Aha!  That seems to make sense!  Cool!

So what you're saying is that because the engine had virtually no load on it, the 48° total advance at 4K was a bit exagerated and will likely be less when driving along.

I'm not able to hear any pinking or detonation when driving.  It's a biproduct of my definately mis-spent youth; it's funny that amongst other things, I used to design and sell hearing aids... 

Having said that, though, I have seen under 8 x magnification and a focused LED flashlight (would today give one of my testes for a decent otoscope...) a few small shiny aluminum balls on the otherwise clean (save for a ring extending about 1 mm upwards from the insulator base) spark plug insulators after a spirited drive.  Just a few, mind you, but I'd rather they weren't there.

I think I'll reduce the mechanical advance from 32 down to 30 and see what happens.  In any event, it's good to know that I really can't use the unloaded total figure for much more than an assurance that the vacuum unit is functioning.  If it persists, I'll look for something that gives around 6° total and see how it goes.

the total advance with vacuum will seem really high, but remember that only occurs under no load / very light load conditions so OK to be a lot more advance than you would have otherwise thought would be OK to have

The key thing is that there is no detonation / pinking under any circumstances.

For instance, when the advance is 48deg and then you apply a load to the engine (throttle open suddenly, vacuum drops and vac advance falls off) the advance will go back to what the mechanical weights provide.

Norm

°Ok, so I really annoyed the neighbors today.

Warmed up the car and let it idle in the garage for a bit.

Disconnected and plugged the ported vacuum advance.  Checked the timing at 1000 rpm - 6° BTDC.  Run the engine up to 4000 rpm - 32° BTDC (distributor has a 13° cam).  Super.  I may drop this to 30 later on, but for now, ok.

Re-connected the ported vacuum advance and checked at 1000 rpm - ignition timing didn't move.  Super.

Run the engine up to 4000 rpm, ignition timing sailed up to around 48° which is where I lost my nerve.  Wow.  So, given that the current vacuum advance unit is the classic small-bore 3-14-8, the '8' indicates total additional advance (distributor degrees), so that's 16° additional total.  48° - 6 (static) = 42°.  42° - 16° (vacuum advance) = 26° centrifugal.  Does the math seem right so far?

So...  how much total advance should/could we expect when the vacuum advance is functioning?  I feel a bit confused now...

Not having access to a dyno or rolling road, I must admit that I'm tempted to blank it off, lock the distributor plate and live with the fact that I'm giving the trees over here a bit more to process...

Oh, and I looked again at the twin HS2's and the stock HS4's - the vacuum advance nipples are pretty-much right at the throttle disk which is probably why idle speed increases when they're connected.  The HIF's have the manifold vacuum nipple a mile away from the throttle disk and the blanked-off pre-drilled hole for ported around 1 - 1½ mm before the throttle disk.  This is where I inserted an additional nipple.

I think that you are asking your question the wrong way around (not "wrong", but, there's another way to look at it): Instead of asking "Should my engine use ported or manifold vacuum?" ask, "What ignition advance does my engine need?"

Once that is understood, then the type of advance (manifold or ported) and the amount of advance (initial, total, and "Hg) are simply the answers.

The fact that there are the two different ways of getting vacuum, they each have their own characteristics which suit different engine setups well, and the fact that there have been lots of different vacuum units manufactured to fit our distributors over the years, neither of these things answers the question of what your engine needs and will provide the best power output with. Any of them can be the answer, and manufacturers have used each for specific reasons through the years, making it seem confusing if one approaches the question the wrong way.

The key to answering that question is either: - look up what came with your engine originally and use that (only useful on totally stock setups) or - experiment and find out*

It is time consuming to do these experiments, but they are the best way to be sure, when working with non-stock setups.

What you want to find out is this: "What is the maximum timing advance your engine can tolerate under all conditions (throttle position, power demand)".

