----- Original Message -----
From: Erik Quackenbush <erik@midwestfilter.com>
>
> The 1147cc engine used in the Herald 1200 produced 39 horsepower, had a >
>single 30mm Solex carburetor, 8:01 compression, and a mild camshaft.
>
> The engine used in the Herald 12/50 produced 51 horsepower, had 8.5:1
> compression, and used used the same carburetor and camshaft.
This is where the difference lies. The early Herald 1147 unit had a camshaft
with 12-52-52-12 timing, whereas the 12/50 unit used the Spitfire's 18-58-58-18
cam.
>
> The Spitfire Mark 1 had twin SU HS2 carburetors, 9.0:1 compression, and a
> hotter camshaft to produce 63 horsepower.
>
> The Spitfire Mark 2 got a new exhaust manifold and a higher compression
> ratio (9.75:1? 9.5:1?) which gave it 67 horsepower.
>
> At some point the base Herald engine was increased to 48 horsepower with
> 8.5:1 compression.
>
> What I can't figure out is when this change (39->48hp) happened.One source
> says it happened at engine #GD001, one says GA80,000, and another says
> GA190,000.
The camshaft change happened first at GA164890HE, but was not universal - the
workshop manual lists several change points backwards and forwards between the
2 specifications - I'm happy to mail you a scan of this page off-list. The
compression ratio was increased to 8.4:1 at GA190224HE, but again was not
universal. The workshop manual does not show any increase in bhp at the
camshaft change, but shows the increase from 39 bhp to 48 bhp occurring with
the compression ratio increase. Other sources (not to hand at present) have
quoted an intermediate 43 bhp output, which we can logically presume was the
8.0:1 compression ratio in conjunction with the 18-58-58-18 cam. I'll try and
locate the source of that information.
> What were the other differences between the 51, 48, and
> 39 horsepower engines? Did the valves get bigger? Did the exhaust manifold
> change?
The 39 and 48 bhp engines used nominally the same manifold arrangements, though
there were changes in the carburation early on which did not affect the
specified power output. The 48 bhp engine was identical to the 51 bhp (12/50)
unit with the exception of the manifold which had a longer and less angular
form on the higher output engine. The exhaust downpipe was correcpondingly
different. All except the earliest US export 1200s were apparently fitted with
the 12/50 manifold and exhaust as standard.
> Rimmer only shows one part number for the manifold gasket so I
> don't think the port size changed but who knows.
The 12/50 has a larger bore at the manifold/downpipe connection so a
correspondingly larger gasket is used - this is the same gasket as used on the
948 Twin Carb engine.
Hope that helps rather than confuses further,
Cheers,
Bill.
--
Rarebits4classics
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