Frankie the Fink

No. Cruising is when vacuum is highest, the vacuum can is at max extension and the car is running at full advance...... What JohnZ said is the car will be at 38* when accelerating or at WOT (reread his post).

And you really can't read the amount vac advance adds sitting in your garage.

Centrifugal advance is RPM driven; vacuum advance is load driven. The car is not under load sitting still...

The reason the high performance engines (like my 61 270hp car) didn't have vacuum advance is because they were expected to be at WOT a lot of the time...not cruising.

Jackfit

That’s what I said.....good thing I can do it rather than say it..lol

alexandervdr

No idea what advance I have at cruising (say 60-70mph). With my 5speed Overdrive gearbox, rpm is below max centrifugal, engine is lightly loaded (so there may be some vacuum advance?)...

Frankie the Fink

It doesn't have to be at 60-70 mph.....any cruise speed (35 mph even) pulls high vacuum, activating the can for more advance, cooler running, fuel economy, etc..

I think you are getting fizzed up about the numbers; its more important how your engine acts....if you can cruise with no detonation or 'trailer-hitching' you are probably fine. Its inconsequential if that is 48* or 52* total advance...

You can "T" off your vacuum advance line with a gauge you can see from the cockpit to note the vacuum when cruising.

Big 'ole American cars like Pontiac used to have a factory gauge in a center console for that....it would have a red/orange area for "Power" (WOT) and a green area for "Economy" (cruise vacuum)...

SWCDuke

Did you read my tuning seminar that explains the advance requirements for different operating conditions?

IIRC you have a 350/290 HP crate engine that has Gen I heads, L-82 cam, and 8.5 CR. Best power advance is in the range of 36-40 as high in that range as possible as long as the engine doesn't detonate.

Get the 28 centrifugal in at 2500 to 3000 with light springs, then with the VAC disconnected, use a dial back timing light to set total WOT advance in the above range a few hundred revs above the point of max centrifugal.

Then connect the VAC (and verify that it is connected to a full time manifold vacuum source) and take it for a test drive. Load it up in the 1200-2500 range to check for detonation.

Given L-82 cam idle behavior of 14-15" @ 750 a 12" B26 is the correct VAC for your configuration.

There is no need to reduce centrifugal advance.

Duke

alexandervdr

Hi Duke , got the B26 in today (to replace an adjustable one). I indeed have short of 15" @750. Found out that the adjustable one was actually broken (although new), adjusting part turned loose. No wonder it drove me nuts ;-) With the B26 (and 35° full mechanical) engine did great , vacuum is fully locked in at idle, very linear acceleration too. I have some upside in mechanical advance. Will test in the days to come

Jackfit

Yes, if you turn the adjuster too far...you break it. I had to read the directions over and over...

Jack

GearheadJoe

The limit for maximum centrifugal advance is set by a pin reaching the end of a slot, as shown in post #5. You reduce the maximum centrifugal advance by making the pin diameter larger or the slot length shorter.

I've read both this thread and another one you started on a similar topic. I'm not sure I agree with some of the feedback you have been getting, so I'm going to throw in my own thoughts on this topic. I realize that ignition timing can seem complicated, so I commend you for persisting in your efforts to understand it.

Let's start by summarizing the end result that we would like to achieve:

1) We would like to have a LOT of advance at idle, because most engines idle better and run cooler with lots of advance at idle. Normally we rely on the vacuum advance to accomplish most of this.

2) We would like the to have the centrifugal advance all-in at about 2500 rpm for best throttle response and torque.

3) Under load, we want the total advance (initial + centrifugal) to be limited to around 38 degrees for stock, non-vortec heads to avoid detonation at wide open throttle (WOT). Note that under load, the vacuum advance drops out.

4) Under light-load cruise conditions where the vacuum advance is active, we want the total advance (initial + centrifugal + vacuum) to be limited to about 48 degrees to prevent what is typically called "trailer hitching." This is sort of a bucking instability that is a sign of incipient detonation. Lars claims that with today's gas (in the USA) it is better to stay under 48 degrees compared to the way most 1960's cars were set up to max out at about 52-54 degrees at light throttle cruise.


So, we have four criteria that we want to meet. What you will find is that if you use a stock centrifugal advance that maxes out at 28 degrees, and a stock vacuum advance that maxes out at 16 degrees, the initial advance has to be limited to 4 degrees to keep the total under 48 degrees for criterion #4. The engine would likely idle better with more advance at idle, but the only way to get that is to increase the initial advance, which would push the light-throttle cruise total over 48 degrees.

To fix this problem, Lars sells a vacuum advance limiter plate that limits the vacuum advance to 10 degrees. That allows you to increase the initial advance to 10 degrees and still meet the 48 degree limit in requirement #4. However, the total advance at idle is not changed by these modifications. The engine still has less advance at idle that it might like to have.

The way to fix this is to limit the centrifugal advance to 20 degrees, limit the vacuum advance to 10 degrees, and set the initial advance to 18 degrees. This combination can meet all four of the above criteria.

In summary, compared to the stock configuration, the optimal configuration for today's (USA) fuels has less centrifugal advance, less vacuum advance, but more initial advance.

