Spark Plug Gaps vs Compression Ratio

[Bob Bidonde]

Rule of Thumb: As the compression ratio increases, the spark plug gap decreases.


What spark plug gaps are recommended for the following using 87 Octane gasoline:

Stock 4.2:1 Compression Head: _________________
5.5:1 Ford Police Head: _________________________
5.5:1 Compression Current Aftermarket Head: ______
6:1 Compression Aftermarket Head: _______________

[alexiskai]

From the Service Bulletins, August 1932. Rather than using compression ratio, Ford used compression test pressure as the rule of thumb. And I guess if you check compression pressure the shade-tree mechanic way, it really is a rule of "thumb."

[Jim Brierley]

All the same gap. I use the same on my 7:1 with no problems.

[Benson]

FWIW

I agree. 55 years ago I tried reducing the gap from 0.035 to 0.030 for my Police Head, 5.2 to one and it did not run very well.

Tried 0.025 and it ran worse.

It did not run as well with reduced gap ... went back to 0.035.

I also have 7:1 head plug gap set to 0.035.

Even the original Thomas 8.25:1 liked 0.035.

[BillCNC]

Originally Posted by alexiskai

it really is a rule of "thumb."

Much more than a thumb, Physics. Molecules packed tighter together resulting in less resistance.

Regards
Bill

[ndnchf]

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This is a good example of how useful the old pressurized spark plug testers are. The pressure to the tester is regulated to match the engine pressure. Then you can play with the plug gap and watch how it works.

I made this short video of mine a few months ago.

https://youtu.be/7EegDUTFsR0

[Ruth]

Steve, that is pretty cool. Thanks for sharing.

[Bob Bidonde]

I have a Model "B" engine with a 6:1 compression cylinder head. Timing is standard for a Model "A", that is it's set at the TDC of the power stroke of cylinder #1. While cruising, the spark lever range is limited to 4 notches down to avoid knocking.

To restore a more useful spark advance range at the lever, the distributor cam needs to be retarded from the standard Model "A" setting. How many clicks of the cam?

[Big hammer]

Last weekend I was watching a tractor show that had an experimental tractor, one of one. It had a 12-1 compression ratio engine, it was run on aircraft fuel 130 octane, it produced 50 hp. Bob I’am guessing that your running 87 octane gas, if you upped the octane to 91 you could then advance the spark lever a couple more clicks. If you set your base timing with the spark lever down two clicks your base timing would be more retarded. This would allow the spark lever to be advanced more, the only thing that I can think of to be able to use the spark lever more like stock is to increase the octane of the fuel.

[BRENT in 10-uh-C]

Bob, aren't you are asking for recommendations for a topic that you have not given enough information on first? Generally speaking, the wider the gap is preferred as long as the electrical spark can jump the electrode under all operating conditions. Wouldn't the intensity and/or voltage output of the Coil be a huge determining factor in sizing the plug gap?

[Jim/GA]

Originally Posted by Bob Bidonde

I have a Model "B" engine with a 6:1 compression cylinder head. Timing is standard for a Model "A", that is it's set at the TDC of the power stroke of cylinder #1. While cruising, the spark lever range is limited to 4 notches down to avoid knocking.

To restore a more useful spark advance range at the lever, the distributor cam needs to be retarded from the standard Model "A" setting. How many clicks of the cam?

Is this why you originally asked about plug gap? Because your engine knocks easily with too much advance? The two are not related, really.

If you set the initial time 2 clicks retarded, then you will use 2 additional clicks to get back to where you are now when it starts to knock. No real benefit there.

Better to find the source of the knock.

1) What spark plug are you using? They may be too hot.

2) How confident are you that your initial timing now is really at TDC? How did you set it? Because if it is really advanced initially, that's why you can't advance much more before it knocks.

[nkaminar]

As usual Brent has the right response. With higher pressure it is harder to jump the gap. But the proper thing to do is to have a hotter ignition system. Racing cars will have a large gap and a very hot spark.

The proper ignition advance is predicated on the best performance from the engine. The fuel/air takes a certain amount of time to burn. The pressure should be the maximum when the piston is at the top. Too advanced and the pressure is slowing the piston down before it gets to the top. Too retarded and the pressure is not doing the best job at powering the piston down.

Higher compression requires less time for the fuel/air to burn so the ignition does not have to be as advanced. Very high compression will cause a low octane fuel to burn before the spark plug ignites it which is like having the ignition too advanced. Or the fuel/air will ignite in the corner of the combustion chamber in an explosive manner after burning starts and the pressure is increased. This is a different issue than having the ignition set at the correct timing.

Unless you are running very high compression ratio the higher octane fuel will be a waste of money. For most engines the higher octane will not change the optimum ignition timing.

[Bob Bidonde]

I am curious to see what you all have to say on this topic. The August 1932 Service Bulletin suggests a variation in plug gaps, but that was relative to the time it was written. Today we have much better gas and ignition coils to work with, so my actual practice is:
>Leave plug gap at 0.032 to 0.035 so the coil has plenty of time to develop its E Field;
>Run 87 Octane gas;
>Time the ignition with the spark lever down 4 notches, so I should get a total of 8 notches to drive with.

[john in illinois]

I check my timing with a timing light. It is set for 28* 2 clicks above full advance at cruising. 5.5 head and 87.

John

[old31]

Would an electronic ignition have any bearing on what the gap would be?

[tinkirk]

Originally Posted by old31

Would an electronic ignition have any bearing on what the gap would be?

I run mine at .040 with electronic ignition

[johnneilson]

Compression, or actual cylinder pressure would be the concern, higher pressure creates more resistance to jump the gap.
The main component to this is the coil, secondary voltage produced and the ability of the rest of the system to deliver (wires, distributor etc).
If you look at the size of the disty cap, and compare with a GM HEI, you will notice the size mainly to house the coil. But, these could jump a .120 gap in the plugs.

Magnetos are another story all together, but the faster you turn one, the more spark you get.

John

[BRENT in 10-uh-C]

Originally Posted by old31

Would an electronic ignition have any bearing on what the gap would be?

Originally Posted by tinkirk

I run mine at .040 with electronic ignition

An electronic ignition system such as the Pertronix Ignitor has no bearing whatsoever on spark plug gap width. All the aftermarket Model-A electronic systems are doing is triggering the spark timing. The amount of width setting (gap) between the electrode to the ground is determined by the Coil windings.