Spark plug gap

[rack]

Hi geraldm,
Very interesting thoughts.
Which reminds me, when a mechanic changed the plugs some months ago, he found 5 of the plugs were copper and one iridium. That time I did not pay any attention on this, did not check gaps or what ever... did not even ask on what cylinder was the iridium plug in.

Geraldm, do you suggest to increase gap on all plugs or just on the plug misfiring?

Do you guys think this was done on purpose, to cure this problem?


As a side note, I switched LPG injectors, but misfire keeps on going on cylinder 4.

Thanks for the help,
Ivo

[geraldm]

Hi geraldm,
Very interesting thoughts.
Which reminds me, when a mechanic changed the plugs some months ago, he found 5 of the plugs were copper and one iridium. That time I did not pay any attention on this, did not check gaps or what ever... did not even ask on what cylinder was the iridium plug in.

Geraldm, do you suggest to increase gap on all plugs or just on the plug misfiring?

Do you guys think this was done on purpose, to cure this problem?


As a side note, I switched LPG injectors, but misfire keeps on going on cylinder 4.

Thanks for the help,
Ivo

Hi Rack No I'm not suggesting you do anything that I have done just that it worked for me . The reason I entered in the thread is just to share how different problems can be solved different ways . Now my propane powered 2 rotor wankle engine motorcycle is a way off center as well as myself and is a mystery to most how it even runs never mind did making the plug gap a tad wider fix my problem ,,. well yes I believe it did and will attempt to explain why ,be patient I wander .

Now each rotor of my engine has 2 oil injectors one for lower rpm's and both for higher . now for my project which goes from 1200rpm idle to WOT in a fraction of a second for 200 ft. and then back to the pits and I do not baby the little dude it must do the job or splode but no holding back . so the oil metering control is set to be 1/3 open at idle just to keep a nice film of oil on the internals thus making a good seal on the rotor apex seals and side seals ( same kinda thing a rings on a piston ) . I have no idea what the compression is as I have never checked and really dont care about compression . What matters to me is the HG of vacuum which is huge @ 27 HG of vacuum at the intake manifold at 1200 rpm idle and holds that with a bouncing needle right up to 2000 rpm which tells me the oil is doing its job and has it sealed as good as can be expected . so with that much vacuum comes also huge compression , which pretty much is the rule if you have good vac then you will also have the same respective with compression . I know this sounds a little bit off but what ever . I believe what was happening to me was the extra high compression blast would actually blow the ignition out before it could light the compressed propane charge . The propane available in most of Canada is 104 octane making a nice volatile mixture but it needs a good wack to ignite it proper . So when you increase the spark gap to a certain point ( In my case I went almost 30% more gap ) it intensifies the spark reason being that the coil is needing to hold back it's charge for just a milli second or so longer but when the secondary coil is finally saturated from the primary windings in the coil with a few thousand volts extra and finally forces the spark to jump causing a much stronger spark It also changes the timing a tad so be ware ( that is why on a weak engine or poor spark plug if the plug wire is held a half inch from the plug it will fire but will quit firing when pushed back on the plug .) I never did put the first plug back in with a wider gap so I don't know if it would have worked or not , when I pull a plug besides for tuning I always throw them away .. I'm sure there are many opinions on the subject and what I have told you could be related to a gasoline powered machine as well .. that is my story and I sticking to it >> my engineer Bumpstart from Aus has probably got something to add seeing as he has been with me on this project for going on 4 years now, so that is my thoughts , also I would bet money that the odd plug was in that hole .enough said have a good one Gerald m
PS Had a 04 gran prix supercharged GTP 3800 went like stink. hope you get it figured out . Yea it seems to me I was around 60 thou for gap and pulled another 10 ? and no more problem , but it might not have had anything to do with it . Might want to look into heat ranges also .I probably only put 5 hours max running time on the dragon this year but after changing gap it ran flawlessly , no more ignition module problems or coil problems , my modules which are GM 4 prong use to get so hot if touched they would just slide the finger prints right off , coils also when I had misfire problems I melted one oil less coil till it was dripping with goo in less than one minute but then again my ignition is way different than most single fire direct fire with a coil and module for each rotor , long story short everything runs reasonably cool now .

[J. W.]

