Jay and piners,
The important thing to consider particularly with carbs is not
to let the velocity change rapidly in any part of the port, from
the ends of the carbs to the valve itself.
Sudden changes in area of the port cause sudden acceleration or
deceleration of the flow of air. This doesnt lend itself well
to the mizing of the fuel, and also doesnt assist in resonant tuning,
where the inertia of the air is exploited for added cylinder filling.
Intake ports like to have a gradual taper in the port size from larger
in the carbs to smaller in the port ends. the key hear is "gradual",
no sudden steps or area changes. Carbs are at a disadvantage already
because the venturi already causes a suden velocity increase, then
decrease on exit. Its important to match the carb exit shape, and
taper the manifolds taper on into the ports. If this is what your
"jetter" did, then great. The diameter is not as important as the
ultimate port shape and rate of taper.
What Holbay did with the H120 setup, defies common logic in several ways.
Small ports, massive cam advance, and big overlap, just doesnt seem to
make sense. But, guess what? It seemed to work, dispite the odd nature
of the changes.
I wonder though whether or not most people who have Holbay setups,
particularly
H120 cams, actually have an H120 spec camshaft.
Why does it matter?
1) H120 setups have long been coveted, most of the pieces to make H120
engines
went to the scrap heap long ago. Many people have appoximated H120
specs
with a Cam regrind, and some well documented head mods.
2) There isnt enough "meat" in a stock camshaft to regrind a H120 profile
without
welding the lobes.
3) Even with the lobes welded, the sprocket keyway is cut for 7 degrees of
advance, which can be re-cut, nor likely accomplished with offset keys.
4) Holbay also sold the "Tornado /E128" cam, which is actually a higher
performance
cam, that uses a far more conventional method to deliver power. There
likely
will be confusion between the H120 and the E128 cam due to that they are
both
Holbays.
Jay, the next time you get into your engine I think it would be prudent for
you to "degree" your cam. That way you know what works well, and others
may accurately emmulate.
Cheers,
Jarrid
-----Original Message-----
From: Jay_Laifman@countrywide.com [mailto:Jay_Laifman@countrywide.com]
Sent: Monday, November 25, 2002 2:45 PM
To: JGross@econolite.com
Cc: alpines@autox.team.net
Subject: RE: Weber intake size and alternator information
I should add that yes, the Holbay head did not have intake runners this
large. My mechanic opened them up to match the Weber throats. And,
again, my car is very tractible and responsive at the low end too. I'm
not convinced that Rootes/Holbay did the right thing in the first place,
especially given the nice running I have.
Taking it one step further, where I'm bound to trip, intuitively, it seems
wrong. The issue is the size of the throats of the carbs for the size of
the engine. It's like blowing with your mouth wide open versus with your
lips puckered. That makes sense to me. But, to restrict the flow after
the carbs have already mixed the atomized gas with the air seems like a
misplaced cure that basically kills the very benefit in the first place.
If the carbs were so incorrectly big, they should have started from the
top with smaller carbs. But, from my experience, I don't think they were
too big either. Again, as I said, with this step, I will probably trip.
Jay
"Jarrid Gross" <JGross@econolite.com>
11/25/2002 12:04 PM
To: Jay_Laifman@countrywide.com, alpines@autox.team.net
cc:
Subject: RE: Weber intake size and alternator information
Jay wrote:
>Someone asked me the size of my Holbay head intake inlet size. That is,
>the size it was opened up to to match the 40 DCOE side draft Webers. I
>can't find that e-mail. Sorry. So, everyone gets the number. I measured
>it at 32.7 mm in diameter.
The 32.7mm runner diameter is quite large. I doubt the stock holbay head
had such large runners, becuase this was contradictive to the strategy
that
Holbay was after in the tuning of the H120 engine. The strategy seems to
have been to keep the port velocities high, and utilize the valve-train
reversion to give higher volumetric efficiencies as lower RPMs.
Larger ports are great at higher RPMs, but probably not so good for the
camming strategy from the H120 cam.
It is my understanding that the stock H120 head used slightly smaller
exhaust ports, and the same sized intakes as compared to stock.
Jarrid Gross
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