As noted above, the way to reduce the maximum centrifugal advance is to either make the limiter slot shorter or to make the limiter pin bushing larger.

One interesting benefit of making the limiter bushing larger is that it effectively makes the slot shorter at both ends. In your other thread you asked about making the centrifugal advance start later but still be all-in at 2500 rpm. Making the slot effectively shorter at both ends can accomplish both goals while retaining the stock springs on the counterweights. This is how it worked out for me on my distributer.

Note that if you add a larger diameter bushing to make the slot effectively shorter, you run the risk of the bushing rubbing on the sides of the slot. So, you may need to widen the slot to ensure there is no interference.

Also note that some of these bushings are a simple press fit and they can fall off if the press fit is not tight enough. That could be catastrophic because it would dramatically increase the maximum advance available at WOT. When I modified my distributor, I used a C-clip to retain the bushing on the pin, and added some epoxy glue for good measure.

alexandervdr

Thanks Joe for sharing. It all make sense, beautifully logical, comprehensible and feasible ;-) The larger bushing solution will work , but need to check how much larger I can go. There is already a rubber bushing installed and can't see how much more gap is left in the slot. Shortening the slot both ends is then the alternative. Will do so in my next pull of the distributor, within a week or so when the shims arrive to reduce gear endplay

L78racer

a small push nut can be used to retain the bushing. just be sure it doesn't interfere with the cam slot.
https://cliphouse.com/PushNuts.aspx

GearheadJoe

With regard to modifying the slot, there is another way to do this that I should mention. It's actually the way that I reduced the centrifugal advance in my distributor.

I'm not sure what to call the part that contains the slot. It has the rubbing block for the points and the pivot posts for the weights, but not much else. I will refer to it here as the "counterweight mount." This particular part is physically interchangeable in a wide range of Chevy points distributors.

I found the attached listing of 1970-1981 distributor specs online, and noticed that some of the 1970's smog era distributors had only 20 degrees of mechanical advance (sometimes only 15 degrees). I thought that simply substituting the counterweight mount from one of these distributors might be an easy and clean way of shortening the slot.

I'm not a very good welder, so with the tools and skill set that I have, it's much easier to make the slot longer rather than shorter. I looked for examples of 1970's distributors that had a counterweight mount with specifications that would provide a good starting point.

If you look at the attached list of distributor specs, you will see several distributors that have shorter slots than your existing distributor (less than 28 degrees centrifugal advance). Some of these distributors were used by the millions in passenger cars.

For example, the 1973 307/115 HP engine used distributor number 1112102. This distributor has centrifugal advance specs of 0 degrees at 1000 rpm and 20 degrees at 4200 rpm. If you search ebay motors using the number "1112102", several of these distributors will likely show up, some of which are quite inexpensive ($15 to $35).

So, I picked out a few distributor numbers that had centrifugal specs that looked like a good starting point, and then looked on ebay to see which ones were available at low cost. I ended up buying two or three different distributors, just so I could examine and experiment with a variety of counterweight mounts.

In my case it was easy to test different combinations of counterweight mounts, springs, and weights because I built my own distributor machine that makes the testing easy. In principle, your engine can be used as a "distributor machine," but this is very tedious and noisy. The type of distributor optimization that we are discussing here requires multiple iterations of testing, modifying, and re-testing, so using your engine as the distributor machine is rather unpleasant.

calwldlife

sounds like me minus the ability to make a test machine.
can you share the machine you made workings?

tbarb

These distributors from the seventies are designed to work with ported vacuum advance.

When you use the weight base cam from one that will allow a high initial advance setting along with manifold vacuum advance keep in mind the very high idle timing will allow the motor to be very efficient but the lean mixture associated with it can make the motor low on power when you put it under load letting the clutch out.

It's also a good concept for using ported vacuum advance because our cars will run fine with 16-18* initial timing espacially if the camshaft is not stock. I guess what I am saying there can be a limit to excessive idle timing. The ported vacuum advance I am suggesting has different initial and centrifugal timing than the factory emission timing.

GearheadJoe

Yes, I have some documentation about the fixture I built.

Attached are both parts of a two-part article I wrote for my NCRS chapter newsletter in 2014. I have submitted a single, more detailed version to the NCRS Restorer magazine, but it hasn't been published yet.

I had a lot of fun trying to design something simple and inexpensive that still has exceptional accuracy. I've always been somewhat in awe of the famous Sun Distributor machines, but I never sat down and tried to understand them until I was attempting (unsuccessfully) to repair an old Sun Machine that I acquired.

My Sun machine was manufactured in 1953, using vacuum tube electronics for measuring rpm and dwell and for triggering the strobe light at the right moment. The distributor speed control is a mechanical arrangement.

Once I figured out the basics of how the original Sun machine worked, I started thinking about alternate ways that the same function could be implemented in a simpler way.

The key simplification that I came up with was to use a dial-back timing light for the "brains" of the fixture. If you start with a dial-back timing light, the rest of the task is a simple mechanical arrangement that will "operate" the distributor in the same way that the dial-back timing light is expecting to see it on a running engine.

