Sorry, my degree symbols seem not to have come out very well. I've
corrected it below.
Phil Bates wrote:
> 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 degrees static, + 20 degrees distributor
> advance ==> 30 degrees. The 25D4 40897 has a curve that is at 20
> (crankshaft) degrees 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 24 degrees, 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 30 degrees
> * run the engine RPM up to 2400
> * Check timing with a strobe for 30 degrees
>
> 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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