I can't really think that high RPMs are that bad in themselves. After
all, the engine was designed for it! Besides the obvious fact that every
stroke of a piston creates an infinitesimal additional increment of wear,
I suspect that most wear is caused during start-up and cold running.
Think about it -- high RPMs are when you have the best oil pressure -- at
start-up you have none for a few seconds. I remember averaging 85 mph on
road trips in the Central Valley of CA, before I had my overdrive. That
sustained high RPM didn't seem to cause any problems, although in
retrospect I don't see how I could stand the noise, vibration, etc! You
should also consider the power curve of the engine -- it really doesn't
come on below 3000 RPM, anyway.
So anyway, the main arguments for OD are noise reduction and improved
fuel mileage, with wear & tear a distant third, I should think.
As for 5 speeds, I think I recall an earlier discussion in which someone
described a Nissan 5-speed conversion. In this installation, 5th was
equivalent to 4th on the stock gearbox, and 1st was too low to be useful
-- not much of an improvement, eh? I would think that in general
installing an MG OD gearbox would be far less hassle (BTDT -- and $$$)
than converting to some kind of "foreign" 5-speed, and would give
equivalent or better results.
IMHO, YMMV, FWIW, etc.
Tab Julius had this to say:
>
>I've been watching the OD discussions, plus the comment from Gerardo of
>being tired of cruising at 5000 rpm (me too). Actually, anything over 3000
>RPMs I know I'm burning up the gas, but I just can't feel that higher RPMs
>are very good for the engine; like the optimal is 2500 to 3000. Of course,
>going red is definitely not good, but what about 4000 to 5000? It won't
>blow the engine, but surely it's taking years off. Or is it not? On the
>back roads it's not a problem, but the 75 mph highways I feel like I'm
>doing my engine a disservice if I actually go that fast for an extended
>period of time.
>
>Wishin' it was a 5-speed...
>
>- Tab
>
>'78 B
>
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.
|