dan0617

Well, you might have seen in the other thread, I ran a 10.66 @ 131 on a 200 shot. I think I'm being conservative on the timing on motor (28 overall and no detonation can be heard), and I retarded 11 degrees for the spray. Anyway, just pulled the plugs and the ground electrodes are melted and there are black specs on the white insulator. Last time I melted plugs I melted an exhaust valve, was easy to diagnose by unhooking the coil wire and cranking the engine, could hear there was a cylinder with no compression. This time I did the same and it sounds if they all have good compression. Thank GOD!

I am thinking the black specs are meaning detonation, and I think the detonation would be from the spark plugs becoming glow plugs rather than from too much timing.

I'm running NGK iridium IX plugs, heat range 8. I melted heat range 5 plugs on a 175 shot. I was sure (although I now know I was wrong) that 3 steps colder would be fine. Now I'm thinking of going to at least 2 more steps colder possibly 3. That puts me to non-resistor type plugs, which I didn't want to run, but ooh well. Ignition box is fine with them, not sure about the rest of the car's computer and electrical systems.

What do you guys think? Anyone here running higher than 11:1 compression, around 9.2 dynamic compression, on 93 octane pump gas, and a big shot of nitrous? What heat range plugs are you using??

I am going to step over to the Autolite AR series race plugs. 2 heat ranges colder than the NGK 8's in Autolite is their AR3932. 3 ranges colder would be their AR3931. Not sure which to go with.

305sbc

iTrader: (3)

You'll need Autolite AR-51 or AR-12,
whichever one fits into your heads.
The AR-12 stock SBC1 taper-seat.
The AR-51 is deeper thread and gasket seat.

dan0617

The AR-3931 is the same heat range as the AR-51 but the 3931 is not a projected seat, and it is a 5/8" hex. But I think you nailed it for the heat range. I think I'll try the 3931's and if they are too cold and start fouling out I'll go to the 3932's.

RBob

iTrader: (1)

If Autolite makes a non-projected tip spark plug they may be a good bet.

RBob.

dan0617

Yes, after countless hours of research and learning, it seems the Autolite AR series will be the best and most affordable bet for me. The AR3931's are a non-projected tip and are 3 heat ranges colder than what I just melted the ground straps off of.

My only worry at this point is that the cc temps are so hot that any plug I put in there will melt. If that is the case I think my only viable options are higher octane (by blending race gas), or a water/meth injection setup.

305sbc

iTrader: (3)

All of the AR series is non-projected tip, and most are nonresistor.
To keep those plugs from fouling at idle & part throttle you'll probably need to put in some work into tune - start-up & warm-up cycle.

With N2O you can almost always make more power by starting the spark later in the cycle. The higher O2 content speeds the burn rate, so reducing your SA will put maximum pressure closer to most efficient crank angle. It sounds like you are running at or near the lean limit already, so I think your best option is to drop your advance some. You run fastest with as little fuel and as little SA as you can get away with.

The black specs are from detonation causing the aluminum of the pistons to melt/burn.

What engine coolant temp are running when you spray the 200 shot? For shots that big it's best to have the engine cool at the beginning of the run as a buffer for the rapid temp rise that will happen. I like to be around 160f at the start so it's still under 200f at the end.

edit: Have you verified that your Spark Latencies are correct?
You may be getting more advance than you think at some point.

dan0617

I'm at 17 degrees overall when spraying, and through the peak torque range I'm at about 14 degrees. I run 28 overall on motor. How much more do you think I should retard the timing? I think the black specs are from preignition from the spark plugs being too hot and glowing and setting off the mix too early. I really don't think it is due to spark advance.

I am, however, going to pull 1 more degree out while running on motor, about 2 more degrees out through the peak torque area, and 1 more degree out on top of that with the retard box while spraying. I don't think I really need to do all this but with my compression and amount of spray I really doubt that a couple degrees of timing will hurt my ET's all that much.

