As I said, I have no personal experience modifying MGA engine. However, a
lack of knowledge has never stopped me from having an opinion in the past
and I see no reason to make an exception this time. These recommendations
are based on my experience and knowledge of large American Iron, the
writings of Lindsay Porter in MGB DYI Restoration (a great series of books,
I have had both MGB and Spitfire versions. Lindsay- write one for the "A"
please), the MGA BBS out of England, and some clippings from MGA
newsletters that Kevin Mardell sent me.
It sounds like your camshaft is a pretty good choice. I have read that
later lifters and pushrods are shorter and lighter than early sets. I have
also seen Crane lifter advertised. I have also seen full roller rocker
sets for the MGA. I don't know if they have a different ration than stock
but, if they do, I would be careful about high cam pressures and valve
spring stack combining them with a particular cam. Matching components is
important. For a street engine, I think you would be better off just
making sure that your stock rockers are in good shape. Remember- any extra
play in the system steals from lift at the valve. Also, spend the time to
degree the cam. I have seen expensive adjustable sprockets that would do
the job. I have also seen offset keys that would probably do the job. If
you could get a lot of dyn time, the sprocket would be the way to go.
Otherwise, buy the offset keys and dial it in and leave it. Ask the cam
manufacturer for recommendations.
With the larger cam, you could probably afford to bump the compression
slightly. Remember, keeping the intake valve open longer steals
compression stroke so true compression will be lower than the "static
compression ratio." The MGA BBS has a great thread on piston selection
where someone recommends Venolia (sp?) pistons rather than stock pistons
because of the lighter weight, better rings, and choices in compression
ratio. Compression can also be raised by milling the head but be careful,
MG heads are weak to start with and removing material doesn't help. Note-
changing piston weight usually requires a dynamic rebalance of the
crankshaft.
Concerning combustion chamber modifications, I have heard two things
related to the B series motor. First is that early combustion chambers
have a fairly sharp peak between the valves that can get hot and cause
pre-ignition and subsequent detonation. Increasing the radius of this and
all sharp edges in the chamber will help with this issue. Second is that
the head design is such that the valve openings are heavily shrouded by the
cylinder walls and that the engines respond very well to larger bores. A
good speed shop should be able to test your block and recommend a maximum
safe overbore size. If you do this, you will again probably need custom
pistons. Also, Lindsay Porter mentions that some people relieve the top of
the cylinder around the exhaust valve to help flow. This is a tricky
business and I would definitely find someone who knows what he is doing to
do this. Other than that, just make sure that the shop does a really good
job on the valves with three angle valve seats. Pocket porting is cheap
and may help also, although I have not heard whether these engines respond
to it.
You could also consider replacing the head. I have heard that later heads
can be make to fit and have larger valves. Also, there is someone on the
Internet that is advertising a crossflow aluminum head for MGB's for around
$1500.00 but I don't remember where. Again, changes like this need someone
who really understands the engine unless you can afford trashing parts.
For instance, I expect that the larger valve would hit the cylinder wall
without relief.
On the intake side, Porter prefers Del Orto's and Webers to SU's for power
output. I thing the SU's look right on the car and would probably stick
with them. Hot rodders always port match their manifolds by opening the
head, gasket, and manifold to exactly the same size. Some people will go
as far as cutting down the butterfly and throttle shaft to increase flow.
The most important factor is careful tuning. If you can afford it, but a
handful of needles and rent dyn time. Otherwise, tune for proper peak
power mixture by running hard, shutting down and reading the plugs. Then
estimate the needle needed by midrange and idle characteristics- whether
they are lean or rich. Also, I do not consider Longflow filters to be high
performance. I would use K & N. If you could figure out some way to
attach a filter and cold air intake to a set of velocity stacks, that would
be the best. I don't think that the standard flat back filters do a good
job of controlling the fuel cloud in front of the carburetor. Again, make
sure there are no restrictions.
For exhaust, a header and low restriction muffler is good. They can be
loud, however. See if you can "test listen" to the setup you want. Don't
forget the standard performance exhaust people like Thrush, Walker, etc.
They make some pretty good products.
For ignition, rebuild the distributor and have is re-curved to add more
advance earlier. This is almost always necessary when doing a performance
rebuild. I have no specific recommendation, however. A coil with higher
output will allow you to run larger gaps on the plugs and, since you are
going to use an electronic ignition, you shouldn't have to worry about
arcing.
Also consider the "feel" of the package. I would recommend a lighter
flywheel to improve throttle response. This will make more difference than
you think.
For reliability- have the crank maganafluxed. Some speed shops will grind
a crank with larger radii at the journal edge and modify the bearings to
fit. This can increase strength. Lighter pistons and new style straight
split rods (I believe they fit) which are also lighter would help with the
forces the crank sees. Fit an oil cooler if you don't already have one and
possibly an early MGB radiator which will fit, I believe and is larger.
Finally, as to what I would do in your shoes. I would by a pre-68 5
bearing MGB engine and the proper electronic tach and drop it in. The cam
and exhaust you have would fit or could be adapted. All of the changes
mentioned above would still apply but I think you could skip enough of the
expensive ones to pay for the other engine and still build more motor than
your transmission can handle. Plus, you would feel a lot less guilty about
throwing a rod though the block.
Good Luck
Bill Eastman
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