Then, based on what those experiments teach you, set up your distributor that way. There has to be a lot of compromise when using a mechanical system, and you want to err on the side of safety (detonation will quickly harm the engine), so when in doubt dial it back, but getting the most advance your engine can allow will unleash some hidden power.

*or have a professional distributor maker do the set up for you, like Jeff at Advanced Distributors. Folks like that have the extensive experience and know how to answer these questions directly

hope this helps, Norm

"...Since our minis are aparently designed for manifold vacuum,.."

I would suggest that this is not so....at least for "all" Minis.

All three carbs that I could check (ie they're not attached to cars so I could actually see the port/throttle plate relationship) have ported vacuum - ie the hole is upstream of the closed throttle plate.

These ranged from the original HS2s fitted to my 1071 (only the left hand (as you look at them) has a vac port), an HS4 off a Moke and an HIF 44 that I was told came from a Metro Turbo. Note that while the S does not have vacuum advance distributor, the carb still has the port.

Cheers, Ian

There's no real need to bench flow test this, which is why that new HIF38 has 2 nipples (giggle...).  When the vacuum advance is plugged into the original nipple, right after the throttle disk (receiving manifold vacuum) the engine rpms rise due to the ignition being advanced.  When the vacuum advance is plugged into the 2nd nipple, ported vacuum, the rpms remain the same as though there wasn't any vacuum advance connected.  To be sure, I even plugged a 'T' connector onto this port and sucked on it - sure enough, rpms rose.

The crux of my question is essentially 2-fold:

1. What is the difference in vacuum unit construction between those designed for ported vacuum and those designed for manifold vacuum.
2. Since our minis are aparently designed for manifold vacuum, if we adopt the ported vacuum source for vacuum advance, shouldn't we also be changing the distributor vacuum unit to a device specifically designed for ported vacuum?

If we consider reversing this paragraph, subsituting our minis originally designed manifold vacuum for ported (it appears that the Healey was designed with ported advance), it reasons that the same is true:

"So - if you have 3 webers on a Healey 3000 never connect your 'ported
vacuum' distributor's vacuum advance to a manifold vacuum port - because the
manifold vacuum port will make the Healey vacuum advance unit operate
exactly the OPPOSITE to how it was designed - and make your engine's advance
curve 'wrong' - and your car will ping and your engines lack of ability to
idle will be a frustration......"

I found this paragraph particullarly interesting:

"i.e. a 'Manifold port' provides vacuum at exactly the OPPOSITE engine
throttle position to 'ported vacuum'. Manifold port vacuum advance was
typically for distributors which had more total mechanical advance, than
those distributors using SU carb ported vacuum - and the total vacuum
advance is usually a much smaller number than 8 - 10 degrees."

Here's a small table I'd compiled for 45D4, as I was originally trying to find advance specs for part numbers commonly available from prominent retailers who don't reveal what the specs are:

When looking quicly at the table, it appears that the total advance is typically under 10 degrees... which leads me to believe that our distributors have relatively high total mechanical advance numbers and that the vacuum source was originally intended to be from the manifold side of the throttle disk and that if we move to ported, so shal we also try to find a ported-type vacuum advance unit for the distributor.

I think a phone call to the folks at British Vacuum Unit would help shed some light on this, if anyone fancies a call to New Hampshire -

A bunch of good questions!

Since the standard vac advance on a Mini is supposed to impove fuel economy when at idle or low throttle, and is to fade back as mechanical advance takes over, it seems to me one would want maximize vacuum and use the port on the manifold side of the butterfly. With the throttle closed, you get maximum manifold vacuum and vacuum advance. As the throttle gradually opens, e.g. under light accelleration, manifold vauum drops graduallyeasing back on the advance and providing a later (but not "late") spark to improve low-speed torque and power, which at no vacuum would be the static timing. As the rpms increase, the mecchanical advance noves ths spar farther forward to get the burn started earler to compensate for the speed the pistons are moving and provide optium burn at the rright point of the stroke.

