Ignition timing and the big valve engine

[iain.hamlton]

My understanding of the timing of big-valve weber engine is as follows:

Static timing 12 Degrees BTDC
Up to 1000 rpm: no additional advance - i.e. 12degs BTDC
1000-2500 rpm: 14 degs of advance fed in linearly giving a total of 26deg at 2500.
above 2500 rpm: no further advance. i.e. 26 degrees.

1/ Do I have this right?
2/ Why does the engine with strombergs have more advance?
3/ Would more advance above 2500 rpm give more or less power? I assume that the timing is optimum for the engine in standard form.

Let's assume the ideal fuel grade - i.e. the 101 octane the engine was designed for.

best regards, iain

[stuartgb100]

Hi,

Can only comment, if you can help with the following:

1. Standard ignition set-up (points, coil, etc)?
2. Standard cams and valves ?
3. What is the distributor number (stamped on the casing) ?

Sorry, but that last will entail a distributor removal!

Regards,

Stuart.

[iain.hamlton]

Thanks for the reply. standard points; standard d-type cams (two grooves); Z head with standard valves 1.56 inlet, 1.32 exhaust, and 41189A distributor- off the car so no problem. I have a 43d4 replacement on it now which is supposed to be same, but that's a long story. My interest is to get the advance right over the range where I use power, that's to say 2500 rpm and up. 26degree?

best regards, iain

[stuartgb100]

For the 41189A:

Static:...........12 degrees
Below 2000:..no advance
2500:............14.5
3000:............16.5
3500:............19.0
4000:............21.3
4500:............23.5
5000-6000....26.0 (max advance)

Miles Wilkins' book (LOTUS Twin-Cam Engine) states that "The current production Lotus/Lucas distributor model is 43D4, which is a direct replacement for 41189 and 41225 only."

However, he quotes a different advance curve for the 41225 !!! ?

As regards Octane ratings, he says:
"All the settings given may need slight adjustment to meet local fuel requirements and, because of the demise of 101 RM octane fuel, the domestic big valve setting should be decreased by 2 degrees, ie. the static set at 10 degrees and the maximum advance at 24 degrees. Use the S/E figures for the 41189A......"

The S/E figures for the 41189A:

Static:...........10.0
Below 2000:..no advance
2500:............12.5
3000:............14.5
3500:............17.0
4000:............19.3
4500:............21.5
5000-6000....24.0 (max advance)

Quoting again:
"The Stromberg engines required more advance, hence a 33 degree maximum (domestic), to overcome the deficiencies of the carburettor."

Quoting again:
"Maximum torque is 113 lb ft @ 5500 rpm."
"125-126 gross bhp @ 6500 rpm."

Regards,

Stuart.

[steveww]

We should get a partition together and see if we can get http://tdcperformance.ca/ to do a Lotus twinc version of their interesting product.

[steveb1]

You might find this interesting, they appear regularly.

http://cgi.ebay.co.uk/Lotus-Twin-Cam-Fo ... dZViewItem

[steveww]

Looks interesting but appears to still use springs and weights for the advance curve. The one from TDC uses electronics for the advance curve.

[iain.hamlton]

I notice the advance curve in Miles Wilkin's (otherwise?) excellent book differs from the workshop manual and what I have measured on both my original and current distributors. The question is not which is right, but which is best?

What curve is used in motorsport?

- Iain

[steveww]

I have had a reply back from http://tdcperformance.ca/ and they inform me that a Lotus/Ford twin cam version is under development.

[stuartgb100]

Miles Wilkins doesn't seem to mention fuel additives (at least, I could find no reference).

For those that do use an additive, the following may be of interest:

Castrol tell me that their ValveMaster Plus (when added in the correct proportion) has the following effects:

Unleaded 95 octane becomes approx 97 octane
Super Unleaded 97 octane becomes approx 98.5 octane
Super Unleaded 98 octane becomes approx 99 octane

Regards.