MTFCA Forum

I don't know...
That looks way too professional, planned out and much more efficient than any Rube Goldberg!!

Thumbs up!

It depends...is it a pencil sharpener?

Humblej wrote: ↑

Thu Jan 01, 2026 4:06 pm

It depends...is it a pencil sharpener?

LMAO

Old Rube would be proud!
Marshall

A follow-up question for the experts: what would the spark plug gap be in free air to simulate a .030 gap at 45-50 psi under compression?

David: Smarter and more experienced folks than me may know all this. I did a search and found this paper where they measured the firing voltage of identical plugs with different gaps under varying compression ratios using air (no fuel, just air). The data is limited but interesting to me. The researchers were interested is establishing ignition designs for WW I airplane operation.

One plug with a spark gap of 0.071" fired at 6,000V in free uncompressed air at 70 degrees F

An identical plug gapped to 0.020" fired at 6100 V in air compressed 3.1 times or approximately 45 psi at 70 degrees F

Here's the link if interested, https://ntrs.nasa.gov/api/citations/199 ... 091116.pdf
"Effect of Temp & Pressure on Sparking Voltage" by L B Loeb & F B Silsbee
Report # 54
National Advisory Committee for Aeronautics, circulated confidentially during the war.

All the best, jb

A follow-up question for the experts: what would the spark plug gap be in free air to simulate a .030 gap at 45-50 psi under compression?

When testing a coil for proper operation a HCCT is typically set to 1/4" with bright clean electrodes
That gap allows for a level of robustness in testing that assures a plug of .025-.032 gap will fire under 4:1 conditions. You are asking for an exact gap value in air to simulate .030 gap under pressure, that would vary for every coil/plug combination. If you want to ENSURE coil/plug operation at pressure then the above answer is what you need and I would expect that this is really the value that you are looking for

this answer is of course contingent on the fact that this what you wish to accomplish. If I misunderstand your question then all bets are off.

I'd limit test gaps to 1/4" to avoid possible coil secondary damage. The shape of electrodes can affect the voltage required to initiate a spark across an air gap, so if you make your own test gap, shaping the electrodes similar to those on an HCCT would be advisable. The ultimate test of a spark plug / coil combination is whether or not it will function consistently in your engine on the road.

Gentlemen, Thank you for your responses, that was the information I was needing. This project started as a way to test various flywheel configurations but has morphed into quite a "show and tell" piece that seems to interest both Model T folks and others. It makes it possible to witness the entire ignition system from magneto to spark plug and see the result. I'm having great fun generating all sorts of data that I have wondered about for many years in real time and under various speeds and conditions.