Bristol's Inline-6 - What was possible?

[Robby B]

Read of the impossibility for Bristol’s 1971-2216cc versions of the BMW M328 engine to be overbored to around 2.5-3.0-litres on an old thread, yet how plausible would it have been for the Bristol engine to have at least been viably overbored to around 70-72mm for approximate capacities of 2244-2443cc or was 69mm bore the absolute limit for the production Bristol engine in 2216cc guise?

The section on the BMW M328 engine in Dr Karlheinz Lange’s BMW – The History of the Engines books does not say much apart from BMW conducting experiments with direct fuel-injection and three butterfly throttles replacing the carburettors (which has been touched upon in some Bristol Cars books), along with BMW developing a 120-145 hp Twin-Cam 2-litre (72mm bore x 82mm stroke) seven main bearing successor under the M318 designation that only reached the experimental stage before being canned as due to the onset of WW2. There were other inline-6 projects BMW looked at to fill the gap between the 2.0-litre M328 and 3.5-litre M335 engines displacing around 2.5-2.6-litres before they too were also abandoned due to either cost or time.

What is of particular interest would be Bristol at one time considering the larger 212kg BMW M335 6-cylinder engine alongside the smaller 160kg BMW M328 6-cylinder engine for possible use in the Bristol 400, albeit reduced to 3-litres with potential for 140-160 hp in sporting form and over 100 hp in a soft state of tune. Frazer Nash themselves were said to be particularly keen on the pre-war BMW 335 saloon, whereas Bristol preferred the BMW 327 grand tourer that became the starting point for the Bristol 400.

However whether because of the extra 50kg weight penalty and any size difference of the M335 over the M328 engines and their potential impact on the Bristol 400 (unlikely given the later adoption of the Chrysler V8 engines) or due to other factors, Bristol ultimately decided not to pursue this vision that had it occurred could have potentially changed the face of the British Motor Industry.

[Stefan]

We’ve never found any evidence in the archives about Bristol considering buying in a larger BMW unit back in the day. ( the 2 litre was developed under the prewar licence agreement with AFN).
And the centre 2 cylinders of the Bristol 6 are already perilously close together, risking gasket failure there if overbore was tried ( though someone did race without liners for a few more ccs)
Bristol had a well planned path towards bigger engines with their own design 3 and 3.65 litres twin cam all alloy engines which was far into development - and one was even destroyed on the test bed.
The Chrysler was (successfully) chosen at short notice when the cost of further development of Bristol’s 160/220 project couldn’t be met, and one Armstrong unit was tried.
One version has it that an auto gearbox ordered by Filton for testing arrived with a Chrysler v8 attached though I don’t know the source.
Yours in Bristol
Stefan and the Bristol Owners Heritage Trust.

[Robby B]

Thanks for clearing things up about the Bristol version of the M328 engine.

The reference to the pre-war BMW M335 and its engine can be found in Chapter 5 of Christopher Balfour's book, however the footnotes come from the Frazer Nash Archives with the relevant section being from George White's 1946 report.

White himself emphasized this was not thought out in detail though wanted to show H.J Aldington that he was development minded, yet he did not reject H.J's plea for the larger engine it was just undermined by external influences ranging from government directives, tax increases, the challenges of the still connected aircraft side that all served to thwart the proposed timetable which included a 3-litre version of the M335 engine.