Which VAC to use??

[Larry T.]

I have both the VC1765 and the VC1810 cans in my garage to choose from. The engine is a 427/425 L72. There have been many posts indicating that both of these are good for the application and I am not trying to beat a dead horse, but since I have both to choose from, which would be more ideal?

Thanks.

[Duke W.]

Re: Which VAC to use??

Well, since you've apparently done a little archive reading on the subject, which one do you think is best matched to L-72 and why?

Duke

[Larry T.]

Re: Which VAC to use??

Duke,

If I only had the benefit of the info I have studied I would probably go with the VC-1810, even though the most frequently recommended is the VC-1765. I have not seen any reason why the advance beginning to come in a little earlier or reaching maximum advance sooner is a bad thing. I would think maybe it is a good thing for the performance of my engine in fact. But again, the VC-1765 is most often recommended. I have assumed that may be due to the fact that the VC1810 is discontinued under the Napa/Echlin brand and the VC-1765 is easier to come by, and does a fine job.

So, did I get it right?

Thanks,

Larry

Well, since you've apparently done a little archive reading on the subject, which one do you think is best matched to L-72 and why?

Duke

[Mike Z.]

Re: Which VAC to use??

Larry-
I assume the vac you refer to is the vac canister on the distributor-correct? According to the copies of Delco Remy blueprints I have, the #093 distributor (65-66 425hp) was delivered with a #360 stamped into the vac. This canister had a max advance of 12 degrees. Reproductions are available from most of the large supply houses with the correct stamping for judging purposes and are very close, but not exact to the original in configuration (have ripples in the front stamping vs. none on the original and are way too shinny). If you insist on buying an aftermarket part, check the advance limit, especially with today's fuels.
Mike Zamora
#12455

[Larry T.]

Re: Which VAC to use??

Mike,

I thought the 236 was the original can. I guess this throws another possibility into the mix. Is it possible that 16 on top of the 36 initial plus mechanical will be too much? Or will 48, with the 360, be too little? I do not mind trying all of them until I get it right, but I would like to start out with the one that "should" work best first. Someone has already tried all of these variations, riding their coattails is fine with me.

Thanks,

Larry

Larry-
I assume the vac you refer to is the vac canister on the distributor-correct? According to the copies of Delco Remy blueprints I have, the #093 distributor (65-66 425hp) was delivered with a #360 stamped into the vac. This canister had a max advance of 12 degrees. Reproductions are available from most of the large supply houses with the correct stamping for judging purposes and are very close, but not exact to the original in configuration (have ripples in the front stamping vs. none on the original and are way too shinny). If you insist on buying an aftermarket part, check the advance limit, especially with today's fuels.
Mike Zamora
#12455

[John H.]

Re: Which VAC to use??

Larry -

What's your manifold vacuum at normal idle?

[Mike Z.]

Re: Which VAC to use??

Larry, the #236 vac (16 degrees of advance) was original equipment on the small block motors. The #236 was never used on a BB of any year model. The #360 used on the 425hp 65-66 motor, was also used on the 67 390/400hp motor, when the 435hp motor switched to #201 (15 degrees of advance). I would not recommend the #236 (with greater advance potential) for your 66 425hp motor. I even recommend to my customers to switch the #201 in favor of the #360 in their 67 cars (due to today's fuel) if not concerned with judging. For more details of these distributors, see my several listing on E-Bay under michaelz505. Hope this helps,
Mike Zamora
#12455

[Duke W.]

Re: Which VAC to use??

Duke,

If I only had the benefit of the info I have studied I would probably go with the VC-1810, even though the most frequently recommended is the VC-1765. I have not seen any reason why the advance beginning to come in a little earlier or reaching maximum advance sooner is a bad thing. I would think maybe it is a good thing for the performance of my engine in fact. But again, the VC-1765 is most often recommended. I have assumed that may be due to the fact that the VC1810 is discontinued under the Napa/Echlin brand and the VC-1765 is easier to come by, and does a fine job.

So, did I get it right?

Thanks,

Larry

Well... John asked a key question a couple of posts above. Checking manifold vacuum will tell us whether it really has the OE cam or some hot rod cam. I can't tell you how many people have said their engine was "stock", but a simple idle vacuum test was definitive evidence to the contrary. It happens all the time!

You also need to specify idle speed with vacuum readings since vacuum increases with increasing idle speed. The L-72 should idle comfortably in the range of 800-900.

Once you know idle speed/vacuum, apply the Two-Inch Rule, which can be easily found in the archives. You only want the "minimum" VAC to meet the Two-Inch-Rule. One that is more aggresive than necessary can promote part throttle detonation.

The fact that these two VACs have 4 degrees more advance than the OE ...360 is of no real consequence. The key point is the vacuum required to obtain full advance.

