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Max Cruise Vacuum Advance On A 300 4.9L

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old28racer
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Max Cruise Vacuum Advance On A 300 4.9L

Post #1 by old28racer » Sat Mar 21, 2020 10:26 pm

What would be a good max vacuum advance timing number for a cruise RPM of 2800 - 3200 with a Air Fuel Ratio (AFR) of 15.5 - 16.0 for best mileage?

My unit is a 1971 F250 basic stock 300 4.9L with full dual 2 1/2" exhaust, headers, Holley 2300 2bbl carb, 30" tires, 3.73 gears, C4 trans,Protronix Ignitor distributor module & Flame thrower coil. Motor has about 10K miles sense rebuild. My base timing is 12 deg & 32 deg with max mechanical advance. Truck is empty (no load) 98% of the time.

Mine is set at 12 Base / 32 mechanical advance & 40 with vacuum advance all in. Is 50 - 55 max vacuum advance to much? or not enough?

I also have this on the Ford truck site but little action. Thanks guys.
Bad Day Racing Is Better Than A Day In The Stands :mrgreen:

pmuller9
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Re: Max Cruise Vacuum Advance On A 300 4.9L

Post #2 by pmuller9 » Mon Mar 23, 2020 12:21 am

You shouldn't need to go past 45 deg total.

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old28racer
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Re: Max Cruise Vacuum Advance On A 300 4.9L

Post #3 by old28racer » Mon Mar 23, 2020 11:26 am

Thanks for the reply Paul. I am not clear on what happens when the vacuum advance is to high? What will I see in the truck operation?

Like I said my max advance (base/mechanical/vacuum) currently is 40 degrees.
Bad Day Racing Is Better Than A Day In The Stands :mrgreen:

pmuller9
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Re: Max Cruise Vacuum Advance On A 300 4.9L

Post #4 by pmuller9 » Mon Mar 23, 2020 11:42 am

If the timing is too far advanced the engine will ping (detonation).

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old28racer
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Re: Max Cruise Vacuum Advance On A 300 4.9L

Post #5 by old28racer » Mon Mar 23, 2020 4:38 pm

That is what I thought, but I was not sure. Mostly flat ground in my area of Southern, CA and with the truck empty 98% of the time (no load) it runs real nice, starts easy first hit of the starter, no ping or knock from 0-55 mph my normal cruise range or on hills.

The Holley 2300 2bbl carb as per my AFR gauge is running rich 13.7 at idle in gear & 11.4 at WOT. My cruise ATR is in the 14.7 range, would like to get cruise in the 15.5-16.0 range for better mileage. I am about 90% complete on milling my cast 240/300 log intake to mount my 2300 carb on. Working on an aluminum adapter plate for the system. Will re-jet down a bit after I try the carb on the modified intake.

First thing is to get the dizzy with the Protronix Ignitor 1 & Flame thrower coil installed. This unit has a single port canister that is adjustable. To start I will set it so at 3200-3500 the max vacuum advance is 40-42 like I am running now. After I get the dizzy dialed in & truck is running good I will rejet carb for a better AFR.
Thanks for the help.
Bad Day Racing Is Better Than A Day In The Stands :mrgreen:

pmuller9
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Re: Max Cruise Vacuum Advance On A 300 4.9L

Post #6 by pmuller9 » Mon Mar 23, 2020 5:32 pm

What rpm are you cruising at?

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old28racer
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Re: Max Cruise Vacuum Advance On A 300 4.9L

Post #7 by old28racer » Tue Mar 24, 2020 12:33 am

Tac is a little shaky but it shows around 2800 with 29" tires, 3.73 rear gears, C4 auto trans. Cruise at 50-55 90% of the time.
Bad Day Racing Is Better Than A Day In The Stands :mrgreen:

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