Stan's Customs

10-4 John I saw the summit description...just wasn't sure, thanks for clarifying.

You mentioned that you've been using them a long time. Do they need replacing often...or do they last as long and are as reliable as a stock one generally speaking.

Thanks again...Stan

Plasticman

16 years still in my 62 (non-HEI version - I still use points), and have not heard of one ever failing.

John

Stan's Customs

Mighty fine...thanks John

Stan

olescarb

I think the main reason GM says not to hook up the vacuum advance is because the vacuum can they have on their replacement HEI distributors has 20 degrees of advance which is too much for todays reformulated unleaded fuel and a engine with fast burn heads. Both Lars and I limit the vacuum advance to 10 - 12 degrees advance, we came up with a limiter plate for a points style vacuum advance but it will not work on a HEI distributor so we use a standard motor products #VC302 unit that has 10 degrees of advance from the vacuum can.

I hope this helps Henry @ oles carb

Stan's Customs

It helps what I'm doing more than a lot......that probably would fix about half the HEI distributors made, including mine...lol.

69427

Technically, no engine needs a VA. Any internal combustion engine will run "adequately" without a load compensated timing curve. It's the lawnmower engine principle for timing curves. Set it so it runs well at WOT at all RPMs, and screw the thermal efficiency everywhere else.

However, if improved fuel mileage and engine drivability/durability is desired, a load compensation timing curve is necessary.

Stan's Customs

#VC-302 is on the way, ...from RockAuto, shipped priority mail....$26.68 including shipping.

Shows to be for a 1980 Hi-Performance 5.7 Corvette...

GearheadJoe

Hi Henry:

I pulled out a copy of Lars' paper that I got from him about a year ago. In the listing of various vacuum cans for the HEI distributor, I do not see the VC302 listed. However, I do see these two:

VC1703: starts @ 3"-6" vacuum, 10 crank degees all-in @ 7"-9" vacuum

VC1852: starts @ 7"-9" vacuum, 10 crank degrees all-in @12'-14" vacuum

It looks like the VC1703 would be best for a big cam with low vacuum, and the VC1852 would be best for a milder cam.

How does the VC302 compare to these other two?

olescarb

the VC302 has the same spec as the discontinued ac-delco d1370a which was all in (10 degrees) at about 10 inches of vacuum. I have not kept up with the borg warner #s since none of our suppliers carry that brand. Henry

Stan's Customs

I also noticed that the VC302 was not in the Lars list.

GearheadJoe

I tried a google search for the VC302 specifications, but I could not find the actual specs listed. However, I noticed that the 1980 Corvette "Special High Performance" engine was a listed application. A search for the 1980 Corvette specifications turned up a GM spec that showed four different distributors used in 1980, but only the L82 distributor 1103291 had a vacuum can that maxed out at 10 crank degrees. The specs for that can are:

Starts @ 3.5" vacuum, 10 crank degrees all-in @ 8" vacuum

By this very indirect inference, these *might* be the specs for the VC-302 can.

So, it appears there are a total of three off-the-shelf HEI vacuum cans that max out at 10 crank degrees (VC-1703, VC-1852, and VC-302).

The choice of which one to use should be based on the vacuum characteristics of the engine. A general rule of thumb is that vacuum advance should be all-in at a vacuum level that is at least 2" below the engine's idle vacuum.

My understanding is that it's not a good idea to use a vacuum can that is more aggressive than necessary to meet this target. In other words, if your idle vacuum is 15", a can that's all-in at 13" is probably a better match for the engine than a can that's all-in at 8".

On the other hand, a configuration that's very bad is one where the idle vacuum is not sufficient to keep the vacuum advance all-in. That makes the vacuum advance wander around at idle that creates an unstable idle.

Stan's Customs

You are right Joe...and I forgot the RULE (2" under max vacuum at idle)

I also was not thinking about the others that are listed...when they said 5 degrees (which was talking about at the distributor..) Those were all 10 degrees at the crank too, and I just overlooked them.

I may wind up with a Crane adjustable yet...I just like stone cold simple if I can get it. Once one is adjusted ..I'll never change it, so I thought I'd skip that part...lol!

Henry says they have good results with the VC32...so maybe I'll get lucky...even though I forgot the RULE. I have 17" of vacuum at 750 rpm idle.

GearheadJoe

Hi Stan:

If you have 17" of vacuum at idle, the VC1852 might be a better match:

VC1852: starts @ 7"-9" vacuum, 10 crank degrees all-in @12'-14" vacuum

In Lars' paper, he implies that it is generally not a good idea to have a can that comes in too quickly.

Stan's Customs

Yep you are exactly right, but the other one is on the way .....so I guess I'll give it a try, just to see exactly how it "does" respond.

GearheadJoe

My guess is that both the VC302 and VC1852 will have the exact same effect at idle. With the extra 10 degree advance, your idle vacuum may even improve beyond where is is now.

If there is any perceptible difference at all, it will probably for the special case where you are cruising at light throttle (maximum vacuum advance), and then you add *slightly* more throttle, which causes the vacuum to drop a bit. The VC302 won't respond to just a slight drop in vacuum, so you might get some transient pinging. The VC1852 will be more responsive to slight changes in engine load.

Of course, both cans will respond exactly the same for full-throttle situations, since vacuum goes to almost 0" for that condition.

Another way to state this is that for your engine with 17" idle vacuum, both cans are functionally identical at idle and at full throttle. They are only different in the way that they respond under light-load conditions.

Please post your findings after your try out the VC302.

Stan's Customs

Thanks Joe...

I will post the findings after I get the exhaust on.

It goes to the muffler shop Monday...I don't like the way my Corvette Central Magnafow mufflers and exhaust pipes fit...need some tweaks from the leadoff to the muffler....and the tail pipe will need swedging and shortening at the muffler end.

Stan's Customs

Well I bought and installed the VC 302..

The one I received only advanced 7 degrees. That's better I guess...but I needed 10-12 .

I did find a VC1852 on Rock Auto...or one that cross references as being the same and ordered it today.

That number is cross referenced # is 4V1061...marketed by Wells I believe.

I'll have it Friday so well see if it advances as spec'd....(starts @ 7"-9" vacuum, 10 crank degrees all-in @12'-14" vacuum).

Stan...

GearheadJoe

If necessary, you can easily file the end of the slot on the vacuum can to add a few degrees to the maximum advance. It's usually easy to *carefully* pull the bent rod out of the slot to get better access to the end of the slot.

TCracingCA

Some of the modern stuff does make maintaining your tune easier, but engines ran with points, vacuum advance decently way prior crank triggering and multiple coil packs! How much money do you have???? Gm will simplify a packaged engine by giving recommendations and some of those are necessary to do, for your warranty! But an enthusiast can go old school or new school or a mix and it can be set up to run nicely!

I think a 1-3/4 size is a real decent compromise size for both of these engines. The rpm power band will shift slightly changing header sizes. The standard sizings for headers for years was 1-5/8 and 1-7/8, so they are streamlining a unit that can live decently in both engine sized worlds!

Stan's Customs

Got the 4V1061(VC1852)vacuum advance from Rock Auto a day early.

There's not a nickels worth of difference in the two advance units.

Both start at 5 " vacuum (and all in at 8" vacuum with a Mighty Vac tester) and only 7 or 8 degrees of additional advance showing at the crank with a Snap-On timing light (using direct manifold vacuum.)

Some timing flutter with a timing light...but both are within 1 degree of each other on my engine.

I guess the specs are not followed to closely by today's manufacturers...so back to the drawing board.

Stan