What Spark Plugs should I use?

fvike · May 18, 2010

Hi, posted this on s b f t e c h .com, but no answers:

I hope you guys can help me with spark plug selection. I rebuilt my engine, a '69 351W, a year ago, and I've been having trouble with dieseling and overheating and pinging ever since. Not a good combo, so I haven't driven much. Anyway, have been playing around with the ignition lately, and gotten it in the ball park, at least. So it starts pretty easily, but have some pinging when under load, such as accelerating up a hill. It can also diesel sometimes. So the next step for me is to go on to carburetor tuning (I know it's lean), but then I thought, it could all be a waste if the wrong plugs are in it.
Here are the engine specs:

* 1969 351W, overbored 0.030
* New Scat cast 3.500 crankshaft
* New Melling standard volume oil pump
* Moroso 9qt roadrace oil pan
* Original connection rods, ARP bolts
* Wiseco Pro Tru forged pistons
* Edelbrock Performer RPM heads, ported (60cc)
* Edelbrock Air-Gap manifold
* BG Speed Demon 750VE carb
* Speed Pro CS-1162R camshaft (Equalent of the Edelbrock Preformer RPM)
* Complete Crane ignition system, with optical/digital distributor

According to Wiseco, the pistons with 0.038 thick compressed head gaskets (like mine) and 0.010 deck height (I don't know what mine is), and a 60cc head, comp ratio should be 10.8:1.
http://www.wiseco.com/Catalogs/ProTru/FordOldsPontiac.pdf

I have been using NGK FR-5 spark plugs, but I'm not sure if they can handle the compression ratio. BTW, I'm Norway, and I use 98 RON octane gas, and it should be the same as your 93 PON octane.
Any input is welcome!

rbohm · May 18, 2010

i would use replacement autolite plugs, they seem to work the best.

blown65 · May 18, 2010

rbohm said:
i would use replacement autolite plugs, they seem to work the best.

X2, I've had great luck with Autolite stuff. No so much with NGK.

fvike · May 18, 2010

I've been looking into Autolite, but I'm not sure what heat range to use. I think maybe 6..

j69302 · May 18, 2010

whatever plug you decide on, make sure you check with your cylinder head manufacturer and get the correct REACH and seal type.

I have edelbrock performer rpm head on my 302 which aside from displacment is pretty similar build to yours, and if I remember correctly they require a 14 mm, 3/4 inch reach, gasketed plug. and I believe they recommend Champion RC12Y if I remember correctly. I went with whatever they recommended and they are working out well. No complaints.

I also had Autolite platinum plug when I had the stock iron head on and those things lasted forever as well.

I have been using NGK FR-5 spark plugs, but I'm not sure if they can handle the compression ratio. BTW, I'm Norway, and I use 98 RON octane gas, and it should be the same as your 93 PON octane.
Any input is welcome!

Also the plus doesnt really care about the compression ratio. However you do need a hot enough spark, which is created by the ignition. The heat range on spark plugs is the amount (or lack of) ceramic insulator around the inside of the plug. Having to little of insulation can result in the plug being a hot spot and cause preignition. Having too much will cause carbon to build up on the plug and foul it.

If you think the compression is causing problem with ignition, you need to upgrade your iginition to a higher power or change the gap. A larger gap (assuming the ignition can still create a spark) will create a hotter spark. 10.8 isnt that high, I would not think the compression is causing any issues.

What gaps are you running?

fvike · May 19, 2010

j69302 said:
whatever plug you decide on, make sure you check with your cylinder head manufacturer and get the correct REACH and seal type.

I have edelbrock performer rpm head on my 302 which aside from displacment is pretty similar build to yours, and if I remember correctly they require a 14 mm, 3/4 inch reach, gasketed plug. and I believe they recommend Champion RC12Y if I remember correctly. I went with whatever they recommended and they are working out well. No complaints.

I tried Champions once, didn't work well for me.

I also had Autolite platinum plug when I had the stock iron head on and those things lasted forever as well.

I've heard great things about them, I'd like to try Autolites.

Also the plus doesnt really care about the compression ratio. However you do need a hot enough spark, which is created by the ignition. The heat range on spark plugs is the amount (or lack of) ceramic insulator around the inside of the plug. Having to little of insulation can result in the plug being a hot spot and cause preignition. Having too much will cause carbon to build up on the plug and foul it.

I looked at the NGK Tech Support site.. found this:

NGK said:
Ignition Timing
*Advancing ignition timing by 10° will cause the spark plug tip temperature to increase by approximately 70° to 100°C.
*A colder heat range spark plug may be necessary if the ignition timing has been advanced to near the knock level. Higher cylinder temperatures near the knock level will bring the spark plug firing end temperature closer to the pre-ignition range.

Compression Ratio
*Significantly increasing the static/dynamic compression ratio will increase cylinder pressures and the octane requirement of the engine. Knock may occur more easily. If the engine is operated near the knock level, a colder heat range spark plug may be necessary due to the resulting increased cylinder temperatures.

