Spark plug choice

by chevellelaguna Sat Mar 21, 2015 9:37 pm

Well, I still have a way to go before I'm ready to fire the motor back up. But.... I was wondering about spark plugs. I have a 12.5:1 355 small block with hogged out 882 iron heads milled to 74cc's ( the pistons are domed forged hyperutectic specific for a 74cc head to yield a 12.5:1 comp.)
I currently run AC Delco R45TS plugs(taperd seat). I've noticed with 36 degrees of timing(running Sunoco94) the electrode actually begins to melt. Not an issue with a higher octane race gas and more timing. My question....many questions..
What brand/style of plug should I try?
I know I have detonation issues... No money to swap to aluminum heads right now. Just want some insight as to some options that may be out there. A couple racer friends are trying to push me into the NGK route... Maybe that's the only option, maybe go waaaay down in heat range, colder plug, I dunno, thoughts?

by dynchel Sun Mar 22, 2015 12:18 am

After I asked the same question last year, I ended up using NGK's. Since I was trying to get the most out of my tune up, I called their tech line & let them pick their right plug for my mismatched combination. I then bought the plugs from amazon. the.https://www.g3gm.com/t8539-performance-tune-up?highlight=performance+tune+up

by Joe73 Sun Mar 22, 2015 11:53 am

Any idea of what the "quench" of the combustion area is?? That might be your detonation issue especially with compression that high.
Was the block zero decked or left stock?
What head gasket are you running?

.040 is optimum quench. More than about .042 and detonation starts being a factor.

Just a thought.

by chevellelaguna Sun Mar 22, 2015 12:11 pm

Interesting about the NGK plugs, did you notice any difference as far as longevity of the plugs? What heat range are you equivalent to in an AC Delco plug?

Joe, the block was surfaced but I insisted on them not zero decking it, just wanted it leveled, my piston to valve clearance was .056" I'm sure that combined with the iron heads and high compression is part of the problem.

by Joe73 Sun Mar 22, 2015 2:19 pm

Ok, so not zero decked. Did you take a measurement of how deep the piston is in the hole? Stock small block chevy is usually .025 in the hole. If you used a common fel pro 1003 head gasket, its compressed thickness is .041. So .041 and .025 in the hole is .066 which would be well in the detonation zone.
If the blocked was just leveled they probably took off .010.
Also, each model of piston comes in at a different depth in the hole.

I agree, part of the problem is that high of a compression and not knowing what the quench is.

Any street car should be running some vaccum advance with an adjustment set to a max of 10-12 degrees. Your in the ball park with 36 degrees of timing.

Were the heads decked as well? 882 heads are 76cc. Unless decked.

by chevellelaguna Sun Mar 22, 2015 4:54 pm

The pistons were .015 in the hole after cleanup, I'm using .041 gaskets. The heads were originally 76cc and I had the shop mill them to achieve 74cc combustion chamber, I sent my intake in as well to have it angle milled to match the heads. That created all sorts of valve train issues, had to get custom length pushrods, I elongated the slots in the head, installed guide plates. 36 is the max I can run on 94, with race gas I'd turn it up to 38-40. One thing I never tried was tightening up the plug gap, always ran them at .045 for some reason. My ignition system has always been healthy, Taylor 10.4mm wires, brass terminal cap, Accel HEI coil, MSD 6AL box. I'll have to try .030-.035 gap once I get better plugs.
Should I try a colder plug?

by 77mali Sun Mar 22, 2015 8:12 pm

I'm running 23 degree iron Vortec RHS race heads on my build but compression is only 9.5:1 with stock 3.48 crank, 4.06 bores w/ flat-top hypereutectic pistonss. They are 76cc chambers & am also using a .41 compressed head gasket.

I don't understand how you're getting such high compression? Did you want that type of compression for racing? You're about the same compression as an L-88 race engine which runs on 105 or maybe even 110 octane. What crank are you running?

