Thanks for the Twinkie stuff, Neil.
One comment - the reason that Twincams burned pistons was not apparent to it's
contemporaries, and the material quoted by you appears to be older factory
stuff.
The real reason that the Twincam engine had warranty problems was an
undiagnosed fuel frothing problem that leaned the engine out and caused the
piston burning problem for which it became notorious. This condition occurred
at two specific engine resonant frequencies that coincided with (I am going
from memory here as I am too lazy to dig out my files) around 2500 and 5500
rpm. The earlier one would normally be passed pretty quickly, but it was in
the cards for the engine to be held for some time at the latter rpm on the
highway and duriing competition. Apparently the vibration had a tendency to
jam the fuel float on the centre pin resulting in starvation at those rpm.
Obviously it wouldn't happen every time and would have been infuriatingly
difficult to diagnose. One of the first tips that made the lights go on for
the brighter lights that were trying to figure this out was that ALL of the
cars that had problems had SU carbs, and NONE of the competition Weber
carburetted cars had these problems, often despite more rigorous use.
The solution is easy - use Webers, or have your original manifold grooved for
'O' rings. If you don't want to bother with the machine work necassary for
this, you can use the special Weber gaskets that are essentially an O ring
attached to a carb gasket. As it happens, the ones from a 45 DCOE also fit the
H6 SU bolt pattern when turned diagonally. You also need double wound
'Lawrence' washers as if you just reef the nuts tight it is as bad as having
no rubber in the setup - they need to be able to flex.
Have a look at Lotus or Alfa engines for similar applications. Too bad they
didn't figure it out sooner!
By the way, a Twincam in good tune will make the sub 10 second 0-60 time - the
others given to the magazines were off colour and for some reason badly tuned
and prepared. Hope they fired the guy responsible.
Having said all that, you are quite right that the engines rev so easily that
many were broken simply by indiscriminate use of a heavy right foot. It is
basically a totally different creature than a pushrod engine, and for anyone
that experiences one it is hard to go back!
What they clearly should have done in retrospect was to fit a rev limiting
rotor, as Lotus did in the Lotus Cortina (these will in fact work in the
Twincam and are calibrated for 6500 rpm). I don't know if these were available
in 58 but it would have prevented half of the warranty claims, and proper de-
bugging before production might have solved the fule problem.
Ironically, the problem never showed on the test bed as they used a different
fuel system than stock. If they had detected it early enough, I would be
willing to bet that we would have had 120 bhp Twincams in 1800 size for the
Bs, at least as an option. Sad, no?
Bill S.
(Real MGs have 2 Cams!)
|