Spark plug heat range recommendation?

[Robbie Racer]

What heat range would you guys recommend for my 621" blower motor? I have been using the NGK-9's when running with 91 octane gas at around 8-9 lbs. boost and the motor seems to run fine. When I put in race gas and bump the boost up to 14 or so, I changed to the NGK-11 colder plug for the 400 hp nitrous hit. I notice now that the motor is missing under WOT conditions with the higher boost set up. I called Larry Peto who told me that he thinks the reason is that the -11 plug is too cold and that may be what's causing the miss. I also have some -10 plugs that I may try the next high boost run to see if that helps eliminate the miss. Any thoughts?
Keith

[HeavyHitter]

RR, I've never run colder than 10's even at 36 lbs of boost. You might try .020 gap if you want to run the 11's.

[Robbie Racer]

Thanks HH. The 10's ought to be fine for me even with the nitrous if you are able to run them with 36 lbs of boost. Is that on alcohol?

[blowngas]

Is that on alcohol?
I would certainly hope so, unless you are running 30 deg ambient air temp!!!

[DEL51]

Autolite makes a race plug with a cut back electrode. I would try them on a one or two step colder plug.You did not include any info on your ignition system.You may be exceeding the capacity of your current system.

[Robbie Racer]

Autolite makes a race plug with a cut back electrode. I would try them on a one or two step colder plug.You did not include any info on your ignition system.You may be exceeding the capacity of your current system.
Thanks DEL. I have a MSD 6AL ignition fired by a MSD crank trigger. I was wondering about the 6AL's ability to fire the -11 cold plugs. :confused: I have enough race gas left for one or two more passes so will crank up the boost again this weekend (no nitrous) and replace the -11 plugs with the -9's and see if the miss goes away.

[ColeTR2]

I notice now that the motor is missing under WOT conditions with the higher boost set up.
Sounds like a lean condition. Be careful you don't hurt the motor. How do the plugs look?

[Robbie Racer]

I didn't get a good plug reading because I didn't shut it off quickly enough after the run and it idles so fat that it loaded up the plugs so I couldn't get a good reading. I will do a fast shut down after a run this weekend and try to get a good plug reading. I didn't think it would be running lean because the density altitude during the dyno session was 4,000' and it was fat then. The air quality last weekend where I ran it was 2,400' so I didn't think I would need to rejet but you may be right. It definately deserves a good plug check. :idea: Thanks.

[cjordan]

I'd run the -10's gapped .025" Worked well on my old 632" NA occasinally spayed motor. That was with an MSD digital 7 programable box. Ran -9's otherwise same gap. Is your fuel system up for 14# boost and 400hp NOS shot? How much timing are you pulling out?

[Robbie Racer]

Thanks CJ. It's missing on the high boost with the -11's even before I hit the NOS. I have a low fuel pressure alarm and it's not going off so I think my fuel system is okay. I do have a separate fuel tank and pump just for the NOS fuel. I have the MSD auto retard box and it's currently set up with a 10 degree retard chip, so it drops from 34 to 24 when I hit the button. I tried 30 degrees and retarded it to 20 but I still got the miss so I don't think it's timing related. It could be the plug gap too as I may have it gapped to wide at .035". I will try a high boost only run this weekend with the -9 plugs. If the miss goes away, I will put the -10's in with a .025" gap and give that a try with the juice the next time out after I refill the bottle.

[cjordan]

RR I think you're on the right track....maybe check for chaffed plug wires and swap MSD box also. If its not ignition it sounds lean.... :confused: Good luck

[ColeTR2]

Did you do your dyno runs with the same boost you are running now?. Anytime you turn up the boost or change the blower pulleys you need to re-jet.

[Robbie Racer]

We didn't rejet from the 9 psi 91 octane dyno pulls to the 110 octane 13 psi pulls and didn't see much of a difference. For example:
9 psi pull (12% over) = 11.6 A/F and .634 BSFC at max power (6,600 RPM)
13 psi pull (33% over) = 11.6 A/F and .642 BSFC at max power (shut off pull at 6,500).
I'm running the same jets now that we had for the dyno pulls for both settings but with the better air quality, I better do a good plug check this weekend.

[ColeTR2]

I don’t know much about dyno’s but when I went from 10% over to 15% my Motor went lean and was poping / backfire at WOT. From 12% to 33% that’s a big jump!! Do you have a boost gage? I have had my motor pop when the plug get loaded up from idleing much I use a NGK 8 but I have a mag.

[Robbie Racer]

When you went from 10% to 15% over, was that with carbs or mechanical fuel injection? I have heard that carbs will automatically compensate for the increase in boost where mech fuel injection may not.
Yes, I have a boost gage and it seems pretty accurate compared the dyno instrumentation with a given pulley combination. You might be on to something with your comment your motor popping when the plugs get loaded up too much from idleing. I do idle a lot and the plugs look like crap but I "assumed" that they are cleaning themselves off at WOT. In the past I have seen the plugs looking all loaded up after idleing but after a WOT, they look perfect. I have been using these same -11 plugs for awhile now, so it's possible they are just going bad too.

