HEI and Spark Plug Reach

[John Rogers]

Bill Robertson and others have pointed out the advantages of High Energy Ignition (HEI) by increasing the power of the coil's secondary voltage output, bypassing the ballast resistors and increasing the spark plug gap size. I have nothing to add to what has already been posted on these three factors.

What does interest me is a fourth factor of Spark Plug Reach mentioned in the 1970s paper (Measuring the Effect of Spark Plug and Ignition System Design on Engine Performance) quoted by Bill Robertson, both in this forum and in Bill's excellent YouTube videos on HEI. In the previous mentioned paper, it was pointed out that increasing the projection of the spark plug gap into the combustion chamber increased the amount of hydrocarbons that were burnt thereby leading to higher engine performance.

The spark plugs that we plan to run in our DeLorean are NGK BPR6EFS plugs. As this code indicates, these plugs have an E rating which indicates that they have a reach of 19 mm into the combustion chamber. I am interested in discovering how far into the combustion chamber spark plugs can reach before they run up against pistons and valves.

I am very interested in hearing from other DeLorean owners as to how far they have found their spark plugs can reach into the combustion chamber without hitting the pistons or valves. Obviously if someone is running spark plugs with a longer reach than 19 mm then this would indicate that that longer reach is possible.

Should someone have one of the heads off their engine it might be possible to measure the maximum reach of a plug that is possible.

Hoping to hear from fellow DeLorean owners regarding their experiences regarding spark plug reach.

[Bitsyncmaster]

Interesting. I run the stock plugs. I would think the stock plugs would already be close to max reach. I guess you can install a longer one and turn the engine over by hand. If it stops you have gone to far.

[John Rogers]

Interesting. I run the stock plugs. I would think the stock plugs would already be close to max reach. I guess you can install a longer one and turn the engine over by hand. If it stops you have gone to far.

Good idea re turning by hand. Hopefully someone has already discovered that a longer reach is safe. As far as I know my plugs are stock as they are simply the plugs sold by DeLoreanGo. Looking forward to getting our car going and using the relays etc purchased from you.

[David T]

The safe way to do this is to put modeling clay on top of the piston and turn the motor over. I doubt changing the spark plug reach will matter much. Indexing the plug so the ground electrode doesn't shield the air/fuel mixture may matter. Any improvement may be so small as to be unmeasurable.

[John Rogers]

Modelling clay is another great idea. I am hoping that someone is running longer reach plugs than I am. I can therefore benefit from their experience. For those interested, the paper that I referred to which investigates whether longer reach plugs improve ignition can be read (courtesy of Bill Robertson) at https://www.johnrogers.com.au/spark_ignition_performance.pdf The relevant pages are pages 51, 52 and summary point 2 on page 60.

[Ron]

Hey John,
Just ignore if I'm wrong but it seems you may confusing extending gap locations with longer reach plugs. The reach of a spark plug is the distance from the seat of the plug to the end of its threads, excluding the electrodes and any extended metal shell.
If you extend the reach much, you are prone to exposing the leading threads, creating hot spots (preignition).....
I don't know of any spec charts that would show what's needed to find what you seem to be looking for.

The NGK Part Finder Site (https://ngksparkplugs.com/en/part-finder) shows the NGK BPR6EFS to have a reach of .708" (17.9832 mm). The OEM Bosch HR6DS plugs, .691" (17.55 mm). [Summit Racing]

FWIW, I'm with David WRT any measurable gains to be had doing this to a PRV.

[John Rogers]

You are right Ron. I have been assuming that reach indicates how far the spark plug protrudes into the combustion chamber. It seems that it does indicate this but not in the best way possible. Below is a photo from the previously mentioned paper that discovered that extended gap location, as well as other factors, "reduced the cyclic combustion variation and the unburned HC." (hydrocarbons). Explanations for this can be found in the previously mentioned paper, for which I included a link in an earlier post. The illustration does indicate that extending the length of thread is not the solution.


70957

I am not interested in persuading people of the correctness of the paper's findings. I am purely interested to discover if the spark from the plug can be projected further into the combustion chamber.