I've had my truck converted for almost 6 years. I've tried a lot of different things to improve it. The number one thing that helps the car from knocking is the amount of gasoline I have in it. A full tank the truck runs smoothly. Even though I was told to keep under a quarter tank I keep between 1/4 to 1/2. The second thing I find that helps is, I run it in gasoline once in a while. I do not know that much about cars but I experiment. I believe that this is because of the sensors in the engine. If you keep under a 1/4 tank they are dry and do not sense anything. With a full tank all the sensors are wet and are sensing the gasoline to make the adjustments. When you drive it on gasoline the sensors pick up the readings and stores it. In propane it might reset itself then not have any readings to adjust too. When you changed the plugs did you remove the battery terminal or do a re-set after words. This might explain it. IF not then try what I am saying and let us know if it worked. As for the gap I do keep it a little tight for example .045 I will set it for .040 but I do not know if it makes any difference. Can someone with more experience comment on this. thanks

[storm]

I've had my truck converted for almost 6 years. I've tried a lot of different things to improve it. The number one thing that helps the car from knocking is the amount of gasoline I have in it. A full tank the truck runs smoothly. Even though I was told to keep under a quarter tank I keep between 1/4 to 1/2. The second thing I find that helps is, I run it in gasoline once in a while. I do not know that much about cars but I experiment. I believe that this is because of the sensors in the engine. If you keep under a 1/4 tank they are dry and do not sense anything. With a full tank all the sensors are wet and are sensing the gasoline to make the adjustments. When you drive it on gasoline the sensors pick up the readings and stores it. In propane it might reset itself then not have any readings to adjust too. When you changed the plugs did you remove the battery terminal or do a re-set after words. This might explain it. IF not then try what I am saying and let us know if it worked. As for the gap I do keep it a little tight for example .045 I will set it for .040 but I do not know if it makes any difference. Can someone with more experience comment on this. thanks

I might be having a brain fog moment, no sleep last night due to severe storms, but from what I read here you are saying just because you have petrol in the tank (even though you are not using it) the engine runs better. If I am correct then your vehicle must have special sensors that detect the amount of petrol in the tank and tell the ECU so the ECU can tell then engine to run better even though you are runing on LPG. Sorry but this just does not happen.

[C3H8]

First things first. Happy new year to all the friends I have on this site but have never met personally. The best of health to you all and happy motoring on one of the best alternate fuels in the world.

There has been a fair amount of information in this post. Some accurate and some, not so much.
Question: What is the main common reason plug gap is reduced on a dry fuel.
Answer: To offset the degreased life the ignition system would experience if left stock and to provide an adequate spark to ignite the fuel.

Let's review history to try to clarify it. In the 1960 to 1980 decades it was common to reduce the gap on spark plugs using dry fuels. The rationale was simple. Wet fuels like gasoline put less demand on the coils to create enough voltage to jump a rotor gap and spark plug gap. Converting to a dry fuel required the coil to put out a higher voltage due to the added resistance of the dry fuel. The issue was not the dry fuel as much as the limitation of the coil output. Most ignition systems also included a ballast resistor which reduced the coil input voltage from 12 volts during cranking to 7 - 9 volts once the engine was running. This increased the life of the coil on gasoline but reduced the maximum coil output.. Some manufacturers that performed factory conversions would lower the gap on the plugs to as little as .018 (International) due to the marginal ignition system capabilities in this period. In this period ignition systems were barely able to achieve 30K volts. Most were lucky to get 25K volts. 22 to 25K volts maximum was common on International and Chrysler/Dodge products.

In the late 70's and 80's some manufacturers (Ford and GM) improved there systems to put out higher voltages. Points became a thing of the past. Ford systems could achieve 32K volts to 35K volts. GM HEI systems could hit 35 to 40K volts. During this period the gap reduction was usually 15 to 20% below the gasoline spec. The only reason again was to offset the added resistance of the dry fuel which put added stress on the ignition system components shortening their expected life. During this period Chrysler/Dodge continued to use ballast resistors and could still only achieve a maximum of 25 to 28K volts. Many aftermarket installers would eliminate this resistor recognizing that it would shorten the coil life but improve the coli output. Chrysler even offered an aftermarket ignition module that would accept 12 volts and modify timing when the alternate fuel was chosen.

In the last two decades distributors began to disappear. Coil over plug systems showed up and these systems could pump out 45 to 55K volts. There was no longer any rotor gap to deal with and distributors all but disappeared. Plugs materials changed and even spark plug wires are less common. These advancements have made it so that plug gaps don't require modification. The extra reserve in the coils can more then compensate for the dry fuel without affecting plug life or the coil durability. In addition LPG injection has been refined so the mixtures are always optimal.

