Just as a little caveat to the OD/NON-OD debate, the J-Type OD unit top gear
drops revs from 1.0:1.0 gear ratio to 0.79:1.0 gear ratio (if I'm
remembering correctly).
And although it doesn't seem like a large amount, the engine revs to do
60mph on my car is somewhere around 3000 rpms. With OD, I could do 60mph and
the rpms should read about 2400.
And at 4500 rpms (which I think is actually pretty screaming for a stock
engine) with 23.5" outside diameter wheels (880 revs per mile), a late model
Differential (3.89:1) and OD engaged, you should be able to do around 99
mph.
The Conversion Component's (in NZ) Toyota Celica 5-speed conversion, has a
5th gear ratio of 0.86:1.0 which would only be around 91 mph at 4500rpm with
the same 23.5" od wheels and 3.89 final drive.
And just to really make you think, I've heard of someone on the list that
had the swiss rear diff (3.63:1), and an engine he could take to 8000 rpms,
Assuming he had Firestone F570 185/70r13's (886 revs/mile) and OD (.79:1),
he should conceivably move his spitfire to around 188.91 mph .
This is obviously a theoretical observance.
(engine RPMS) divided by (trans gear rate) divided by (final drive rate)
divided by (tire rotations per mile) multiplied by 60 (# minutes in hour) =
Movement in miles per hour.
Terry L. Thompson
'76 Spit 1500 (which has never done more than 75 mph)
Maryland
http://www.access.digex.net/terry_t/index.html
----- Original Message -----
From: Joe Curry <spitlist@gte.net>
To: Laura Gharazeddine <Laura.G@141.com>
Cc: <spitfires@autox.team.net>
Sent: Tuesday, May 23, 2000 1:22 PM
Subject: Re: So, who wants to sell me a Spitfire OD tranny?
>
> If you do any highway driving at all (and in LA, I can't figure out how
you can avoid it), you would really benefit from an overdrive.
> Sure, it is one more thing that can fail, but it is something that will
take a lot of wear and tear off the engine. I smile every time
> the OD kicks in and I can watch the revs drop by 500 rpm. That will
increase engine life and for my money is well worth the added
> possibility of additional problems.
>
> But then Dave wouldn't get to see you as often! :)
>
> Joe
>
> Laura Gharazeddine wrote:
> >
> > None of my Spits have had o/d. With Nigel, we've talked about it. Then
one of the guys from my local Triumph club said he could get me (a used) one
at a good price. I thought about it. Talked about it with Dave. Talked about
it with the English guy who works for Dave. In all, I talked to about 5
different people and what it came down to is that it's a nice thing to
have-but not a necessity and more importantly-it'll be just *another thing
to break or go wrong*.
> >
> > That last bit, coming from individuals I respect and who hadn't confered
with each other convinced me not to do it.
> >
> > This year, at least!
> >
> > Laura G.
>
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