8 replies to this topic

[stevegrabba]

Morning guys,

 

If you do a google search or look for ignition timing by vacuum gauge readings, there are 101 articles out there and some great videos.  Those who set their timing this way generally have done for years and swear by it and it makes perfect sense by our mechanical minds and logic.

 

Once you have slowly advanced your timing to get the highest stable vacuum reading (Taken from inlet manifold) and then retarded by 1/2 inch of mercury pressure that is supposed to be the ideal 'true' ignition timing setting.  So I have done this on many occasions, on different engines over the years and never cease to be amazed at what the resultant timing I have when I check it again using a strobe.

 

For example I'm just working on my 78 SA Mini GTS at the moment.  According to book the timing should be 8 deg btdc at 850 rpm.  However, using the vacuum method it takes it all the way up to around 20 deg btdc, which is quite a difference!

 

Throw in mechanical advance from the dizzy and its probably upwards of 40 deg of total advance once over 3500 rpm.

 

Does anyone else use the vacuum gauge method and what timing figure do you finally arrive at?  Respecting to the age and wear and tear of the car etc...however how does that compare to manufacturers figures?

[Wim Fournier]

Never heared about this approach, but think it is very interesting to dig in. I'll go and Google.

I think it has much to do with the timing of the valve opening times and gas flowing of the individual engine.

Piston speed is the highest half way between bdp and odp and than the inertia of the inhaled gas leads you to the very moment to produce a spark. Wonderfull.

[mini13]

yeah....  8 deg advance is for with no vac advance attached, so you are likly to be pulling 15-18 with it attached and timed as standard.

[stevegrabba]

yeah....  8 deg advance is for with no vac advance attached, so you are likly to be pulling 15-18 with it attached and timed as standard.

With twin HS2's its a ported vacuum connection to the advance and retard unit on the distributor, so Zero vacuum and zero advance at idle.  That threw me off last week too...From the port connection the vacuum isn't created until about 2000 rpm or there abouts. 

Edited by stevegrabba, 24 October 2017 - 11:32 AM.

[stevegrabba]

Thinking out loud....if I connected to an inlet vacuum it would be sufficient to advance the ignition to the 'sweet spot'...ie 8 deg static + 12 deg from the advance and retard unit.  Hence hitting my 20 degree that was determined by vacuum gauge timing.  I've then got about 22 to 24 deg of mechanical advance to give me about 30 to 32 degree of total advance which is in the right ball park.  Vacuum of course never counts towards total timing....  Food for thought.

[6joshh6]

Thinking out loud....if I connected to an inlet vacuum it would be sufficient to advance the ignition to the 'sweet spot'...ie 8 deg static + 12 deg from the advance and retard unit.  Hence hitting my 20 degree that was determined by vacuum gauge timing.  I've then got about 22 to 24 deg of mechanical advance to give me about 30 to 32 degree of total advance which is in the right ball park.  Vacuum of course never counts towards total timing....  Food for thought.

Vacuum advance tends to work well to time an engine because a properly timed engine will pull more vacuum as it is more efficient, meaning the throttle needs to be open less. As such using a vacuum gauge is like working backwards; you set the ignition advance to what the engine wants instead of an arbitrary figure. Bear in mind however that as the engine is working over a range of speeds, what is best at one speed, won't be good at another speed. 

 

I think it must be stressed that you need to make sure your distributor is in absolutely tip top condition before playing with the timing if you want to stand a chance of getting it right. I would double check everything is nice and clean and without slop as otherwise you will have spark scatter which will lead to poor running and potentially detonation if you aren't very careful. It would also be worth checking your vacuum advance is working properly, its a nice easy job and I've found nothing affects how the car runs more than the state of the ignition system. A tip you probably already know but its with reiterating is that if the car struggles to turn over, but turns over fine with the ignition retarded, it is is too advanced and you need to sort it out. Normally this is a symptom of stretched springs which have just enough slop to advance the car too much at idle meaning the distributor doesn't advance enough at higher rpms.

 

I can sympathise with you on this as I used to have a city e 998 with a fairly high compression engine, it would take a bit more advance than the book figure and ran much better providing you had good fuel but ultimately after a few years of tinkering and trying various things I found my distributor had stretched springs. Putting in some new springs made a world of difference and cost very little. I also found a considerable improvement in mpg; I only wish I'd done it sooner. 

[stevegrabba]

Appreciated thanks.  Great minds think alike and when I initially timed her up using a strobe there was quite a bit of scatter, so I put a brand new distributor on and put the old one on one side for me to recondition.  It stabilized the strobe readings massively.  I have checked the vacuum advance and it does start at the right pressure, and pull a full 14 degree by the final pressure.  Again it was brand new, however I checked it anyway.

 

A lot of what I have read on the net have said that timing by vacuum gauge is one of the best ways to take into account 'wear and tear' on a car, and it is direct feedback of what is mechanically happening with the engine.  It is something that I used to do 25+ years ago, and is in the grey matter somewhere...I'm just a bit rusty these days.

[Spider]

I have a very old Auto Magazine Article (somewhere!) that goes not only in to this but also (limited) engine diagnostics by Vacuum Gauge.

 

I'll add though, that this really only works if the valves and seats are in good order, the cam is worn and with standard cam profiles. Certainly going to a warmer profile throws this well out the window,

[carbon]

Morning guys,

 

If you do a google search or look for ignition timing by vacuum gauge readings, there are 101 articles out there and some great videos.  Those who set their timing this way generally have done for years and swear by it and it makes perfect sense by our mechanical minds and logic.

 

Once you have slowly advanced your timing to get the highest stable vacuum reading (Taken from inlet manifold) and then retarded by 1/2 inch of mercury pressure that is supposed to be the ideal 'true' ignition timing setting.  So I have done this on many occasions, on different engines over the years and never cease to be amazed at what the resultant timing I have when I check it again using a strobe.

 

For example I'm just working on my 78 SA Mini GTS at the moment.  According to book the timing should be 8 deg btdc at 850 rpm.  However, using the vacuum method it takes it all the way up to around 20 deg btdc, which is quite a difference!

 

Throw in mechanical advance from the dizzy and its probably upwards of 40 deg of total advance once over 3500 rpm.

 

Does anyone else use the vacuum gauge method and what timing figure do you finally arrive at?  Respecting to the age and wear and tear of the car etc...however how does that compare to manufacturers figures?

You have highlighted the main issue with this timing method. You will likely get it running much better at idle, but depending on the mechanical advance in the distributor the timing could be well out at higher rpm.

 

Running 40 degrees advance would not be good for a 1275 A-Series, it could start making expensive noises...