All that being said there are times when a certain brand of plug may work better then another or a reduced or increased gap may help. These times will usually occur on a modified engine or in a turbo application. The only way to determine this is carefully reading the sparkplugs. In addition using an ignition analyzer would help to see if one brand works better than another. The optimum situation would be observing the ignition system while on a dyno which unfortunately is costly and many times not even available to many of us.

[Steptoe]

Excellent explanation...

In the late 70's and 80's some manufacturers (Ford and GM) improved there systems to put out higher voltages. Points became a thing of the past. Ford systems could achieve 32K volts to 35K volts. GM HEI systems could hit 35 to 40K volts. During this period the gap reduction was usually 15 to 20% below the gasoline spec. The only reason again was to offset the added resistance of the dry fuel which put added stress on the ignition system components shortening their expected life.

The above omits a an essential bit of info to illustrate "resistance" even more
Early 70s was the introduction of high volt HEI type ign....the voltage rather than amps, which would have achieved the same... but increase amps, get 'hit' with a spark u DO die
I digress a little....political pressure and fuel shortages of the time had manufactures going less emissions better economy... and ECI was universally adopted....ECI created lean mixtures, lean mixture means higher resistance across the gap....closing gap would help sort of...answer was higher volts. High voltage/ HEI where NOT an upgrade on points.. they where new technology to solve the problem of high resistance across spark plug gaps.
The resistance is determined by by type of fuel (just like different materials conduct electricity different) the mixture and the gap....AND the gap between cap pins and rotor (wear)
Essentially .. basic high school physics and common sense to anyone who has fixed a dirty connection on anything to get it to work.

[J. W.]

I've had my truck converted for almost 6 years. I've tried a lot of different things to improve it. The number one thing that helps the car from knocking is the amount of gasoline I have in it. A full tank the truck runs smoothly. Even though I was told to keep under a quarter tank I keep between 1/4 to 1/2. The second thing I find that helps is, I run it in gasoline once in a while. I do not know that much about cars but I experiment. I believe that this is because of the sensors in the engine. If you keep under a 1/4 tank they are dry and do not sense anything. With a full tank all the sensors are wet and are sensing the gasoline to make the adjustments. When you drive it on gasoline the sensors pick up the readings and stores it. In propane it might reset itself then not have any readings to adjust too. When you changed the plugs did you remove the battery terminal or do a re-set after words. This might explain it. IF not then try what I am saying and let us know if it worked. As for the gap I do keep it a little tight for example .045 I will set it for .040 but I do not know if it makes any difference. Can someone with more experience comment on this. thanks

I might be having a brain fog moment, no sleep last night due to severe storms, but from what I read here you are saying just because you have petrol in the tank (even though you are not using it) the engine runs better. If I am correct then your vehicle must have special sensors that detect the amount of petrol in the tank and tell the ECU so the ECU can tell then engine to run better even though you are runing on LPG. Sorry but this just does not happen.

You are missing what I am saying. It is not the sensors that measure the amount of fuel in the tank. There are all kind of other sensors in cars now, which adjust the engine based on its readings It will read the exhaust and adjust the air to fuel mixture allowing the car to run smoothly. E85 also measure the gasoline to alcohol mixture there are also a lot of other sensors. When you run your car on propane it bypasses a lot of these sensors So it does not have any reading to base these adjustment on. What I am saying is, when tank is low the sensors may not be getting a reading from the fuel to make adjustments. When I run the truck on gasoline the computer picks up the readings and adjust the engine then it stores the readings when I switch to propane the computer still has the readings stored in it and is adjusting based on those readings.

When the tank is full the gas is high enough for these sensors (not the one that measures the amount of fuel in the tank) to get a continuous reading and my engine runs smother. I know the propane set up has a computer in it to adjust for the engine with out using these other sensors. But I do know if my tank is full my truck runs smother. You do not have to believe me and you do not have to try it and you do not have to ridicule me. I have had my conversion for almost 5 years and I do know this works for me.

[C3H8]

I don't think you were being ridiculed. He and probably many of us are just puzzled about the rationale. You have not mentioned the type of vehicle or conversion system. Is it injected LPG or is it using an older mixer system? Your initial question on the gap. Gapping at .040 should be fine. Typically on the newer ignition systems it is not required but it won't hurt anything.