[HeavyHitter]

Keith, the 36 lbs was on gasoline, 118 octane ERC, ice bath intercooler. intake temperatures never got over 90 degrees F. Idling kills. If I have to idle around without getting on plane before shut off I'm changing plugs. Usually 3 get fouled to where they won't clean up.

[ColeTR2]

When you went from 10% to 15% over, was that with carbs or mechanical fuel injection? I have heard that carbs will automatically compensate for the increase in boost where mech fuel injection may not.
Yep!! injection
Maybe you could try warming it up on the old plugs then take it and change the plug out on the lake and then do some WOT passes.

[Robbie Racer]

Thanks HH. Were you using a PSI blower to get up to 36 psi on gas? The ice bath intercooler is a great idea. We used to pack dry ice around the tunnel ram intake and use it to cool the fuel on my Pro Gas Jet boat but I never thought of using ice to cool an intercooler.

[Robbie Racer]

Yep!! injection
Maybe you could try warming it up on the old plugs then take it and change the plug out on the lake and then do some WOT passes.
Yeah, that's not a bad idea. We also did that on the drag boat and it worked well.

[HeavyHitter]

Thanks HH. Were you using a PSI blower to get up to 36 psi on gas? The ice bath intercooler is a great idea. We used to pack dry ice around the tunnel ram intake and use it to cool the fuel on my Pro Gas Jet boat but I never thought of using ice to cool an intercooler.
No blower a pair of TV71 turbos into a 540. It was a 30 gallon tank that took up what would be the passenger seat on the AA sport modified BoCar that held class land speed record until the engine was put into a better aero package, which currently holds the record.

[Robbie Racer]

Awesome. Congratulations on the record!! What is the record speed for that class?

[RACER52]

keith I would would have to agree that the Autolite semi-gapless plugs are a good choice.
I have ran them always, no comparison cause my shit is slow and usually dont run. but no matter how far I have been off on the fuel mapping and always at 12-14lbs boost the plugs have never faultered.
Racer

[SUI-CY-COLE DIMARCO]

get a lighted magnifying glass and check the color ring on the porcelin,,,thats the only way,,every motor is different.........

[Badburn]

Heres some food for thought. I had a high compression 548 inch nitrous motor a while back that was real hard on plugs, I ran the ngk 10's mostly, but they would load up and eventually foul after about 3 or 4 passes. I found that by using the 8's or even 7's that the plug would last longer before it started acting up. Not the ideal heat range for a nitrous motor. I was running a MSD 6AL with all the retarding goodies. When I changed to the MSD digital 7 ignition, all of my spark plug problems disappeared. I could run the same 10's for 15 or 20 passes. Point is, that I think the MSD 6 series ignitions are marginal on a high HP motor like yours. Hope this helps. :cool:

[SUI-CY-COLE DIMARCO]

is this badburn with the red camaro?

[Robbie Racer]

Troy, your shit runs great given the size of motor you are running. Let's put my motor in your boat and beat Rock-it-man to 150. :D Do you know what the Autolite part number is equivilent to a NGK-R5671A-10?
Dimarco, I do have a good lighted magnifying glass. I just need to learn how to read plugs better. :confused: I will try to see if I can get a good close up picture after a WOT run tomorrow and post it if it turns out, to get opinions on what you guys think of the plug reading.
Badburn, that is very interesting. If your -10 plugs were fouling after 3-4 passes, my -11 plugs are probably toast after the 15-20 passes that I have on them now. That is probably my biggest problem with the miss but I will know more after tomorrow's high boost test run on the -9's.
Thanks for all the feedback.
Keith

[Badburn]

yup ;)

[ColeTR2]

I will know more after tomorrow's high boost test run on the -9's.
How did it run??

[Robbie Racer]

How did it run??
Well, you know what they say about the best laid plans. We went to the Riverboat to have lunch, then met up with Brett to participate in the Hot Boat photo shoot and by the time we got back on the water, there were too many rogue rollers from the big RV boats to make any WOT runs safely. It might be 10/14 on hookey day before I get another chance to make the test runs now. :confused:

[ColeTR2]

there were too many rogue rollers from the big RV boats to make any WOT runs safely.
I hate when that happens!! keep us posted!

[Robbie Racer]

I hate when that happens!! keep us posted!
I was able to make several WOT runs last Friday at the Hooky Day event on the delta with the new -9 plugs. The new plugs seemed to have solved my miss problem. It ran 121 mph on 91 octane gas at 8.5 lbs boost (with 4 people on board) and 130 mph on race gas at around 13 lbs boost (no juice) with 2 people on board at 6700 RPM with my lab 32 prop. The air quality was pretty good at around 1900' and the water was very flat. I will probably try the -10 plugs when I use the nitrous again instead of using the -11 plugs. I am now sure that they were the cause of the miss.
Thanks for all the suggestions.
Keith