I pondered many of these questions when I rebuilt the 18V engine in my
71BGT about fifteen years ago. First, you can definitely tell from the
crown of the piston if its low compression - its got a significant dip (the
high 9:1 compression pistons also have a dip although slight).
The first thing I did was change to the 9:1 pistons plus I decided to make
a few other changes to bring the engine more to a late 60s spec including
dual timing chain. Then I tried to figure out the best advance curve and
even thought about changing the weights and some other modifications. A
local dynamometer shop claimed they could make changes for me. But as
confusing as all the curves were, I decided to stick more to the original
specs which meant acquiring the an older late 60s distributer from a junk yard.
So my advice is just to stick to the 18GH specs as you mentioned, getting
the parts for that engine including the distributer.
As far as timing, I always just time by the road - just adjust until it
drives best. I used to do it that way when I was too poor to afford a
timing light. After I bought one, I found it just didn't work as well as
the ear and the feel.
David
At 04:46 PM 1/28/01 -0500, ValveCurtain@aol.com wrote:
>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
David Councill
65 B
67 BGT
71 BGT
NAMGBR 9-4173
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