Some thoughts in response to Susan's MG mods to her 1622:
<<<. You lose quite a bit of torque below 5000. You have to ask yourself how
often are you above 5k rpms? The breathing is very poor at low rpms. I decided
to go with a Peter Burgess modified head which will produce very near as much
power as a Xflow with MUCH more at low to midrange speeds. A little porting
and a 5 angle valve job. S-B inlet guides and hardened seats too. I plan on
using stock size valves, for better low end power, but switching to the
Rimflow design.
***This all makes sense except for the statement about the X-flow head. I've
used these for years on MG's on the street andon the track,and in my TVR race
car as well, and they produce more, not less torque than the stock head low
down. I have the curves somewhere with the info. They also produce more power
at top end, than anything except a fairly radically ported iron head with
large valves, which will equal the X-flow.
I wonder if the guy supplying the head either wanted to sell you a CI head or
maybe didn't have any hard info on the X-flow?
I'm using roller rockers from Cambridge Motorsport for 2 reasons. One: the
stock ratio is about 1.42:1, I say "about" because every rocker will very a
little bit..... normal during production runs. and Two: the CM rollers have a
ratio of 1.625:1, higher lift, more fuel in/out= more power (with the right
cam). The rockers have less friction and a correct ratio ---- essential for
accurate cam timing.
***Especially good choice if your head has been ground - you will help to
avoid some of the guide wear you'd otherwise have.
I chose a Kent Cams 717 gring as giving me the best combination of torque and
power. Good torque comes in at about 2500and continues up til 5500. HP peaks
out at 5600. All in all a good range. The duration, off the top of my head, is
270.
***Sensible choice for a street cam, especially given the bore size of the
engine.
For carbs I will stick with the stock 1 1/2 SUs, but modifying them to flow at
the 13/4 rate (about 200cfm or so I think). Easy to tune and I some around
too. Again the engine will look totally stock from the outside.
***I'd go with the 1 3/4", but chacun a son gout. The result will be about the
same if the H6 carbs were unmodified.
With the stock fan you will lose a few HP over the electric one but for me
it's stock again.
***Right - no big deal at street rpm.
Stock cast iron exhaust manifold, unless someone can convince me, on a dyno,
that a header will help on my specification engine. I have a NOS Abarth system
that I'm putting on, a period modification.
***Again, right on the money. The B headers don't fit a left hand drive car
(hit the steering column), and the stock A header flows respectably well
anyway. Probably only around 3-4 bhp to be gained at your state of tune with a
lot of expense.
Ask me about designing and building Twincam headers some time.
ARP fasteners, Crane pushrods, usual work to the block, crank and rods. Piston
type and manufacture is not decided at this time. Cast pistons do not expand
as much as forged but forged are stronger. If thats an issue below 7000 rpm
is an unanswered question. Compression will be between 9 and 9.5:1.
Stock oil pump shoud do fine but will be checked against specs. Some of the
rebuilt ones are not within blueprinting specs.... easy to check and a cheap
fix anyway.
***Using ARP fasteners is relatively cheap insurance. The pushrods aren't
strictly necessary as the stock ones aren't weak, and the slight extension of
valve bounce rpm isn't a factor for you. With the cam you have, a 6000 rpm
redline is fine. Compression is also sensible given these days gas ratings,
although 10:1 should be about the max with pump premium. I assume hardened
seats? You may want to 'double' port the oil pump - easy and quick and gives
a bit more volume.
You don't need forged pistons for what you are doing, but if you do pay strict
attention to clearances. Some of them expand _less_ than cast. On a normal
cast, allow something like .004 for a free running engine. I had a buddy that
built a 3-main B at stock clearance (about .0015 isn't it) and seized it when
running a Judson blower on it. A little extra might give a chance of piston
slap, though I doubt it (ever drive an original Lotus Cortina - half of them
slapped until they warmed up as they came from the factory), but too little
can be disastrous!
Just my thoughts
"Midget" Susan>>
Let us know how you like it. Oh yeah - do balance the engine, it's worth it.
Bill S.
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