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.

 

[pmuller9]

https://www.holley.com/products/ignition/ignition_boxes/street/parts/6523?srsltid=AfmBOoomekkUpthS_Y4_Jjzr-xPdhfaZSVmgaoFeGdDg5yN7TQgCVyDb

 

[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.