Idling at Start-up and advance/retard

[Paul Spencer]

Following James's post in the Derby forum, I was wondering what others do with their WOs in a couple of circumstances.

When I bought the car, the previous owner said that he always warmed it up for five minutes at idle in the garage in summer, ten minutes in winter. I decided that was not a good thing, and now drive off as soon as normal oil pressure is reached. Obviously, I don't over-rev or let the engine strain until it is fully warm.

Something Chris commented on in the other thread was the advance/retard. I usually run on full advance, but retard a little in 30mph limits when the revs are down. When I was at William Medcalf's once and the car was idling, he reached in and fully retarded the ignition. I know of other long-term and experienced owners who whack it up to full advance as soon as the car starts and leave it there. Retarding at low revs makes sense to me, but I wonder how necessary it is with modern fuels.

[Robert Clifford-Wing]

Paul, I am a newcomer to WOs but when starting our 3 ltr I have the timing set in the mid position. Once the oil pressure has risen after a minute of so I fully advance the timing; this increases the revs and appears to allow the engine to run more easily. It will be interesting to hear from others who are more experienced.

[John Murch]

In the past I also started on full retard and essentially brought it up to full advance once started, like Rob it seemed to run best like that.
However since my recent (well a year or so ago) engine rebuild with 4.5 block, but still with the Phoenix 3ltr crank, I find the necessity to retard while running slowly.(to avoid pinking while accelerating from lowish revs) I put this down to the significant increase in compression ratio on the new configuration.
John

[Chris Card]

Maybe there is more than one right answer.

John, with his higher compression ratio needs to retard.

In my case, I have set the idle speed with ignition advanced to normal driving position, having sterted with it retarded. If, having sterted, I then retard the ignition, the idle speed drops too low and becomes lumpy.

Q. Should the idle speed be set with ignition retarded?

Chris

[Dan Suskin]

Yes Chris, it should be set with the engine retarded.

[Robert Clifford-Wing]

[Dan Suskin]

Well that;'s the thing Rob. The car should be set to idle properly fully retarded, and then driven with the timing advanced as appropriate for the driving conditions at hand. Fully advancing and just leaving it there is not the best way to drive either.

The car will not predetonate at the same degree of advance under all conditions - depends on speed and load.

On a difficult climb, more power is achieved by retarding the ignition than by advancing it.

[Paul Spencer]

[Dan Suskin]

Paul,

Under all conditions of driving, you want the maximum power from the fuel explosion to occur immediately after top dead center. If maximum power occurs before TDC, you are fighting against an ascending piston. If too late after TDC, you've lost most power as the piston is already far down the cylinder.

As you speed the engine, the piston is obviously moving faster - but the explosion of petrol takes about the same amount of time to achieve maximum power. If you fire it at the same time as when the car is retarded, the piston will have moved way past TDC already. So, you need to fire it earlier so the max flame front is achieved at about TDC. Firing advanced allows less fuel/air into the cylinder, but that's a loss you have to take.

When climbing a tough hill, the engine is turning over more slowly, so the mixture must be fired later (retarded) in order not to fire before TDC. Additionally, by retarding the ignition you allow a greater amount of fuel/air into the cylinder - that increases power significantly.

[Paul Spencer]

Thanks Dan. The first part of your post was what I already knew, and just relates to retarding when the piston is moving more slowly. I'm not sure how retarding the ignition allows more mixture in.

[Chris Card]

Dan - many thanks for that explanation. Undoubtedly I have been driving lazily in that I seldom move the Advance/Retard when driving, having set the idle speed when fully advanced.

It also explains why William Medcalf "reached in and retarded the ignition" on Paul's car.

Next week, after I get back from Namibia, I will readjust my idle speed to a smooth ticker when fully retarded.

Chris

[Dan Suskin]

Paul

As you know, advancing the engine speeds it up too - the faster it runs, the less time there is during the intake stroke for gas/air to fill the cylinder - both due to time, bit also because less vacuum is generated to pull the mixture in.

Remember, you are only running retarded when going slowly - that allows more time for the mixture to be brought into the cylinder.

If you set it too retarded, the mixture will still be burning when the exhaust valve open - possibly burning the valve.

[Dan Suskin]

Chris

OFF TOPIC

I might be in Namibia in December - there's a 100km run through Sossusvlei I'm considering.

[Chris Card]

Dan, when is too retarded, too retarded?

