DMBZ6A 3.8 Distributor Vacuum Advance 54412348

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rfs1957
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#1 DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by rfs1957 » Mon Jan 11, 2021 1:57 pm

I was looking this up to work out what a true original distributor "map" for the MegaJolt would look like.

It will be of no immediate interest to anyone, but as I have the info fresh to hand I thought I would put in on the Forum for anyone who ends up searching for it in the future.

Image

The DMBZ6A distributor is shown (in the Bentley Publishers copy of the original ?) as having a vacuum-retard specification 54410415 ; this is the internal suck/advance specification of the 40617A vacuum-unit itself, which translates into the corresponding "degrees-per-inches-of-suck" figures which are given alongside, see the table.

Martin Jay, the Distributor Doctor, is puzzled by that number 54410415, as his Lucas info for that period gives 54412348 ; the latter is referred to as a 7-14-8 which means that between 7" and 14" of mercury, the vacuum advance rises from 0° to 8° of distributor movement, which is 0° to 16° at the crank.

Image

He suggests that these units are only ever linear, as they are just a diaphragm operating against a spring, which makes Jaguar's "curved" specification rather odd ; I have plotted the points given in the Bentley book for the 54410415 in pink, and the 54412348 in green - forgive the scribbles, some of this was done on the 'phone.

The Bentley figures are not that different, at their lower end, but would potentially have given 2 or 3 degrees more for a given manifold pressure.

If anyone can throw in any other sources of data for this, please join in.
Last edited by rfs1957 on Mon Jan 11, 2021 2:12 pm, edited 1 time in total.
Rory
3.8 OTS Cream 877393 Built May 28th 1962
1978 Mini Van
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#2 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by mgcjag » Mon Jan 11, 2021 2:08 pm

Hi Rory...im confused by your vacume unit number 54410415.....in your text you quoting the same number and not the 2438 number you have on your graph needs editing i think....Steve
Steve
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#3 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by rfs1957 » Mon Jan 11, 2021 2:13 pm

Well spotted - think I've corrected it ??
Rory
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#4 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by mgcjag » Mon Jan 11, 2021 2:19 pm

Hi Rory...3.8 vacume curve/graph came up recently on this 123 thread...Christopher Storey gave some goog calculations.....worth looking at and comparing to yours....see post 75& 78 http://forum.etypeuk.com/viewtopic.php? ... f&start=60
Steve
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#5 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by rfs1957 » Mon Jan 11, 2021 2:43 pm

Yes, Christopher's numbers are indeed the same as my pink curves, and as the book.

What is curious is that Martin Jay has no record of that Lucas reference, and that by all accounts the suck/advance curve cannot be a curve but only a straight line - unless there were two springs in series ??

Maybe the "curve" was a reflexion of what the engine would tolerate.
Rory
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#6 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by mgcjag » Mon Jan 11, 2021 2:52 pm

Hi Rory..i just looked at the 3.8 parts catalogue in the knowledge base section. ....it shows the DMBZ 6A uses the 415 vac unit from engine R1001-R9999 and RA1001-RA1381........after these numbers the 22D6 with vac 54415894....so parts catalogue does confirm service manual........you could search the bulletins for any changes...Steve
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#7 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by mgcjag » Mon Jan 11, 2021 3:01 pm

Hi Rory....have a look here....British vacume they give an explanation of the 2 vac unit numbers 415 and 348 https://www.britishvacuumunit.com/services.html
Steve
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#8 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by rfs1957 » Mon Jan 11, 2021 3:49 pm

What an oracle you are.
Fantastic.
Will delve deeper.
Rory
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#9 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by rfs1957 » Fri Jan 15, 2021 9:10 pm

Yes, I had an interesting exchange with BritishVacuumUnit in the US.

Usual friendly, enthusiastic, and reactive, US approach.

According to them both 54410415 and 54412348 had indeed exactly the same behaviour, being 7" - 14" - 8°, and were just different shapes and had different fittings.

0 advance until 7" of mercury "suck", then rising linearly to 8° at 14" of "suck".
Rory
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#10 Re: DMBZ6A 3.8 Distributor Vacuum Advance 54412348

Post by mgcjag » Fri Jan 15, 2021 9:28 pm

Nice one Rory.....it all adds to the knowledge........Steve
Steve
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