Hp Super C
New User
Has anyone used a Bosch blue coil on their Farmall? It claims to put out 50,000 volts.
- Thread starter Hp Super C
- Start date
Janicholson
Well-known Member
Coils only putout voltage that fires the plug. 50,000 is lots. 20,000 is more than needed in our world of tractors. If the primary resistence is around 3 to 4 ohms it will not overload the points. If less, it will not be a long lived proposition. Unless there is something very wrong, it does not need voltage it will never reach. Jim
Hp Super C
New User
I checked the specs on a stock coil and it’s 1.5 ohm. Same as the Bosch I found. The reason I’m looking for more spark is I put a 3.25 stepped head piston kit in, 240 head and manifold, and a bigger carb. I’m thinking I can get a little more out of it if I get a hotter coil and open up the plug gap.
Janicholson
Well-known Member
If you are pulling with it fantastic. The plugs are probably firing in a 150 or so compression pressure. maybe a bit more you probably know. Gapping a .025 plug to .035 changes the required voltage. Plugs fire as soon as they are presented with enough voltage to jump the gap, and no more. I spent my first 30 years in the automotive industry. and I have seen engines needing 25,000 volts to run at 7500 rpm in cars, and 15,000 in motor cycles.
Looking at diagnostic equipment. So at 7 volts running and charging it would draw about 4.6 amps. if running 14.6 volts charging, it is well into 8 amps without resistor. With resistor of similar ohms, you are back to the 4 amp OK level. A dyno will tell you the answer. Seat of the pants maybe not. If it misses at all when in operation it could help. especially if the original coil had a path to ground before it reaches plug firing voltage. Good luck with it, I like increasing performance until things break. Jim
Looking at diagnostic equipment. So at 7 volts running and charging it would draw about 4.6 amps. if running 14.6 volts charging, it is well into 8 amps without resistor. With resistor of similar ohms, you are back to the 4 amp OK level. A dyno will tell you the answer. Seat of the pants maybe not. If it misses at all when in operation it could help. especially if the original coil had a path to ground before it reaches plug firing voltage. Good luck with it, I like increasing performance until things break. Jim
Hp Super C
New User
The compression ratio on it is somewhere between 8-8.5:1. The stock 240 had 7.3:1 with flat pistons. The step on the new piston is .2” and the piston is a few thousandths taller from the wrist pin, plus the bigger bore. I’m trying to get it over 40 HP to the PTO.
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