Ferret Ignition Timing?

[sexton]

(This is a repeat of my post on the MLU forum as I want to reach the biggest audience I can. Hope that's OK.)

 

Another Ferret question: I found the ignition timing of the Ferret I am working on at 25 deg BTDC. It still idled reasonably well. I corrected it to the spec 0 deg BTDC and maybe it idled a bit better.

 

I can't help feeling 0 deg BTDC is too retarded with modern petrol.

 

Retarded timing can cause hot running and it definitely reduces power and increases fuel consumption. Typical static timing seems to be 10-15 deg BTDC on other vehicles, albeit more modern ones.

 

Combustion rate with 60 year old combustion chamber design and a 6.4:1 CR must be a bit sluggish so I suspect the engine would benefit from some more ignition advance.

 

I don't expect us to try hand start the old girl so I don't see that as a significant issue.

 

So I am considering advancing the timing to 10 deg BTDC to see if it makes any noticeable difference. I think the 25 deg added by centrifugal advance to give a total of 35 deg should still be safe, what with modern petrol having lots of detonation resistance compared to old-time petrol.

 

Has any one tried this? Any comments?

[marmon]

10 or 15 degrees would be fine the only thing you have to watch out for is kicking back against the starter while cranking and pre ignition ( pinking) any sign of either of these and back the timing off a little bit at a time until it stops, I'm sure you have, but check the centrifugal advance is well lubricated and the springs are still tight enough. You'll get more horsepower with more advance up to a point.

[sexton]

Thanks. I was surprised there was no kickback when starting it with the 25 deg advance I found it with. I'll give it a try.

[paulob1]

Thanks. I was surprised there was no kickback when starting it with the 25 deg advance I found it with. I'll give it a try.

 

there is a very specific reason why they went for 0 degrees its the same on the B81, some one once explained why it was but I also cannot see why a 15 degrees before tdc would not be better...combined with a Jolly engineering electronic ignition i would not be surprised if you could get 10 mpg out of them...worth a try...

 

I am wondering if it has something to do with the side valve arrangement...will try to find out...

 

 

I have the jolly gear in two of my b81's and they run very very sweet compared to the points in the saracen and ferret, although I have to say the ferret engine is superb...

[guykay]

Don't keep us in suspense. What was the very specific reason the timing was set to 0 degrees?

[Richard Farrant]

Regarding angles of timing, this can vary on different engines and cannot be used as rule of thumb generally. It is all to do with con-rod length and stroke. There is a point when the piston stops at TDC momentarily and this can effect the firing point, all dependant on the ratio of rod length and stroke as I think that lag increases in some circumstances.

 

Don't forget also that TDC timing point is static and the centrifugal advance will kick in as soon as the engine starts rotating, I forget the degrees on this with out checking.

Edited May 3, 2015 by Richard Farrant
spelling

[Sean N]

The ignition advance is set to give the correct flame front propagation so that the fuel burn in the combustion chamber exerts the maximum force on the piston without risking damage; at its crudest, you don't want the flame front chasing the piston down the bore, but you don't want it trying to push the piston back the way it came either.

 

This can vary considerably from engine to engine as Richard suggests. Engine design can differ radically from generation to generation; engines designed in the '50s and '80s are very different things and put together in very different ways, so you can't necessarily make parallels between the two. If I remember correctly, it was very unusual to see static advance of more than 6 to 9 degrees before the '80s.

 

I suppose an engine set up to run on any old muck down to about 80 octane might be happy with the timing advanced slightly when running on good quality pump fuel, but there may also be other issues.

 

You need to be very careful with over-advancing ignition just as you do with over-retarding it. You might think that if you over-advance and it starts easily, you don't get kickback and you don't hear pinking, that's job done; but there are other things to be aware of. For example, if you set a high static advance, you significantly increase the total advance (static + centrifugal + vacuum), which can lead to high speed detonation (high engine speed, that is) which you won't hear but can be very destructive very quickly.

[paulob1]

the B series engines are half side valve, as I mentioned there is a very specific reason but damned if I can remember...the two things that spring to mind, and they are guesses on my part so may be totally wild, are:-

 

1. that the b series engines were meant to run without the water pump working, ie boiling water...so in order to keep combustion chamber temperatures to a minimum they had them firing at TDC chasing the pistons down the bore..as was said earlier... the explosion, when you ignite petrol takes a few moments to reach full capacity...hence why we have the engine firing before the piston is at the top so when the piston does reach TDC full power is acting on the piston as it goes down, the vacuum advance allows for the higher rotation speeds and the higher piston speeds, so that this keeps happening....ensuring that the engine achieves max power as the revs go up. the B series made big power for those days, a 40's designed normally aspirated engine producing over 200 bhp at 3750 rpm was unheard of and with tests, they got the B81 close to 300 bhp and still relatively reliable, if not fuel economic...and remember this is still a part side valve engine...

