Someone correct me if I am wrong, but don't the GT40P heads have the spark plugs angled out 90 degrees from the head, and the GT40 heads have the spark plugs angled at around 45 degrees? Wouldn't the GT40 heads be more user friendly when it comes time to install headers?

Last edited by Ron68 (4/17/2025 1:40 PM)

Nos681 wrote:

In case you have slept since then. 😁

https://fyi.boardhost.com/viewtopic.php?id=11979

Yes I have Hi-Po manifolds on my car with the GT40P heads. Was just curious as to what plug people were using on their GT40p heads I hear the regular GT40 heads use a different plug.

I miss Josh-Kebobs Post 

RTM wrote:

I'm not sure why but I'm thinking there was a shorter plug one could run?

I am just trying to see what plugs go in them  not a tight fit or anything I dont think I just wanted to grab a new set to go in it but was wondering what people were running in theirs with those heads. I mean I can take one out and go get some just like them but I am not sure they are the right ones. I think the ones I have in their now are Autolite 104 copper they come gapped at 0.050

Last edited by BILLY WALTON from GEORGIA (4/17/2025 7:37 PM)

I thought that 3-bars designated the GT-40 heads and that the -P heads have 4 bars in the casting.  Regardless, my information indicates that both heads used the same plug in the Explorer V8 - AC Delco R42LTS or equivalent.

Last edited by John Ha (4/18/2025 5:30 AM)

don't know much about spark plugs, but do you think Dale Earnhardt would use a gimmick plug on his Cup car?

TKOPerformance wrote:

Stock plugs are probably double platinum, which IMO aren't ideal for really any application.  They use them in OEM applications so they can claim the engine will go 100k without needing plugs.  I run regular old copper plugs in anything NA, especially with a carb.  You will have to change them much more often, but the engines seem to run better with them.  In turbo cars I run iridium plugs.  There's no real trickery in plugs beyond proper heat range.  The idea that this material or that electrode design makes more power, gets better mileage, etc. has been roundly debunked.  The gains cited are typically within the margin of error of the testing parameters.  In his book Smokey Yunick spoke at length about all the stuff he tried to improve combustion by trying all kinds of outlandish ignition system parts and methods.  He came to the conclusion that so long as the mixture is ignited at the correct time the method makes essentially no difference.  Hotter spark, etc. is really just an improvement in reliability.  It also may become more important as compression ratio rises, and the engine spins higher RPM. 

I just got a new set of Autolite 104 coppers  which is what I have in there but its been running so crappy but I wasnt sure if that was what I should have had in there and what they needed to be gapped at  cause I no longer have the explorer pistons in there they looked like soup bowls...I went back with forged flat tops...I just got my Carb back from Steve and got more some valve covers with Baffles so I wanted to change these out in case the extra fuel bath and oil they been getting messed them up.Also put my new 22 gallon tank in  right before I sent Carb to Steve.
 

Last edited by BILLY WALTON from GEORGIA (4/19/2025 11:11 PM)

I went back with Autolite 104 coppers

Regarding plug gap, it depends on your ignition system.  High energy ignitions typically run in the 0.040-0.045 range.  Without a hot coil or some kind of amplifier you would want less.  More like 0.030.  These are generalities and some experimentation may be needed to get your specific setup dialed in. 

Another thing to consider is heat range.  A stock engine wants a stock heat range, but with higher compression and higher performance you may need to move away from the stock heat range.  If its fouling and the carb is tuned right go one step hotter.  If the plugs are burning up you may need to go one heat range colder.  Again, this may take some experimentation. 

TKOPerformance wrote:

Regarding plug gap, it depends on your ignition system.  High energy ignitions typically run in the 0.040-0.045 range.  Without a hot coil or some kind of amplifier you would want less.  More like 0.030.  These are generalities and some experimentation may be needed to get your specific setup dialed in. 

Another thing to consider is heat range.  A stock engine wants a stock heat range, but with higher compression and higher performance you may need to move away from the stock heat range.  If its fouling and the carb is tuned right go one step hotter.  If the plugs are burning up you may need to go one heat range colder.  Again, this may take some experimentation. 

It has the set up like a 85 GT I guess the blue strain ign box and distributor and reg 40,000 coil so you think maybe .035 they are gapped at .050 from factory and explorer are saying 0.054 but they have those dished pistons and I have the Forged Flat Tops in it now.
 

Last edited by BILLY WALTON from GEORGIA (4/21/2025 6:43 AM)

Duraspark ignitions had 0.044” or 0.045”

Nos681 wrote:

Duraspark ignitions had 0.044” or 0.045”

True, but all were probably lower compression that what he's running.  Assuming its a 302 with a 61cc chamber and a flat top piston with 2 or 4 valve reliefs, reasonable compressed gasket thickness and typical 302/5.0 deck height of near 0 he's running like 9-9.2:1 compression.  So I'd run a little less gap to ensure the plug fires against the added pressure.  I'd also experiment a bit though.  A lot of it comes down to the coil and how much dwell is built into the system.  We used to run as much as 0.060" on 5.0s with a really good ignition system (Screamin' Demon coil and a good TFI module).  The stock stuff was junk.  You'd get spark scatter above 3,500RPM with it once you did some tuning and maybe added a set of headers and a decent exhaust. 

I think I'd run 0.040 and see how it acts.