Warning ! Small spark plug gap + hei = preignition + engine damage
March 23rd, 2015, 07:25 AM
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Location: Matamata, Waikato, New Zealand
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Warning ! Small spark plug gap + hei = preignition + engine damage
The purpose for me relating this episode of “Life with the Cutlass” is to inform other people who may be considering making the wrong choices I made of the consequences.A bit of payback for all of the help I have received in the last year and a half, maybe.
The car has been doing 13-14mpg with about 65% steady highway time and as I had noticed that the interior of both tail pipes were looking fairly sooty I figured that the car might be running rich.
It was also pretty common for me to get a good whiff of petrol fumes when I left the engine running and walked around to the back of the car to shut the top gate before taking the car out.So I removed the sparkplugs (AC Delco R45S) and was a bit surprised to see that the colour was a nice light brown not sooty like I was half expecting.I then cleaned and gapped the plugs to 0.030”.A gap I had found was the right one for my car via the internet.At the time I did notice that the feeler gauges felt pretty loose in the gap before adjusting but because I hadn't gapped plugs for a very long time I didn't think too much about it.
It was about this time that I figured that the electric choke would be a good next point to look at as it may have been staying on a bit long. As I live in a mild subtropical climate (mostly) and it's very easy to change the choke settings back I had no qualms about doing this.I then altered the choke to come off at the quickest possible setting. Because the engine often revved too high (900-1000rpm) when placed in Park or Neutral when returning the car to the garage after a run I backed the fast idle speed right off.The overly fast engine speed in Park problem is still there but that's for another day.
I then proceeded to adjust the primary mixture screws, to lean them out a 1/8th turn each at a time.I merely listened for any change in RPM as an indicator but as nothing changed I made a number of adjustments to each screw.
Taking the car for a test drive all seemed fine until I switched the engine off and nasty preignition occurred.This happened each time I turned the engine off and rather than let this happen I started stalling the car in Drive as I knew that this condition was very bad for engines and could cause serious damage if left..
To fix the issue I started tweaking the mixture screws back to the rich side because I knew a lean mixture was the prime cause of this problem.At the back of my mind I wondered about the spark plug gap because my car had been converted to a big cap AC Delco HEI system at some stage and I knew that there was supposed to be a 50,000volt spark involved.I asked a few experienced car guys if they had heard of too small a plug gap causing preignition but nobody had heard of such a thing.
Along with someone known as a bit of a tuner I got out my vacuum gauge setup and we had another go.The reason I didn't use the gauge prior to this was that everytime previously I could only get 6-9 inches on the gauge and nothing changed when adjusting carb settings.I knew that warm cams can cause low vacuum but I had also read somewhere that Old's V8 engines often had this problem and was the reason that the early 442s didn't have power assisted brakes.
This time however there was heaps of vacuum, well into the low-mid twentys which was odd considering we were using the exact same setup as we had previously.Another day..... again.
We proceeded to enrichen the mixture and after a while managed to get rid of a slight miss at high RPM that we had heard.Taking the car home and switching off I still had preignition but it was a lot less.I also suspected that we had now gone more rich than I had gone lean.
Thinking about this and as I still had the reduced spark plug gap at the back of my mind I spent quite a bit of time trying to find information on the internet regarding this but it wasn't until I found a couple of lines on a spark plug manufacturer's website that went something like
“You should use the hottest spark plug you can before experiencing preignition” that I knew I was onto something.
Not long after that I found the information I was looking for.It was here at http://www.442.com/oldsfaq/ofign.htm#IGN Tuning under Change Plugs if you are interested in looking further I found what I had been searching for.It reads :
Change Plugs:*
When I converted my 1970 Cutlass to HEI, it ran badly until I regapped the plugs from 0.030 to 0.065. I had newish platinum plugs and figured I'd give the old ones a try.
If you have converted to HEI, you might notice a shortage, or absence of spark plugs for your Olds engine. An appropriate substitute is AC-Delco R46SX. When using R46SX plugs, the book says to gap them at .080" but try .060". It make a difference in power with no harmful effects.
I found out today that the SX series of AC plugs has been superceded by the SZ series and a local NAPA store was able to order them for me.
With the MSD you might try getting something a bit heavier duty, perhaps platinum or if they make the "truck plug" version of the R46SX.
If you are using the MSD 6AL box, I would recommend NGK 5670-6 spark plugs. They are like the NGK X45 but a little better. These plugs are set at .040" gap. Make sure your timing is set at 36° total. The inital advance should be around 18° if you installed a recurve kit. If not, buy the kit, and put on one light spring and one medium spring, and that will get the timing close.
Basically you run the largest gap possible. The larger the gap, the greater the voltage required to jump the gap. The greater the voltage - the sooner the leakage - the sooner the cross-fire etc.
