 45* Drove the car in cruise mode; 45-70 mph and once I'm at steady speed I'm at 18-19" Vacuum, 13.9 14.3 AFR. No ping with hard acceleration.
Tomorrow I'll slowing adjust the vacuum advance max limit and see how the car reacts.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
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- 25,833
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Now youre talking.Im so glad youre sneaking up on it. I hope your engine responds favorably and doesnt make me look bad. Im pretty confident you wont be disappointed.
Too bad about the box cause its an excellent tool.
Too bad about the box cause its an excellent tool.
ken5124
65 Valiant Signet
Any updates? this is goood information!
mderoy340
Well-Known Member
I'm up to 52* total at cruise. Vacuum at cruise does not change so I'm down to fuel mileage checks when I take trips. Glad I took the time to dial in, car runs great. Before 15 mpg, after 17.5
skep419
5.9 Magnum 4 speed afficionado
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