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spyder
United Kingdom
14 Posts |
 Posted - 07/03/2012 : 08:33:53
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As an aspiring Triple M car owner  (posting in "Restoration Costs" thread), I have been doing some revision on pre-War models and am intrigued by the J5 which is mentioned. My understanding is that it was stillborn, offering no advantage other than reduced engine capacity enabling entry to up-to-750cc racing classes. But, if one were built today, I wonder how a short stroke unblown J with lightweight body would go as compared with the other J models? Any forum enthusiasts interested in speculating about the pros and cons of creating a 'new' J5 ..  |
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David Allison
United Kingdom
665 Posts |
Posted - 07/03/2012 : 10:04:27
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Spyder
The J5 was a ruse to attempt fooling the handicappers - supercharging was still new technology and the benifits were still unproven in the eyes of many (indeed Fred Dixon still refused to admit the superiority of forced induction until the late 30's) - also the races were long and it was thought that an unblown car could run longer without stopping and negate the improved performance given by the blower.
Of course experience proved otherwise and by the time the J5 and K4 were announced the accepted view was that it was better to "run with the hare" at the front of the grid.
MG did not build an unblown racing car again until the NE - this too was because Aston Martin and Riley successfully lobbied the RAC to ban forced induction in the belief that there was no way that MG could build a car to compete in such a short time.
The NE was much quicker than Aston Martin and Riley would have liked - plus had a very large fuel tank and was very reliable allowing MG to win their 3rd TT.
An unblown 750 cc lightweight J special would give similar performance to a well set up unblown J2 - although you would arguably have slightly better brakes.
The unblown 750 is a pleasant if rather breathless beastie and you do need to rev the nuts of it to make it go - those with long memories can possibly remember when Fooz used to run the C type unblown to great effect in handicap races?
Pros - It would be nice to see an unblown 750 MG racing
Cons - Be content with being the mobile chicane
Big Pro - You will take scalps of those less able to drive - Mike Hawke used to delight in taking blown cars scalps whenever possible in his unblown J2, indeed he once managed to beat NA 0307 at a very wet sprint at Colerne in the early 80's.
Regards David |
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spyder
United Kingdom
14 Posts |
Posted - 07/03/2012 : 11:34:22
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Thank you, David. That's fascinating. (My intended use is for road fun only as I don't have the talent to take on anyone on a track - let alone play David to their Goliath). Interesting that you mention Freddie Dixon, remarkable tuner of Rileys. I think I recall his favouring multiple Amal installations and, while never having had first-hand experience of them, I've always wondered why they've never caught on with tuners in more modern times (apart from lack of originality, of course). Commercially, in the late 1958s, Speedwell (endorsed by Graham Hill) offered a twin Amal kit for the Mk 1 AH Sprite but that didn't take off. Now I am wandering off topic... |
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David Allison
United Kingdom
665 Posts |
Posted - 07/03/2012 : 15:13:06
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Dixon worked with Amal carbs because that is what he knew most about - in the mid thirties he designed a 3 choke SU carb which he fitted in pairs to his racing cars for track use.
However the SU design was prone to fuel surge problems on sharp left handed corners and Dixon moved to triple SU's on the 1750 Riley engine and a pair of carbs on the 2 litre.
Dixon was a motor cycle rider and fettler before he moved to cars - hence his knowlege of Amals.
Mel Jones had a J type special which used to go very well on 4 Amals - it went even better with a blower though!
The reason that the Speedwell kit didnt catch on was cost - a pair of MGA 1 1/2" carbs was cheaper from your local scrap yard than a pair of Amals!
Personally I would build a standard J2 if you are planning only road use.
The shorter stroke engine needs lots of revs to make it go and doesnt make for an easy touring car.
A standard J2 goes more than well enough and is both pleasant and tractable for most motoring tastes.
Plus the "sans porte" style racing body will get on your passengers nerves - if you think it difficult to maintain modesty with the suicide doors - watch em moan as they have to clamber over the side!
The J2 is probably the prettiest sports car ever made and for its time goes very well indeed.
With modern engine internals the engine produces decent power and is easy to drive too - the handling compliments good drivers and flatters the less able.
The 80 mph J2 is now a reality (if you are brave enough) - plus modern drum developments even make the 8 inch cable brakes tolerable too.
In short - J5 no J2 yes please!
Regards David |
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spyder
United Kingdom
14 Posts |
Posted - 07/03/2012 : 15:24:30
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Really says it all!
Much obliged.
I think I see a way forward...... |
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