We were recently doing some engine dyno testing of a 3/4 race "fast road" TR-4
engine destined for a Morgan. We had what I thought were some very interesting
results.
The engine is an 87 mm bore Hepolite piston engine with standard size
valves, 10:1 CR, 280 degree seat to seat cam (similar to the Competition Dept
"D" cam), a "pocket ported" head with a 3 angle valve job. The testing was
done with both HS-6 carburetors on a long manifold (these carbs and manifold
were dead stock and in perfect shape with the float bowls mounted on rubber
using MG-B mounting insulators) and with a fully race prepared set of H-6
carbs on a race prepared short inlet manifold. The engine had "Tri-Y" style
headers and was run both with an open exhaust and with a straight through
English style muffler.
The first and most important thing that we found was that the first day of
testing we could not get the engine to repeat power runs accurately enough to
learn anything at all until we modified a 45 DCOE Weber "soft mount" and put
it in between the HS-6 carbs and the inlet manifold to insulate the carbs from
engine vibration that apparently was upsetting the fuel delivery to the
engine. As soon as this was done, we were able to start to get some testing
results.
The next surprise was when we added velocity stacks to the HS-6 carb set
up. We tried the old aluminum parabolic stacks that I had seen on the
Competition Department TR-4s, and also a set of stacks that I had made many
years ago that look like Weber Velocity stacks. Either stacks just killed the
power above 5500 rpm. I mean it just took a nose dive, whereas without any
stacks the power would peak at about 5400 rpm and then stay almost level until
6,000 rpm. Unfortunately we did not have a set of "stub stacks to try.
Apparently the 13.2" length of the intake and carbs is very much a "tuned
length, at least on a 3/4 race engine. At least that is what I concluded. What
a shame, because the Velocity stacks look so cool!
We then found that with no stacks, the stock HS-6 carb setup made a higher
torque peak by almost 8 ft. lbs. than the race prepared H-6 setup.Above 5,000
rpm the H-6s on the short manifold was slightly superior. The H-6 carbs did
not need the "soft mounts" and the "weber like" velocity stacks worked well on
them.
It made very little difference whether the muffler was on or the engine
had a straight open exhaust! But remember the muffler we were using is a very
free flowing unit, we have used it on engines making up to 150 hp and this
engine was in 120s.
Our best runs made 123 hp with a very broad peak that gave over 120 hp
from 5,000 to 5700 rpm and 138 ft lbs of torque with over 130 lbs. from 3300
to 4700 rpm and with the peak around 3900 rpm.
All of the testing was done on 91 octane pump gas. The best power was with
34-36 degrees total advance measured on the outside of the flywheel.
Regar
ds,
Greg Solow
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