timing curve?
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Senior User
Joined: Nov 2002
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From: ca
timing curve?
I have practically a carbon copy of the hotrod magazien 400m build except I have headers and a 1406 600cfm eddy carb.
anyway Today I finally got my adjustable vacuum advacne kit. It consists of an adjustable advance diaphram and different sized sprigns for the mechanical advance. Ok my qustion is should I set it to achieve full advance at 3800 3500 3400 3300 or 3200 degrees? It says the advance still starts around 800. Oh this thing will be running on good ol' regular unleaded and is a daily driver.
anyway Today I finally got my adjustable vacuum advacne kit. It consists of an adjustable advance diaphram and different sized sprigns for the mechanical advance. Ok my qustion is should I set it to achieve full advance at 3800 3500 3400 3300 or 3200 degrees? It says the advance still starts around 800. Oh this thing will be running on good ol' regular unleaded and is a daily driver.
Postmaster
Joined: Aug 2001
Posts: 4,270
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From: Delaware
You could check what is in there now with a delay timing light. Since you know how it runs now, you could determine which way to go.
Edelbrock carbs are calibrated lean for economy from the factory, this will affect the way the fuel burns. Lean mixtures will detonate more easily than rich mixtures.
All these factors come into play, so the best way to get it right is to put it on a Dyno with all the sensors for Oxygen, Timing, and RPM.
Edelbrock carbs are calibrated lean for economy from the factory, this will affect the way the fuel burns. Lean mixtures will detonate more easily than rich mixtures.
All these factors come into play, so the best way to get it right is to put it on a Dyno with all the sensors for Oxygen, Timing, and RPM.
Cargo Master
Joined: Dec 2004
Posts: 2,232
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From: NE Texas
You want it set on the short slot. This will limit the mechanical advance in the distributor, allowing a little more initial timing.
You don't need a "delay" timing light to check your advance. Use a regular timing light and tachometer. As you rev it up, note the various timing marks for each 500 RPM and write it down. You will probably have to clean your balancer and put white paint in the markings so you can read it easier as the mark moves up.
Good luck,
Doc
You don't need a "delay" timing light to check your advance. Use a regular timing light and tachometer. As you rev it up, note the various timing marks for each 500 RPM and write it down. You will probably have to clean your balancer and put white paint in the markings so you can read it easier as the mark moves up.
Good luck,
Doc
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Posting Legend
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Check out the distributor specs in one of my threads here for a 1971 400. It is a good basis to start from on your modified engine.
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From: Albuquerque
Is the engine running already with a different dizzy? If you are below 3,000 ft altitude I would use this as a starting point but you still need ot evaluuate and tune if it knocks.
10 degrees initial + 10 degrees vacuum with total limited to 36 in by 3400rpm. This may be a bit agressive for regular gas.
10 degrees initial + 10 degrees vacuum with total limited to 36 in by 3400rpm. This may be a bit agressive for regular gas.
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Senior User
Joined: Nov 2002
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From: ca
also how much is the vacuum advance supposed to come into play?
Right now it brings the engine up to like 30 degrees advance really early, does this mean that I need to adjust it?
whats a good ball park figure for the advance that the vacuum does and at what rpm range should it come in?
Right now it brings the engine up to like 30 degrees advance really early, does this mean that I need to adjust it?
whats a good ball park figure for the advance that the vacuum does and at what rpm range should it come in?
Cargo Master
Joined: Dec 2004
Posts: 2,232
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From: NE Texas
The vacuum advance does not advacne according to RPM, it advances as the vacuum increases. That's why they call it the vacuum advance. You will have full vacuum at idle and at cruise speed. It is an economy device. When you cruise the vac comes up, thus adding some extra advance for better fuel economy. When you are accelerating, there is little or no vacuum and it does not come into play. This is why you disconnect and plug the vac line while adjusting timing.
As far as your 30 degree total advance "really early," you need to be more specific. At what RPM is it at 30 degrees? I would think that about 3500 or 3800 would be about right.
Good luck,
Doc
As far as your 30 degree total advance "really early," you need to be more specific. At what RPM is it at 30 degrees? I would think that about 3500 or 3800 would be about right.
Good luck,
Doc
Posting Legend
Joined: Sep 2001
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The vacuum level will depend on whether he is using ported or manifold vacuum. In stock form these engines used ported vacuum when at normal operating temperature. They used full manifold vacuum when cold or when the engine was hot (overheating).
There is another post here with some comparative vacuum levels for ported and manifold vacuum at different engine speeds
I favor using full manifold vacuum all the time on a modified engine but you may need to make adjustments to the vac advance unit. The specs I listed in another thread are for a factory distributor with ported vacuum.
There is another post here with some comparative vacuum levels for ported and manifold vacuum at different engine speeds
I favor using full manifold vacuum all the time on a modified engine but you may need to make adjustments to the vac advance unit. The specs I listed in another thread are for a factory distributor with ported vacuum.
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From: ca
hmm then perhaps something is wrong here. Because if I have the vehicle at idle I have around (10 degrees) advance then when I hit the gas (with the vacuum advance hooked up) the timing peaks out to (around 35 degrees) right away.
If I have it disconnected it gradually rises as the engine picks up speed.
It doesn't seem to matter if I have the vacuum advance adjusted all the way in or out.
As for the driving symptoms, I have a good crisp power and uhh engine sound right off idle but then as it picks up the engine tone changes and It doesnt pick up speed very fast (since i don't have a tach hooked up ill just say it starts to fall off around 25 mph.) this crappy performance continues as the engine speed increases.
If I have it disconnected it gradually rises as the engine picks up speed.
It doesn't seem to matter if I have the vacuum advance adjusted all the way in or out.
As for the driving symptoms, I have a good crisp power and uhh engine sound right off idle but then as it picks up the engine tone changes and It doesnt pick up speed very fast (since i don't have a tach hooked up ill just say it starts to fall off around 25 mph.) this crappy performance continues as the engine speed increases.