My air fuel ratio while spraying the 200 shot is around mid 11's. Do you think that is on the edge of being lean? I can richen it up to bottom 11's or high 10's AFR. Isn't that too rich though, or is that good when spraying a 200+ shot? Maybe I should go up a step or a 1/2 step in fuel jet and make it that much more rich, I would assume richer would help the combustion chamber cooling a little.

My coolant temp is in the ballpark of 200 when I start the run, and is about 205 at the end of the run. On a motor pass it is 200 at the beginning but goes down to 190 to 195 by the end of the run. I usually run about 185 but it creeps up to 200 while waiting in the staging lanes. I'm running a 180 stat and want to keep it because this car is 99% street driven. It is a weekend cruiser that is a serious sleeper.

I do not know if my spark latencies are correct. I do know that there is no audible detonation, and I know that when not spraying my plugs look really good, no signs of detonation. Wouldn't a spark plug that was in the process of melting light the mix off early and cause preignition and the subsequent black flecks that I have? I'm pretty sure that is what is going on.

RBob

iTrader: (1)

This isn't true. Many of the AR series plugs are what Autolite calls the "power tip." Which is a projected tip plug. The AR51 and AR3923 are both projected tip plugs. While the AR3931 is not.

There is a bunch of Autolite spark plug info at this link:

http://autolite.com/carcare/techSpecs.php

RBob.

305sbc

iTrader: (3)

Well, your timing doesn't sound very high and your AFR doesn't sound lean.
If it was me I would go back and double verify your timing with another known good timing light, and swap WBO2 sensors to verify that as well. If all is correct then it doesn't really make sense that all of your cylinders are getting the detonation. Was it all cylinders or only some?

You are right about your sparkplugs overheating and causing preignition. Too much spark advance can cause this to happen. More advance increases peak cylinder pressure and temperature because the A/F charge has expanded more before it becomes compressed fully. The compression creates more pressure and therefore higher temperature. It makes more power that way, but the heat can get out of control and build up in sharp edges or protruding metal like those on a spark plug. Things like that in the chamber just can't heat-sink quick enough.

The engine makes maximum heat when at WOT. Adding the 200 shot makes even more heat. I would try not to run at 200f before spraying. All your gauge shows you is the average coolant temp. The metal temp around the chamber is far greater, and though the aluminum of the heads can heat-sink fast enough, the steel parts can't. It may not be the lone cause of your detonation, but it's a contributing factor.

Robert, I will get back to you about the AR plugs. I have tons of pics of mine that aren't projected tip, but I can't post them here. I will pm.

dan0617

For the plug type, if you go to www.sparkplugs.com and type in each plug we are talking about in the search bar it will bring up a picture and a description. The power tip is shorter than most projected tips, but it still sticks out some. The non-projected tip is completely up in the plug body.

I agree with about everything else you said. I honestly don't think I'm getting any detonation from afr or from sa. I believe the NGK heat range 8 plugs were still too hot of a heat range, and the ground straps got so hot that it caused preignition, causing the black flecks on the insulators. By the end of the run the ground straps were so hot that they melted.

The NGK8 heat range is similar to an autolite AR3934. Do you think something else caused the plug to melt (like too much timing making too much heat), or do you think that 11.4:1 compression and a 200 shot (rated at the wheels) was just too much for that heat range of plug? That is what I need to figure out. I put in the AR3931's but if I end up melting them I'm liable to melt a piston too in order to find out the answer to the question.

dan0617

Well, I installed the AR3931's and upped the fuel jet big time. The colder plugs actually leaned out my NA tune almost 1 full point. Worked that out, then sprayed it. It pulled HARD. Wideband read 9.6 to 10.3 for the whole pass. Plugs look a little rich but not like you would think. Unfortunately I had to drive about 5 mins after the pass before I could pull any plugs. At least there was no signs of melting and no signs of detonation on the plugs, so I'm good there even if I'm still unsure if I'm rich now or not. Seems way rich on the wideband but I've done alot of researching and this is what I've found out:

Nothing.

