Could you Chaps give me your opinion on what I should do!
What? You expect opinions out of *us*? Dear boy, whatever were
you thinking?
I want to build a fast, town drivable motor for my Midget.
Town driving is the key. More in a bit...
Should I stay with the 1300 displacement and tune it to the max OR
should I get a 1500 block and crank and tune it to the max. I intend
getting new pistons and everything else is interchangable.
With 1500 lower half i will have a very high CR (about 10:1 I estimate).
That's kind of a shame, because it's going to be trouble to run it on
pump gas, and if you intend to drive this around town you'll want to
fill it up at your convenience. Not much trouble -- I don't know about
the TR head, but the A series head will run about 10:1 before it starts
pinking badly.
So what is best for a torquey (not to be confussed with a seaside resort in
SW UK) fast street motor which I rarely winde up past 5K revs??
There Is No Substitute For Cubic Inches (or centimetres, I suppose).
A street motor needs to be flexible, reliable, and not too touchy. You
want decent low-end and midrange so that you can squirt through holes in
traffic, yet with some top-end left for screaming through the twisties
when you can get out to the country roads. You don't want an engine
that lumps or stalls at idle, because you'll be stuck in traffic from
time to time. You don't want an engine that runs too hot, because it's
a pain to have to replace head gaskets all the time. You want a mild
engine that gets most of its go through increased torque, and the biggest
prescription for that is to increase displacement. Now that that's out
of the way, let's make a shopping list for your new 1500.
The advantage to the high compression ratio is that it will add throttle
response, as well as about 4% per point of increase. Going with a mild
overbore is probably worth doing; on the A Series engine that ought to
be in your car :-), an overbore not only increases displacement and
compression ratio, it also improves breathing around the valves (in
particular the exhaust valves, which are somewhat restricted by their
proximity to the cylinder bores).
Go with a moderate cam; it's fun to have the engine come up on the cam,
but you don't want one like mine that really starts to have fun at 4000
if you don't want to hold it to 6500 or 7000 all the time. Go with a
cam that has a duration of less than 280 degrees (mine is a 285), and
talk to a Spitfire expert for valve lift. Do note that the high-
compression cylinder head means that you are going to have vastly
reduced valve lift before your valves meet your pistons. Be sure to
measure carefully or do a test assembly before you fire it up.
Be moderate in your carburetion; too big a carb will take away from your
part-throttle response and general drivability. A pair of 40DCOE Webers
with appropriately small chokes for high intake velocity will be a lot
of fun. Alternatively, a pair of HS4 SUs (1-1/2") should be a good
choice, since the SU's constant-velocity feature will help the low-end
responsivenss and the size should be about right for a mildly hot 1500.
Get a good exhaust header and feed it into a properly sized system. Find
a header tuned for the Spitfire's firing order and go for one with a long
section between the manifold face and the collector to maximize low-end
response. Don't go too big with the tubing or you'll end up with low
gas velocity in the pipes.
Balance everything in the engine, but you know that. Lighten the flywheel
as much as you think you dare; it'll really improve the car's willingness
to rev, since it has less mass to pull around with it.
Oh, and *do* fit an oil cooler; Mark Bradakis finally explained why
Midgets that use the Spitfire engine tend to eat rod bearings at an
alarming rate (not enough cooling of the engine in the tight Midget
engine bay).
Try to find a Bugeye differential; you can get gear ratios up to 4.56:1
for the rear. A 4.3 (early Sprite) will *really* make the car punchier
to drive, and I think the axle housings are still the same. Do use the
later halfshafts, though.
Then you'll need to work on lowering the car back to its intended ride
height and getting rid of those awful bumpers... Are you sure you don't
want to just buy a nice '67 Midget in the first place? :-)
--Scott "Okay, okay, just kidding" Fisher
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