AutoRail engines
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Author:  J.J.Horst [ Wed Mar 01, 2006 1:18 pm ]
Post subject:  AutoRail engines

I have always heard that Bugatti had 25 T41 engines made, and of course only 6 cars were build. The engines were later used in the AutoRail. Does this mean that the first AutoRails had double ignition-engines, as opposed to the later ones that had single ignition?

What further differences were there between the T41 and the AutoRail engines? Different compression ratio? Camshafts or other changes?

And last: How difficult would it be to upgrade an AutoRail engine to double ignition, or is this just impossible?

Author:  Herman [ Wed Mar 01, 2006 2:17 pm ]
Post subject: 

Something that I can remember, is that some prototype or prototype drawing had a double ignition, not with spark plugs side by side, but on both sides of the engine.

In your book is a picture of a T41 engine at Retromobile. If you can post the photo, we can at least see if there are visible differences...

Author:  J.J.Horst [ Wed Mar 01, 2006 2:34 pm ]
Post subject: 

That would be surprising, double ignition at two sides of the engine! All Twin Spark Bugattis have the ignition at one side.


See above the inlet / spark plug side of an Autorail engine.

Most obvious differences have to do with the external fittings, like the different inlet manifold / carburettor, and a different cooling system and exhaust. I'm especially interested in the more principal differences to the basic engine.

Anxious to learn more!


Author:  rivaaquarama [ Wed Mar 01, 2006 7:50 pm ]
Post subject:  Spark Plugs on Royale engines

Spark Plugs on Royale engines

Below a picture of the Train engine used for the Esders roadster as build at the Museum in Mulhouse. It is clearly visible that this engine has one spark-plug per cylinder… At least on the right side of the engine.


I think it is not so easy to add another plug on the same side where first one plug was located. The reason for this is that if a single plug is used, it will most probably be located in the middle of the cylinder. If another plug is added to the same side it will be placed outside the center…. and it will not fit with the same depth into the ignition room of the cylinder. An extra plug on the other side of the block could be an option…. What complicates this is that all wires going to the plugs should have the same length…. Otherwise you can have problems with the timing of the sparks…. A plug close to your outlet valve is also not that logical…. Of course depending on where the valves are located… If I remember correctly there is a drawing of the Royale engine in the book “The amazing Bugatti’s” Unfortunately I do not have this book and therefore can not make a scan…..

Author:  rivaaquarama [ Wed Mar 01, 2006 9:22 pm ]
Post subject: 


This picture I found in the Bugatti Magnum…. It is clearly visible that there are plugs on both sides of the engine….. Was this design ever build? Or was it only a prototype…. or one of Bugatti’s brain waves….

Here a picture of the engine in Royale #41111... Two plugs on the right side....

Author:  J.J.Horst [ Thu Mar 02, 2006 8:50 am ]
Post subject: 

Thanks Martijn!

I had never seen this drawing of the double ignition, with spark plugs on both sides of the engine! What is the designation of the drawing?

As we all know, the autorail engines have single ignition, the Royale T41 double ignition on one side, so may this drawing have been for the Prototype Royale engine, which was bigger (14.5 litre as opposed to the 12.7 litre for the "production"models) and may have differed in other ways?

As the T41 engine was derived from the T34 Aero engine, we may take some info from that. In the drawing below it seems (though difficult to see) that there is a 2nd hole for a spark plug, in between both banks. (so double inginition on opposite sides)


Maybe we can conclude that the prototype T41 engine had double ignition, on both sides, that probably this gave problems, maybe due to overheating as a result of the presence of the exhaust, and that a redesign was made with double ignition on one side, as we know it.

Author:  Herman [ Thu Mar 02, 2006 10:18 am ]
Post subject: 

The drawing that Martijn took from the Bugatti Magnum is the same as the one I was talking about.
If I recall correctly, it said "prototype Royale Engine".

Author:  Greg Morgan [ Tue Jan 13, 2009 4:14 pm ]
Post subject: 

Going back to that photo I found and gave to the Bugatti Trust, it shows the only known pic of the prototype engine. Although there are detail differences here and there, it is apparent that the prototype had two plugs per cylinder in the same manner as the production engines.
Whether the prototype had a larger capacity to the production cars is debatable- it certainly had the same wheelbase as the production car, contrary to what Bradley stated back in the day.
The legend that Ettore built 25 engines and did'nt know what to do with them is also I believe a story put about by Bradley in his biography on Ettore.
Personally I think that Bugatti only made six or seven engines with twin plug.
One thing that is interesting though is why it was decided that the Railcars would be fine with just one plug per cylinder- surely not just a cost cutting exercise?

Author:  Lazarus [ Tue Jan 13, 2009 5:01 pm ]
Post subject:  Royale motors

It is very bad practice to put the plugs on the cold side of the engine [ the induction side ] Keeping the plugs hot on the exhaust side is far better. Bugatti made this mistake with most of his engines.

Author:  Lazarus [ Tue Jan 13, 2009 5:03 pm ]
Post subject:  PS

At 186.000 miles per second it really does not matter if some ignition leads are slightly longer than others :lol: :lol: :lol:

Author:  J.J.Horst [ Tue Jan 13, 2009 5:03 pm ]
Post subject: 

Much easier to replace though!

Author:  Uwe [ Wed Jan 14, 2009 10:23 pm ]
Post subject: 


Author:  Greg Morgan [ Thu Jan 15, 2009 12:43 am ]
Post subject: 

How many plugs on that photo?

Author:  Uwe [ Thu Jan 15, 2009 11:55 am ]
Post subject: 

Is to be recognized very badly :(

Author:  Herman [ Thu Jan 15, 2009 9:16 pm ]
Post subject: 

I tried to enlarge the photo, and sharpen it a bit, but I am not 100% sure that it has 2 plugs per cilinder.

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