Topic: Vacuum Advance

[Pinhead]

why would you use vacuum advance with a unit utilizing programmed and programmable advance curves? actually, don't answer that. i'll gladly read it in another thread... this one is long enough as is.

From an article at GoFastNews.com called Distributor Curve Science Simplified, David Vizard states that, while vacuum advance has negligible effect on power, it definitely has a positive effect on combustion.

Too many street rodders see that the pro racers use distributors without vacuum advance and figure if it’s good for the winning car it’s good for them. Unfortunately this is not so. Although a drag race car may not realize much in the way of an advantage, none of it performance related, it’s about the only type of race car that falls into this category. Here’s how the puzzle fits together.

When you back out of the throttle the manifold vacuum goes up. This means the compression pressure at the end of the compression stroke is way lower. When a spark timed for a normally fast burning charge (as it is at WOT) fires under these circumstances it is going to occur far too late for the pressure to be at it’s peak at 15 degrees after TDC. This means that less than optimal use of the energy content of the fuel has been made. That in turn means burning more fuel to get the level of power being demanded by the driver at that particular moment. By having vacuum advance pull in appropriately more timing you can let out of the throttle more and consequently cruise on less fuel.

Also you may have based a cam selection on the basis of minimal negative idle impact. This may have limited your choice of cam to one of say a 280 degrees (advertised). If this was to be paired with a distributor with no vacuum advanced as most competition ones are then rest assured you have probably given away about 10-15 hp. If a vacuum advance is used the ignition timing at idle can be optimized where-as without vacuum advance it cannot. Assuming a realist street engine idle speed a ‘mechanical advance only’ distributor is barley pulling in any advance. What with say 12 degrees of initial and at most 5 degrees mechanical the total timing at idle would be only 17 degrees. In practice we find that a big cammed V8 needs about 50 degrees for best idle along with the lowest idle fuel consumption. I have seen some engines require as much as 55 degrees yet these engines – with cams as big as 260 to 270 degrees at 0.050, would idle tolerably well at 1000 rpm – sometimes less. The same goes for cruise. The vacuum advance can pull in the desired advance which at about 2500 rpm and a quarter throttles can be as much as 50 degrees. Normally an engine with mechanical advance only would have no more then about 25 to 30 degrees max. It does not take a diploma in rocket science to see that cruise fuel consumption is going to be much higher with the timing as much as 25 degrees out!

[kghost]

Why are ya on about Vacuum advance?

[Pinhead]

For one reason: Combustion Efficiency.

At part throttle, light load, there is much less pressure in the chamber (when compared to WOT) and therefore the charge burns much more slowly. This requires additional vacuum advance to optimize the combustion timing. With a purely mechanical advance mechanism, you have to greatly compromise between WOT-optimized ignition timing and cruise-optimized ignition timing. If you only tune for full-out WOT on the dyno, gas mileage will suffer. If you only tune for cruise-speed, the engine will ping at WOT. Vacuum advance greatly reduces the compromise between the two.

55mpg is terrible for our bikes considering the horsepower level, aerodynamic drag, and weight. More-optimal ignition timing, IMO, is one of the keys to substantially increasing cruise gas mileage.

[kslrr]

The vacuum advance only comes into play when the throttle position is changed.  When cruising (throttle at steady state), the mech. advance is all that is needed.

[Pinhead]

I have to cordially disagree; go ahead and read the linked article.

[kghost]

Ya worried about fuel economy at idle?

Think you'll get a couple more hp at top end?

[Pinhead]

No and maybe (although, personally, HP isn't my goal as long as i don't lose any). Vacuum advance isn't only about idle quality.

Let's do an experiment... Install a vacuum gauge to the manifold side of one of your carbs. Take your bike out on the highway at a steady speed, say 65mph. Note what your gauge says at this speed; you'll notice that not only do you have vacuum when sitting in the parking lot, but you'll have an appreciable vacuum going down the highway as well. This is where the advantage of vacuum advance comes into play; light load, part throttle, steady state cruise needs the "extra" advance (versus WOT) in order to optimize ignition timing.

[kghost]

Yes and no....

Your thinking is correct but theres a few other factors to consider

First and formost is what works for a water cooled iron block enginbe doesn't always work for an air cooled aluminum engine

But good luck with whatever your on about. 

[eurban]

How are you going to alter the timing with a vacuum canister on a SOHC?

[Pinhead]

How are you going to alter the timing with a vacuum canister on a SOHC?

I haven't decided exactly how vacuum advance could be implemented with the stock ignition. The simplest way I can think of would be to install a Dyna2000 programmable ignition, taking advantage of the retard wire. I would put the WOT timing curve on the retard map, and using a hobbs switch, ground the retard wire above a certain vacuum level. That way, under heavy acceleration the ignition would be in the retard mode (which is your WOT timing curve) and then go to the advanced curve while cruising. This won't give a true curve like a vacuum canister would, but it should help with stead-state cruise mileage.

[BobbyR]

With 55 MPG I would not bother with all that time and expense.

[mystic_1]

I don't know, seems to me like if someone wants to try a new idea, they should go for it.  It may not accomplish the intended goal, or even anything useful at all, but experimentation and unconventional thinking can be a very good thing.

mystic_1

[bradweingartner]

I like it if for no other reason than that I like tinkering and seeing how I can improve things.

I'm thinking if you essentially allowed the points plate to move freely, locked the mechanical advance unit and then hooked a spring pulling one way on the points plate and the vacuum canister pulling another you'd be there. Would probably want to tap all the intakes for the vacuum.

I'm new-school though and prefer all programmable electronic. Even less compromise than the vacuum. I'll have something in the next couple months I hope.

[Soos]

Interesting article there pinhead.

Having access to not only a dyna2000, but the CurveMaker software as well...
To say the least this has piqued my interest.
Now if I only had a Racing only 550 or 650 cam to play with to test this....
That would be something to make a ungodly hot cam be able to idle, and run well at low rpm's.


Have to go search the internet now before more half assedry on my part though, as the 650 is running well at the moment.
Knowledge is power..... And ignorance is bliss, so off i go happy as a lark, in search of power....


l8r

[eurban]

Vacuum advance is not going to replace the lower RPM torque loss from a "race" cam.

[Soos]

I wouldn't expect it to, but if it would assist the motor in being able to reliably idle at 1200-1300 RPM, why not try?



l8r

[MRieck]

FJ1100 and 1200's had vacuum advance systems.