'66 427/425 UPDATE for Duke Williams

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

I would like to add my "two" cents here and share with you the following vacuum readings I got from my L-72 engine this weekend. To start with, its an original rebuilt engine with a clone type factory camshaft. The engine has about 5,000 miles on it and I run 5 gallons of 111 octane racing fuel and top it off with Texaco 93 octane. Using a timing tape and a timing gun, and after I drove the car for 40 minutes, please note:
1) Static timing is 10 degrees with the vacuum can line blocked.
2) When I reconnect the vacuum can (original can)to the port opening on the list #3247 Holley carb, timing goes up to around 26 degrees at idle.
3) When I connect the vacuum can line to the port opening located on the rear top of the intake manifold, the timing is 40 degrees.

So, with the factory set-up, as noted in #2 above, I am getting the 26 degrees at idle, which Duke indicated was needed to keep temperatures down. Given this, I see no point in replacing my orginal vacuum can.

Also, how would the car perform if I ran it with the vacuum can line tapped into the intake manifold port oening which resuled in the 40 degree reading at idle? And if this results in better performance, then why didn't GM use this source for vacuum? Thanks

I now believe that my heating problem at idle relates to the radiator and plan to purchase a new core from DeWitt's.

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

As I said in a companion post, I believe the ported vacuum on L-72 was California emission inspired, and for whatever reason, Chevrolet sold this configuration to all 50 states.

From you data, I conclude that your vacuum can is providing about 30 degrees of advance. The OEM spec is 15 degrees at 15.5", so I would conclude that your vacuum can is either way out of spec, or the wrong one is installed.

I don't think I've ever seen a vacuum can speced at 30 degrees.

Duke

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

Duke,

Its the orignal vacuum can that came with the car. Okay, point taken, I don't know how it put out the 30 degrees of advance to get to 40 degrees of advance when taping the intake manifold. Can these things mis-function in such a manner?

Let me ask you this. With the correct factory set-up, that being the vacuum can connected to the carb port, I am getting about 26 degrees at idle. Isn't this the range you recommened to be correct for the L-72 at idle? Thanks

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

Yes, 25-30 degrees seems to be the timing that minimizes EGT. If your vacuum can is really contributing 30 degrees you are way overadvanced at cruise, so you should check the can against spec with a vacuum pump and replace if it is out of spec.

Another issue could be the centrifugal advance. When you attach the vacuum can to full vacuum, engine speed will increase, which might cause the centrifugal to deploy. You should check your centrifugal curve. You can also temporarily tie it up with a rubber band, to check total idle timing.

Duke

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

Duke, Good point; with the vacuum can line attached to the intake manifold opening and getting full vacuum, the centrifical advance must be kicking in for me to get a reading of 40 degrees. This is probably why the vacuum line was not designed by GM to run off the intake opening!

It seems that my reading of about 26 degrees at idle (with vacuum line connected), as you noted, is a good one and that the vacuum can is okay

One more question. Is is even possible for a vacuum can, like my orginal one, to put out more vacuum than its designed maximum output? I understand that when there is a problem with a vacuum can its usually with the diaphram (like rupture) and consequently the can will not be able to put out its designated amount of vacuum but not he other way around. Thanks

[Everett Ogilvie]

A few things to check...

Your timing at idle is 10 - at what idle speed? To check your mechanical advance (centrifugal), disconnect the vacuum can and start slowing your idle down bit by bit, to see if the timing retards (to see if your centrifugal is coming in at whatever your current idle speed is).

What vacuum can is it? You say it is the one that came with the car - the stock, original can is a 360 and is marked so. If it does not say that it has been replaced.

It is possible to have a combination of parameters (idle speed, timing, mixture) such that with a "stock" setup you can just be on the verge of the stock 360 can starting to contribute to advance at idle (but not nearly to the levels you describe). With the vacuum can connected to the standard location, slow the idle down with the idle screw and see if the vacuum contribution stops. That is the point at which the throttle butterfly "covers" the timed spark port in the venturi.

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

"This is probably why the vacuum line was not designed by GM to run off the

intake opening!"

No, that likely has nothing to do with it!

Diaphrams do break. Without running my own tests I have no idea why your vacuum can appears to be out of spec. From what you have provided, the can is either out of spec or there is an error in your test procedure.

If you have a vacuum pump, tie up the centrifugal advance and test the vacuum can by pumping it down and checking the increase in timing.

Duke

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

Duke, Bear with me I'm a banker not a mechanic. However, no to beat a dead horse here, but I believe that my vacuum can is functioning properly. As you mentioned, my vacuum can is designed to povide 15 degrees of maximum advance so if my static timing is 10 degrees, with the vacuum line hooked up, I'm getting about 26 degrees of advance at around 900 to 1000 RPM. The numbers add up correctly unless I am missing something here. John

[John Lokay]

Re: A few things to check...