 

2. the combustion chamber shape, wide range of fuel qualities and the fact that it was designed to run at full power and full revs for over 200 hrs or more meant that pre ignition could occur at high engine temperatures, so better to run them firing at tdc.

 

I sort of remember reading about harmonic balances too, at something like 3300 rpm they were all in conflict or in balance, cant remember but perhaps with too much advance it induced additional in balances .

 

As I say guesses but I do have a book on the subject and will give it a read to see if I can find out the real reason...

[Richard Farrant]

Paul,

Only the civilian versions of the B Range have vacuum advance, the screened ignition military engine rely only on centrifugal advance.

If I remember correctly the B range were developed from a Bentley engine design from pre-war (Maybe why they where given B title) and in that period of time, vehicle engines were given a Taxation Rating called RAC h.p., which was worked out by bore area;

b²n over 2.5 b = bore diameter in inches and n = number of cylinders

 

To sell vehicles it was necessary for them to be in as low a taxation rating as possible and to achieve this the bore diameters were designed as small as possible which meant the stroke had to be longer. When this happened on a large engine, there was not enough room to get two valves of sufficient diameter in the headspace so the answer was to have inlet over exhaust, ie side exhaust valves and overhead inlet.

 

My recollection was that W.Robotham, a Rolls Royce director was working on this engine in the Forties to develop it in to multiple cylinder configuration for military use as it was an existing design and as we know now, hampered by the RAC rating which was holding back engine development.

 

So in my opinion the side valves were not done to enhance the engine power at all.

[101 Ron]

In the research I have done on the B81 Rolls is the ignition timing is retarded static ,to allow hand crank starting without kick back.

Soon as the motor runs.........ie idle speed the ignition timing is normal and then though to normal ignition advance at higher speeds.............I understand the distributor advance to be set up that way.

I understand to static advance the B81 more than TDC will over advance the motor overall.

The combustion chamber on the B series is long and narrow with a long cylinder stroke.

The side exhaust valve was purposely done that way for better exhaust valve cooling and the long side valve like combustion chamber with long stroke to give broad power delivery over the rev range as the F head design breathes reasonably well too.

It must be remembered too that the B series designed for indifferent fuels and temperature extremes.

I would be happy to be corrected............it is what I have around with my research so far on the B series motors.

Ron.

Edited May 3, 2015 by 101 Ron

[101 Ron]

It should be noted before the days of pollution control Vacuum advance was for improved fuel economy during light loads and cruising speeds as more ignition advance could be had under light load conditions.

Light cruising load rarely happens with military vehicles and the little bit of extra fuel burn was not a worry to the army and therefore vacuum advance was not needed.

Edited May 3, 2015 by 101 Ron

[101 Ron]

Just one another point about the B series and what I have noticed about my B81, that it is the easist motor I have ever owned petrol/diesel to start hot or cold...........retarded static timing may be a factor in this ?

Edited May 3, 2015 by 101 Ron

[paulob1]

In the research I have done on the B81 Rolls is the ignition timing is retarded static timing to allow hand crank starting without kick back.

Soon as the motor runs.........ie idle speed the ignition timing is normal and then though to normal ignition advance at higher speeds.............I understand the distributor advance to be set up that way.

I understand to static advance the B81 more than TDC will over advance the motor overall.

The combustion chamber on the B series is long and narrow with a long cylinder stroke.

The side exhaust valve was purposely done that way for better exhaust valve cooling and the long side valve like combustion chamber with long stroke to give broad power delivery over the rev range as the F head design breathes reasonably well too.

It must be remembered too that the B series designed for indifferent fuels and temperature extremes.

I would be happy to be corrected............it is what I have around with my research so far on the B series motors.

Ron.

 

 

Ron that sounds entirely plausible......

 

Richard, I never said they built the side valve configuration to improve performance, I understand that over head cams were considered but that they would make the engine too noisy, this was when it was being considered for a car engine not in its development into the military engine...but I am struggling with my memory on this...need to read up on it again to refresh my memory...

[Richard Farrant]

Ron that sounds entirely plausible......

 

Richard, I never said they built the side valve configuration to improve performance, I understand that over head cams were considered but that they would make the engine too noisy, this was when it was being considered for a car engine not in its development into the military engine...but I am struggling with my memory on this...need to read up on it again to refresh my memory...

 

Paul,

I was not implying that about the side exhaust valves, it was from the original Bentley car engine design of mid-Thirties and a necessity in order to get as large an inlet port and valve as possible. They were hampered at the time with the RAC rating formula for bore and stroke.