Some say that the nitrous AFR is much lower, the bigger shot you are running the lower the AFR needs to be. By calculating my N/A hp and my nitrous HP, it shows I should be running around 10.0.

Others say that as long as gasoline is the fuel that 12.7 is what the wideband should say, no matter what the air source is, because the oxygen is being read after the burn. If it were being read before the burn then I should look for a richer AFR the more nitrous I burn.

Still others say that a wideband is totally worthless for nitrous use, especially once the shot size gets up over 150. They say that the burn and exhaust is so hot that the gauge becomes inaccurate.

So I have no idea. I just know I didn't melt any plugs this time and had no signs of detonation so I should be safe. When I get to the track again I'll try to shut it down right after the run and coast back to the return road and pull a plug or 8 and see what they look like.

305sbc

iTrader: (3)

WBO2 readings aren't really that much different while on N2O, even big shots. Still, keep in mind that the WBO2 is only reading a collective account of free O2 that's in the exhaust, wherever you've placed the sensor, and not a direct reflection of what is happening in the chamber.

Running significantly reduced spark advance will have much more effect on the WBO2 reading of the exhaust. Less advance will give a richer reading.

What it sounds like to me is that you have a heat issue in your chambers, which could be caused by a number of things, including sparkplugs. Advanced timing aggravates a temperature problem. Compression of a gas makes temperature rise. The earlier you start the spark before TDC the more gas there is to be compressed because more combustion has taken place before peak compression. This makes the temperature spike. This is how you can melt the plugs, even with a relatively rich AFR.

You may be making more dynamic cylinder pressure than you think, or you could be getting more actual spark advance than you think. There could also be another factor that is either keeping the metal temperature high around your chambers, or causing hotspots in the chambers. You may have fixed the problem by changing plugs, but keep your eye out for other causes too.

Some engines just have a combination of parts that make them extremely efficient at making dynamic cylinder pressure. Things like mild camshafts and/or high-swirl cylinder heads can do this. If this is the case, then reduced spark advance and/or increased octane when spraying is probably the best answer, and could result in even better performance.

If you actually have a thermal management issue in your chambers, then you should look into causes and solutions.
Sometimes you might be surprised at the strange things that can cause problems like this.
I do cylinder head reconditioning, so I see some things you might never think of. Valve guides wear out quicker on the exhaust side due to heat, and sometimes due to higher valve lifts. If you have a lot of slop in your exhaust guides, the valves will not seat true, and over time will beat up the valve seats causing them to leak. With the exhaust valves not seating/sealing properly they can effect combustion in two ways: by overheating to cause preignition, or a leak can expose exhaust flame/heat to the air-fuel charge. Engines that use a lot of N2O need a larger exhaust to make more power when spraying. Any extra backpressure when under power/N2O will show up as a problem if your exhaust valves aren't seating perfectly.

Just throwing out some ideas/info.

dan0617

I am checking into some things, and you gave me some more ideas of things to look at, but for now my problems seem to be solved by running 9.6 to 10.2 on the air fuel ratio. My plugs were too cold, so I went 1 step hotter and I think I'm there now on a 225 shot.

I honestly think that since the wideband sensor is calibrated in clean air that the air fuel ratio displayed while spraying nitrous is not correct. If 34% of the power is coming from nitrous, then you would need to calibrate the 02 sensor in 66% air and 34% nitrous. Since that isn't possible you need to calculate accordingly to find the air fuel ratio that the wideband should display. Nitrous has more oxygen than fresh air. By calculating, mine it works out to about 9.8 to 10.0. I'm now running 9.6 to 10.2 through the whole run on nitrous and my plugs look perfect. Not much color on the ground strap at all though so I might need to go another step hotter.

I also now take into account that I'm running a fuel that is 10% ethanol so I run 0.5 richer when on motor and I figure that in also when calculating what AFR to run on the spray. All seems to be working out great this way.