In a message dated 95-07-11 20:25:27 EDT, you write:
>How much compression? 10.5:1, 11:1?
>More compression ratio needs more octane.
>Too much octane reduces power, but doesn't
>hurt the engine. Too little octane can really hurt.
>Some genius with a dyno once claimed to =
>prove that a 10:1 engine ran best at 98 to
>100 octane. The 93 octane at the pump today is meant for the 9:1 de-tox=
ed
>motors of today.
>You may need something more on the order
>of 100 to 110 octane. Try airports, race tracks or octane boosters.
>If that doesn't fix it, then try timing.
It's the standard high compression engine for the time. I have seen the
compression ratio specified at 8.8:1 vs. 8.0:1 for low compression. I do=
n't
have a compression gauge, so I haven't checked it myself, but it translat=
es
to 160 lb/sq.in. vs. 130. Maybe I should buy a compression gauge. The
rebuilt engine was bored .030 over, but that should only have a small eff=
ect
on compression. =
>Seems to me if you have a problem under hard
>acceleration at 4000 RPM, you'd want to check the
>timing at 4000 RPM. (Or 3 to 4.5). Maybe one of the
>books you have has a distributor advance curve
>that you could compare against. Get a dwell tach
>that's easy to read while you're looking at the strobe, degree off the
pulley
>and check it with, w/o vacuum advance plus steady state and under
>acceleration (big vacuum changes). =
After checking the tach, it was really at about 2500 rpm, but only really=
bad
under hard acceleration from 2500 to 3000. I am borrowing a dwell tach f=
rom
a friend but don't know how to use it, hopefully he will tell me. As to =
the
distributor curve etc. seems I have a lot of conflicting literature. Th=
e
Complete Official MGB 1962-1974 shows a static timing of 10=B0BTDC, but =
under
the Distributor centrifugal advance it shows:
"Cranckshaft degrees/speed (vacuum pipe disconnected)
6=B0 at 600 rpm
6=B0 at 700 rpm
9=B0 at 900 rpm
15=B0 at 1600 rpm
20=B0 at 2200 rpm"
Also shows:
stroboscopic ignition timing at 14=B0BTDC at 600 rpm
the curve in the back of the book shows a steady increase in distributor
advance from 0=B0at 350 distributor revs /min (I know this means 1500
crankshaft rpm) to 4=B0 at 400 dist.rpm another line from there to 10=B0 =
at 1200
dist. rpm =
from this I read maximum advance of 10=B0 at an engine speed of 2400 rpm.=
Is
this not correct?
Now, if the static timing is 10=B0, how is it that at 600 rpm, with about=
6=B0
advance (spec is 6, the curve is about 5.8), the stroboscopic timing shou=
ld
be 14=B0?? Shouldn't it be 16=B0? Or if that spec 6=B0 ,600 rpm is corr=
ect for
stroboscopic timing, how is it that a distributor with a total of 10=B0 a=
dvance
hits 20=B0 at 2200 rpm if it is at 6=B0at 600?? =
I have a lot of MGB books, and none are fully consistant.
If you're still interested, see my other post about the springs. Oh yes,=
and
I am going to the airport for low lead aviation fuel as soon as this tank=
is
low enough.
Phil Bates
67 MGB
75 Jaguar XJ12C
52 MGTD replicar (VW)
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