Bottom line, with a stock rotor, your pretty much stuck with the stock gap. HEI systems can take up to a .060" or even larger gap. Most engines are good for about .035". Higher cylinder pressures will require higher voltages to jump the gap - you might need to upgrade wires to accomplish this without cross-fire.
I have experiemented with gaps - going from .025" to .060" on a stock system. The car basically ran the same. With the .060" gap it sometimes misfired at idle and high rpm. The factory manual calls for .035" and .045" works fine. Without dyno testing I couldn't tell what was optimal.
I guess the standard ignition system rules apply - if it fires all cylinders under all conditions than don't mess with it (save your time and money).
Don't reduce the resistor spark plug gap to that of the non-resistor plug gap (from .060" to .030"). This prevents the HEI from providing its best advantage, a hotter spark. Electricity will jump to ground as soon as it can. The smaller gap reduces the voltage in the spark to no better than the points setup.
The AC Delco R45S sparkplugs currently in my engine are a basic resistor type (the R),have an extended tip (the S) and have a heat range factor of 5 (the 5) although I have seen them advertised for sale as “Heat Range : 4”.Perhaps the 4 rating came about because I had seen elsewhere that a spark plug's possible heat range being from 0 to 9.
So what was obviously (now) happening was that, due to the large 50,000 volts delivered by the HEI a spark was created that was so powerful in the small gap that the electrode end of the spark plug became so incredibly hot that the fuel and air mixture was able to repeatedly spontaneously combust in the cylinder causing the damaging condition known as preignition.
What can be learned from this? I learned plenty like the old do one thing at a time,listen to the little voice in your head,there is nothing new, do your research,consult real experts.There are many more - unfortunately.
However the most important technical thing is that you must use a much bigger spark plug gap than “factory spec” when you have an aftermarket HEI system installed in your car. Not can have bu[B]t must have!
So later on today once I have the alternator back together and working (A.D) I will be changing the spark plug gaps to .060”.I will then go for a drive and when I get back most likely get the vacuum gauge and screwdriver out and see if I can keep the vacuum high but go a bit leaner on the mixture.I will update this post as soon as I have made the changes.
Cheers
The car has been doing 13-14mpg with about 65% steady highway time and as I had noticed that the interior of both tail pipes were looking fairly sooty I figured that the car might be running rich.
It was also pretty common for me to get a good whiff of petrol fumes when I left the engine running and walked around to the back of the car to shut the top gate before taking the car out.So I removed the sparkplugs (AC Delco R45S) and was a bit surprised to see that the colour was a nice light brown not sooty like I was half expecting.I then cleaned and gapped the plugs to 0.030”.A gap I had found was the right one for my car via the internet.At the time I did notice that the feeler gauges felt pretty loose in the gap before adjusting but because I hadn't gapped plugs for a very long time I didn't think too much about it.
It was about this time that I figured that the electric choke would be a good next point to look at as it may have been staying on a bit long. As I live in a mild subtropical climate (mostly) and it's very easy to change the choke settings back I had no qualms about doing this.I then altered the choke to come off at the quickest possible setting. Because the engine often revved too high (900-1000rpm) when placed in Park or Neutral when returning the car to the garage after a run I backed the fast idle speed right off.The overly fast engine speed in Park problem is still there but that's for another day.
I then proceeded to adjust the primary mixture screws, to lean them out a 1/8th turn each at a time.I merely listened for any change in RPM as an indicator but as nothing changed I made a number of adjustments to each screw.
Taking the car for a test drive all seemed fine until I switched the engine off and nasty preignition occurred.This happened each time I turned the engine off and rather than let this happen I started stalling the car in Drive as I knew that this condition was very bad for engines and could cause serious damage if left..
To fix the issue I started tweaking the mixture screws back to the rich side because I knew a lean mixture was the prime cause of this problem.At the back of my mind I wondered about the spark plug gap because my car had been converted to a big cap AC Delco HEI system at some stage and I knew that there was supposed to be a 50,000volt spark involved.I asked a few experienced car guys if they had heard of too small a plug gap causing preignition but nobody had heard of such a thing.
Along with someone known as a bit of a tuner I got out my vacuum gauge setup and we had another go.The reason I didn't use the gauge prior to this was that everytime previously I could only get 6-9 inches on the gauge and nothing changed when adjusting carb settings.I knew that warm cams can cause low vacuum but I had also read somewhere that Old's V8 engines often had this problem and was the reason that the early 442s didn't have power assisted brakes.
This time however there was heaps of vacuum, well into the low-mid twentys which was odd considering we were using the exact same setup as we had previously.Another day..... again.
We proceeded to enrichen the mixture and after a while managed to get rid of a slight miss at high RPM that we had heard.Taking the car home and switching off I still had preignition but it was a lot less.I also suspected that we had now gone more rich than I had gone lean.
