c2 timing

[Robert T.]

I'm trying to use an old distributor (66 425 TI) on a new engine (454ho 425 hp). I have no clue what I am doing so bear with me. I'm trying to set the initial timing. I set it for 8-10 degrees w/o the vacuum advance. I have about 12 in-hg at idle. When I plug the vacumm advance back in I don't see any advance. I then hooked up a vacuum pump to the advanced pod and found that I didn't get any advance until about 15 inches. (It's a 201 pod). So now what. Was the vacuum on the original 427/425 higher than the 12 I'm seeing, thus activating the advance at idlle. Do I have to get a different pod to work with this 454ho. Or is this situation all right, that is no vacuum advance at idle.

Any help would be appreciated. Thanks.

Bob

[Timothy B.]

Re: c2 timing

Robert.

Make sure the vacuum line is not hooked to ported vacuum on the carburetor. If you have 12" idle vacuum then you need to install a #236 (B28) vacuum can. This is a easy change and this control will yield 15*@8" vacuum. You will notice a increase in idle speed and throttle responce and probable a cooler running engine at idle.

What's the distributor part # as the specs may allow 12-14* inital advance.

[Barry K.]

[Duke W.]

Re: c2 timing

The device is called a vacuum advance control or VAC.

The "201" is not correct for L-72 - either the part number or the match to the engine's vacuum characteristics. Yours also appears to be malfunctioning. The "201" rod should start to pull at about 8" and achieve maximum at about 15.5".

Also, the OE carburetor VAC port is "ported", so there is no vacuum signal at idle.

Search the archives for VC-1765 (NOT VC-1820) L-72 L-71 to find out what to do.

Your idle vacuum should be closer to 14"@900 (idle revs MUST be stated for the vacuum reading to be meaningful). Does it have an OE equivalent camshaft?

Duke

[Robert T.]

Re: c2 timing

Tim, Barry,

Thanks for the quick reply and for the great part numbers.
Is it a general rule of thumb that the vacuum should be all the way in at idle. Can I just advance my initial timing some.

Thanks so much for helping me out.

I am connected to manifold vacuum

[Robert T.]

Re: c2 timing

Thanks for the additional info Duke. LI Corvette supplied the TI distributor with the 201 attached. For want of a better stating point I told them to set this up for a 66 425 engine. I guess I'm at fault because this is not really a 66 engine. It is made from the Gen VI block so everything bolts up to it and it looks period correct but it really isn't a 66 427.

[Timothy B.]

Re: c2 timing

Robert,

You can't just advance your inital timing without knowing total centrifugal advance that's built into the distributor. If you provide the distributor part # someone may know the specs and then you have a starting point for inital advance. Inital + centrifugal = total WOT advance and that usually should not exceed 36-38*. Vacuum advance is in addition to this but when the throttle is opened and vacuum drops the timing is instantly retarded to the distributor settings.

If you have the interest, there is lots of info in archives and it's very helpful when tuning a engine to understand this.

[Robert T.]

Re: c2 timing

Thanks again Tim. I will search the archives. I believe the distributor number is 111093. It was set up by LIC. You tell them what engine it is going on and they set it up accordingly. I just bought it direct from them.

[Duke W.]

Re: c2 timing

So I have no idea what your engine configuration is. I hope you do. You have to understand some details of the configuration, have a target idle speed in mind and know the manifold vacuum at that idle speed with about 25-30 degrees of total idle timing if you want to go with full time advance, which I recommend.

I also have no idea if these parts houses really know how to set up a distributor. In your case the "201" VAC was not OE to L-72, so I have no idea what you have, and I doubt if they do either. Parts is parts to them and they don't understand spark advance map requirements!

I've explained the whole L-72, L-71 spark advance map issue a zillion times, so do the search. Also search "two-inch rule".

I think Barry also has "Lar's paper" on his website, which has a section authored my me. You need to do some studying so you know how to get the spark advance map set up to your specific configuration if you want the best overall across-the-range engine performance and fuel ecomony.

Good luck with your project.

Duke

PS. I corrected an error in a previous post - VAC part number to search.

[Barry K.]

Re: c2 timing

Duke, I thought the VC1765 was probably the stock equivalent but since he said his vacuum was only 12" is why I figured the VC1810 would work better.

[Duke W.]

Re: c2 timing

If 12" is the final idle vacuum reading at what he deems to be the best idle speed with either ported or full time vacuum advance - whichever he choses - then you are correct.

But as of now the camshaft is not verified nor is the carb IDed, so we don't know if the carb is set up for full time or ported vacuum advance, and no idle speed was stated. A higher idle speed will increase manifold vacuum, so will full time vacuum advance.

There are too many unknowns to make a recommendation, but once he runs the tests and gathers the data he can select whatever VAC is best using the "Two-Inch Rule".

The trouble with many of these types or situations is that we know nothing about the details of the configuration - often just a vague reference, so what we recommend based on proven analysis may not be correct unless we have good data.

The best we can do for questioners on the board is point them to the educational resources that will give them the tools they need to understand and analyze the situation and make their own sound decisions.

Lars' paper is an excellent place to start, and there are many discussions on the board about how to set up a spark advance map.

Duke

[Robert T.]

Re: c2 timing

Thanks again for the info. I realize this is a tough issue to help with because of all the variables. I've been collecting the articles you guys reference and they are very helpful.

One thing of interest that was reported here is that the 201 was not OE to the L72. LIC has got some splaining to do about that.

My first goal was to understand the vacuum advance and how do you decide what is "right" for a particular engine.

[Wayne M.]

Re: c2 timing

.....One thing of interest that was reported here is that the 201 was not OE to the L72. LIC has got some splaining to do about that..

The 1111093 distr should be factory equipped (per Delco Test Spec's) with the 1115355 vacuum advance. This VAC starts 2 degrees earlier and needs 4 inches Hg less for full advance than does the 1116201.

[Duke W.]

Re: c2 timing

The 201 was OE on L-71 and L-78. L-72 is as stated above. L-72 and L-71 also have ported vacuum advance, which should be changed to full time, and it's simple to do. The OE L-72 VAC is okay to use with full time vacuum advance, and the VC-1765 (stamped B20) is equivalent in spec. The 201 on L-71 (and L-78) needs to be changed to VC-1765 (or equivalent) to meet the "Two-Inch Rule", which summarizes how to select a proper VAC for any engine configuration. For OE cams I've previously posted summary recommendations for all OE engine configuration. If an unknown or non-OE equivalent cam is installed, use the "Two-Inch Rule".

It's almost as simple as selecting the proper engine oil.

[Robert T.]

Re: c2 timing

Thanks for the additional info Wayne, Duke. (sounds like an old western)

Sounds like I definitly need a new "lower" can. I'm pretty sure that I have the can connected to manifold and not ported. I see vacuum there, (about 11-12 inches) at idle (700 RPM).

Sounds like the 201 is not correct, and the one I have appears to not to be functioning correctly. Hooking a hand vacuum pump to it while the engine is running, I don't see any advance until I get to about 15 inches, and even there it is only about 2 crank degrees.

Some other data I have taken;

Mechanical Advance starts at 900
It is full in at 3500
at WOT and no VA I see 38 degrees total
To get the engine to not hesitate, bog, backfire I have set the initial timing at about 10. Again no vacuum advance when I connect it.


Duke, Thanks for taking the time to write all those articles. I didn't realize that that was you I was talking to on these posts. They really help.

Thanks guys. I haven't been able to get any knowledgeable help locally and these posts are a God send. THANKS!!

Bob