Chris

PS when you say a 100km run through Sossusvlei in December, what kind of "run" are you talking about? December is getting hot hot, especially in the desert.

[Dan Suskin]

Chris

I don't know when retarded is too retarded. That's why I set timing per the recommendations.

I mean run as on two feet. It'll be hot, but much better than my April one - that was 250 km in the Sahara at temps around 140 F, and carrying a backpack with all my belongings. This one is only 100km and I only have to carry water. Not certain I'm going yet.

[Dan Suskin]

In an effort to try demonstrate the effect of timing on power, I took my 1902 Delahaye up a steep hill today - easier to see in a one cylinder car as I don't have any hills the Bentley will struggle on close by.

In the video, the car is retarded as much as I can without engaging the decompression lever. Then you'll see me advance the ignition to a point where you can here it knocking (pinging, pre detonating), after which I pull it back to be as far advanced as I can without knocking. You'll hear then engine slow and start to struggle a bit. Then I retard it again and you'll hear the engine smooth out and speed up.

https://youtu.be/N19nOTz03KU

This shows that driving with ignition retarded is proper under the right circumstances.

[Chris Card]

Dan - I believe that exercise (especially that kind) can endanger your health!

The desert scenery around Sossusvlei is spectacular, there is unlikely to be water in the vlei, and the sand will burn your feet - enjoy!

Chris

PS Your video is most informative - thanks.



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[Dan Suskin]

Thanks Chris. I've been to Sossusvlei many times. Love it.

[Chris Card]

BACK ON TOPIC

With regard to setting the idle speed when retarded, it makes absolute sense when you consider that that is what is done with more modern automatic advance and retard systems.

Chris

[Christopher Carnley]

When the static ignition timing is 45 degrees BTDC, I don,t really understand some of the arguments,and experiments with single cylinder engines never applied to multi- cylinder ones, in practice.

How much can the spark be retarded until it approaches TDC in the Cricklewood engines, magneto energised?

Igniting fuel after TDC is completely wasteful, and leads to overheating,and because there is no centrifugal advance, it is common sense to advance the ignition on load.

Ignited petrol/air burns rapidly, it does not explode, that is unless it pre-ignites, explodes and blows holes in the pistons.

[Dan Suskin]

It's true there no explosion - my laziness in typing. It's actually a progressive flame front.

Timing principles are the same irrespective of the number of cylinders - a multi cylinder car is just a bunch of single cylinders firing in sequence.

[Chris Card]

Having thought about both CC's and Dan's posts I compared them with my own experience of an overdrive combined with a higher than normal back axle ratio.

My diff is 3.3, instead of the normal 3.53, which makes the car a bit over geared with the overdrive. I have found that with the overdrive engaged, I have to retard the ignition slightly for best performance.

Not scientific, but it does seem to back up Dan's comments.

Chris

[Christopher Carnley]

I still don,t understand the overall thesis with the ignition timing at 45 degrees BTDC, surely any advance and retard at such a figure is academic, when the ignition sequence never goes over the top and the piston descending.

Of course one should retard when hand cranking.

The Derby engine with static ignition timing at TDC will kick back if there is a mere 5 degrees advance at start up.

[Robert Clifford-Wing]

[John Murch]

On full retard my car does not kick back on starting, but most certainly will with only a modest amount of advance. I find that full advance is only beneficial if at over 2000rpm, certainly at under 1000 rpm it pulls better when nearly fully retarded. A third gear overtaking manouver will sometimes involve well over 3000rpm (full advance) when it is deliciously smooth and powerful. Consequently very tempting to use the revs.

[Chris Card]

Now back in the UK, I've adjusted the idle speed with the ignition fully retarded, and the difference is remarkable.

I previously retarded the ignition for starting and then immediately advanced it to my normal running position, having adjusting the idle speed while fully advanced. I was aware that the tickover was a bit harsh and lumpy, but took that to be 4 cylinder normal.

Having reset the idle speed with ignition retarded, the tickover is much smoother, the pickup is good and I advance as required when on the road.

It's only taken a mere 21 years of ownership for that penny to drop! Thanks guys.

Chris

[Dan Suskin]

Wonderful Chris!

[John Murch]

How pleasing

[Christopher Carnley]

What do you estimate are the degrees of ignition timing, BTDC, fully retarded, when the "book" setting for"normal running" ignition is 45 BTDC?