In case you didn't know, 18G and 18GA are 3-main bearing
engines. The 18GB is a 5-main engine same as the 18V engines (it
just has a smaller flywheel). Once into 5-main engines there is very
little difference in the mechanical parts through end of production,
but the devil is in the detals.
From 18GB through end of production there was very little change in
the lower engine, and only minor changes in the cylinder head. I
recon just about everyone will advise to avoid 1975 and later North
American configuration with single carburetor and catalytic
converter. If you want 5 main bearings and dual carbs, and are not
overly concerned about emissions, then look at 1965 through 1967 GB
engine configuration. After 1967 the flywheel was larger mating with
the 4-synchro gearbox. There were various additions of emissions
control components starting 1968, which you can omit, although the
evaporative emissions system (carbon cannister and related piping)
and the anti-run-on valve are friendly and no harm to performance.
The 18V engines (all of them) used lighter weight tappets with longer
pushrods, single valve springs, and single row timing chain. The
lighter tappets with longer pushrods are good, but if you slip in any
kind of upgrade camshaft I recommend going with the earlier dual
valve springs and dual row timing chain. For some cam upgrades you
need heavy duty dual valve springs (and higher compression) to take
advantage of more power into higher engine speeds (beyond 6000
rpm). Read the specifications from the cam manufacturer.
For 1972-1974 (and later years for European market) the "L" cylinder
head had one size larger valves, better porting, and smaller volume
combustion chamber for higher compression (but not Notrh America
after 1974). From 1972 on there were eyebrows in the engine block to
clear the exhaust valves on full lift. For pre-1972 blocks you need
to add the eyebrows to accomodate the "L" head, or any cam with
increased valve lift.
If you would like to follow this recipie, you can start with 18GB
engine and inatall 18V type tappets and pushrods. Or you can start
with 18V type engine and install dual valve springs and dual row
timing chain. End result is the same. Chose any MGB cylinder head,
1962-1967 if you want no air injection ports, or 1972-1974 for the
"L" head (notes above). After 1968 you get to plug or cover unused
air or water ports. Once you get this basic mechanical configuration
down, you can think about performance upgrades as a separate subject.
The reason MGA guru knows something about all this stuff is because
poeple put these engines in MGAs fairly often. I have all of these
features in my MGA, plus a Crane fast street cam, except I stopped at
1625cc with 3-main bearing MGA block. I also run the early MGB
flywheel and stock MGB clutch parts. It is a hoot to fly, and not
particularly expensive to buiild. If you want to go much beyond
100-bhp, then cost is almost exponential with power output, and
reliability can decline accordingly. Anything over 100-bhp requires
some more high performance input. 120+ is race engine territory,
probably not street friendly, and way beyond my area of expertice.
Barney Gayloed
1958 MGA with an attitude (and a house full of rally and autocross trophies)
http://MGAguru.com (with thousands of pages of MG Tech information)
At 03:08 AM 11/29/2012 +0000, Windows Live Team wrote:
>I would like to build a strong MGB street engine. What are most people using
>the early 18GB or the later 18V. .... Lorne Fritz
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