I tried this procedure several years ago with mixed results. I have an
early TR-4 motor for my 62 Plus-4 and a VERY late TR-4 motor in my 68
(built in '67, shipped in '68). Thinking about the differences in the SU
bodies and manifolds, I did exactly as outlined below. My logic followed
that the larger, shorter, straighter ducting in the early manifold would
allow the motor to breath easier and therefor aid performance.
Unfortunately the car became sluggish at slower speeds and startup
acceleration. After some discussion with some wiser heads, it was noted
that the later manifold produces higher flow velocity and increases the
breathing past the valve whereas the larger diameter and shorter manifold
didn't 'ram' the air into the combustion chamber. The porting mentioned
would reduce some of the affect however the best addition to the short/fat
manifold would be larger valves to complete the breathing ease.
Since the longer, smaller diameter manifold was produced on the later
engine, I guess Triumph DID know what they were doing. Since then, I've
put things back and enjoyed it ever since. Performance tuners would likely
offer several other secrets and tips but so far I'm satisfied.
>>> There were at least two types of SU carbs fitted to TR3's & 4's & Morgans
>>> over the years. The early type is HS6, which has a relatively long
body. It
>>> attaches to a rather stubby cast aluminum alloy intake manifold. This
setup
>>> is the same diameter as the later (TR4A) HS4 carbs, which have a short
body
>>> and are attached to a longer, more gently curved intake manifold. (This
>>> manifold probably also has a tapped hole to receive the crankcase vapors
>>> via the valve cover from the PCV system. The early one has a boss for the
>>> breather connection, but it may not have been drilled yet, or it may
have a
>>> socket head plug screwed in.) The overall length of each setup is about
the
>>> same from the jets to the intake ports on the engine block. If you marry
>>> the HS4 carbs to the shorter manifold, you save almost an inch of overall
>>> length. I had the faces of the short manifold machined down about 1/4" on
>>> both the engine side and the carb side, with the result that I can now fit
>>> some thin K&N air filters under the hood. I might have given up a little
>>> idle smoothness, but overall performance doesn't seem to be affected. I
>>> also spent a bit of time reshaping the inside of the manifold to better
>>> align with the ports on the engine and the carbs, and also smoothing the
>>> curves inside. I used a Dremel tool for this. Be sure to lubricate the
>>> cutting head with candle wax or something to keep the alloy chips from
>>> clogging!
>>>
"Entropy Happens!"
Michael D. Miles, PE Consulting Design Engineer
(503) 292-1234, FAX: (503) 292-1105, email: mdmiles@teleport.com
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