I have to disagree with Ron about the side exhaust running cooler, the exact opposite in my experience with the burnt valves that we used to get on them. This has always been a problem with side valves.

[sexton]

The comments that the mechanical advance on these engines comes in sooner than more modern engines and advances timing even at idle makes a lot of sense to me. That would give the best of both worlds - easy and safe hand cranking and a normal advance curve when running.

 

Since I can't find any info on the advance curve rpm vs. degrees, what I need to do is temporarily splice in some unscreened plug wire on #1 and watch what happens with a timing light.

[101 Ron]

Paul,

I was not implying that about the side exhaust valves, it was from the original Bentley car engine design of mid-Thirties and a necessity in order to get as large an inlet port and valve as possible. They were hampered at the time with the RAC rating formula for bore and stroke.

I have to disagree with Ron about the side exhaust running cooler, the exact opposite in my experience with the burnt valves that we used to get on them. This has always been a problem with side valves.

Richard you are correct about exhaust valves getting hot on side valve motors and I wondered this myself.

In fact very high outputs with side valve motors tends to distort the cylinder bore with heat from the exhaust valve.

One of the people who helped designed the B series worked out the exhaust valves could be made to last longer and cooler if a F head configuration was used with a wider than normal water jacket between the valve seat and cylinder .

During design much adtension was paid to cooling water flow around the exhaust valve seat.

It should be noted one of the reasons why the combustion chamber is so long on the B series is the need for the cooling passage between the valve seat and the cylinder.

The B series as far as I am aware as given little trouble with exhaust valves,but I never worked on them in field and as Richard implies that may not have been the case.

One of the reasons for the B series high fuel use is that long combustion chamber and the heat losses into the cooling system overall..

Edited May 3, 2015 by 101 Ron

[101 Ron]

Richard as you implied for tax reasons English motors of the time had small bores and valves.

Another reason for a F head design on the B series was the need to keep the bores small to keep overall length of the block short as possible on the bigger 8 cylinder motors.

I find the B series motors interesting from a design point of view .

The more I learn about them and why they designed things the way they did......................every part of these motors was well tested and reseached.

Edited May 3, 2015 by 101 Ron

[Richard Farrant]

Richard you are correct about exhaust valves getting hot on side valve motors and I wondered this myself.

In fact very high outputs with side valve motors tends to distort the cylinder bore with heat from the exhaust valve.

One of the people who helped designed the B series worked out the exhaust valves could be made to last longer and cooler if a F head configuration was used with a wider than normal water jacket between the valve seat and cylinder .

During design much adtension was paid to cooling water flow around the exhaust valve seat.

It should be noted one of the reasons why the combustion chamber is so long on the B series is the need for the cooling passage between the valve seat and the cylinder.

The B series as far as I am aware as given little trouble with exhaust valves.

One of the reasons for the B series high fuel use is the that long combustion chamber and the heat losses into the cooling system overall..

 

Hi Ron,

Good for your input here!

My working experience of RR B range spans over 40 years now on all types of military vehicles as well as some fire engines. We did get burnt exhaust valves and cracked valve seats, but one recurrent problem with all engines was head gasket failure due to the proximity of the exhaust valve to the narrowest part of the gasket between certain pairs of cylinders. I think on the B60 it was 2 & 3 or 4 & 5.

 

regards, Richard

[sexton]

Just an update on what I found checking the timing with a timing light. With the static timing set at 0 deg BTDC (engine shut down), it advanced to ~10 deg BTDC when idling around 600 rpm in neutral. So the mechanical advance kicks in at very low rpm. This may be as per design to make hand cranking safer as posted above or it may just be because the springs are weak.

 

I must admit, I don't like the idea of advance kicking in at such low rpm because it makes the idle more unstable. By that I mean if the idle slows down for some reason (engaging gear for example), the timing retards and reduces the idle speed even further.

 

Anyway, I found the happiest medium was setting it at 15 deg BTDC in neutral, engine idling at 650 rpm. Then engaging a gear drops the idle to ~450 rpm and the timing to ~12 deg BTDC. This provided a lower rpm drop when engaging a gear and a much more stable idle in gear.

 

I think a relevant point here is this Ferret has low compression. I haven't measured it yet, but it's too easy to turn over with the crank handle. (Winter project, maybe.) So overly-retarded ignition combined with a very low dynamic compression ratio at idle makes for very poor combustion. The eye-watering exhaust fumes at idle reduced significantly when the timing was advanced. I suspect the low compression is also responsible for what seems like a large rpm drop when loaded by being put in gear.

 

Malcolm

[attleej]

My experience of a B81 in a Leyland Martian was that it was easy to start on the handle. Just as well because the starter would regularly cut out due to the over-heat cut out device in it.

 

John