G'day everyone, just after some opinions. I've owned a 2WD 1990 F150 with the 5.8 EFI and C6 for 18 months or so. Just realised the PO used spark plugs with an approx 0.750 reach, whereas the stock plug should be around the 0.460 length.

It's been driving fine (apart from a KOER code 18, that's a different struggle) but I wondered if the longer reach would be better, worse or indifferent for everyday operation? I found barely anything online, mostly stuff pertaining to high performance drag and race motors. Whaddaya reckon for our daily driven stockers?

 

I think for daily drivers, as long as the reach isn't so deep as to hit the top of the piston, you will not notice any significant difference.

In theory, you want the spark to occur in the middle of the richest part of the compressed air/fuel mixture. That's where the fuel is most easily lit to start the combustion. This would be determined by the flow pattern of the incoming A/F mixture; by the valve locations and angles, and the overall shape of the combustion chamber. Also, you'd like the spark to occur in that ideal location magically out of nowhere, with no plug electrodes in the way to block A/F flow.

In practice, some of those conditions are not possible, although the newest engines can get close. Most manufacturers now have much better control of the flow inside the combustion chambers so they put the tip of the plug exactly where they want it. This allows them to better control the ignition process, in the pursuit of that last bit of fast and complete combustion, timed at the right moment, to get more usable power out, while leaving a cleaner exhaust.

One thing that some racers do is index the plugs; they make a mark on the insulator to tell them where the ground electrode is, and they can shim the plug so that when it's torqued to spec, the ground electrode ends up where they want it. If that provides any combustion improvement, it's probably a very small amount that most of us daily drives will never notice.

 

Yeah that's what I was thinking. I did read somewhere the extra body of the plug in the chamber could potentially create an area which when hot would cause pre-ignition. But again, probably more with high performance engines and not our old trucks.

 

I'd be concerned about the protruding threads of the plug getting filled with carbon, so when attempting to unscrew the plug, the cylinder head threads could be damaged or the plug break.

 

So turns out after a bit of research, my 1990 Aussie spec 5.8 has E6 heads, hence the longer thread on the plugs.

 

Using an extended "power tip" style plug would be a lot different than using one with the threads sticking out into the combustion chamber. With that said I do think that a projected nose plug is superior in some applications. On my Mercury Flathead it calls for an Autolite 216 but I use a 437. I've done that on 8N tractors before too and I think that the plugs tend to stay clean better if the engine gets started cold a lot and doesn't really get enough high load running.

 

That would explain it.... although that is an unusual combo that wasn't used in north america. I was intending to ask what heads were on the motor as the only small block heads that require long reach plugs are the GT40p and E6SE/TE heads. Have you confirmed that you indeed have E6 heads? There is a capital letter S in the top outside corner of the E6 heads and the letter P on the GTP heads in that location, these heads also have 4 vertical bars on the end face which may not be visible on an assembled engine in a vehicle. GT40p heads would make more sense on a 351, the E6 heads are terribly restrictive although they do make for a very torquey motor, I may be the only other FTE member that has ever owned a 5.8 with E6 head mounted on it.

 

I think that E6 was intended to be a high swirl design and a lot of stuff was changed. I think that the valve lengths are different too.

 

Yeah I got all excited, I thought the PO had put on GT40 heads which were on some of our 90's models Falcons over here, I believe. But no, there's a definite "S" cast into the heads. I actually read some of your previous posts regarding your E6 heads and I guess I'm content with the fact they're a bit torquier and more fuel efficient.

 

Yes they had a very deep kidney bean shaped combustion chamber that produced fantastic swirl effect but badly shrowded both valves, so even when heavily ported as my copies were they still didn't flow as well as untouched E7TE castings.

 

I seem to recall the '86 302 engines with E6 cylinder heads had no valve reliefs on the pistons.
What were the pistons like on your 351 (aka 5.8)?

 

It was a '030" rebuild with aftermarket pistons that had valve reliefs, I had plans to swap on aftermarket heads with bigger valves at some point so the E6's were just an experiment in the mean time.

 

Sorry, I meant the Australian 5.8. I'm curious what pistons it came with.

 

Haven't had the heads off Beans, I'll let ya know whats underneath if I do.

 

Hmm, so you're saying if I were to swap my E6 heads to E7, I'd need new pistons?