Question about vacuum gauge readings and the impact of a cam for those
Roadster mechanical gurus with real world experience (I've got the
books). Info: U20 engine, Mikuni carbs, 37mm chokes, hotter than Solex
cam, high compression (tested fine too), ported head, pretty much new
top to
bottom, 110 miles. Attached vacuum gauge to intake manifold through the
use of 4
vacuum tubes installed where the manifold plugs normally go on a solex
manifold (if you are familiar... in other words, about 2 inches from the
head), all tied together with t-fittings etc... should be a good
signal. At idle, I get a rapid oscillation between 10 and 15 in of
vacuum. It
steadies out some if I rev the engine, and has improved some with
carburetor
adjustments, but it is still there. Vacuum never goes above 18-20, and
only goes near that high while decelerating when driving. Often it is
running about 5-8 while driving, still with some oscillation. I am not
familiar with vacuum gauges so much in the real world. According to the
book, I should have between 18-20in of vacuum, and it should be steady.
In contrast, my Ford Probe is rock steady when checking from the intake
plenum. This oscillation suggests (again, according to the book) that I
have worn valve guides, but that seems unlikely because 1) they are new,
2) they have been re-checked and found fine and tight, and 3) they were
done by someone who knows his stuff. Also, I seem to be burning some
oil, again. The only thing that is common to this and my last motor
(which did the same oil burn thing) is the head and carbs, so it makes
me wonder what the hell is going on.
Questions: Could my hotter cam cause this vacuum oscillation? Could my
carbs be causing this due to worn parts? How much internal crank case
pressure should there be in
these motors, and will additional venting be helpful (in general) or in
regards to this or oil burning? Any other thoughts? Any experiences
similar out
there? Any known problems or quirks with valve guides and/or stem
seals? Thanks!
On another note, despite the above problems I can confirm that water
injection does in fact work. I am still in the process of calibrating
and refining it, but the car does not knock, even low down and if I
really get on it. This is on 92 octane fuel in a 11.7 to 1 compression
engine at 18 BTDC. The car pulls like an SOB and just wants to go.
Turn off the system, and it is knock city. The system is designed to
activate at 8in of vacuum and over 2000 rpm, and then increase output as
load increases. The only problem so far is that, with my above vacuum
problems, I am going through too much water because of the poor signal.
Still, I am very pleased with how smooth the power is. Anyway, thanks
for any insights, experiences or suggestions.
Greg Burrows
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