John makes some very good points.
It is extremely important when planning modifications to understand
how the car as a system works. It is also very important not to kid
yourself, if you really want performance increases. The other side of the
coin is that one modifies for personal enjoyment, and who can tell what
bizarre aspirations may lay in the hearts of the owner.
Example 1. High lift long duration "go fast" cam in a street car. Sitting
at a stop light, the engine lopes, shaking the car giving the impression
that there is a caged animal under the hood. The owner smiles at Mr. Normal
sitting in his Chevy Stationwagon in the next lane.
The light turns green. With a roar of intake noise, our valiant
owner stomps on the gas and dumps the clutch. The engine immediately bogs.
Raw fuel dribbles into the cylinders as the mixture velocity in the oversize
intake tract slows to a crawl. Fuel falls out of atomization and washes the
oil off the cylinder walls. In a screech of tortured piston rings and a
belch of hydrocarbon smoke the valiant engine labors to launch the car from
the intersection with the help of the flywheel, which being aluminium isn't
helping much. Slowly the tach needle climbs to 4,000 rpm. Suddenly in a
rush huge gobs of power push the driver back into the seat. This is it!
The sports car life. 7,000 rpm and the exhaust is screaming setting off
burglar alarms along the road. Speed shift into second. Suddenly the rush
slows down again as the big gap in the gearing drops the engine off the peak
torque range. With teeth clenched the foot is again dropped and the engine
climbs into power again. The surge of power, smell of unburnt fuel and
raucous intake noise bring the dreams of Fangio alive.
Meanwhile the tail lights of Mr. Normal's Chevy fade into the
distance.
Repeat ad infinitem. The factory built a very nice tractable car. To
improve overall performance plan on spending lots of money and taking lots
of time.
Technology, quality and material improvements are the only places to look
for real performance increases without major drawbacks.
Kelvin.
> -----Original Message-----
>
> The mechanic is right... maybe.
> You didn't specify what weber or "how much mild upgrade" cam
> you have. If
> it's around 265-275 overlap, you first problem, all else
> being equal, is that
> you and everybody else he's talking to likely have too much
> carb. A stockish
> B motor does very well on stock needles on the HS4s with a
> "torque grind" cam
> and the 270's range are fine on #6 or #7 needles. 90% or
> more B weber
> installs are too much carb for not enough cam, and the low
> end goes away in a
> sea of unburnt gas.
> The other major issue is cam timing. Almost every cam
> I've seen comes
> ground a few degrees off time. If not installed properly
> (finding spec with a
> degree wheel etc) you will almost always end up with a cam
> that ends up
> top-endy at the expense of bottom end. The only possible
> reason I know why
> this happens is that the typical DIY cam buyer wants a
> wiz-bang increase in
> revvy power and will complain if all that shows up is more
> torque and a
> bigger power band...even though that's really what you are
> buying at this
> level of upgrade.
>
> __________--------------------------original
> post------------------___________________
> From: Enrique Claure <spanlab@ceibo.entelnet.bo>
> Subject: Camshaft bull s...
>
> Hi list, I took my MG to get the weber tuned. About four race
> cars were
> waiting their turn for the dyno. I was very impressed at all
> the fancy
> equipment they have. The mechanic in charge told me that I
> had to find out
> the correct valve clearances for the cam (which I did) and
> also told me
> that these mild cam upgrades tend to reduce the engines low
> end power at
> 1st and 2nd gear and are designed for speed on the track
> rather than fast
> acceleration. I find this hard to believe since acceleration
> at the race
> track is always important. Is he bull shitting me? Enrique
> SPAN Inc HeadQuarter
> PO BOX 6524
> La Paz Bolivia
> South America
>
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