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RC
Joined: 25 Mar 2007
Posts: 2637
Location: Australia
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 Posted: Tue Mar 16, 2010 11:00 am Post subject: |
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| flosho wrote: | | Anyone able to clarify? |
Sure.
The vac adv scale is expressed as vacuum like most dizzy graphs do. The units of measurement, torr, m bar, "Hg, is the VACUUM from atmospheric pressure. Standardised at 101.3 Kpa but varies with weather conditions and altitude. So the ABSOLUTE pressure (baseline 0) from these readings becomes atmo - vac, in whatever units.
The Kpa measurement as we know from modern EFI tuning maps is an absolute reading, from 0 which negates the ambient atmospheric pressure. This is read by the MAP sensor, manifold ABSOLUTE PRESSURE, not VACUUM (from atmo).
The 3D map appears to be derived from the euro spec dizzy.
As I can`t find the Euro vac curve in what must be the retarded US FSM, this is a US spec illustration that also includes the retard function (2 port vac diaphragm canister) so just disregard the retard trace below the 0* line, since Euro versions are advance only. Also units are mbar, so convert to torr, but will suffice for a general explanation.
Trust this helps explain the difference between VACUUM and PRESSURE, as I know many of our readers here get confused with the concept and terminology. |
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flosho
Joined: 01 Jul 2004
Posts: 3160
Location: Eau Claire, Wisconsin
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| Posted: Tue Mar 16, 2010 12:29 pm Post subject: |
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RC, I'm still a little cloudy on this, but perhaps that is due to me over analyzing the whole thing.
First, is just appears to me that there is no additional advance after ~400mbar and its steady until redline?
Secondly, the second two charts you posted, RC, do those work on top of each other? So.. for instance.. At 200mbar at 1800 RPM the total advance would be ~15 degrees? (Using the top line/plot from each graph)
5.5* at 200mbar on the first graph
10* at 1800 RPM on the second graph
Max advance is 12.5 + 6 or 18 degrees?
Or am I just not understanding these graphs for some reason. Seems no matter how I look at I see the same thing.
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RC
Joined: 25 Mar 2007
Posts: 2637
Location: Australia
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| Posted: Tue Mar 16, 2010 2:07 pm Post subject: |
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| flosho wrote: | | First, is just appears to me that there is no additional advance after ~400mbar and its steady until redline? |
Well looks like the VAC advance starts at about 120 mbar (or torr, as its unclear what the units are in the 3D graph) which is 12Kpa less than atmo = 88Kpa (using atmo rounded at 100Kpa to KISS) then increases to a maximum of 5* at around 220 mbar. So actually theres no more advance after 220 mbar or 78Kpa.
| flosho wrote: | | Secondly, the second two charts you posted, RC, do those work on top of each other? |
Yes. Sorry I just take some things for granted. Should have said that the centrifugal or mechanical advance (based on RPM only) and the vac advance are cumulative (added together).
The graphs I posted are just examples from the FSM, similar but not the same as the Euro NA curves. IIRC they are included in Haynes, along with others.
looking at the 3D graph the mechanical advance is on X axis, front RH side with RPM graduations. Vac adv is the Y axis, left side, with the pressure units (mbar or torr) expressed as value BELOW atmo. Under load at or near WOT simply follow the "front or RH" line from 0* to 15* with the RPM. The OE dizzys have vac adv coming in quickly and is all in by 78Kpa, cruising or light throttle. The total advance then follows the rear line from 5* to 18* depending on RPM. We both know that in practice we get lower Kpa than this on light throttle and theoretically can tune more advance for better fuel economy than with a now obsolete dizzy using vac advance.
| flosho wrote: | | Max advance is 12.5 + 6 or 18 degrees? |
Yes, AT THE DIZZY or cam. Or double that at the crank, 36*. Thats just the ADDITIONAL advance available from the dizzy. Initial advance is compounded, so if initial fixed advance is 6* then at full mech & vac adv the TOTAL adv = 42*. Euro curve initial is 10* but thats at idle, 900 RPM or whatever, and allows for the weights to take up slack and stabilise. Even so, a total 46* at high RPM, low Kpa is acceptable, although unlikely to occur in practice. look at some of the MS tables.
| flosho wrote: | | Or am I just not understanding these graphs for some reason. Seems no matter how I look at I see the same thing. |
Maybe you are over thinking things and overwhelmed. Happens to everyone at some time. Go to bed on it and will open to you in a new light tomorrow, hopefully.  |
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