Ignition timing in degrees

[Sakura]

I want to ignition time my RGS with a timing disc. I believe there are computer programs to change a linear measurement to degrees but numbers and computers are not my forte. Spec is: Carillo con rods -6.5" between centres, 9:1 comp ratio and I will run on 99 octane.
My guess is 35/36 degrees? I would be grateful if someone runs thenumbers. TIA.

[chaterlea25]

Hi Sakura,
I found that 33/34 degrees is a good figure to use, especially because of the (crap)fuel quality available
I seem to recall that the BTDC figure equals 36 degrees ?
relying on a rapidly fading brain

John

[Sakura]

Hi Sakura,
I found that 33/34 degrees is a good figure to use, especially because of the (crap)fuel quality available
I seem to recall that the BTDC figure equals 36 degrees ?
relying on a rapidly fading brain

John

Thanks John,
I think the con rod length seems to have an influence on timing. 33/34 degrees is a fairly common figure on several engines and probably won't cause auto ignition (pinking), which, of course, is very destructive. I'm no longer after peak performance, but I would like to think it's timed optimally.

[JulianS]

The BSA factory data book for 1963 gives 33 degrees for the RGS.

If your Carillo rods are 6.5 inches between centres, like the R and R rods, then they are 40 thou (1mm) longer than the original BSA rods and the Thunder Engineering rods (6.469 inch centres) which will increase the compression ratio.

[limeyrob]

After a lot of reading or period and current advice and mindful of modern petrol I settled on 33 deg.  I used both a deg wheel and vernier and I think (arrgh memory) it was 5/16"

[RichardL]

Sakura,

You should check out both of these topics:

https://www.a7a10.net/forum/index.php?topic=2710.msg17772#msg17772

At the above you will find this:

1951 BSA A10  ZA10 8083         
Piston position in degrees BTDC, and inches         
Measured using degree wheel on crankshaft, and dial indicator on piston (head removed)         
         
BTDC (in inches)         Degrees
Fraction   decimal     
 9/32       0.281                31
 19/64     0.297                32
 5/16       0.3125              32.6
 21/64     0.328                33.3
 11/32     0.344                34
 23/64     0.359                35
 3/8         0.375                36
 25/64     0.39                  37
13/32      0.406                37.2
27/64      0.422                38
7/16        0.4375              39
29/64      0.453                40

Then, there is the significantly interesting dynomometer results (even if not for your specific engine) found here:

https://www.a7a10.net/forum/index.php?topic=1292.0

Richard L.

[Sakura]

The BSA factory data book for 1963 gives 33 degrees for the RGS.

If your Carillo rods are 6.5 inches between centres, like the R and R rods, then they are 40 thou (1mm) longer than the original BSA rods and the Thunder Engineering rods (6.469 inch centres) which will increase the compression ratio.

Hi Julian,
      I am aware the Carillo rods are 1/32" longer than A10 standard. I queried this with Fred Carillo in 1990 and his reply was :"everybody wants more compression".  Well actually I didn't and had to replace the 9-1 pistons with 8-1 to stop pinking but they were bought and paid for. Also, there was no left hand bleed hole for the left side. His reply was that the rods were narrower and allowed enough splash. I never had a problem in 8k miles so that was probably correct. Today's rods are possibly better but Carillos were the best then.

[Sakura]

Sakura,

You should check out both of these topics:

https://www.a7a10.net/forum/index.php?topic=2710.msg17772#msg17772

At the above you will find this:

1951 BSA A10  ZA10 8083         
Piston position in degrees BTDC, and inches         
Measured using degree wheel on crankshaft, and dial indicator on piston (head removed)         
         
BTDC (in inches)         Degrees
Fraction   decimal     
 9/32       0.281                31
 19/64     0.297                32
 5/16       0.3125              32.6
 21/64     0.328                33.3
 11/32     0.344                34
 23/64     0.359                35
 3/8         0.375                36
 25/64     0.39                  37
13/32      0.406                37.2
27/64      0.422                38
7/16        0.4375              39
29/64      0.453                40

Then, there is the significantly interesting dynomometer results (even if not for your specific engine) found here:

https://www.a7a10.net/forum/index.php?topic=1292.0

Richard L.

