[scott70]

At what rpm do you like to see the timing start to advance?

[Schurkey]

Originally Posted by scott70

At what rpm do you like to see the timing start to advance?

Centrifugal advance? At least a hundred RPM higher than curb idle. If there's a throttle-kicker for the A/C, a hundred RPM higher than that.

Point being, I want the centrifugal advance to NOT start when the engine is idling in neutral under real-world conditions--A/C on, or off, engine hot, etc.

Getting some centrifugal advance at FAST idle is fine--even desirable, when the engine is cold, choke on, fast-idle cam engaged.

Vacuum advance is another story. Some engines benefit from vacuum advance at idle, some don't. And the reasons aren't all the same--I've seen engines have wicked off-idle throttle response because opening the throttle to move away from the stoplight suddenly adds ten or twenty degrees of vacuum advance from the ported vacuum connection, and the tires chirp. Other engines have unstable idle rpm because the manifold vacuum is constantly changing the timing...which affects the manifold vacuum...which changes the timing. And either way, ported or manifold vacuum to the vacuum advance, changes the throttle position for a given idle speed, which changes the relationship between the throttle plates and the transition slots in the idle air/fuel passages.

Tuning the vacuum advance is just as important as tuning the centrifugal advance, on any vehicle that's mainly driven at part-throttle. But you need to tune the amount of advance, the "starting" vacuum, the "all in" vacuum; and whether ported or manifold vacuum works better in your application.

[many birds]

Originally Posted by Schurkey

Centrifugal advance? At least a hundred RPM higher than curb idle. If there's a throttle-kicker for the A/C, a hundred RPM higher than that.

Point being, I want the centrifugal advance to NOT start when the engine is idling in neutral under real-world conditions--A/C on, or off, engine hot, etc...

How do you accomplish this? Factory weights can be found, but where can you source factory springs? The existing boneyard units' springs may be and often are worn. Using factory weights with the aftermarket springs allows the timing to come in too early. I've gone as far as cutting and shortening the aftermarket springs, but even then it's less than ideal.

[70GS455]

I'm in the camp of the best timing will be what the engine wants. We started off on the dyno with the "all in" by 2800 or whatever but quickly found out through testing that this was not optimal. We found that simply increasing total timing did increase top end hp but reduced mid range tq, and vice versa. So the optimal curve that maximized tq and hp was 38 deg total but was finished advancing at 5500 rpm. This was done by using the MSD weights and the stiffest springs out of 2 ppls collections we could find. This was a BBB but the results should correlate.

Advance stops were screws added to the HEI in the pic.

Video of the timing marks during a test. The line on the balancer is 30 deg.
https://youtu.be/IIjkqDrJlhA

Sent from my SM-T817V using Tapatalk

[scott70]

Originally Posted by Schurkey

Centrifugal advance? At least a hundred RPM higher than curb idle. If there's a throttle-kicker for the A/C, a hundred RPM higher than that.

Point being, I want the centrifugal advance to NOT start when the engine is idling in neutral under real-world conditions--A/C on, or off, engine hot, etc.

Getting some centrifugal advance at FAST idle is fine--even desirable, when the engine is cold, choke on, fast-idle cam engaged.

Vacuum advance is another story. Some engines benefit from vacuum advance at idle, some don't. And the reasons aren't all the same--I've seen engines have wicked off-idle throttle response because opening the throttle to move away from the stoplight suddenly adds ten or twenty degrees of vacuum advance from the ported vacuum connection, and the tires chirp. Other engines have unstable idle rpm because the manifold vacuum is constantly changing the timing...which affects the manifold vacuum...which changes the timing. And either way, ported or manifold vacuum to the vacuum advance, changes the throttle position for a given idle speed, which changes the relationship between the throttle plates and the transition slots in the idle air/fuel passages.

Tuning the vacuum advance is just as important as tuning the centrifugal advance, on any vehicle that's mainly driven at part-throttle. But you need to tune the amount of advance, the "starting" vacuum, the "all in" vacuum; and whether ported or manifold vacuum works better in your application.

yes centrifugal.At 850 rpm idle I have about 15 initial As you slowly rev it the mark doesnt start to move until about 1300 rpms. Didnt know if there was a optimal or if that was ok. Totals out around 2500.

[Cliff R]

I just about made a career out of removing the lightest distributor springs known to man from Buick and Olds 455 distributors. Not sure exactly why, but zero exceptions every single one that comes in here for "carb work" has distributor issues. This is NOT optimum for most of these engines as mentioned above. The mechanical advance should start right off idle, but NOTHING in at idle and advance smooth and steady with engine RPM's.

As a coincidence I recently had a discussion with a very experienced engine builder and we both agreed that these big engines typically do not like, want or even respond to bringing in all the mechanical advance right off idle. It's something difficult to evaluate on a street engine or even on the dyno because we typically don't start pulls on them low enough in the RPM range to see how the advance curve effects power output......

[67Fbird]

Originally Posted by Cliff R

I just about made a career out of removing the lightest distributor springs known to man from Buick and Olds 455 distributors. Not sure exactly why, but zero exceptions every single one that comes in here for "carb work" has distributor issues. This is NOT optimum for most of these engines as mentioned above. The mechanical advance should start right off idle, but NOTHING in at idle and advance smooth and steady with engine RPM's.

As a coincidence I recently had a discussion with a very experienced engine builder and we both agreed that these big engines typically do not like, want or even respond to bringing in all the mechanical advance right off idle. It's something difficult to evaluate on a street engine or even on the dyno because we typically don't start pulls on them low enough in the RPM range to see how the advance curve effects power output......

lol...the best carb adjustment I ever did was re-curve the distributor..

