Hi Gang,
A few of you have shown that you use your cars
on the street and on the track. That's way cool.
I hope to put my B-GT on the track this summer at
Hallett as well as on a number of hot road rallies in
the spring. But that is not why I am writing today.
I have been studying everything I can find on ignition
advance curves and am comfortable with how the
system works. Here's a distillation of the research
info.
+ The early cars use centrufugal AND vacuum advance
mechanisms.
+ Static timing varies but not by too much
+ Compression ratios are listed as 'high' and 'low' but little
is published about HOW the ratio differs or how it is
changed -- or the absolute ratios, for that matter.
+ Most of the differences in advance curves are based upon
knee jerk reactions to constantly changing emissions law
limitations, not performance.
All of this brings me to a few conclusions regarding my
1970 MGB-GT.
1. I need to know if my car is high or low compression. The
piston crowns in my '70 B-GT, the car in question, have the
same crown geometry as those in my '79 MGB. The heads
seem to be roughly universal, for the most part. Therefore,
how is the compression ratio changed? Piston crown? Head
gasket thickness? Something else?
2. I don't intend to let my B-GT go untuned and as there is no
emissions testing in Oklahoma, I don't give a wet flip about
emissions. Therefore, I am not interested in the "correct"
ignition timing for my car but rather in the BEST ignition
timing for the car.
The distributor that is supposed to be on my car is a 25D4
# 40897. It has a centrifugal advance described by the curve
below. The vacuum advance maximum is 20 degrees +/- 2 degrees.
RPM Adv. in crank degrees (not including the static timing)
0 0
600 10
1625 24
3000 30
The distributor that I have on the car is a 25D4 #41228. The
vacuum advance maximum is 20 degrees +/- 2 degrees.
Advance is given in crank degrees but does not include the
static advance setting of 10 degrees BTDC. The vacuum
advance maximum is 20 degrees +/- 2 degrees using the
5-17-10 advance module (Vacuum advance starts at 5psi
depression and reached a maximum 10 degrees distributor
angle or 20 degrees crank angle at 17psi depression.).
RPM Adv. in crank degrees (not including the static timing)
0 0
600 3
700 6.5
900 9
1600 15
2200 20
Knowing all this, the question that remains is;
What is the best mechanical advance curve for an MGB,
using an 18GH engine used for aggressive street driving and
the occasional rally (assuming something about the
compression ratio queried above)? From this information,
if some kind soul can provide it, I can choose the best
combination of advance springs for optimum performance.
Thanks,
Rick
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