Here's a question for the techno-freaks and exotica-nerds. All others
hit DELETE now. Please, no flames. I'm too old to put up with them.
Since I'm a budget racer, I have stock gears in the transmission, stock
crank, a 3.7 rear end, and limit my rpm's to 6000 (except at the ends of
Road America straights) to make my engine live. I've also used what I
thought was a relatively mild camshaft.
Using the stock gearbox causes me to come out of low speed corners like
#5 at Road America in third gear at about 3500 rpm. For the past two
years, we've fought the problem of engine breakup between 3500 and 4000
rpm. We tried all the ignition advance and carburetion tricks (dashpot
oil, springs, needles, float levels, fuel pressures, ad nauseum). This
year we FIXED THE PROBLEM by changing intake valve lash from the
specified .018 to .022. The problem truly vanishes. It is a genuine
solution, for we can turn the problem off and on with it.
Now the question is, why does this fix the problem?
The obvious answer is that for some reason this engine just does not
like its little intake valves opening so early, and increasing the
intake lash makes them open later. SOOOOO..as the fellow says
--inquiring minds want to know --. When we took the engine out for
winter freshening, we measured the intake valve events and found that
this .004 delays the intake opening by 5 degrees, but it delays it only
one degree at .050, which, we're told by the cam grinders, is where
"effective" flow (hence, effective duration) truly starts.
NOW the question is (and I really don't know the answer) what is
happening in those first .050" lift that is causing this breakup?
The amateur engineer with the most plausible explanation will be forever
enshrined in the Useless Trivia Hall of Fame, with Special Speed
Reader's Medal for slogging through this lengthy message.
--
uncle jack, saying thank you to el nino
TR4 Rallye Replica vintage racer
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