AU NZ Boost retard by using a vacuum advance unit.

[aussie7mains]

Ok people, ive been driving my lpg blown crossie for a while, but Ive only recently given it full throttle. When i did i got pinging when the manifold pressure approached 60 inches hg (15psi). Ive been running 25degrees BTDC fixed timing. So now it would appear that I need a pressure retard to stop this, I think 12 degrees retard would do it. So my idea is to fit a vacuum advance to my Bosch dizzy and connect this to the suction side of the supercharger. Then set the dizzy to 12btdc, the vacuum unit would pull this to 25 degrees at light loads and low manifold pressure but when vacuum gives way to boost this would drop back to 12, QED reducing pinging. Thoughts on this reasoning? I dont know of any electronic setup which could do this, other than the more comprehensive total control ECUs, which i dont want. See the piccie for reference.

 

[DON]

Sounds logical . Any way to test your idea before you start drilling? Maybe just retard base timing ? Manuel retard manually just before WOT?
Is a petrol upgrade possible?
Just a few thoughts from a novice

 

[powerband]

"everything old is new again ... "
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... This was done with the Corvair Spyder where the Distributor was set with an initial advance of 24 degrees BTDC. This 24 degrees gave pretty good performance until about 3000 RPM... , At this point, the pressure retard device - used instead ofvacuum advancedevice - retarded the spark about 10 degrees at 2psig boost. At about 3800 RPM ..., the centrifugal advance came into effect ( another 12 degrees advance by 4500rpm). (Kudos' - MacInnes)
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The stock mid-70's Ford DSII distributor already has a total centrifugal advance limit of a mild 26 degrees - 2X what the D4xx has listed (2X crank RPM). "Locking Out' centrifugal advance is simple enough but controlling total advance Vac/Centrif for performance on and off boost is needed.
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on the blow-thru Holley 250 log motor with @ 8.7 SCR, the 26 degree limit performance is supplemented by allowing for vacuum advance line to intake to add advance until boost when the pre-determined/pre-set centrifugal limit is reached. Works good thru @ 10psig when more control will be needed to prevent detonation at higher boost just like aussie7's mentions. . Water/Alcohol at higher boost levels seems a reasonable non-ECM type option for OCCASIONAL boost bursts.
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With or without added controls or solutions, building for full 1+ Bar (>14psig) boost level performance is a whole other investment. But with modern ignition controls and careful AFR management, the little Falcon Six will get you there. ( For how long is your variable )
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have fun
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[powerband]

Forced Induction learning curve can be dramatic, with inadequate ign. control , a Draw-Thru BOP 231cid Turbo setup got log 250 wound up quickly aproaching 1Bar boost rates until stock piston lands blew off with too much enthusuiasm.
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have fun
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PS -... don't quite understand why MacInnes mentions that the Corvair engine at @ 3800 RPM added the centrifugal advance timing up to @ 12 degrees at @4500 RPM's. Once RPM's up - spooled up [?] to manifold boost pressure - timing advance was/can be added without dangerous detonation ? ?.
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have fun
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[drag-200stang]

I would put a gauge where you are thinking, to see what is really happening under all throttle conditions, the vacuum may more under boost than one would think. Water meth would help. Had mine on a cheap adjustable Hobbs pressure switch I got from AK Miller. Low buck ford 70s windshield reservoir with pump and a electric shut off valve so as not to suck fluid when not under boost
Added bonus, the blower should run more efficient.

 

[aussie7mains]

Sounds logical . Any way to test your idea before you start drilling? Maybe just retard base timing ? Manuel retard manually just before WOT?
Is a petrol upgrade possible?
Just a few thoughts from a novice

If i retard timing it gets hard to start. petrol "upgrade" No, the fuel consumption is high, and at $1.70/litre, it would also involve a tank, pump, filters, carby etc, and would be quite costly. I would probably have to run a single DCOE weber and remake my intake piping, big job. And driving it would become expensive. Right now im considering reducing manifold pressures, by either, reducing blower speed, it runs at 2.95;1 to crank speed by coming down to a 2.5:1 ratio, OR, making it into a 250, this is easy enough given I have most of the parts. This would lower the manifold pressure by 200/250 so to around 12psi. I dont have pistons that would give low enough compression. Its more work of course. I could of course just go NA, but I do like the blown torque.

 

[aussie7mains]

I would put a gauge where you are thinking, to see what is really happening under all throttle conditions, the vacuum may more under boost than one would think. Water meth would help. Had mine on a cheap adjustable Hobbs pressure switch I got from AK Miller. Low buck ford 70s windshield reservoir with pump and a electric shut off valve so as not to suck fluid when not under boost
Added bonus, the blower should run more efficient.