Modern vehicles, as you indicated have several sensors but most of them are in no way connected to the level of the gasoline in the tank. The tank could include a few sensors depending on the manufacturer. These would be a level sensor, Pressure sensor for the purge system, and an methanol or ethanol content sensor to modify the timing and fuel strategy depending on the methanol content. None of these sensors should have an impact on the way an engine runs no matter how much gasoline is in the tank.

As indicated you say your vehicle runs better when the gas tank is full. Can't argue with that. You own it and drive it everyday. The rationale in keeping at least a quarter tank is that typically gasoline pumps are left running all the time in todays conversions. The pump runs cooler if it is submerged completely since it is usually working at the maximum pressure. Industry recommendations are to keep the tank at least 1/4 full while operating on propane. There is no relation to this on how well the engine runs.

If your engine is knocking (pinging) on propane with a partially filled gas tank I would be concerned with several things. The first being excessively lean fuel mixtures since you indicate a full tank alleviates this. Your explanation would point towards the possibility that gasoline is able to get into the intake when the tank is full and not when the level drops. This could be for a couple of reasons. Filling the tanks allows more vapours into the purge canister and the vapours are then drawn into the intake is one explanation I can think of. I would also be concerned with the knock sensors as they should retard engine timing to the point of eliminating the knock. Why aren't they doing this? It should be looked into. This over advanced timing could damage your engine and it could also be the cause of your rougher running engine.

The bottom line is that the engine runs smoother on a full tank and this indicates gasoline is being burnt along with propane when the tank is full. Only a thorough investigation would reveal the reason for this.

[storm]

You are missing what I am saying.

Nope I know what you are saying and this reply confirms it.

I don't think you were being ridiculed. He and probably many of us are just puzzled about the rationale.
.......
Modern vehicles, as you indicated have several sensors but most of them are in no way connected to the level of the gasoline in the tank. The tank could include a few sensors depending on the manufacturer. These would be a level sensor, Pressure sensor for the purge system, and an methanol or ethanol content sensor to modify the timing and fuel strategy depending on the methanol content. None of these sensors should have an impact on the way an engine runs no matter how much gasoline is in the tank.

You are right it wasn't ridicule just puzzled (after a long night thus the brain fog comment) and I should have worded my reply better. Huge storms, tinder dry landscape (not now it has greened up but certainly that night it was brown and dry as) and the possibility of wild fires (which happened in 2 states from the same weather system) make for a very sleepless night.

It is not the sensors that measure the amount of fuel in the tank. There are all kind of other sensors in cars now, which adjust the engine based on its readings It will read the exhaust and adjust the air to fuel mixture allowing the car to run smoothly. E85 also measure the gasoline to alcohol mixture there are also a lot of other sensors. When you run your car on propane it bypasses a lot of these sensors So it does not have any reading to base these adjustment on. What I am saying is, when tank is low the sensors may not be getting a reading from the fuel to make adjustments. When I run the truck on gasoline the computer picks up the readings and adjust the engine then it stores the readings when I switch to propane the computer still has the readings stored in it and is adjusting based on those readings.

I'm a mechanic by trade and a teacher by profession, I have made my living from the automotive industry for the last 30 years both working in it and teaching it. What you are describing does not happen in a properly functioning system. The issue is, and C3H8 hits it on the head with his reply which I quote below, there should be no good reasons for the engine to run better just with the level of fuel in the tank being increased unless you are running 2 fuels simultaneously. These sensors are able to help the ECU adjust the engines running parameters regardless of running LPG or Petrol.

When the tank is full the gas is high enough for these sensors (not the one that measures the amount of fuel in the tank) to get a continuous reading and my engine runs smother. I know the propane set up has a computer in it to adjust for the engine with out using these other sensors. But I do know if my tank is full my truck runs smother. You do not have to believe me and you do not have to try it and you do not have to ridicule me. I have had my conversion for almost 5 years and I do know this works for me.

No one has said it doesn't work for you, what is being suggested is your system has a problem.

As indicated you say your vehicle runs better when the gas tank is full. Can't argue with that. You own it and drive it everyday.
.....
The bottom line is that the engine runs smoother on a full tank and this indicates gasoline is being burnt along with propane when the tank is full. Only a thorough investigation would reveal the reason for this.