Vacuum Question

[morgan]

So big news first. Up, running, MSD 12V. Made it to work this morning - 30 miles of smoooooooth running Happy bunny.

Spent last night with timing gun getting her in the zone.
Very impressed with the MSD supplied stuff. Chose blue advance stop and 2 x blue springs, set initial to 14deg. At 3000rpm on the nose she was all in at 36degrees. Lovely.

Then I connected the vacuum.
Bad move. BANG!BANGBANGBANG!! pop crackle BANG BANG - phut! Oops...

Vacuum off, sweet as you like. Driving her with vacuum off at the moment and she is lovely.

So - this mornings question to the gurus is "Vacuum. WTF ?? " Is it an either/or thing ? (i.e. you have to lock the mechanical advance if you run the vacuum ? Only conclusion I could come to ! )

MSD curve sheet. Very helpful.

[Pete]

Are you connecting it to the Spark Port (used mostly for emissions control), or full manifold Vac - in which case you are probably over-advancing.

36 Degrees is probably too high on todays petrol, we only run the race car at 34 nowadays, even on super unleaded and octane booster / anti-knock additive....

[Blue]

Agreed, if you are making a lot of vacuum you could well be advancing to the point where it's trying to fire on the next cylinder, look how close together the terminals are on the cap and you can see the problem. Also agree on 34 degrees maximum these days, I never run more than that.

[Pete]

If you have got the car "in the Zone" then why did you connect the vac at all?

I am not trying to be an arse, but what are you hopping to achieve with vac?

Are you trying to get more initial and then advance eat idle? If that is the case, you will have to re-time the ignition to suit.

If not, then leave the vac disconnected.

[Dave-R]

I would buy a Dyson every time.

[morgan]

Thanks chaps. Not taken as being an arse - I am learning all the time.

What am I trying to achieve ? My understanding is that vacuum was best used for road cars and provided the ability for a more variable timing curve which depended on engine load. For instance - If driving up a hill and the car labours more the vacuum would change and the ign would adv/retard into a sweeter zone for whatever the engine was asking for at the time ; the vac would move the base plate.
Mechanical advance is just that - engine speed = advance degrees. The end.

So in answer, I was under the impression that a softy road car like mine would be happier with vacuum. Although, I confess, she runs lovely now so not sure why other than I thought it was best practice...

No obvious pinking at 36deg, but will knock it back a couple. Chose 3000 rpm springs as that pretty much all she does anyway

Connected vacuum here -

[Les Szabo]

I never ever used vacuum on my cars and 3 out of 4 were driven on the road, but in essence you are correct, vacuum will if timed right give you a cleaner exhaust and perhaps a bit better mileage as Dave has said many times, I'll take his word for it.

Leave it as it is, if its sweet, and if it don't pink at 36 with the fuel you use and your CR, combustion chamber design, (ally or iron headed?) leave that as well, why retard it if its running in the sweet spot.....34 is not a magical number that all engines have to be run at.

Les

[Adrian Worman]

The reason why you would still want to use vac on any street car is to ensure complete combustion under light engine loads, cruise, overrun etc.

Although it's probably slightly outdated info now but Mopar Perf recommend adding vac to a street dist to achieve an overall combined advance figure of 52deg on a B/RB engine.

Seems wildly excessive but I'm runnin that on my 440 and that must be making some reasonable HP numbers to get where it is.

The van can is adjustable thru the little hose nipple, find a small Allen key to fit inside and locate in the adjuster and back off the advance.

I have to agree with the others that 36deg is a little high for a fairly stock engine. I think I would have experimented with a higher initial number than 14 deg as well, prob 18 initial and 34 total.

If your running tall gearing in a heavy car you'd want all the mech advance in later, say 2800-3000, a light car with low gears could have all its advance in by 2500-2600.

But, as is usual with old engines and new fuel, you have to experiment and give your engine what it 'likes'.
Dave Robbo does an excellent tech piece about setting timing and the reason why you need so much initial adv to run modern fuels

[morgan]

Thanks chaps.
Still not really wiser on the vacuum, but seems to be doesnt really matter anyway. More a 'nice to have'.