I have manifold pressure gauge from a WW2 bomber, its has two needles, one upstream one downstream so I can see whats going on, its a twin screw blower so quite efficient as it is, however cooling would be nice. Its manifold pressure absolute and inches Hg. The engine is under boost condition quite a bit even when just beetling around. Heres the gauge.

 

[drag-200stang]

May not be a problem but it does not take much oil vapor getting sucked in under boost to add to the problem. Maybe disconnect the pcv for a test. If it helps a separator may work. Every little bit helps. I assume that the air fuel ratio is keeping up and where you want it.

 

[427John]

Speaking of the Duraspark II ignition referenced earlier, the OEM (Motorcraft brand) blue grommet module has a 4 degree start retard feature that could be used for this with creative wiring, the yellow grommet module has 2 4 degree retard features 1 start retard and 1 wired thru the 3rd pigtail to a thermal or barometric switch, the barometric version was used in high altitude areas and I'm guessing the thermal setup was used in the southwest. Years ago back before the common availability of N2O controllers guys used them for retards when using nitrous. There is uncertainty on whether or not the aftermarket replacement modules have those features. The 2 wire pigtail that supplies power to the module has 1 wire that supplies power to the module in the run position of the ignition switch (red wire) and the other supplies power when the switch is the crank position (white wire) this one triggers the start retard feature, you can supply power to the red wire and use the white wire to trigger the retard, on the yellow grommet box you can also use the 3rd pigtail to trigger the other retard for a total of 8 degrees.

 

[aussie7mains]

Speaking of the Duraspark II ignition referenced earlier, the OEM (Motorcraft brand) blue grommet module has a 4 degree start retard feature that could be used for this with creative wiring, the yellow grommet module has 2 4 degree retard features 1 start retard and 1 wired thru the 3rd pigtail to a thermal or barometric switch, the barometric version was used in high altitude areas and I'm guessing the thermal setup was used in the southwest. Years ago back before the common availability of N2O controllers guys used them for retards when using nitrous. There is uncertainty on whether or not the aftermarket replacement modules have those features. The 2 wire pigtail that supplies power to the module has 1 wire that supplies power to the module in the run position of the ignition switch (red wire) and the other supplies power when the switch is the crank position (white wire) this one triggers the start retard feature, you can supply power to the red wire and use the white wire to trigger the retard, on the yellow grommet box you can also use the 3rd pigtail to trigger the other retard for a total of 8 degrees.

Not seen in Australia, we used Bosch stuff mostly.

 

[Frank]

Right now im considering reducing manifold pressures, by reducing blower speed, it runs at 2.95;1 to crank speed by coming down to a 2.5:1 ratio

This seems like the simplest and least expensive place to start.

I could of course just go NA, but I do like the blown torque.

NO DOUBT.

 

[pmuller9]

I dont know of any electronic setup which could do this, other than the more comprehensive total control ECUs, which i dont want. See the piccie for reference.

Would you consider an electronic ignition controller like the MSD Part # 6523 which would operate off of absolute manifold pressure?

 

[aussie7mains]

Would you consider an electronic ignition controller like the MSD Part # 6523 which would operate off of absolute manifold pressure?

These are over $800 here, and the coils to suit are $157, so your looking at $1000, too much. My actual ignition system i have is entirely adequate so far. So no, not really. I am leaning to slowing the blower down, so ill have to find/make a pulley to do this. Its currently 60mm, so Ill ggo to around 75mm.

 

[427John]

What was your initial CR before boost? Over here in the early days of turbo's without full on ECU's with knock sensors and timing control the CR was only the ~8 to 1 range and they still limited boost to 9-10 psi, examples being the Ford 2.3 turbo's,the previously mentioned draw thru 231 Buick, the earlier Corvair Spyder and the Olds turbo 215 had the benefit of much better gas than today and the Olds still used a water alcohol mix injection.

 

[aussie7mains]

What was your initial CR before boost? Over here in the early days of turbo's without full on ECU's with knock sensors and timing control the CR was only the ~8 to 1 range and they still limited boost to 9-10 psi, examples being the Ford 2.3 turbo's,the previously mentioned draw thru 231 Buick, the earlier Corvair Spyder and the Olds turbo 215 had the benefit of much better gas than today and the Olds still used a water alcohol mix injection.

Its 9;1, but I run LPG, propane for you guys. Ive found a 75mm pulley for the blower, so im going to adapt that up as a first stop.