DX6A Distributor advance curve - 401

[p5b401]

Restoring 401 and have decided to add an ignition ecu. I've done this on several other cars and am well into the computer and electrical side of it.
I'll be locking down the mechanical advance weights, converting the points to electronic and adding vacuum advance to the ecu which effectively eliminates the 'I' knob on the dash.

What I'm hoping to find are details on the advance curve other than 5 degrees static and 13 degrees max advance - presumably at 3500 rpm as this will be the safest start to the tuning.

John

[Julian Caples]

Hi John

Here's a starting point
Static timing set to 8 degrees BTDC (Crank)
Initial advance starts at 1000 RPM
16 degrees at 2000 RPM (Crank)
26 degrees at 3000 RPM (Crank)

aim for a total advance of 34 degrees (26 plus the original 8)
some engines do better with a bit less static (say 6 degrees) some with a bit more (up to 10) anything more than 10 is excessive

the Bristol 'I' knob only retards the ignition - it was designed for use with post-war pool petrol and other dubious fuels in europe (ie to compensate for really bad fuel) my advice is disconnect it - it only leads to trouble (because people want to play with it)
a vacuum advance will not replace the 'I' control - it should only provide additional advance under partial load conditions
be very careful with a partial load advance feature the engines are sensitive to over advance and you can burn out pistons quickly

what ECU/programmable unit are you using?

regards

Julian

[Geoff Dowdle]

John,
I don't totally agree with Julian's comments .
I believe the only way to find out the advance curve needed for a particular engine is to dyno tune it on a "rolling road " . Assuming the mixture is correct through the range you plot your advance curve by adjusting the advance to get maximum BHP at different rev ranges .
Starting at 1000 rpm in 500 rpm increments up to 5000rpm
Then build that advance into the distributor or electronic unit .

In my experience with many Bristol sixes over 40 years I've found the following curve to work well .
Static/idle 10 -12 BTDC
3000 rpm 36 BTDC
5000 rpm 43 BTDC
Most 85 series engines are now running 8.5 comp pistons with material shaved off head & block giving 9.0 plus .
Cams are often upgraded to later 405 type
In Australia most owners run their cars on 98 octane Unleaded fuel .
All these variables will effect the amount of advance needed.

If you want your engine to have the correct advance curve the only accurate way is to dyne tune it .

Standard 400 & 401 distributors had 18 - 19 deg advance (that is 36 -38 deg at the crankshaft plus the static gives between 43 - 45 at max rpm)

When I last dyno tuned my 400 with a reasonably standard spec engine , the operator wanted to start at 5 BTDC (it would barely run ) , he did not want to exceed more than 30 deg BTDC . It kept making BHP up to 43 deg BTDC at 4700 rpm . The carbies are jetted correctly with a mobile gas analyser , it does not over head , idles and performs very nicely.

I'd be interested to hear of others experiences .

Geoff

[p5b401]

I really appreciate the info given talking more real world than the little info I found on line and in the workshop manual.

I acquired this car in 2004 and have only recently started the restore now I'm retired. The previous owner said he had the engine reconditioned and carbys rebuilt around 2000. I knew the garage owner where the work was carried out and he confirmed the work had been done but he has since passed away and there is no record of what deviation from std anything was (bore sizes etc).

Geoff, you mentioned some people used 98 octane but given these cars were made when UK petrol was 92 at the very best. I say this because I had intended to use 91 which I would think would be the manufacturers original spec given the available fuel conditions.

Having said all that, yes a dyno is the ultimate answer but for now I just want to get a good starting point, then tweak it a little from there until the motor runs nicely.

John

[Julian Caples]

I agree that the best way to find the best curve for any given engine is a dyno

My suggested curve was a starting point and "safe" - you won't burn piston crowns at partial load

a curve that continues to rise to 5000 rpm seems excessive with modern fuel but YMMV

English fuel in period (1946-1951 or so) was very poor - octane 70-72 (and even poorer in some european countries) - hence the 7.5:1 CR initially.

any modern fuel exceeds that level of anti-knock

Regards

Julian

[Geoff Dowdle]

John,
Where are you located Perth , Western Australia or Perth UK ?

I suggest you do a compression test , that will give you a idea as to the compression ratio . Unless the original 7.5 pistons were used again ,which is unlikely after 50 plus years , my guess is the engine is probably fitted with 8.5 pistons with material off the head & block so could be running as high as 9 - 9.5 .
I don't like running my lawnmower on 91 octane fuel besides a Bristol.
If you are in Perth WA a compromise would be a minimum of 95 octane without any ethanol. For the limited driving I do each year (most of it is spirited country runs) the extra cost of 98 is not a consideration.

Julian's curve is a good starting point but with my experience on the dyno it needs a lot more than 34 degs if you are interested in taking the revs above 3000 rpm.
---------------------------------------------------------------------------
The Lucas advance curves I have are

Part# Max Range Int Range Adv Start ECM Curve
400/1 47-50 40109 18 -19@2250 8-10@1200 300-450 369

401 51-52 40235A/B 12.5-13.5@1750 5.5-6.5@1000 600-680 471
403/5 40345A 17 - 19@2300 8-10@1450 325-450 520
These advance figures are distributor degrees & revs and must be double for equivalent in engine revs .

------------------------------------------------------------------------------
Have you had the engine running ?
I suggest you get the car restored and running on the Lucas points distributor .
You probably have more important jobs to do during the restoration that will eat up your cash.

I assume your proposed ECU is negative to earth , so consider problems with your positive to earth radio & clock if fitted.

I have been using fully adjustable Mallory twin point distributors for over 30 years without a problem . The bob weight stops are adjustable and colour coded springs of various lengths & tension are readily available to get the curve required . On my distributor testing machine they will run up to & over 6000 engine rpm and still work perfectly . The 60 year old worn or new Lucas are a different story.
I think Julian has a modified Bosch distributor that work well but are not easily adjustable if required.
To me the beauty of a points distributor is that they can be adjusted/repaired on the side of the road if there is a problem.
When your ECU packs it in you can not move ,except on the back of a tilt tray truck .

Geoff

[p5b401]

I didn't see you last reply from July - which is when my PC went bung losing email and all sorts.
Many thanks for the advice, experience is always appreciated. The car was fitted with a Mallory twin point distributor and had a spare good conditions lucas in the box of parts which came with the car.
Given the probable lack of roadside technical support for these cars I've left the points distributor in place but am still running an ecu for timing purposes. In the unlikely event that the ecu fails and simple concealed switch will revert to standard point action and although there will be no mechanical advance, it will at least get me home.

John

[Geoff Dowdle]

John,
I have been running a Mallory twin point distributor in my 400 for 30 years without a problem . I can not remember but maybe one set of twin points in that time & parts are readily available from most spare parts shops. They are very reliable probably more so than a ECU .

I fitted a second hand Mallory to a friends 403 which he did not touch (even remove the cap ) for about 15 years and about 60,000 miles .

The advance curves of these Mallory distributors are fully adjustable and the twin points gives a greater dwell angle and better spark . I have tested them to 7000 rpm without any variation of the spark .
Most Lucas distributors (even new ones) on the same testing machine show points bounce between 3500 - 5000 rpm . They are very erratic and poorly made .

Geoff