11-08-2021, 03:04 PM (This post was last modified: 11-08-2021, 03:46 PM by Mike Costigan.)
Looking at the technical specifications published in sales brochures over the years, the official power output of the Seven is given as follows:
1922-23 10bhp at 2,400 rpm (696cc engine)
1923-32 10.5bhp at 2,400 rpm (747.5cc engine, up-draught carb)
1933-35 10.5bhp at 2,400 rpm (side-draught carb and revised exhaust manifold)
1935-36 13.5bhp at 3,000 rpm (presumably higher-comp 1A684 cylinder head)
1936-38 17bhp at 3,800 rpm (3-bearing engine with high-comp cylinder head)
Which raises a few interesting questions (well, it does to me!).
Did the 7.5% increase in engine capacity in 1923 really only provided a 5% increase in power?
Doesn't the 1933 side-draught carb and associated manifold not provide a bit more power?
If the earlier engines are rev'd to 3,000rpm as quoted for 1935-36, how much more power is achieved?
Similarly, if revs are increased to the 3,800rpm used for the 3-bearing engine, how much more power do the earlier engines produce?
Mike, if you recall a couple of years ago I did an analysis of the variations between a collection of Low Compression heads dating from 1923, there is quite a difference in the combustion chamber area as well as the cross section of the transfer port depending on what year and casting type. Obviously Austin were messing with things over the years, quite why seems strange because some versions would theoretically perform better than others, even if only subtle variations. I think the quoted outputs are likely simply a marketing tool but it would be interesting if one could be bothered to carry out dyno tests on otherwise identical engines today. Probably all somewhat academic and of little real time use to most seven owners today but still something one or two of us may find interesting.
11-08-2021, 09:20 PM (This post was last modified: 11-08-2021, 09:21 PM by Bob Culver.)
I raised this question when I first discovered this Forum years ago and suggested that all the engines to the 3 bearing must have about the same max bhp based on performance as per contemporary road tests and present day owners (although all could have been non standard). Such heresy from a rank outsider, and colonialist to boot, caused a furore (rivalled only by my later observations about lapping)!
RN updraught seem to at least match the RP and often better the more streamlined early Ruby! There was a cr increase in 1933 but the top end performance may have been reduced by more constricted breathing. (Alan supplied bhp figures for about 1935) The Williams Specials book lumps them all together at 13 bhp but the publication is riddled with errors.
At the time the makers may have considered it more prudent to quote hp at then normal operating revs and not as driven by men young in the 50s and 60s.
I have a bhp curve for a late marine engine but this may not exactly match the car version..
(11-08-2021, 09:32 PM)Dave Mann Wrote: Given that you run your motor at 50 mph Mike I think it's producing more than 10.5bhp at 2,400 rpm (747.5cc engine, up-draught carb).
Yes, Dave, that's my point: 50mph is approx 3,200 rpm, so is my 10.5bhp engine actually producing something like the 13.5 bhp at 3,000 rpm of the 1935 engine, in which case what was the advantage of the side-draught carb and 'better' cylinder head? More significantly, I regularly take the car to 30-35mph in second, which equates to 3,500-4,000 rpm, so the engine is probably developing 14-15 bhp, not that much less than the 3-bearing high-comp engine!
When engine power was discussed previously, I graphed various figures quoted and came to the conclusion that power output was approximately linear with RPM (within rounding errors and the extra from a bit of additional compression and breathing) for all normally aspirated engines. The Jamieson engines were something else.
This is really interesting - are these at the flywheel? Has anyone with the earlier models validated the RPM that the engine 'feels' to peak at using the good old 'butt dyno'?
Its usually a combination of Cam profile, inlet and exhaust profiles that shape the power curve. I cant imagine a '22-'34 engine topping out so early, especially given Mike's response.
