How much cutting and welding is required? The gussets at
the rear of the engine compartment must be removed, but this can easily be
done with a reciprocating saw. No other cutting of the tunnel is required,
so the dashboard will not need to be removed. Stock control arms, shocks
and springs, hood, etc can all be reused without modification. There are
several holes that get drilled for frame rails and other mount points. The
NA’s require 4 small tabs to be welded onto the rear subframe assembly (to
match the NB setup). However, the NA Miata’s rear subframe can be removed
with about 6 bolts, and the welding can be done separate from the car (or
at a welding shop).
What steering changes are required? The lower steering knuckle is
replaced with a slightly longer version, which moves the steering rack
forward about an inch. The stock steering cylinder is behind the center of
the front wheels, so the true effect of moving the cylinder forward should
make absolutely no difference to steering feel or bump steer, and does not
add any additional joints or increase the joint angles.
Why did BF choose the 8.8 differential? BF considered both the 7.5
and 8.8 Ford differentials, and a variety of other IRS setups including
the Getrag/Cadillac and Supra. The Getrag was eliminated from
consideration because of the lower torque rating, limited choices of gears
and posi combinations, and it’s recall and failure history. The Supra rear
ends are too hard to find. The 7.5 differential would be a good choice,
but the supply of these is also limited. BF chose to use the bulletproof
8.8 center section and developed a unique mounting system to maximize both
the strength and ground clearance. The stock 8.8 outer CV will not clear
the shocks, so remanufactured half shaft assemblies are used with the Ford
7.5 CV’s and a new custom length 4340 shaft. Our new half shafts have a
short outer boot to clear the shocks. This rear end arrangement offers a
higher overall torque capacity than either a Getrag or a Ford 7.5
differential system can deliver.
Why are new rear hubs needed? New OE Mazda hubs are custom broached
to fit the larger outer CV’s, and new wheel bearings are also supplied
with the kit.
Is weight and balance affected? Yes, but not very much. The engine
subframe is about the same weight as stock, and the engine is only a
little heavier than the 4-cylinder. The transmission and rear differential
are both heavier than stock, so some of the extra weight is shifted
rearward again. Then net result is a shift of a few percent toward the
front – not enough to make a huge difference in handling.
Is power steering available? BF recommends eliminating the power
steering system, but it is possible to keep it. Some re-plumbing will
likely be required as the stock lines may interfere with the engine
subframe. There is currently no plumbing kit available for these
modifications.
Is air conditioning available? BF does not currently offer a kit to
enable the AC system, but it is fairly easy to add, and there are other
sources that do offer a retrofit kit.
Do your kits fir both NA/NB steering cylinders? BF kits currently
are made for the NB steering cylinder. NA steering cylinder kits are
coming soon.
What about front sway bar clearance? The stock front sway bar would
run through the balancer pulley, so it must be shifted forward a couple
inches. The engine subframe kits include relocation brackets to move the
sway bar forward. The stock sway bar can be used, but the arms should be
lengthened about 2” on each side. BF supplies brackets to extend the
Racing Beat aftermarket sway bars to the stock (or aftermarket) end links.
What about shock, spring, brake, and wheel clearances? BF
recommends using either stock shocks or a performance shock that will
allow up to stock heights, depending on how much ground clearance you
want. Big brake kits work fine. Wheels and tires up to 16x8 and 245/35-16
will fit without modification.
What about ground clearance? Ground clearance depends on many
things, including exhaust height, shocks and springs, etc. The BF system
will keep everything at or above the scrub line – the rest is up to you.
Any wheel hop issues? BF supplies new Energy Suspension bushings in
the nose of the differential and heavy urethane bushings in the rear to
help reduce any tendency to wheel hop. Differential braces are also
supplied to tie the rear subframe into the floor and main frame.
What about a radiator? The stock radiator is too small to cool the
big V8’s, so a new one is needed. A wide variety of choices exist,
including Miata racing radiators and custom made radiators. BF does not
current offer a “bolt in” radiator…yet.
Shifter location? The T56 shifter will fit right in the Miata’s
body hole, slightly forward of center, but without any modification to the
body or boot.
Any questions about kits, parts or installations are welcome. Contact Bishop by phone at 604-970-3454 or e-mail and we will respond promptly. If you're in the neighborhood, then drop by the shop in North Vancouver BC and we will gladly give you a tour of our facilities, but beware – there may be a Monster in our shop when you visit! We really love to share the fun with other Miata and MX-5 owners who want to take their cars to the next level of performance, appearance and handling!
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