1968 427/435 rebuild, spark plugs

[Scott R.]

Hi folks, we are almost ready to mount the engine in the dyno, all stock L-71, components/ new pistons and rings. bearings, rod bolts, cam springs and valve seals. Please advise on correct spark plugs, sunnen tune up card list AC 44 N. All other parts of engine are rebuilt as correct as possible. Scott

[Chris H.]

Re: 1968 427/435 rebuild, spark plugs

Scott I believe the chassis manual mentions 43s. In my ‘69 L71 I run 45XLSs. 45s seem to be the preferred plugs.

[Duke W.]

Re: 1968 427/435 rebuild, spark plugs

I don't think the 44N and OE 43N are available any more. The first number in the AC numbering system, 4, means a 14mm thread. The second is the heat range. A higher number is hotter.

Despite the OE plugs being heat range 3 they are too cold for normal road driving. Heat range 5 is a better choice. The suffix letters indicate geometry features and all current AC plugs are resistor type carrying the prefix, R.

If you plan on doing power pulls on a lab dyno I recommend heat range 3, but swap them out with 5 for normal road driving.

IIRC the N suffix plugs only have a half threaded reach, but the current XL and XLS are full thread. The additional prefix S indicates an extended tip which will increase CR slightly due to removing 1-2 cc from chamber volume, which is not significant.

So AC R43XL or R43XLS for the dyno and R45XL or R45XLS for normal road use.

I have no idea who makes AC plugs nowadays, but it's not GM... probably the lowest bidder. I prefer NGK and Denso.

Both have online cross reference charts, but don't include the R prefix, which will yield non-resistor plugs as were the OE plugs.

Duke

[Scott R.]

Re: 1968 427/435 rebuild, spark plugs

Hi All, Motor ran on Dyno, started first turn ran excellent !!! we did a couple of pulls ran to specs 435 HP / 450 torque. Muscle Machine owner, Mark said he never runs a motor where he does not have to fool around with something !!! Scott oh, have a Z06 new alternator belt #303409 , but it is not long enough to fit, is it the wrong one ? is there a different one for 435 hp ? Scott

[Mark E.]

Re: 1968 427/435 rebuild, spark plugs

Did it peak at 435 hp or did you stop when that was reached? What rpm?

[Scott R.]

Re: 1968 427/435 rebuild, spark plugs

Hi Mark, I decided to not to exceed 6200 RPM, so when we got the 2 pulls done the graph showed almost exactly what the GM graph showed in the sales brochure. Since this is not a race motor I did not the need to try for more even though the motor builder said it got plenty more power. Scott Does anyone know where to get the grey 45 degree spark plug boots that are the correct length, 2.5 inches long ?