| Author | Message | ||
     Greg_e |
This may end up like an oil thread, but here goes nothing: On my little 125cc miniGP bike, I was running a 9:1 piston last year and the plug looks like it was in the correct heat range. It was an NGK "regular" nickel plated copper plug. I just bought one step colder "regular" plug for the 13:1 piston upgrade as I've read that you should go colder with big increases in compression ratio. But would I be better served by putting in an Iridium plug and would it follow the same heat range designations as the "regular" plugs? I did send an email to NGK support asking the same basic question, but thought it was worth asking here as well. | ||
| Tootal |
I would use the cheaper plug until you know you're in the right heat range. It's a race bike so a new plug at the beginning of each season is easy. The Iridium would last longer and is the reason I use them in my Uly since they're such a PITA to change. On a race engine I wouldn't spend the money. As an example, my BMW M3 engine actually states not to use them but then it's 1989 technology. | ||
| Greg_e |
Yeah, I'll play with the cheap plugs until I know what I've got. Here is what NGK said: "Thank you for your inquiry. By upgrading to an Iridium CR8HIX, you will notice that the throttle response will be quicker, and since it takes less voltage to jump the gap you will have more reserve in the coil so less wear and tear on the coils." Real or bunk? It should take the same energy to jump the same gap, shouldn't it??? Heat tolerance and length of life would be the only things I would expect to get from a material change like this. As is, I kind of expect I'll need two different heat ranges, one for hot summer and one for cold spring and fall. The difference being like 90 or 100 degrees F and 50 degrees F. And of course a couple different jets to handle the same changes. | ||
| Ourdee |
I think the bigger gain would be a cheap plug indexed so that the open gap was pointed at the intake valves. ning ning ning...this could be on the test. | ||
| Greg_e |
NGK has that as one of their FAQ... They said that you can only be certain indexing works by trying several different rotations on a dyno and looking for that last 1%. They said some engines run better pointing towards the intake, some better pointing towards the exhaust. I would think having the open side facing the combustion chamber would be best as it then has no obstruction for the gas to get around the electrode. But my engine is off to the side, not centered straight down which would also have an effect on the burn. And yes I thought about indexing mine, and may draw a line on the insulator so I can see which way it faces without having to pull the head for each plug change. I may still do this and see what happens as I did note a less than optimal burn pattern on the head when I was doing the work. Need to get some spare compression washers/gasket things. | ||
| Fast1075 |
I ran standard NGK plugs in my dragbikes. I only changed them when I had the engines down for refresh. I did keep a box of plug gaskets and changed them every so often (I did a leakdown on the engine after every event). I data logged the engines and got no misfires at 11K with a heavy nitrous load. With that much compression, start two heat ranges colder than stock. Your plug viewer will tell the tale. Jet for a smoke ring 1/4 of the way up the insulator. If you have good combustion chamber shape and you have actual quench area, you should be good. (I've never seen inside one of those motors). Do the rules allow slotting the cam sprocket and playing with the cam timing? If so, pull the lobe center to 109/110 and see what happens as long as you still have good valve clearance on the approach. | ||
| Greg_e |
The rules are pretty much unlimited as long as I stay 125cc, or I start to stomp on everyone else, which may take a while with my lack of skills. I think I can get a cam timing adjustment sprocket, was thinking about that and thinking retarding it slightly might make better power and clear the exhaust valve a little. Did a bunch of reading on plugs and indexing and stuff. Looks like I need to cut the ground strap down a little so that the end of the strap is down the center line of the electrode. And that I should probably try to orient the open side of the plug towards the center of the combustion chamber. I'll get a picture up in a few so you can see the piston and head. (Message edited by greg_e on May 02, 2017) | ||
| Greg_e |
Here is the piston, exhaust is the smaller (top of image)  The circular indent is the clearance for the sparkplug which is kind of forming its own combustion chamber. And head, exhaust has the small line from the solder when I checked the clearance  | ||
| Mbest |
Yeah, I'll play with the cheap plugs until I know what I've got. Here is what NGK said: "Thank you for your inquiry. By upgrading to an Iridium CR8HIX, you will notice that the throttle response will be quicker, and since it takes less voltage to jump the gap you will have more reserve in the coil so less wear and tear on the coils." The platinum and Iridium plugs have a lower "break-over" voltage than standard plugs. That means that less voltage is required to initiate a spark. While that's handy in it's self, you do end up with a less energetic spark at the lower break-over voltage. In our race engines, both auto and bike, we gap the exotic metal plugs at a larger gap than the std plugs. A larger gap takes more voltage to jump, but the Iridium makes it easier to fire. So we end up with a hot high voltage spark in a wider gap than stock while still using and not taxing the stock ignition system. mike | ||
| Greg_e |
Thanks. Been learning about spark plugs, but not a lot of concrete info out there pertaining to the ground strap. What I can find is a lot of people talking about cutting the strap back to be even with the centerline of the positive electrode. Most this is said to unshroud the arc to give better flame propagation. When I look at racing plugs, they pretty much all have this "truncated" ground strap and are said to be better for high performance engines, but the trade off is shorter plug life from erosion. So I'll definitely cut the strap back in my plugs. I did order some washers for indexing, might as well do that too, even if the gain is minor, every little gain in racing until you blow it or get regulated up is good. The other side effect of truncating the strap is that it can withstand a little more heat, or cool down a little faster to prevent ping. Both things are probably good for my specific use. Going to be a long second post on this upgrade on my website. | ||
| Phelan |
On the 90" Firebolt, I had to use indeximg washers to space out the plug, as it was getting nicked by the 1.980" intake valve. | ||
| Greg_e |
And that's the second reason for the washers that gets mentioned a lot. Did you double plug that motor? | ||
| Mbest |
Thanks. Been learning about spark plugs, but not a lot of concrete info out there pertaining to the ground strap. What I can find is a lot of people talking about cutting the strap back to be even with the centerline of the positive electrode. Most this is said to unshroud the arc to give better flame propagation. When I look at racing plugs, they pretty much all have this "truncated" ground strap and are said to be better for high performance engines, but the trade off is shorter plug life from erosion. So I'll definitely cut the strap back in my plugs. I did order some washers for indexing, might as well do that too, even if the gain is minor, every little gain in racing until you blow it or get regulated up is good. The reason for trimming the ground strap back to the centerline is because the spark jumps easier from an edge or corner than from a flat surface. That is in effect, also reducing the "break-over" voltage. So now you can increase your gap up until you are at your original spark levels but now with a wider gap. mike |
