Usually it is like this. You don't want too much advance at idle, or
you won't Idle well, and won't have room to advance for acceleration.
THerefore you have centrifugal advance. Generally speaking, and for
power, not effeciency, once you start advancing, the quicker the advance
goes in the better, and there is a maximum that is useful - usually
about 300 or so. Hence 8-10 0 static, + 200 distributor advance ==>
300. The 25D4 40897 has a curve that is at 20(crankshaft)0 at
2400(crankshaft) rpm (see page 456 in The complete Official MGB - needs
a little translation from distributor to crank 0's and rpm). I believe
this is with the vacuum adv. off. There are other distributors with
other advance curves, and The complete Official MGB even shows the 40943
with 240, so more can be tolerated under the right circumstances. So,
the best way to measure this is:
* Remove vacuum adv. line
* get your harmonic balancer marked for 300
* run the engine RPM up to 2400
* Check timing with a strobe for 300
If you want more advance, mark and set it for more, and if you want a
double check of the springs, you can go below and above 2400rpm; below
2400, you should have less advance, above you should have no more
advance. If this isn't the case, your springs are screwed up somehow or
your breaker plate isn't moving properly.
Generally (again) you can advance the timing as much as you want as long
as you don't ping on hard acceleration (which takes some driving). As a
matter of fact a lot of people tune by ear with just that in mind -
advance until it pings, and back it off just enough to prevent the
pinging, and this is probably the most effective way to set your timing
if you have a good ear.
As for compression ratio it was changed by pistons, i.e. by crown shape,
flat or dished. Ratios were 8.0:1 (low compression) or 9.0:1 (high
compression). You can get more with different heads, gaskets and
pistons, but 9:1 is pretty good for performance. Better cams will also
offer better performance if you want it.
Barney Gaylord - chime in on dual point distributors and power at this
point, I've already put in too much here.
Phil Bates
ValveCurtain@aol.com wrote:
> 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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