That is exactly what I experience, the ignition timing has to be near knock level to run any good. So I know it's a little lean, going to work on that tomorrow. Gonna lend me a A/F sensor. But if I still experience knocking after that, I'm considering going to a colder plug, heat range 6.

If you think the compression is causing problem with ignition, you need to upgrade your iginition to a higher power or change the gap. A larger gap (assuming the ignition can still create a spark) will create a hotter spark. 10.8 isnt that high, I would not think the compression is causing any issues.

I have a complete Crane Ignitions system, HI-6 box, Firewire cables, and optical/digital distributor. Optima Red Top for the juice. I've had a little alternator trouble, but found it had a bad ground. Added another ground wire just days ago. Hope it works. It's a one-wire setup, so it was supposed to ground thru the block.

What gaps are you running?

Never paid any attention to that, whatever is standard on the NGK FR5.

Thanks for the input. I'm going out to the garage and have a look at them plugs

TeamEntity · May 19, 2010

nobody mentioned motorcraft. there a solid plug, coppers are fine. plus whats your total timing set at? I would bump it a few degrees and keep testing till it seems to stop detonating. wouldn't hurt to richen it up a tick too if you think you've gone too lean. Your plugs will tell you everything you need to know.

MustangEngineer · May 20, 2010

Motorcraft Platinum plugs. That's what comes it from factory. Just make sure its platinum plug. Regular plugs want last as long. Bosch Plugs are a big mistake stay away from them.

tx65coupe · May 20, 2010

I say only run Motorcraft or Autolite plugs.

From what I understand it is a bad idea to run platinum plugs with aluminum heads. Does anyone know why that would be? Our 97 Cobra has platinum plugs in it?

I don't think that setup originally came with platinum plugs. I'll definitely agree that BS fancy plugs like Bosch +4 etc are a waste of cash.

fvike · May 20, 2010

Looked at the plugs yesterday, they were brown/tan. Very nice. But that is cruising. I think it's lean at WOT. Gonna head out later this evening with an AF sensor and find out. Plug gap was not much, so opened up to .045.
Oh, and they were in heeat range 6, not 5 as I thought.

TeamEntity · May 20, 2010

well the plugs sound good, a pic would help. I say advance the timing 2 degrees and see how she acts. O.E. plugs in the day were copper. Unless you use this car every single day like a d.d. copper will work fine for yah and Is my recommend choice. Plus on the average of 1.99 a plug you can try out a few different sets with your timing and not break the bank.

fvike · May 21, 2010

Did a little tuning yesterday. First had a drive, it was lean all the way trough the band, except cruising. Around 16. But the plug gap had helped, the engine was smoother, and a lot less interior resonance. So I upped the jets from 73/83 to 75/85. It's a Speed Demon 750 vacuum sec. Drove it again, it was around 14.2 tor 14.6 on cruise, and a steady 13.1 at WOT. So sat the timing to 12 degrees static, (had 8). But it was getting late, So I'll continue another day. Probably need to jet it once more, and the target for the ignition is 14 degrees. Just taking it step by step.

TeamEntity - Plugs looks to be okay, They are NGK V-power plugs. Just tuning left I think

65FBE2 · May 21, 2010

The timing should be 34 total. what ever it takes initial to get 32-36 total at around 2800-3000 rpm. make sure you disconnect the vacuum advance when checking

TeamEntity · May 21, 2010

sounds like your getting in the ballpark. great job. Good to hear.

fvike · May 21, 2010

Thanks, it's fun when it works I'll get back to you on any progress..

fvike · May 24, 2010

Did a bit more tuning today. Jetted up to 77/87, and then it was very nice on WOT, from 13 to 12.4 @ 5000 rpm. It got richer as it climbed in rpms. It was a bit lean on cruise, around 15-16. So I changed the primary jets up to 79. It was nice on cruise too. 14ish - 15. Still got a dead spot at idle, but I still haven't set the mixture screws. Gonna do that, and try setting the timing to 14 degrees. Could possibly add a bit more vacuum too.

TeamEntity · May 24, 2010

hey sounds awesome. running a tad lean while cruising is fine actually pretty good. and under load climbs up. thats really good. specially for power adders later on and keeping things cool and lubed up. I would not mess with the jets anymore until you adjust the mixture screws. Peace.

jerry S · May 25, 2010

Frode:

You want either the autolite 3922 or the 3924. Buy them both and give them a try. One or the other is exactly what you need

regards,

Jerry

fvike · Jun 1, 2010

So, final tune is 79/87 jets and static timing to 8 degrees. Have no knocking, and the car has never run as good as now. It runs real good, and pulls strong thru the register. Starts easy too. Opening the gap on the plugs had a real big impact.

TeamEntity · Jun 1, 2010

awesome dude. glad she's running great. I may not know all but tuning i do know. Peace.

What Spark Plugs should I use?

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