Regarding plugs, RHS spec's a 14mm with a .708 reach. They want a gap of 0.60 and so far so good but I've not run the motor in the car under any load so it may change. What gap did you start with? If you're @ 0.60 try them out @ 0.045 and then 0.30. Of all else fails I'd try and find a comparable set of heads to yours on RHS/Comp Cams or another similar manufacturer's site & see what they recommend for a plug & gap.

by Joe73 Sun Mar 22, 2015 8:28 pm

Just curious. Do you happen to have the make and part number for the pistons your running? Seems real high compression for a 74-76cc head.

Before I tore my engine down I was running some high dome 11:1 (speced with 64cc chambers) with my milled 76cc heads.
Basically if a piston compression ratio is speced with a 64cc head and you use them with a 76cc head, you lose 1.5:1 in compression.
So with my combo with 11:1 and a 76cc head, I had 9.5:1.

To have 12.5:1 with a big chamber head would have to be a "huge" dome. And I havent run into that.

by Dinomyte Sun Mar 22, 2015 9:17 pm

Guys - is there a dictionary of terms on the site? I don't understand what the " "quench" of the combustion area " means. I was getting really interested in this thread. . . . and what I could find on-line about it wasn't exactly clear.

by 77mali Sun Mar 22, 2015 9:37 pm

http://www.superchevy.com/how-to/engines-drivetrain/94138-piston-head-clearance-guide/

by chevellelaguna Sun Mar 22, 2015 9:54 pm

Joe, I bought the pistons new in the early 90's you are right about loosing compression with utilizing smog heads and a large combustion chamber. I bought these because they were exclusively for a 12.5:1 compression with a 74cc head. They have since been discontinued ( I'd have to go they some paperwork to find the details, if I still have it)
They have a HUGE dome as far as I remember the dome was taller than 3/8" may have even been 7/16".They didn't sit deep in the bore either. I remember the engine shop freaking out about the dome on them when I dropped them off with the block to get bored and to get them pressed on the rods.

by chevellelaguna Sun Mar 22, 2015 10:06 pm

77mali, the shop that machined the block had a forged crank they sold me, I picked up 5.7 "pink rods" race gas was cheap then so I didn't care about fuel, I wanted to make as much power as I could WITH iron heads so I went crazy with the compression. I threw A LOT of money in this motor and figured I could always get aluminum heads later( they were crazy money at the time)
I set the plug gap at .045" and never really messed with it because it ran incredibly strong. I didn't mind putting new plugs in it every month.
Better stuff out there now... Will try tightening up the gap, and see what happens.

by Dinomyte Sun Mar 22, 2015 11:38 pm

77mali wrote:http://www.superchevy.com/how-to/engines-drivetrain/94138-piston-head-clearance-guide/

Thanks, that made much more sense.

by bracketchev1221 Mon Mar 23, 2015 6:26 pm

Plug gap is not a major issue for power. You can run .035-.040 it doesn't matter. I like the Ngk vpower plugs for performance engines. I use them in my 540. I would be a little concerned about detonation with a hyper piston. Especially if its damaging plugs.

by Joe73 Mon Mar 23, 2015 7:08 pm

I agree with Bracketchev. Hypers dont like detonation.

by bigredlaguna Mon Mar 23, 2015 11:32 pm

I would definitely try a colder plug first, then perhaps e85 for fuel if you have an extra carb to modify for alcohol.

by BBMALIBU Thu Apr 02, 2015 1:32 pm

At one time I had a 396/402 BBC with 12.5-1 compression. I used AC Delco R43XLS for that motor.

I believe the general rule is: higher compression = cooler spark plug (and vice versa)

If I were you, I would start with R43TS and work your way up to find the best heat range for your motor.

Once you find it, if you want to use another manufacturer (NGK, Autolite, Champion, ect…) you could simply cross reference it.

by BBMALIBU Mon Jun 15, 2015 2:34 pm

So what sparkplugs are you using?

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