Ignition advance settings - an experiment

[Lebro]

Following on from a brief diversion in the "betty is dieseleing" Topic, I did a small experiment tonight.

 

Attach my zenon timing light (Halfords EQUUS 416677) to No. 1 plug lead, set timing light to 0° advance.

 

Start engine, remove vacuum pipe from rear carb (I am running HS6's).

 

Check timing at high revs.

 

The most advanced the timing got was 35° BTDC.

 

Adjusted timing using the vernier on the distributer to 30° BTDC max advance

 

Check timing at slow (800RPM) revs = 10°BTDC

 

While looking at timing marks (stamped on my crank pulley by me) suck as hard as possible on vacuum pipe

Timing advanced by 15°

 

Set my timing light to 30° advance, re-check at high RPM - timing shows 0°BTDC, so advance feature on gun is spot on.

 

Check tickover RPM on gun, & TR tacho - both read 800.

 

So, my timing light at least seems pretty accurate, & I am now happy that my timing is nominaly correct.

 

Bob.

 

 

( Expecting response from Peter !)

[Peter Cobbold]

Bob,

Excellent, nothing beats real measurements!

That 15 deg extra advance on the v-a capsule is about right, giving a total 45 BTDC at 3000 rpm. Your method is a quick and simple way to check the capsule.

 

The only information lacking is how your 'sucktion' relates to the venturi depression created around the butterfly edge when at cruise, That depends upon the air velocity (and density IIRC) and cannot be replicated in an unloaded engine - it would rev to destruction with a cruise-level of air flow.

Connecting a vacuum gauge to that little nipple at the butterfly edge will give a measure of the depression while driving at cruise. Then use the same gauge to measure mouth sucktion.

 

 

So how much improvement in fuel consumption would 45BTDC make compared with 30BTDC? That info is not easy to find but Heywood's textbook ( fig 15.3b) shows a 7% penalty for being 15deg retarded, and about 10% for 20deg. So its around 3mpg, ballpark.

That lost flywheel energy, when running retarded at 30BTDC, goes out the exhaust pipe as extra heat and the exhaust valves run hotter. Heywood, for once, doesn't tell us by how much.

 

 

Lesson over....................for now.

Peter

Edited July 22, 2014 by Peter Cobbold

[Lebro]

Peter.

On long runs, when not "pushing it" I can get around 35MPG, which I am quite happy with.

I have no reason to think that the suction from the carburetter is wrong - why would it be?

If I get realy bored, I may dig out a vacuum gauge, & do the above test !

 

Bob.

[Peter Cobbold]

Peter.

On long runs, when not "pushing it" I can get around 35MPG, which I am quite happy with.

I have no reason to think that the suction from the carburetter is wrong - why would it be?

If I get realy bored, I may dig out a vacuum gauge, & do the above test !

 

Bob.

Bob,

The venturi effect changes with the square of air velocity between the take-off bore and the butterfly edge. Its a highly localised dynamic effect**.

If the carbs are not a standard fitment the air flow may not, or may be, enough to pull the diaphragm fully. Bigger carbs make less venturi suction for the same air flow. If the carb has come off a engine with no v-a capsule the drilling may have be wrongly positioned- a few mm out will affect the venturi.

But 35mpg suggest it's working well !

Now you can set about leaning the miture and advancing some more.... to 36mpg.Nothing like experiments - with measurements!

 

Peter

 

** The yellow and red regions are where flow is fastest, pressure lowest - so the 'vacuum' used for the capsule is very localised, its not the 'bulk' manifold depression.

http://www.friendship-systems.com/about-us/newsletter/frameworker-2011/projects/butterfly-valve-6.png/image

[ctc77965o]

This is the way you time the early Jag V12s, 30-something deg BTDC, revs falling thru 3000rpm...argument being that the timing being correct under these conditions is far more important than what it is static or at idle.

[TIMS]

Guys

 

This area of engine management has felt like somewhat of a black art to me, but nothing ventured....

 

So I have read up the instructions on my timing light and had a go.

 

(Car is a CR Pi car with TH5 cam, timing set some time ago and not fiddled with by me)

 

Firstly I did the piston stop to piston stop check on TDC and found that the markings on the pulley are out by 2 degrees to the pointer on the timing chain cover.

 

Then, with tickover at 700 rpm I used the advance function on the light to tell me that the current setting was 8 deg BTDC. I adjusted the distributor to bring this to 12 deg and the car sounded much better. Tests on the road will tell.

 

Then at 3,000 rpm and 4,000 rpm I read off the advance using the timing light and got 24 deg and 26 deg. Is this about right?

 

Thoughts (other than why did you not do this earlier to which the answer is - the car is running very nicely thank you!!)

 

Regards

 

Tim

[Peter Cobbold]

Tim

Depends how that light works.

If you have now got a spark at 12 + 26 = 38BTDC at 4000rpm that's about right.

If you have the spark at 26BTDC at 4000rpm its needs more centrifugal advance.

I think the book value is a round 32-34BTDC at 4000rpm, but a few degrees makes little difference to power ( 4deg out is about 1% down from best).

Peter

[Tony M]

Hi Lebro. Do you happen to have an image of the instructions for that particular timing light Equus 416677. I have inherited one but without any instructions and I cant find anything on line as its quite old. Many thanks

Regards

Tony

[Tony M]

Hi Lebro. Do you have any instructions for the operation of that particular timing light EQUUS 416677 ? I inherited one without any instructions and I cant find anything online. Many thanks

Sorry about the double post but I couldn't delete it.

Edited August 16, 2021 by Tony M

[Lebro]

Wil take a look, but I doubt it.

What don't you understand, it's all pretty self explanatory I thought.

Bob

[Lebro]

Well, surprisingly I do have the instructions ! I will scan them & post later today. or if you PM me your email address I can send the scans to you there.

Bob.

[Lebro]

Equus 416677.pdf

There y' go

 

Bob.

[FatJon]

If it's of interest, here's my ignition table which runs very nicely. It ends at 4800RPM as there are no changes beyond that point so better to have good resolution further down.

The engine is a standard 1970 PI 150BHP but with a nicer exhaust manifold and system plus throttle bodies and electronic injection/ignition. It's still a work in progress on fuelling strategies but runs lovely and feels very smooth and well mannered. It pulls in plenty of advance at high vacuum so I can go quite lean on the over run and moderately lean on cruise. No pinking on standard UK unleaded and 25MPG average unless I get heavy footed then the low teens are easy. There are some areas outside the normal running zone that are untuned best guesses, eg full throttle at 600RPM, let's face it, who goes there? 

Because it uses wasted spark with each plug firing twice per revolution it has some weird behaviour with modern dial back timing lights that one needs to be aware of when adjusting the timing. There are a few areas that might benefit from tweaking but as Peter said, a degree or 4 makes little difference in normal running off idle. Getting the fuel right is much more important.