[ta man]

Best part of this hobby is searching for the best tune up for your own car. All trial and error and Saturday afternoon tinkering.
I have no doubt the scrap value of light distributor springs, Mutha thumper Comp Cam camshafts and messed up Quadrajet carbs have aided in Cliff's retirement fund.

[Cliff R]

If folks made GOOD choices with these things nothing would ever end up here. Instead I turn down close to 10 jobs a day so there are still a LOT of folks out there buying parts that don't work and putting combo's together that don't make the grade.

Looking back at all the cars I've owned, driven and raced I can only think of one that liked, wanted, needed and responded well to a super quick timing curve and a LOT of timing at idle speed. That engine was an SBC 283 bored .060" over with 12.5 to 1 forged pistons, the best flowing heads Chevy ever released on a production small block, and a flat solid cam with over 250 degrees duration @ .050" on a 106LSA. Even with all that compression it was lucking to made 5-6" vacuum at idle 20 degrees initial timing in it and it wanted ALL the timing on it by 2500rpm's.

What amazes me about the topic of distributors, mechanical and vacuum advance is that even with all the good info out there these days milliseconds from the Google Search line very few folks really understand it, even a few well educated "gurus" who post all over the Internet on the subject.

Folks, we "advance" timing simply because increased engine RPM require lighting the compressed mixture earlier in the cycle because the events are happening quicker. The more cycles per second the earlier the timing needs to occur. Out past peak VE things calm down and we just don't have to keep advancing the timing till it starts to drive the pistons back down the way they are coming up. So for every engine combo there will be a "sweet spot" at higher RPM's when the engine is heavily loaded.

Most of these N/A engines will like similar timing curves, but there is no "standard" that applies to all of them.

Vacuum advance is required on a street engine (at least if you want best efficiency and to effectively burn a leaner mixture) and increasing initial timing and bringing in full mechanical advance sooner does NOT make up for the 15 degrees or so that is MISSING at light engine load. As much as this is true I'm still AMAZED that some very knowledgeable engine builders and even shops dedicated to building engines for this hobby do NOT use it.......

[scott70]

Where is there ported vacuum for the dist on a 70 manual transmission 70 qjet?

[steve25]

I don’t have a picture of your 1970 4MV model Q-jet, but if it had a ported vacuum take off it would be in the throttle plate , not the body of the carb and it would be on the front passenger side of the carb almost below the fuel inlet boss.

[Cliff R]

1970 factory Q-jets do not have a vacuum port for the distributor, at least not a direct one.

That year they were using transmission spark control of some sort, which didn't work well then and most of those components were removed shortly after the cars left the dealers lot.

Those parts are pretty difficult to come by these days if one is trying to put one back to "stock" for a restoration project. Can's imagine anyone would want to use it.

These days I get a LOT of requests to add ported vacuum to those carburetors and a select few others that didn't come with it. Just had one yesterday where the owner was unable to get the idle speed below 1100rpm's with the throttle plates completely closed unless he unplugged the Vacuum Advance from it's manifold vacuum source.

When ported vacuum is added correctly, like the factory did it we get FULL MANIFOLD VACUUM to the distributor with the slightest movement of the throttle right off idle. At that point the only real difference between manifold and ported vacuum to the distributor is no timing added idle and coasting with ported. In ALL other scenarios they are the same, even though some self proclaimed distributor tuning experts will try to tell you otherwise.......

[steve25]

This then of course opens up the question of since these type of vacuum take off points in terms of a factory duel plane intake where there only seeing the vacuum level at idle of 4 cylinders, not all 8 as when you run a single plane intake.

In light of this then what compensation is needed , or should we be appling when using a single plane intake?

[scott70]

Originally Posted by Cliff R

1970 factory Q-jets do not have a vacuum port for the distributor, at least not a direct one.

That year they were using transmission spark control of some sort, which didn't work well then and most of those components were removed shortly after the cars left the dealers lot.

Those parts are pretty difficult to come by these days if one is trying to put one back to "stock" for a restoration project. Can's imagine anyone would want to use it.

These days I get a LOT of requests to add ported vacuum to those carburetors and a select few others that didn't come with it. Just had one yesterday where the owner was unable to get the idle speed below 1100rpm's with the throttle plates completely closed unless he unplugged the Vacuum Advance from it's manifold vacuum source.

When ported vacuum is added correctly, like the factory did it we get FULL MANIFOLD VACUUM to the distributor with the slightest movement of the throttle right off idle. At that point the only real difference between manifold and ported vacuum to the distributor is no timing added idle and coasting with ported. In ALL other scenarios they are the same, even though some self proclaimed distributor tuning experts will try to tell you otherwise.......

I guess I should of asked you add one back when you rebuilt mine.

[Kenth]

Originally Posted by steve25

This then of course opens up the question of since these type of vacuum take off points in terms of a factory duel plane intake where there only seeing the vacuum level at idle of 4 cylinders, not all 8 as when you run a single plane intake.

In light of this then what compensation is needed , or should we be appling when using a single plane intake?

You need to take a look beneith the throttle plate to understand from where the vacuum to the ports comes.

[steve25]

Sorry, stupid me I forgot about the connecting channel!

[Cliff R]

"I guess I should of asked you add one back when you rebuilt mine."

I will add them if asked to. You have to drill pretty deep into the main casting to intersect the correct hole coming up from the baseplate. A little tricky to get the source location exactly where it needs to be in the baseplate but I'm pretty good at it these days......