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Unit twin quoted power outputs #724102
02/03/18 12:11 am
02/03/18 12:11 am
Joined: Dec 2017
Posts: 107
England
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ferretjuggler Online content OP
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Do the quoted power outputs of the late model unit twins have any connection to reality ?
Or where they a politically motivated figure conjured up by BSAs sales people ?
I've always noticed how BSA would always quote a couple of horsepower more than the direct Triumph equivalent.
Today I was looking at some sales leaflets for the 1971 BSA range.
The new OIF Lightning is quoted at 52 Bhp and the OIF Firebird as having 54 Bhp.
I'd have thought that the high level pipes with their unequal length headers would have actually reduced power, despite the straight through absorption silencers.
Could the Firebird be that little bit quicker ?
Or just marketing hype ?

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Re: Unit twin quoted power outputs [Re: ferretjuggler] #724113
02/03/18 2:04 am
02/03/18 2:04 am
Joined: May 2004
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Sydney Australia
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BSA_WM20 Online content
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Pipes could easily account for 2 Hp.
While the lengths are different, what is important is the actual length of each pipe from valve seat to to end of the pipe and weather this is a pulse node to not


Bike Beesa
Trevor
Re: Unit twin quoted power outputs [Re: ferretjuggler] #724122
02/03/18 9:27 am
02/03/18 9:27 am
Joined: Jan 2014
Posts: 3,515
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NickL Online content
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I daresay that if you messed about with the standard 650's enough you could get that HP at the engine.
I doubt very much if the one's on the showroom floors actually produced that though.
I don't think they were alone in quoting imaginative figures.



Re: Unit twin quoted power outputs [Re: ferretjuggler] #724220
02/04/18 4:56 am
02/04/18 4:56 am
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Posts: 3,599
ca, us
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DMadigan Online content
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Since the number quoted is usually crankshaft horsepower it takes some guesses at the losses from where it was measured.

Re: Unit twin quoted power outputs [Re: BSA_WM20] #724242
02/04/18 1:43 pm
02/04/18 1:43 pm
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ferretjuggler Online content OP
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Originally Posted by BSA_WM20
Pipes could easily account for 2 Hp.
While the lengths are different, what is important is the actual length of each pipe from valve seat to to end of the pipe and weather this is a pulse node to not


Thinking about it, I think that high level system was probably designed at Umberslade.
The boffins there were quite good at the theoretical stuff like pulse nodes, so maybe it does give a boost.

I read a 1969 (I think) road test which slated the Firebird and it's fangled exhaust system for ruining the performance of the twin carb engine.
But that was just conjecture, another fault could have been causing the problem instead.

Yet another road test got 115mph from a Firebird
That one blew its gearbox apart at maximum speed!

Re: Unit twin quoted power outputs [Re: ferretjuggler] #724298
02/04/18 10:25 pm
02/04/18 10:25 pm
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Posts: 444
new jersey usa
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pushrod tom Online content
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Dyno testing showed me that the Firebird type pipes (1 3/8 vs 1 1/2 diameter) produced highest torque and hp at a lower rpm than the normal size found on all other A-65's. This would be usefull for off road or street riding however it also lowered the max torque and hp by a fair amount. A great way to tailor the bike to your riding requirements! Cheers, PRT


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