Evertt,

It's the 360 vacuum can which came with the car and my 10 degree static advance is at about 950 RPM. Would like to confirm two things with you: 1) doses the 360 can put out maximum advance of 12 degrees or 15 degrees? The more I thought about it I believe its 12. 2) At idle, does not the engine produce maximum vacuum and vacuum falls off as RPM increases?
Thanks

[Everett Ogilvie]

Re: A few things to check...

John, the spec on the 360 can is 12 degrees. Original cans will be marked with; MS 360 12. As far as vacuum goes, it is perhaps more accurate to think of throttle position, rather than rpm, although sometimes the distinction is subtle. Often, vacuum is highest at idle, or just off idle. It is also possible to run at higher rpm than idle, with the throttle barely cracked open, which results in very high vaccum at a steady cruise.

[Everett Ogilvie]

360 Vacuum Can Characteristics

Here are a few specs from one 360 can I have tested;

Zero advance below 10" of vacuum, the unit just starts to move at 11" of vacuum, full advance at 16" of vacuum (12 degrees). You can see that this can starts late, and comes in pretty quickly.

I am sure these units will vary a bit, but they should all be fairly close to these specs.

[Duke W.]

Re: '66 427/425 UPDATE for Duke Williams

In your 7:01 post you said that you are reading 26 degrees total idle timing with the vacuum can connected to the carburetor port. As Everett said, this port does not provide full manifold vacuum at idle and you would need 15.5" to pull the plunger on the OEM can all the way.

In the same post, you said the idle timing increased to 40 degrees when you connect the vacuum can to the port on the manifold. (I assume you have and eighth-inch adaptor threaded into the manifold at this opening.)

The other consideration is centrifugal advance. OEM it starts at about 900 and if someone has installed lighter springs it might start earlier.

The best way to test the vacuum advance on the car is to tie up the centrifugal and use a vacuum pump on the vacuum can, then read the increase in timing with increasing vacuum up to the point where is stops advancing. Likewise, you can test the centrifugal by dropping the idle with the vacuum can disconnected, to the point where you are sure there is no centrifugal, then bring up the revs and check timing increases until it stops advancing.

There are two independent timing systems on the car, plus the initial. WOT timing is initial plus centrifugal, and the total WOT timing is initial plus maximum centrifugal. Idle timing is initial plus vacuum, plus, in some cases, a few degrees of centrifugal if you have low tension springs. In order to test any one of these systems, you need to ensure that the other is not affecting your measurements by temporarily disabling it. You can disable the vacuum advance system by removing the signal line from the can, and the centrifugal can be temporily disabled by tying it up with rubber bands. Also, in order to test the vacuum can you need at least a vacuum gage using the engine as a vacuum pump, but a Mighty Vac or similar tool is the best way to test the vacuum can.This way you should be able to get a good vacuum versus advance data set for the vacuum can.

Duke

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

Everett, great post, I have a few questions: 1) concerning the gas you use, what do you use and what is your mix of racing fuel to pump gas?; 2) are you still using the #360 vacuum can or did you instll the vacumm can Duke was talking about (I think it provided 16 degrees at 8 inches of vacuum); 3) if I want to set total mechanical advance I assume I would want it all in by 3000 rpm. So is it as simple as reving the engine to 3,000 rpm and turning the distributor until the 36 degree mark on the timing tape matches up with the 0 mark on the timing tab/ Thanks

[Duke W.]

Re: A few things to check...

Because of all the valve overlap, SHP engines do no generate much manifold vacuum, - typically 10-12" at 800 to 1000 RPM idle speed if they are set up with the ideal total idle advance and mixture. Increasing engine speed with no load will generate more vacuum.

Timing affects both idle speed and vacuum. The more idle advance at a given idle speed the more vacuum the engine will create - up to a point, so more idle timing will allow the engine to "bootstrap" itself up to the highest possible vacuum reading, and this is why the vacuum can should offer full advance at 2" less than the maximum vacuum that the engine can generate at idle.

Anything that varies advance at idle will affect idle speed, which affects idle vacuum. It's a positive feedback loop that leads to idle instability, so the best set up is a vacuum can that provides full vacuum advance at idle (the most important criterion) and a centrifugal advance that doens't start until above idle.

Duke

[John Lokay]

Re: '66 427/425 UPDATE for Duke Williams

Duke, thanks for the post. That's right I had 26 degrees of advance with the vacuum line hooked-up at about 900 rpm. I had 10 degrees of initial plus the vacuum advance (12 degrees max provided with the 360 can) with some mechanical advance kicking in.

You got me thinking again about the vacuum can advance unit. This summer I hooked up a vacuum gauge to the port on top of the manifold and got a maximum reading of 15 inches. With that said, maybe the engine is not producing a sufficient amount of vacuum at idle to get the full 12 degrees of advance out of the 360 can. This leads me back to one of your earlier posts about changing over to the vacuum can that produces 16 degrees of maximum advance at 8 inches of vacuum. This could be the way to go, do you agree? Thanks