The other thing you should do to your L-72 is convert the ported to full time vacuum advance, which has also been discussed in depth over the last ten years.

There are posts from me in the archives that list three VACs - one of which is best suited to all pre-emission engines or emission-controlled engines converted to full time from ported vacuum advance. Knowing idle vacuum and the Two-Inch-Rule gets you the correct VAC. A pure guess gives you a one-third change of getting it right, but it's an easy process to follow and get it right without guessing.

Duke

[Joe R.]

Re: Which VAC to use??

Larry,

Check this thread out, it has the answers:

https://www.forums.ncrs.org/showthre...+l-72&uid=3869

Also, you might want to change from ported to full time advance, your engine will run cooler. Here's the scoop:

https://www.forums.ncrs.org/showthre...unnin&uid=5749

I plan to run the VC1765 VAC with the mod. to the carb. for full time advance on my L-72.

Joe

[Larry T.]

Re: Which VAC to use??

Well... John asked a key question a couple of posts above. Checking manifold vacuum will tell us whether it really has the OE cam or some hot rod cam...

Between 14 and 15", it should be consistent with OE

You also need to specify idle speed with vacuum readings since vacuum increases with increasing idle speed. The L-72 should idle comfortably in the range of 800-900.

750 is what I have tried to use. It is a little rough so I will take it up closer 850 to 900.

Once you know idle speed/vacuum, apply the Two-Inch Rule, which can be easily found in the archives. You only want the "minimum" VAC to meet the Two-Inch-Rule. One that is more aggresive than necessary can promote part throttle detonation...

I always read the 2" rule as "at least", I never read about "part throttle detonation" when getting too aggressive

The other thing you should do to your L-72 is convert the ported to full time vacuum advance, which has also been discussed in depth over the last ten years.

That was the first thing I did

There are posts from me in the archives that list three VACs - one of which is best suited to all pre-emission engines or emission-controlled engines converted to full time from ported vacuum advance. Knowing idle vacuum and the Two-Inch-Rule gets you the correct VAC. A pure guess gives you a one-third change of getting it right, but it's an easy process to follow and get it right without guessing.

Now that I understand that meeting the 2" rule is based on the minimum VAC that does it and is not "at least" the answer is clear. Thanks.

Duke

Larry -

What's your manifold vacuum at normal idle?

John, Thanks for the question. And I really appreciate the articles you have written on the subject of timing and vacuum advance.

I should have indicated in my earliest post that I was set up on manifold vacuum and that I had about 14" of vacuum at about 800 RPM. That would have made it easier for you to help. I will be putting the VC1765 in tomorrow along with changing the springs in the distributor. I was not getting all in to 36* until about 3300 RPM. I am afraid I am not getting enough initial advance though as it is coming in at about 12-14*. I will be re-checking everything tomorrow after the spring and VAC change. I may be posting again for help solving that problem, but I will double check first.

Thanks to everybody that has responded.

[Terry M.]

Re: Which VAC to use??

Larry,

I don't have near the tuning knowledge of Duke or John H -- but I would change only one thing at a time and drive the car after each change. If you change two things and you continue to have problems, or have a different problem, you will not know which item caused the issue.

But then that is just my take on things mechanical.

[Larry T.]

Re: Which VAC to use??

Terry,

That is very good thinking and I appreciate the input.

Larry

[Timothy B.]

Re: Which VAC to use??

Larry,

If your engine pulls 14" vacuum at 800 RPM the VC-1765 control is a good choice (15*@12").

Be careful when setting inital timing with light springs because the centrifugal advance could be coming into play at lower RPM.

Consider timing the engine for total advance with a dial back light or mark the balancer at 36* and remove the springs so you don't have to free rev the engine so high.. Set total at 36 and let the inital fall where it will. If the inital does not end up in the 10 to 14* range you may want to have the curve checked on a machine..

[Larry T.]

Re: Which VAC to use??

Larry,

If your engine pulls 14" vacuum at 800 RPM the VC-1765 control is a good choice (15*@12").

Be careful when setting inital timing with light springs because the centrifugal advance could be coming into play at lower RPM.

Consider timing the engine for total advance with a dial back light or mark the balancer at 36* and remove the springs so you don't have to free rev the engine so high.. Set total at 36 and let the inital fall where it will. If the inital does not end up in the 10 to 14* range you may want to have the curve checked on a machine..

Thanks for the info. Lars Grimsrud recommends trying to be at 18 initial and 36 total, without vacuum hose connected. He says 12 initial is very low. He suggests trying to idle at 34* once everything is hooked up. These figures are ideal, but he suggests getting close. Do you have any thoughts on this?

[Tom L.]

Re: Which VAC to use??

At 18* initial timing you may have trouble starting a hot engine, I did and went back to factory specs. No further problems. Have fun!!