Lotus Elan

Twin-Cam Distributor variants, Advance Curve Data etc.

PostPost by: stuartgb100 » Wed Sep 14, 2005 8:19 pm

Hope you will not mind, but if it's going to be useful, it might be more easily accessed if it's in a new post with a more suitable subject heading.

Here's a sample of the Lucas ignition data which is specific to the Lotus Twin Cam engine, as opposed to the Elan family:

Quoting direct:

"No fewer than 5 different distributors were used on the twin-cam engine, all with varying degrees of advance. Since they all appeared to function on the same basic engine, how could they work correctly with as much as 12 degrees difference in advance between them? Unfortunately, once again, the various workshop manuals are misleading, and the earlier manuals are wrong.

Before unravelling the complexities, and explaining why different distributors were used, one particular aspect must be cleared up, namely that Lucas distributors 23D4 and 25D4 were current at the same time. The 25D4 had a vacuum retard mechanism, while the 23D4 did not. The early 25D4 (as stated in the workshop manuals) is not the same as the late emission/European 25D4. Distributors used by Lotus on the twin-cam engine are shown in the table below."

23D4 ref 40953 introduced 18/Sept '63 Ignition advance (Distributor degrees) 11-13 degrees. Fitted to Standard Weber, Elan, Lotus Cortina. All domestic (UK) Strombergs on Elan, +2, Europa.

23D4 ref 41189A introduced 6/March '67 Ignition advance as before 6-8 degrees. Fitted to all S/E engines on Elan, +2, Europa, L/Cortina S/E, Escort twin-cam, all domestic Big Valve engines.

23D4 ref 40930 introduced early '62-63 (ceased early '64) 12-13 degrees
1498cc T/cam, early 1558cc T/cam on Elan, L/Cortina.

25D4 ref 41125A introduced 3/Jan '68. 6-8 degrees. Federal exhaust emission Strombergs only -- all types, Big Valve Dellorto, domestic and european,ECE 15 regulations.

25D4 ref 41225 (vacuum retard removed). 6-8 degrees. European, late-model domestic Big Valve engines, pre ECE 15 regulations.

The current Lucas model is the 43D4, which is a direct replacement for the 41189 and 41225 only.

The static ignition-timing settings listed in the different manuals vary, but the true static settings are as follows:

1498cc/early 1558cc Weber (B type cam) dist ref 40930 7 degrees BTDC

All Weber standard (B type cam) 40953 12 degrees

All Stromberg domestic (UK) C/D type cam 40953 9 degrees

All S/E Weber (C type cam) 41189A 10 degrees

All Big Valve Weber/Dellorto domestic 41189A 12 degrees
(with a D type cam)

All Stromberg Federal emission 41225A 5 degrees
(with a C/D/E type cam)

All European specification Dellorto 41225A 5 degrees
to ECE 15 regulations
(with a C/D type cam)

All Big Valve Dellorto, domestic and 41225 10 degrees
European, pre ECE 15 regulations


Now to detail: the advance curves for all types. Please note that in the manuals, Lotus got confused between crankshaft rpm and distributor rpm. Distributor rpm is half crankshaft rpm; in some cases, static settings have been added, in others they have not! the following tables are the final settings, ie. static settings have been added, so maximum advance is exactly as it says..........."

to be continued ASAP.........

40953 Standard Weber (B type cam)

distributor
rpm
Below 500 ............ 12.0
Below 1000 .......... static, no advance
1000 .................... 17.6
1500 .................... 22.5
2000 .................... 28.0
2500 .................... 28.8
3000 .................... 29.8
3500 .................... 30.6
4000 .................... 31.5
4500 .................... 32.5
5000 .................... 33.5
5500 .................... 34.5
6000 .................... 35.5
6500 .................... 36.0
....... maximum advance


40953 All Stromberg Domestic (C/D type cam)

distributor
rpm

Below 500.............. 9.0
Below 1000 static,
no advance
1000 ..................... 14.6
1500 ..................... 19.5
2000 ..................... 25.0
2500 ..................... 25.8
3000 ..................... 26.8
3500 ..................... 27.6
4000 ..................... 28.5
4500 ..................... 29.5
5000 ..................... 30.5
5500 ..................... 31.6
6000 ..................... 32.5
6500 ..................... 33.0
maximum advance


41189A ................All S/E Weber........................All Big Valve
distributor ............ (C type cam) ......................Weber/Dellorto
rpm .................................................................(D type cam)

Below 2000 ................. 10.0 ................................... 12.0
................................. static, ................................ static,
............................. no advance ......................... no advance
2500 .......................... 12.5 ................................... 14.5
3000 .......................... 14.5 ................................... 16.5
3500 .......................... 17.0 ................................... 19.0
4000 .......................... 19.3 ................................... 21.3
4500 .......................... 21.5 ................................... 23.5
5000-6500................... 24.0 ................................... 26.0
............................. maximum advance ........... maximum advance


41225 ................. European Dellorto only *
distributor ............... (C/D type cam)
rpm

Below 1000 ................ 10.0
................................. static,
................................. no advance
2000 .......................... 15.0
2500 .......................... 17.4
3000 .......................... 19.6
3500 .......................... 21.8
4000 .......................... 24.2
4500 .......................... 26.6
5000-6500 .................. 29.0
............................ maximum advance

* Also some very late domestic (as a replacement for 41189).


