When using manifold vacuum for the advance can. Is there any
difference from the back of the manifold and the carb base vacuum
sources? I'm getting light throttle pinging with the 1810 can and the Lars setup meathod. Heavy throttle ok. The 1810 may be too much,
opinions?

 

What carb are you using?

Usually there are too classical ways to provide vacuum signal to the can.
-Use full or actual manifold vacuum pressure.
-Use a ported signal that meters and regulates when the vacuum signal is open.

I am assuming you have a small block. You can tap of any source on the carb that is FULL vacuum all the time, or you could use a free nipple on a manifold source. Either will work.

If you have low RPM detonation you may need to back the initial timing back a tad. You may have to experiment with it to find the honey hole.

If you wind up backing it up more then you like and performance starts to be sacrificed you may have to add a few gallons of Avgas or some kind of race gas. A 25% mixture should do just fine.

My L-76 will ping under heavy load at very low RPM’s. But I am never in this situation so it is not a factor. Under normal operating conditions and WOT runs, there should be no audible detonation.

Mark

 

What's your engine configuration? SB? BB? Cam? Carb.?

Duke

 

Info on the motor- 350ci, Sig Erison cam 286 duration, .440
lift (cam is vintage about 1970 so specs are from the old
way of rating) 11 to 1 CR. initial advance 15, 34 at 2500.
1810 can to maifold vacuum. Carb is 670 Holley street
avenger. Compression test shows about 165 PSI (about the
same as it was the last time I checked it in 1983). It runs nice
and cool and has good power. I think that reducing the
vacuum advance a little will get rid of the light throttle ping
that seems to happen when both vacuum and centrifical are
in play.
Thanks for the quick replys
Bob

 

Vacuum can selection should be based on idle manifold vacuum with about 25 -30 degrees of total idle timing, so if your manifold vacuum is only 10" the VC 1810 is correct. If it idles at 14" you can use the less aggressive VC1765 can, which yields full advance - 16 degrees - at 12".

You also need to look at your centrifugal curve and initial timing; 15 degrees initial is VERY AGGRESSIVE. Also you say the total initial plus centrifugal is 34 at 2500. Does it increase with increasing revs or is this the true total. If it is the true total it means your distributor only has 21 degrees, which is typical for seventies vintage distributors designed for emission controlled engines. The total initial plus centrifugal you want is 36-38 and get it all in as quickly as possible without detonation. If the dist. only has 20-21 degrees maximum centrifugal, then you are forced to run excess initial timing, which won't work with today's fuel. You want to reduce initial to about ten, then file the slot to achieve 24-25 centrifugal, then experiment with different spring sets. If if still detonates with total centrifugal delayed until 3500 or more, grind out the slot to provide 28 centrifugal and run the initial at 8-10

The most aggressive OE distributor was the '64-'65 SHP/FI, which provided a maximum of 24 degrees centrifugal at 2350. The initial timing recommendation was the range of 10-14 degrees. This is a good starting point for a high performance engine with a cam similar to the OE mechanical lifter cams (10-12" idle vacuum at 900 with 25-30 degrees total idle timing - initial plus full vacuum advance), but such an aggressive curve may not work with modern lower octane fuels, so you have to juggle initial, total centrifugal, and rate of centrifugal advance to find a setup that will work with your engine and available fuel octane while still providing about 36-38 degrees WOT advance at high revs.

If you open the throttle slightly to accelerate from low revs, manifold vacuum usually drops to near zero and then increases as revs increase, so a too aggressive vacuum advance may not be the issue. Driving with a vacuum gage connected so you can read vacuum from the cockpit can be very illuminating.

Duke

 

Thanks for the info Duke. I know that the 15* is high and the
max centrifical is low. This distributor started out with more
than 30* travel. I like the sharp throttle responce of higher
initial setting so I reduced the slot by a bunch so it would work.
Vacuum at idle is about 13-14 inches so the 1810 can seems to
be about right. Without vacuum advance it used to run about
210* around town. The 1810 has running about 180*, much better! Tonight, I reduced the travel of the vacuuum advance
can by about 5*. A quick test showed good results. The max centrifical advance numbers are determined by the removed spring method. Just to clarify, by light throttle I mean 2nd
gear at 2500 and just a little more throttle and it would ping,
more throttle and it would stop. Is correct to say that the faster the centrifical advance the better as long as there is no detonation (to limit of 36*)?
Bob

 

I would say so. You probably noticed a "seat of the pants" power improvement when you increased your initial. If so, what you felt was an increase in torque. How much timing you can run and at what rpm will depend on several factors, including vehicle weight, gearing, cam timing, fuel, etc., etc.. While some figures are considered "ideal", they aren't ideal in all applications.

As far as setting total without the springs installed, I have seen a few cases where that had resulted in an inaccurate reading. In most of those cases the springs were incorrect and would not allow the weights to move freely until a much higher rpm. In at least one case the problem was gummed up and corroded weights that didn't want to advance all the way once the springs were added. Just to be safe, I'd check total with the springs installed.

 

Yes, the general rule is to get the centrifugal in as quickly as possible with detonation as the limiting factor. If we could still buy high octane premiums from the sixties, your centrifugal curve would probably be okay. Today's octane won't allow as much advance at low revs, which is where detonation is most prevalent so we have to slow centrifugal and/or add centrifugal at higher revs while reducing initial.

I still think you should increase the centrifugal and cut back on initial. It might reduce low end torque, but it's a compromise we must accept or pay the price of blending in high octane race gas or avgas.

Finding a combination of initial and centrifugal while leaving he VC1810 can alone is your best bet IMO. Finding the "sweet spot", where you have the maximum WOT advance at as low an engine speed as possible without detonation is a matter of trial and error, but the best place to start is the '64-'65 SHP/FI map.

Use a dial back light and bring up the revs (vacuum can disconnected) until it stops advancing to verify the point of max centrifugal and total WOT advance. Even if you have to rev it to 5000 it won't hurt anything. You're on the right track, keep working at it.

Duke

 

Thanks guys for the great info about this subject. I am close
to the final compromise. As far as the total advance test, I have reved it up and the numbers are correct. I am going for
12* initial and 36* at 2500 plus 10* vacuum (manifold connected). Before joining the forum, I felt like I was working
in a "vacuum" (pun intended).
Bob