I have not seen this topic addressed and I hope someone out there can answer my questions. I have a set of Trick Flow Twisted Wedge Heads on a 355 SBC with TRW forged flat top pistons. When I bought the car('37 Chevy Coupe) it had NGK YR 5 plugs. Distributor gear failed and ate up the bearings so I had it rebuilt. Trick Flow was kind enough to send me the tech info on the heads and I noticed two things that caught my eye and thus my questions: Trick Flow calls for NGK FR5(19 MM reach). The YR5 is a 11MM reach plug. What difference does this make and will I be able to tell the difference? The gross valve lift recommended is .501 for this head with my pistons. I have been running a Comp Magnum 292, 244 degrees duration @.050 and .501 lift with the YR 5 with no problems. Will the FR 5 create any issues?
You should be OK clearance wise, just be sure to use a gasket-seat plug. You can always have someone turn the engine over by hand, get #1 to TDC, and measure the distance from the plug hole to the piston. Now will moving the electrode further into the combustion chamber affect performance? Possibly, and quite possibly beneficial. http://www.carcraft.com/techarticles/spark_plugs_tips/index.html
I've experimented with a few different plugs in my Performer RPM heads with 11:1 replacement 350 pistons, and have not had any clearance issues, even with a long plug.
Thanks for the update. I will sleep better tonite! The main advantage of the Twisted Wedge design, as I understand it, was to unshroud the intake valve a bit and if the spark plug is firing back inside the head a bit, it seems that combustion efficiency would be reduced. Does this make sense?
An extra 8mm of reach will definitely get the plug into a better position to light the charge. If Trick Flow recommends a 19mm reach, then I suspect that the shorter YR5 plug has the gap somewhere above the combustion chamber in the plug hole. Not by much, but it's not out into the chamber where they want it to be. If you really want to get nuts about spark plug gap positioning, do a search on "indexing spark plugs".
If I'm understanding Carlover right the 19mm is the length of thread? If so a 11mm reach is putting the electrode up inside the threads, leaving threads exposed. Squirrel, or someone PLEASE correct me if I'm wrong. Use the reach plug that is recommended by the head manufacturer.
You need 3/4" reach, that is likely 19mm, gasketed plug like a nkk 5671a-7 or -8 unless you want projected tip (very common, these are used more in modern motors) then it'd be ngk 5672a-8 or -9 as the heat ranges for these are typically colder.
There may also be the difference of tapered seat and gasket seat, so the actual difference in exposure is not really 8mm.
The Jalopy Journal