Will give car a bit more stick and see if pinking surfaces.

Might try adjusting to black adv busing and silver springs then - see if Ades 18initial with 34ish changes anything.

Engine seem remarkably willing. Drove down to garage last night to set timing and it was off the scale (like 30 deg) initial and still started/ran lovely (slight kickback, but started and ran perfectly).

Maybe its voodoo.

PS - Ade. Will try and get round pick up clutch later.

[Dave999]

when you are cruising at 70 mph and the throttle is nearly closed

the vaccum is high across the ported vacuum port

but you are basically running on the idle circuit due to the tiny slit you are dragging the air through

air that is dargged through mixed with petrol in a high vacuum stretches

making an already lean mixture leaner

if you stretch the air you spread out the o2 molecules and the atomised fuel further apart

so although you still have an explosive mixture

its a bit rubbish think of and arsonist in a forest with a fire break every 3 trees rather than every 3 miles. he anit gonna have any fun with only 1 match

and being a bit rubbish the tiny flame fronts from the spark take longer to jump from place to place

so the burn is much slower

in light of this, its way better to kick off the flame earlier, and way earlier than you would if you have the peddle pinned to the floor to accelerate.

if you do this the flame front covers the full piston and it gets the limited Bananarama! is due from the horrible mixture its working with

with no vacc advance you are setting the flame off later hence a proportion of the high vaccum THIN poor mixture doesn't get burnt and you get less bang for your buck and pollute more

in this case (no vacc advance) in order to maintain your CRUISE you will have to press the acceslerator down a 1/4 of an inch more and hence use more fuel and run the car with the throttle open just a bit wider which produces a better less stretched mixture which burns better and is more appropriate for the timing you have with the vaccum advance disconnected.

its all about allowing a car with your foot off the peddle run further without input

a conected vaccum advance reduces engine breaking when you come off the gas...

and makes cruising easier and cheaper cos you can run a gnats fart of fuel and air when the piston speed is high and the vaccum is high due to closed throttle

normally when the piston speed is high the vaccum is low due to wide open throttle
and there is no vaccum advance necessary becasue the carb is working to its fullest extent to atomise as much fuel as it can for the air its gulping down


thin strecthed mixture needs gobs of advance to set it off
if you are going to set it off you may as well do it at the best time

your vacc can is part of your engine load sensor its kind of an old school MAP sensor



Dave

[MilesnMiles]

Dave999, superb Really good description in layman's terms!

[Dave999]

ta much

think it make sense

another way to put it would by dynamic compression ratio is small due to piston speed and high vaccum

i.e the number of molecules i can compress with the throttle closed and the piston moving this fast is tiny in comparrison to what i could with the throttle open

if there is less in there

it heats up less
it burns slower cos the molecules are universes apart rather than stacked like eggs in a box

therefore is less likley to detonate

and a spark at a time that would blow off the piston crown on good fulsome charge of fuel air mix won't with the dribble it has to work with

Dave

[TW71]

Dave , you should write a book on motors, you have a gift there!

[morgan]

... and if it helps, Although car runs sweeeeet now she is
a) Thirsty , yes throttle on to hold 70.
b) Very stinky on the idle. Like you could run a car on the fumes coming out the back

So - I'm still not clear on why is went nuts when I connected it, although Ade tells me it is adjustable. Will read some more and try again.

If I could get her running as well as she is now AND remove the stinky a bit then she'd be perfect.

[Pete]

I think you can pull in another 6 degrees at idle using Vacuum IF (and it is a big IF) your convertor is loose enough.

As you pull IN more timing, you ADVANCE the torque curve, so the Convertor will lock up earlier in the Rev range.

IF you have a tight convertor, and you attempt to pull in more timing by use of the Vacuum, when you drop it into gear, the extra timing will fall out as the revs drop; so it will be a useless exercise........

Your safest bet is to re-set the timing so you have 34 Degrees full advance and around 20 degrees initial, that will sort the "smell" out.......