In actuality if your earlier head had larger intakes it, all things being
equal, would have had better flow. Velocity is much more a product of intake
port size than valve size, at least until valve sizes get unreal. And
although turbulance is a good thing in the port, you want the valve to offer
as little restriction and turbulance as possible, so as to avoid
back-pressure into the port. In this sense bigger can be better.
One variable is that the later head design has a more efficiant combustion
squish area which, regardless of valves, is going to give better low rpm
performance.
The slightly bulletted guides are typical of aftermarket bronze guides..
Your early head, if stock, should have had 1.56 inch intake valves, the later
head *should* have 1.625" intakes, so it's curious to me if they appeared
smaller, are the alum..head suppliers doing something different?
According to dyno and airflow figures published by Peter Burgess in "power
tune MGB" , the only valves that show any real significant low-rev fall-off
are the 1.69", which are an aftermarket size never used by the factory.
The single best thing you can do to a B motor for more performance is a good
work-over of the head, including light porting, port-matching, a first-class
3 angle valve job, and a light clean-up of the combustion chamber (sharp
edges removed, chamber floor flattened) This work with a mild "RV" grind or
"light road" cam will show large improvements in performance with no
sacrifice in drivability or reliability, with almost all the power gain
coming in the midrange.
IF you cam this motor at ALL, you'll need to copy the factory relief cuts in
the block to clear the exhaust valves. It's a good idea anyway, the late head
breathes a whole lot better with those cuts made and it's how the head was
intended to work.
Cheers
>original segment-----------------
The new head has a late-head
pattern combustion chamber and the smaller intake valves and slightly
bulleted bronze guides, an interesting contrast to my early iron head
with big intakes. Can't directly compare performance, since I had a
sticky exhaust valve which prompted the whole project. Does seem to
breathe well, and the smaller intake valves probably contribute to
higher velocity in the RPM range I run. I'd love to find a rolling
road dyno locally and see what the engine does.
Chris Attias
'64 MGB
'84 Alfa Romeo GTV-6
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