Recommend Spark Plugs

[169mph]

I tried searching the forum first but didn’t find what I was looking for. What spark plug do y’all recommend for my 427IR?

I appreciate the help, Mike

[jwoodard]

Why not get replacements for the ones that are already in it? Just pull one of the plugs and take it to the auto store. Or, alternatively, query the engine manufacturer for the recommended plug.

[169mph]

I’m the second owner. The plugs were replaced at 1000 miles by the original owner with NGKs. I had issues with these plugs in the past (ceramic cracked). I checked Roush’, iconic’s, speedfanatix’ websites and this forum with no luck. I believe it came with Autolite ###?

Appreciate your help

[twobjshelbys]

Whatever Roush recommends. The Cobra part is irrelevant. The 427IR is a bored/stroked Windsor with a 10 (10.5?):1 compression.

[spdbrake]

Autolite 3924 or AR3924 (cutback ground strap)

[169mph]

Originally Posted by spdbrake

Autolite 3924 or AR3924 on (cutback ground strap)

Thank you spdbrake. I don’t have the backdraft binder that came with my car. I don’t have the engine build sheet or the dyno number

[jwoodard]

Autolite 3926

[twobjshelbys]

Roush 427R Sparkplugs

[169mph]

I was wrong. The plugs were not NGKs. They are autolite 605 but not 3926. Does anyone else use these plugs and why?

Thanks, Mike

[eschaider]

Roush dynos their crate motors, like yours. When they go through their tuning process, on the dyno, they will use different heat range plugs until they get the kind of power curve they want.

If you call them they will share this information with you as the owner or even as a potential new owner of one of their engines. At that point you will have the plug that worked best for Roush on the dyno. If you wish to experiment with hotter or colder heat ranges from there, you will have an excellent starting point to work from.

[patrickt]

I know we're not talking FEs, but I will tell you my spark plug story anyway. My engine builder recommended Champion RC9YC5 Spark Plugs, which are one step colder than the plugs Edelbrock recommends for the street (RC12YC5) for my aluminum heads. The opinion of some FE geniuses was that the Autolite #3924 plugs are better with Edelbrock aluminum heads (like mine) and everyone said I should try them. They are one step “hotter” than the Champions I was using and, apparently, with a MSD box, like mine, they would have me singing and dancing. So, I replaced the Champions with the new Autolites (regular 3924, not the “AR” or “XP” plugs). Perhaps it was the placebo effect but they did seem to run a little better. So, I'm still using them to this day. The moral of the story is to go ahead and try a different manufacturer and try moving up and down a bit in the heat range. All spark plugs are not equal and the only way you'll know for sure is to try them.

[BDR0572]

As a young professional Tech i was taught when changing plugs in a ford engine use Autolite plugs and I have to Amit they where right .I have only removed 1 set of premature failure of autolites in the now 40+ years i have been doing this

[spdbrake]

Originally Posted by 169mph

I was wrong. The plugs were not NGKs. They are autolite 605 but not 3926. Does anyone else use these plugs and why?

Thanks, Mike

I can understand the 3926 being recommended by Roush as they do use more oil so would be a better choice than a 3924 which is one step colder.

The Autolite 605 you found installed I don't understand. That is a tapered seat plug and reach is only 0.679" vs the called for plug 3926 being 0.750" reach and gasketed.
https://www.summitracing.com/parts/atl-605
https://www.summitracing.com/parts/atl-3926

I can imagine the recessed spark is not real good for your combustion.

[169mph]

I ordered a set Autolite 605s. Shortly after purchasing my car it started backfire when decelerating. My header collector flanges were loose. After replacing fasteners and sealing flanges the problem when a way. That was over year ago. My car started to backfire again. Not as often or as loud as it was before. I checked the flanges and no leaks were found. But I found two pin hole leaks on my side pipes. I assume these holes are causing the backfiring. Regardless I need to fix the side pipes. I’m curious to see what plugs look like.

Thanks, Mike

[spdbrake]

I think you missed my point the 605 plugs you have installed are too short and have the the wrong mating seat for your heads. Most likely the cause of your exhaust popping due to incomplete fuel burn.

The conical (tapered seat) plugs are usually found on GM engines. The 605 plug you are running do fit 4 cyl Ford Rangers and some V6 Mustangs Taurus cars though.

[169mph]

Originally Posted by spdbrake

I think you missed my point the 605 plugs you have installed are too short and have the the wrong mating seat for your heads. Most likely the cause of your exhaust popping due to incomplete fuel burn.

The conical (tapered seat) plugs are usually found on GM engines. The 605 plug you are running do fit 4 cyl Ford Rangers and some V6 Mustangs Taurus cars though.

I didn’t miss your point I missed your entire response. I planned to call Roush next week. I compared the 605 to the 3926 on Autolite’s website yesterday evening and I snipped the comparison because of the differences you stated. I was hoping Roush would point me in right direction. I did confirm that the plugs installed are the plugs the engine came with. Strange?

Thanks again

[spdbrake]

Good plan Mike,

I was making my recommendation based on someone else's post, which I don't normally do since it could steer you wrong.
It is quite possible Roush counterbored the spark plug holes to accommodate the 605 plugs.

[169mph]

Good morning. Just talked to Roush. At some point they switched to a different head design for the 427ir and these engines run Autolite 605s. I don’t remember the guy’s name I talked to but he was friendly and more than willing to help. I told him that I didn’t have any paperwork on my engine and he offered to send me a copy of the paperwork they filed on my engine. I appreciate everyone’s responses.

Thanks, Mike

[FredG]

The binder spec sheet for my 427r engine calls for the Autolite 605's. Engine was built in 2013.

Fred