41225A ................. All Federal Stromberg
distributor .............. and European ECE 15 spec.
rpm ....................... (C/D/E type cam)
.............................. Vacuum retard capsule fitted

Below 2000 ................ 5.0
.............................. static,
.............................. no advance
2500 .......................... 7.5
3000 .......................... 9.5
3500 .......................... 12.0
4000 .......................... 14.3
4500 .......................... 16.5
5000-6500 .................. 19.0
............................ maximum advance

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 settings should be decreased by 2 degrees, ie the static set at 10 degrees and maximum advance at 24 degrees. Use the S/E figures for the 41189A European and domestic 41225 may also require dropping back to 8 degrees static, 27 degrees maximum. Do not reduce the static setting by more than 2 degrees on any distributor.

NB: Camshaft identification:

B-type (standard) no groove on cam
C-type (S/E) one groove
D-type (Super S/E, Sprint) two grooves
E-type plain

To be continued.......
stuartgb100
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PostPost by: stuartgb100 » Thu Sep 15, 2005 5:42 pm

Hmm ........ no real point in continuing this.

Have just discovered that I'm quoting from a book that's still in print
(I thought it was out of print, having bought mine in 1988).

This being the case, I had better promote it ........

The title is "LOTUS Twin-Cam Engine"
Author: Miles Wilkins

It is simply excellent. Very detailed stripdown and rebuild sections, with
many (albeit black and white ) photos.
Also much history etc.

Sadly, I do not believe the 'bible' I used to use when rebuilding TC's IS still available:

"Tuning Twin Cam Fords" by David Vizard, published by Speedsport.

..... but if anyone knows differently, it's well worth a look.

NB: The data for 800-odd distributors is from a completely different book
also (I believe) out of print.

And Miles' book seems to be listed in your book's section.

Could have saved a few hours typing there!

Regards.
stuartgb100
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PostPost by: neurosur » Sat Nov 09, 2013 4:52 pm

Miles Wilkins' excellent book has been reprinted and is around ?25 new, but there is an error that has carried through all the book editions.

In the detail advance curves for all types of distributor Miles' book gets confused between crankshaft rpm and distributor rpm just as the Lotus manuals do. Distributor rpm is indeed half crankshaft rpm and the following tables reproduced from the book are all wrongly headed "distributor rpm".

The tables list crankshaft rpm up to 6500. A "distributor rpm" of 6500 would destroy a twin cam engine (crankshaft rpm would be 13000).


40953 Standard Weber (B type cam)

distributor
rpm
Below 500 ............ 12.0
Below 1000 .......... static, no advance
1000 .................... 17.6
1500 .................... 22.5
2000 .................... 28.0
2500 .................... 28.8
3000 .................... 29.8
3500 .................... 30.6
4000 .................... 31.5
4500 .................... 32.5
5000 .................... 33.5
5500 .................... 34.5
6000 .................... 35.5
6500 .................... 36.0
....... maximum advance


40953 All Stromberg Domestic (C/D type cam)

distributor
rpm

Below 500.............. 9.0
Below 1000 static,
no advance
1000 ..................... 14.6
1500 ..................... 19.5
2000 ..................... 25.0
2500 ..................... 25.8
3000 ..................... 26.8
3500 ..................... 27.6
4000 ..................... 28.5
4500 ..................... 29.5
5000 ..................... 30.5
5500 ..................... 31.6
6000 ..................... 32.5
6500 ..................... 33.0
maximum advance


41189A ................All S/E Weber........................All Big Valve
distributor ............ (C type cam) ......................Weber/Dellorto
rpm .................................................................(D type cam)

Below 2000 ................. 10.0 ................................... 12.0
................................. static, ................................ static,
............................. no advance ......................... no advance
2500 .......................... 12.5 ................................... 14.5
3000 .......................... 14.5 ................................... 16.5
3500 .......................... 17.0 ................................... 19.0
4000 .......................... 19.3 ................................... 21.3
4500 .......................... 21.5 ................................... 23.5
5000-6500................... 24.0 ................................... 26.0
............................. maximum advance ........... maximum advance


41225 ................. European Dellorto only *
distributor ............... (C/D type cam)
rpm

Below 1000 ................ 10.0
................................. static,
................................. no advance
2000 .......................... 15.0
2500 .......................... 17.4
3000 .......................... 19.6
3500 .......................... 21.8
4000 .......................... 24.2
4500 .......................... 26.6
5000-6500 .................. 29.0
............................ maximum advance

* Also some very late domestic (as a replacement for 41189).


41225A ................. All Federal Stromberg
distributor .............. and European ECE 15 spec.
rpm ....................... (C/D/E type cam)
.............................. Vacuum retard capsule fitted

Below 2000 ................ 5.0
.............................. static,
.............................. no advance
2500 .......................... 7.5
3000 .......................... 9.5
3500 .......................... 12.0
4000 .......................... 14.3
4500 .......................... 16.5
5000-6500 .................. 19.0
............................ maximum advance
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