Hi,
Let's assume that an engine builder is somewhat knowledgable and has built
10 or more engines...
In this case an engine has been increased from about 10:1 compresion to
about 12:1 compression. The heads were modified with new valves. valve
guides, 3-angle job and all that stuff. The rest of the motor is vanilla
hi-po stuff like forged pistons (.040 over) and balancing. Head was ported
by an amatuer, but adequately performed.
The cam in the car is very mild, about 270 degrees duration, seat-to-seat.
There was minor wear on the lobes, but because no replacement cams were
available to make the owner's timeline, the old cam and lifters went back
in.
Okay - so the engine is installed in the car and run up a few times.
Throttle response is quite snappy, as can be expected with a high
compression engine. Interstingly, there are minor mis-fires at small
throttle openings and pretty much only when the motor is cold. But other
than that, it runs seemingly okay. Ignition is perfect - 42 degrees of
dwell, 10 BTDC static, 33 BTDC at 3000 and rock solid. NO spark scatter.
So, we strap the car to the dyno.
Before the rebuild, there was 122.8 HP to the wheels. Static compression
was about 240 psi on all 6 holes. But there was significant leak-down on
two cylinders (30% or more) - hence the tear-down.
On the dyno this time, we got (after about 8 runs) a max. of 128.47 HP.
Most of the HP gain was due to carb jetting as the motor was PIG RICH
(running 11:1 air/fuel under load) to start. We got that down to close to
12:1 but ran out of smaller main jets (Weber DCOE 40s). Static compression
was about 200 psi and there is no signicant leak-down (less than 4%
anywhere).
Okay, so we have insignificant power gain despite lots of internal mods
including the compression ratio boost. The power gains chould have been in
the 15 HP range min. Where did the power go and what was the fix?
regards,
rml
p.s. I already know the answer
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