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Chaps,
I wouldn't get too excited about Holbay heads. Although they do have a better combustion chamber shape the smaller inlets ports are a real limiting factor (and opening them up to alpine size isn't really viable). A much better bet is to get a standard alpine head modified (just copy the combustion chamber shape to be similar to that on a Holbay head for a start). Actually I've got some pics i could post (if i could work out how) both of a Holbay head and of a Chirs Draycott fast road alpine head.
The cam in the holbay engine doesn't really have a great profile - there are much better ones out there that have oodles more torque at low speed and a more linear power delivery - I got one from Kent camshafts (UK), I know cat cams (UK) also do some profiles.
As for rejetting carbs, this is a fact of life if you are after performance increases. A trip to the rolling road is normally every penny. Even a stock motor (especially a 40 year old one) will normally see power gains from some gentle fettling.
I believe the Holbay engine was used in the Hillman Hunter that won the London to Sydney marathon (maybe even developed for it).
Almost there!
Current spec:
Engine rebuilt with +030 flat top pistons (needed to bump up CR with modified head - std hunter 1500cc pistons used from http://www.speedyspares.co.uk/
Lightened Flywheel: 8.5kg
Kent camshaft (rally spec)http://www.kentcams.com/
Fast road unleaded head: from sunbeam classic spares (Chris Draycott)
Quad motorbike carbs (yamaha R1) spaced by Bogg broshttp://www.boggbros.co.uk/
R1 fuel pump
Engine balanced
Gave 115bhp at 6000rpm with the bike carbs and 98bhp on strombergs.
but am only running 9.5:1 CR (unleaded fuel)
Still need to sort out air filter arrangement. More to follow when i get around to uploading some more photos (have worked out how to post them now).
Hi,
Nice modifications SunbeamSam! Congrats on the Webers too Jeff!
Sorry in advance for this long post in defense of the Holbay!
The original development work carried out by Holbay for the Rapier H120 and Hunter GLS resulted in the 1725cc engine producing around 122bhp. In this state of tune it wasn't thought tractable enough at lower speeds for a production car. To fix this the inlet port sizes were reduced to roughly the same size as the exhaust ports. This, along with a milder camshaft improved torque and drivability. The final production spec was 105bhp.
For what it's worth (and I am of course biased) I think the power delivery, drivability and economy (as well as reliability) from the Holbay spec engine is excellent. I have an all original 133,000 mile Rapier H120 that has never had anything more than the valve clearances adjusted. I've never had to touch the Weber carbs at all; it seems to always stay in tune as long as you keep the points well adjusted. My Dad (who I can't remember ever being without a Rootesmobile) has had various H120s too and hasn't had serious trouble with any (if you can discount a broken rocker post!).
Anyway, my point is that the Holbay set up might not produce the ultimate power, but it does give a significant boost and makes the car a joy to drive, even in heavy traffic. The Webers make a wonderful sound and give great pick up. I'm hoping the Holbay head, cam, carbs, pistons and distributor I brought back with me from Scotland will be a nice (reliable) improvement on my standard (1725cc) SIV set up. I just need to get the inlet manifold sorted out. I'm guessing that the 'off the shelf' (left hand drive) Weber manifolds that are available are designed for the standard inlet port sizes, not the smaller Holbay ones. I may have to modify the standard Holbay manifold. Any ideas? What manifold did you use Jeff?
Thanks Micheal!
I never thought of the master cylinder being the issue (probably because I don't have one on my car at the moment!). If it's possible to use a straight manifold then I can probably get away with modifying the Holbay one that I have (as I have seen done back in the UK).
I'm now wondering how I can get round the master cylinder issue. I wonder if it is just the reservoir that fouls? If so perhaps I could use a remote reservoir?
-Stuart