Hi list,
The last time I ran my 69 Sprite on the chassis dyno was March this year and
due to a couple of problems I never saw the power I was looking for.
Since that session I have fitted a new electronic ignition system and coil
(which the tacho doesn't like!) to cure the high rpm misfire. I also have
the base section of the 39mm Full Radius rams machined down by 2mm to
increase the clearance between the ram and air filter case lid (K&N).
Before taking the car to the dyno it did feel up on power and did pull over a
genuine 110mph a few weeks back (still pulling but I had to start braking for
a roundabout).
Unfortunately while the session showed that power is up 6 bhp on the March
sessions there is still a problem with the engine. Turns out I have valve
bounce at about 6k but only on the back 2 cylinders (or one of them). This
drops the power by 4 bhp when in fact it should still be climbing (engine has
286S cam). I don't have the time to pull the head and strip it so will most
likely just keep the revs below 6K until the summer - not a problem given
that it is making over 100bhp before that rpm, and car has a 5 speed box.
One interesting part of the session was to see what difference a swap from
3.5 to 5.0 aux' vent' had on power. A bit of bind because it meant pulling
the carb while paying for dyno time (the eventual time/bill was 2 hours and
£90). With the aux vent swap came a decrease in main jet size to balance
things out - a drop from 185 mains to 160 mains. The 5.0 aux' vent's did not
make more power but the mixture was still bang on and neatly illustrated the
relationship between aux' vent size and main jet (also validates the Weber
jet selection size apparent anomalies in DV yellow edition). It is possible
though that with more power the 5.0 might have shown up a power gain - I'll
never know till I get a decent cyl head from A.P.T. As an aside one run was
down with no air filter (K&N) and power recorded was the same without as with.
A less encouraging part of the dyno session was that after a decade the car
has an engine that still isn't making any more power but ought to be even
with the current unleaded head which has smaller valves than the earlier lead
only head I had fitted. At least the power is further down the rev range
which to me indicates that with the valve springs on this head sorted or an
A.P.T head the spec of the engine would set a new record power output for the
car.
Finally for completeness the dyno mileage was 9 miles, most of which was
between 5 - 6,500 rpm.
Daniel1312
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