Good timing

[spyke]

Hi chaps,

Getting somewhere at last!
Just about to time her up ,but as my motor is a bit non standard what do you think a good before tdc figure would be.Im doing it with a vernier before the head goes on so not in degrees please.
Spec:
A10 with A7 alloy head , just under 8:1 comp ratio,356 cam, 376 monobloc.

Cheers Spyke

[Brian]

5/16"

[Alan @Ncl]

Hello Spyke & Brian

I was curious why you suggest 5/16 inch (about 32 degrees) rather than the traditional 3/8 (35 degrees) for an A10.  Is this because the A7 head (which is presumably of a smaller capacity) will make the actual compression ratio higher than the nominal 8:1 so you are trying to avoid pinking?

Just being nosey.

Alan

[trevinoz]

Alan,
            If you have the time and patience, go trawling through the posts of Orabanda who has had some engines on a dynometer with varying advances.
You will see that maximum power occurred between 30 - 32 degrees btdc.
Trev.

[orabanda]

As Trevor mentioned, I have spent considerable time (and money) dyno tuning most of my A10's (three still to do, when time permits).
The timing was checked by strobe, and the optimum settings were:
Plunger GF: - 30 - 30.5 degrees
Iron head swinging arm GF (2 off): - same as above
Super Rocket: - 32 degrees.

Check my old posts for more detail.

Richard

[Brian]

I'm a bit rough so I still use the stick down the plug hole method and 5/16" is about right. Any more advance and they ping with the lovely high quality fuel we get these days.

Mine are all have iron heads so cant say if you can run any more advance with a recycled saucepan head.

Ah, iron, real metal 

[Alan @Ncl]

Thanks for the updates.  Very interesting results and makes you wonder why they always recommended 3/8?  Dynamometer results must surely be definitive.  Is it anything to do with the implications of modern fuels?

Alan

[trevinoz]

What also makes me wonder is why BSA increased the advance to 13/32" with the high compression pistons.
Surely they must have had plenty of feedback from service providers and customers that the engines were pinging badly.
Trev.

[Alan @Ncl]

Good point Trev (noted you'r in New-Newcastle, which is a bit warmer than the original, as I recall from my visit down there during Merchant Navy days in 1968)!

Looking at my 1958 BSA service sheet its interesting that the A7 Shooting Star (with CR = 8:1 compared to the standard with CR = 6.6:1) also had 3/8 advance compared to 5/16.  So it seems for the designers the recommended advance generally increased with compression ratio on the sports bikes. Its hard to see the logic unless they were anticipating higher engine speeds or slower initiation of combustion perhaps?  Also wonder if its anything to do with the valve timing moving further forward on the sports versions (see diagram)?  Either way, it does not seem to match up with Orabanda's research?

[bsa-bill]

Surely they must have had plenty of feedback from service providers and customers that the engines were pinging badly.
Trev.

Did they ping badly on the petrol of the day remembering that then petrol came in grade going up to Five Star (burnt a couple of valves on my Flash, possibly from using Five Star

[Alan @Ncl]

That is what I was wondering Bill?  I have my bike timed at 3/8 and, if I ever get it to start, am wondering if I will need to push it back a bit.  It never used to Ping on this setting when it was on the road 15 years ago but fuel was different then.  Still does not necessarily explain why the optimum power on Orabanda's dyno was at 5/16 though.  I think he was saying push it back to 5/16 to maximise power, not just to avoid Pinging.  I always believed you could usually get a bit more power with a bit more advance but would not do it because of potential damage (and the unseemly racket).  Perhaps I was wrong here?

Alan

[orabanda]

Hi Alan,
The experienced bike mechanic who owns the dyno, predicted that the optimum advance for my A10 would be from 28 - 30 degrees, based on his work with other bikes, such as Triumph.
I thought he was a long way off the mark, but he wasn't!

We could advance the A10's 2 - 3 degrees more before they pinged, but there was no point (pardon the pun!).

At 30 degrees for a GF, the power was at maximum, and the torque curve was also max, and at the lowest revs, which suits me for round town riding. Advancing to 31 degrees cost me 0.5 HP and 2 ft lb of torque, with the max torque occurring 100 rpm higher revs. However it wasn't pinging.
 
Richard

[Alan @Ncl]

Thanks Richard
This is fascinating stuff and I am very impressed by your 'scientific approach'.  Interesting too that the measurements supported the gut feel of the practical and experienced mechanic.  Can you remind me what compression ratio your GF had?  I guess also it was a standard cam with the timings as in the book (i.e. as shown on my diagram)?  Like you, I prefer the torque low down, if there is an option.

Another surprise to me is how much power seems to get lost in the transmission?  I think BSA claimed about 35hp for the engine so if you are getting only about 22hp at the rear wheel, where has it all gone? Obviously ends up as heat eventually but presumably this is in the gearbox and chains which suggests rather low mechanical efficiency which you would think was enough to fry them?  Is that fairly typical of old bikes do you think (or new ones, come to that)?

Alan

[MG]

I think BSA claimed about 35hp for the engine so if you are getting only about 22hp at the rear wheel, where has it all gone?

In BSA's marketing department 

[Alan @Ncl]

Plausible answer MG. It seems 12-13 hp (about 9.5kw) could provide an ample supply of hot air, even for a Marketing person!

Alan