I just looked at the HS4 that came on my Mini (probably from the original 998) - it has one port - after the butterfly. And looking at the HIF44 I bought from Bill Elliot, it also has the one port - after the butterfly (the crankcase vent port is before the butterfly, where it would draw more only as the engine rpms and blow-by would increase. I have another new HIF44 on the car and I think its porting is the same.

So, why would an economy feature like a vacuum advance want to be ported before the throttle where it would only see vacuum afterthe throttle opens?  As Ian says, performance Mini engines didn't/don't use/need vacuum advance at all.

Now to complicate matters, the Healey article referenced by Jeg says

"The 'other' sort of vacuum setup is where the distributor vacuum pickup was
tapped directly from the intake manifold (not the carb throat at the
butterfly) - and is usually found (usually) on post 1970's cars - when car makers started to get
'green'. In this scenario, with a 'manifold port'- Vacuum is highest at
idle, and under deceleration - and vacuum reduces under throttle opening -
down to no vacuum at wide open throttle on the road. When you open the
throttle the manifold vacuum drops - i.e. the 'manifold port' has the strong
vacuum reading with the butterfly closed - i.e. at idle and under
deceleration."

That would be most of our Minis.

Now it gets worse... the Healey article says "the distributor vacuum pickup was tapped directly from the intake manifold (not the carb throat at the
butterfly)" Which makes me wonder: What does the original carb-mounted "manifold" port (plugged in Jeg's photo) really see? Does it just see the true amount of vacuum in the manifold, or does it get the Venturi effect beause it is locaetd where the not-quite-closed butterfly is blowing intake air at high velocity but low volume across said port opening? Bearing in mind the throttle plate does not actually close completely, the two ports in Jeg's photo "see" the air flowing at the same rate and would have the same Venturi effect on them.

Quick! Somebody bench flow test this and tell us if it makes much of a difference!

Paging Marcel Chichak!

Hi jeg,

Every Mini (that I have ever seen) used carb (port) vacuum... unless of course they had "performance" engines in which case they had none. 

I have tried manifold vacuum when I ran Webers but now understand (from the article you cited) why that did not prove very satisfactory.  After reading the article I'm not sure why anyone would bother with manifold vacuum .. maybe if you were concerned about being super green and your car spent a lot of time in traffic??

I would suggest you look at it the other way 'round..  if you have a port vent then you get a advance unit that matches.  There's no need to look for a "Mini" unit - just get one that provides the output that you want.  Mine's a Ford product but the DNA seems compatible.

These days you wouldn't bother with the clockwork stuff as programmable ignitions are readilly available.  However, if its what you've got (like me) then the old systems work fine.

Cheers, Ian

Image Gallery

I've been doing a bit of reading on vacuum advance units lately, and now, being a bit confused by it all, need to know if the vacuum advance units designed for our minis are designed as manifold vacuum units or ported vacuum units.  I understand the functional differences in the vacuum nipple take-off sources, being before or after the carb throttle plate and have modified several HIF carbs to accomodate either type of vacuum advance.  So now, since we've all modified our carbs for ported vacuum, are we sure that the vacuum unit canisters are designed for this?

I've found documentation that certain vehicles (1963 - 1970 MG w/45D4, vacuum unit Lucas Nr. 54424170 with 5-13-10 as seen here //mgbmga.com/tech/mgb21.htm ) use specific ported-type vacuum advance units, and a stellar description of funtional applied differences as applied to a big Healey found here:  //www.ahexp.com/phorum/read.php?13,19012.  Other models use a manifold vacuum type canister. 

Of course, I haven't been able to find this sort of information as it relates to our cars.

(Another fantastic site is here:  https://www.britishvacuumunit.com/home-page.html - if someone wanted to give them a call and ask, it would save me a few dollars...) 

Edit:  The image with the screw blocking the original drilled manifold vacuum source is courtesy of AC Dodd as copied shamelessly from another forum...  The image of the bi-nippular HIF38 basking unabashadly on the messy workbench is mine.