The resulting fixture is smooth and quiet, just like the Sun Machines. And, I think it is more accurate and easier to use. Both the advance and the rpm are displayed digitally on the dial-back timing light.

GearheadJoe

I have been influenced by Duke Williams and John Hinckley to believe that ported vacuum advance is never a good idea. It was just a trick used to reduce emissions at idle to satisfy the EPA. Those smog engines were notorious for running hot at idle.

In any event, the type of vacuum advance used on those 1970's distributors does not prevent the use of some components of their centrifugal advance mechanism. The centrifugal and vacuum advance systems operate completely independent of each other. Neither of these two systems even "knows" about the existence of the other one.

I agree that there can be such a thing as too much initial advance, causing the car to bog at tip-in of the throttle when a load is applied.

However, I think this points to one of the benefits of adding vacuum advance at idle. At idle the engine likes to have a lot of advance, but at tip-in when a load is applied, the engine can not tolerate as much advance as it can with no load. When properly set up, the vacuum advance drops out immediately at tip-in when a load is applied.

calwldlife

thank you very much.

Plasticman

Regarding Dial Back Timing Lights (from a previous post of mine):


Beware that the adjustable advance lights are not necessarily accurate. I have an Actron CP7528 (Inductive pickup with the advance dial). I compared it's readings with a distributor on my Sun Machine, to check it's accuracy (firing a separate ignition coil to fire the Actron strobe/circuit).

The Actron readings showed a consistent 2 degrees of error at all positions above zero, as tested @ both 500 & 1000 dist. RPM on the Sun (1000 & 2000 crank RPM). In other words, 2 separate sets of tests.

When the Actron's advance dial was set on "0", Sun agreed "0".
Actron set on 10, Sun read 4 dist. (8 crank).
Actron set on 20, Sun read 9 dist. (18 crank).
Actron set on 30, Sun read 14 dist. (28 crank).
Actron set on 40, Sun read 19 dist. (38 crank).
Actron set on 50, Sun read 24 dist. (48 crank).
Actron set on 60, Sun read 29 dist. (58 crank).

In each case, if I added 2 degrees to the Actron setting as a correction factor, it would fire at the correct point.

Examples:
Actron set at 12 deg. would actually be 10 crank deg.
Actron set at 32 deg. would actually be 30 crank deg.
Actron set at 62 deg. would actually be 60 crank deg.

While I agree that 2 deg. of error is not a lot, it still can matter.

And your results may vary......

I also tested my other 2 timing lights at the same time (both non "advance" type). An older Craftsman Inductive pickup, and a cheap Equus Inductive pickup unit. Both fired at zero, and worked fine at all RPMs.

The Actron was the brightest, and the Equus the dimmest.

Plasticman

tbarb

Don't get me wrong, I use manifold vacuum advance on my car but a properly set up ported vacuum advance will work very good. The ported advance referred to by Duke and John are different than what I would use. I am talking about initial timing approx 16-20* and this will allow the piston to compress the idle mixture a bit longer and create more heat for a cleaner idle. I am not talking about the 4-8* initial timing settings of the emissions era.

I believe the high hp Mustangs used a ported vacuum as well as the high hp big block Corvettes in the sixties, there are two schools of thought on manifold vs. ported vacuum advance but once again I am talking about a high initial timing setting with ported advance. If you run a big camshaft with lots of overlap and low idle vacuum the extra heat from compression is needed to gas the fuel mixture or the idle can be rank.

Grampy

One of the benefits of going to the EFI darkside is that all the theory is still the same but the tool is so much easier to use. Just type how much advance you want at any given manifold pressure/RPM and you got it. Want to check just turn on data logging. Need to change, just plug in the laptop.

GearheadJoe

Yes, there are conflicting views on the usefulness of ported vacuum. It just seems to me that ported vacuum advance does not provide any benefit in terms of performance or idle characteristic.

When using manifold vacuum, the problem with vacuum advance is that it will be all-in under two different operating conditions:

1) Engine at idle
2) Low-load cruise

For this example, let's assume the worst-case light-load cruise condition where the centrifugal advance is also all-in. We know that the total advance at this low-load cruise condition (initial+centrifugal+vacuum) has to be less than about 48 degrees to prevent trailer-hitching. This is the first constraint that affects how much vacuum advance we can tolerate.

I'm not sure what the constraint is for maximum advance in the idle condition, but it's probably more than 30 degrees. In my experience setting timing on an engine at idle, the engine always seems to want more advance to help it smooth out, especially if it has a big cam.

So, let's say we set the initial advance at 18 degrees after revising the maximum centrifugal and vacuum advance to meet the 48 degree limit at light cruise.

If we use ported vacuum advance, the advance at idle will be just the 18 degrees of initial advance. If we use manifold vacuum, the advance at idle will be larger by the amount of all-in vacuum advance (typically an additional 10 degrees).

I think most engines would prefer 28 degrees total advance at idle over 18 degrees. When I say "prefer" I mean that the engine would idle more smoothly and would also run cooler at idle.

So, I don't see a reason why it would be beneficial to have 18 degrees advance at idle instead of 28 degrees.