Thanks Richard,
         All that info is based on the unit engine, different in several ways to the RGS engine. However, an interesting read. In the event of no definitive reply, I'll probably settle on 34 degrees

[RichardL]

Sakura,

You should check out both of these topics:

https://www.a7a10.net/forum/index.php?topic=2710.msg17772#msg17772

At the above you will find this:

1951 BSA A10  ZA10 8083         
Piston position in degrees BTDC, and inches         
Measured using degree wheel on crankshaft, and dial indicator on piston (head removed)         
         
BTDC (in inches)         Degrees
Fraction   decimal     
 9/32       0.281                31
 19/64     0.297                32
 5/16       0.3125              32.6
 21/64     0.328                33.3
 11/32     0.344                34
 23/64     0.359                35
 3/8         0.375                36
 25/64     0.39                  37
13/32      0.406                37.2
27/64      0.422                38
7/16        0.4375              39
29/64      0.453                40

Then, there is the significantly interesting dynomometer results (even if not for your specific engine) found here:

https://www.a7a10.net/forum/index.php?topic=1292.0

Richard L.

Thanks Richard,
         All that info is based on the unit engine, different in several ways to the RGS engine. However, an interesting read. In the event of no definitive reply, I'll probably settle on 34 degrees

Sakura,

Not sure what you are saying. Are you saying that the above information is for a unit A65, or a plunger with the gearbox mounted to the crankcase? If the former, that flies in the face of Orabanda (who created this list), who must own about 10 A10s and built more. If the latter, the plunger A10 engine and the RGS engine share the same rod centers and stroke, so this list should apply to both. I would agree that it would be off for your Carillo rods.

If I'm wrong, someone (maybe you) will let me know. Always glad to learn, but I tried to do the right research when I read your response.

Richard L.

[Sakura]

The unit A10 engine was all iron, whereas the RGS engine was stronger, alloy head and hotter cam. I imagine that might allow for more advance?

[limeyrob]

The advance is a matter of how early the spark has to fire to get the most power out of the combustion so while the fuel can be igniting before TDC the peak pressure to do work needs to be on the power stroke.  Flame speed goes up with pressure but its always a flame never an explosion.  Fuel makes a big difference and so does combustion chamber shape, a bad shape needs more advance.  Across all BSA engines the combustion chambers are pretty similar on the OHV engines. I suspect for an A10 an iron head with bigger valves is the strongest set up.
I can't find the source but see often quoted that Eddie Dow recommended 5/16" (about 33 deg) and I've gone with that.  My engine has the 356 cam and 9:1 (or thereabouts) and ally head and I'm expecting good power at modest revs so there's no benefit in pushing too much advance.  30mm carb, 3" intake stack and 2:1 so I'm hoping for decent cylinder filling.
I read recently that touring cars run about 30 deg and F 1 about 50 deg but F1 is at 18,000 rpm, for a road going A10 we want good power and combustion from around 2000 - 4500 with a wide ratio 4sp box.  I run manual advance so I don't want to be making find adjustments in the 3000 - 5000 rev range, I want to get to full advance and stay there.  Another reason to go for a 5/16 / 33deg setting.

[Jules]

thats a good document you sorted out Richard  but surely the piston to deck dimension will depend on your particular engine build characteristics??
eg that listing may work for a standard GF engine build, but once you add in piston height variation (ie Compression ratio), deck height variation (ie after machining) and now conrod length, I'm not so sure of its relevance to other engines??

[RichardL]

I think there is some confusion in this discussion between engine performance for spark at a particular BTDC and the basic geometry of the stroke. It really does’t matter if the piston is one inch tall or three feet tall. The chart describes the relationship between rotational angle of the crank and the position of the top of the piston with respect to that angle of rotation.

[RichardL]

One more thing. Please don’t give me any credit for the work that went into creating the degrees chart or expertise in applying it. Our member Orabanda developed the chart in conjunction with his extensive dynamometer testing under various conditions, including spark BTDC. One would learn a lot reading his 1186 posts, but, particularly, those found at the two links I posted earlier.

[limeyrob]

Richard, that's the way I see it.  Its 5/16" from TDC for 33deg regardless of the height of the piston crown from the barrel top.  Even rod length / gudgeon pin variation won't make much difference as there only a limited amount they can be changed.  The rod length has the most effect in the 90 - 45 range but by the time the piston is at 5/16 / 33deg the rod is heading for vertical and at 0 / TDC the rod length has no effect on timing.