Thinking about this and as I still had the reduced spark plug gap at the back of my mind I spent quite a bit of time trying to find information on the internet regarding this but it wasn't until I found a couple of lines on a spark plug manufacturer's website that went something like
“You should use the hottest spark plug you can before experiencing preignition” that I knew I was onto something.
Not long after that I found the information I was looking for.It was here at http://www.442.com/oldsfaq/ofign.htm#IGN Tuning under Change Plugs if you are interested in looking further I found what I had been searching for.It reads :
Change Plugs:*
When I converted my 1970 Cutlass to HEI, it ran badly until I regapped the plugs from 0.030 to 0.065. I had newish platinum plugs and figured I'd give the old ones a try.
If you have converted to HEI, you might notice a shortage, or absence of spark plugs for your Olds engine. An appropriate substitute is AC-Delco R46SX. When using R46SX plugs, the book says to gap them at .080" but try .060". It make a difference in power with no harmful effects.
I found out today that the SX series of AC plugs has been superceded by the SZ series and a local NAPA store was able to order them for me.
With the MSD you might try getting something a bit heavier duty, perhaps platinum or if they make the "truck plug" version of the R46SX.
If you are using the MSD 6AL box, I would recommend NGK 5670-6 spark plugs. They are like the NGK X45 but a little better. These plugs are set at .040" gap. Make sure your timing is set at 36° total. The inital advance should be around 18° if you installed a recurve kit. If not, buy the kit, and put on one light spring and one medium spring, and that will get the timing close.
Basically you run the largest gap possible. The larger the gap, the greater the voltage required to jump the gap. The greater the voltage - the sooner the leakage - the sooner the cross-fire etc.
Bottom line, with a stock rotor, your pretty much stuck with the stock gap. HEI systems can take up to a .060" or even larger gap. Most engines are good for about .035". Higher cylinder pressures will require higher voltages to jump the gap - you might need to upgrade wires to accomplish this without cross-fire.
I have experiemented with gaps - going from .025" to .060" on a stock system. The car basically ran the same. With the .060" gap it sometimes misfired at idle and high rpm. The factory manual calls for .035" and .045" works fine. Without dyno testing I couldn't tell what was optimal.
I guess the standard ignition system rules apply - if it fires all cylinders under all conditions than don't mess with it (save your time and money).
Don't reduce the resistor spark plug gap to that of the non-resistor plug gap (from .060" to .030"). This prevents the HEI from providing its best advantage, a hotter spark. Electricity will jump to ground as soon as it can. The smaller gap reduces the voltage in the spark to no better than the points setup.
The AC Delco R45S sparkplugs currently in my engine are a basic resistor type (the R),have an extended tip (the S) and have a heat range factor of 5 (the 5) although I have seen them advertised for sale as “Heat Range : 4”.Perhaps the 4 rating came about because I had seen elsewhere that a spark plug's possible heat range being from 0 to 9.
So what was obviously (now) happening was that, due to the large 50,000 volts delivered by the HEI a spark was created that was so powerful in the small gap that the electrode end of the spark plug became so incredibly hot that the fuel and air mixture was able to repeatedly spontaneously combust in the cylinder causing the damaging condition known as preignition.
What can be learned from this? I learned plenty like the old do one thing at a time,listen to the little voice in your head,there is nothing new, do your research,consult real experts.There are many more - unfortunately.
However the most important technical thing is that you must use a much bigger spark plug gap than “factory spec” when you have an aftermarket HEI system installed in your car. Not can have bu[B]t must have!
So later on today once I have the alternator back together and working (A.D) I will be changing the spark plug gaps to .060”.I will then go for a drive and when I get back most likely get the vacuum gauge and screwdriver out and see if I can keep the vacuum high but go a bit leaner on the mixture.I will update this post as soon as I have made the changes.
Cheers
March 23rd, 2015, 08:08 AM
#2
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Join Date: Dec 2010
Location: Milwaukee, WI
Posts: 192
We need way more info about the car. Model year to start, and then a list of what has been done to it (ignition and carb changes specifically).
Does the engine "run on" after the ignition is turned off? If so, that takes a completely different troubleshooting path vs. pinging while being driven.
Does the engine "run on" after the ignition is turned off? If so, that takes a completely different troubleshooting path vs. pinging while being driven.
March 23rd, 2015, 11:19 AM
#4
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Join Date: Aug 2013
Location: Nashville
Posts: 177
I would think that running a smaller gap would allow the plug tip to run cooler, correct? That's my understanding, but I may be wrong.
In my experience, pre-ignition is mainly caused by a hot plug, sharp edges in the chamber, excessive carbon deposits, localized cooling problems, retarded timing, etc... Plug gap, while interesting, wouldn't be my first choice in the trouble shooting list. I would be leaning towards a spark plug that's too hot (in terms of heat dissipation). I run a high compression 330, and I'm running the coldest plugs possible. My gaps are 32 thousands with an HEI.
This is an interesting conversation, maybe someone needs to school me on this, and I need to widen my gap?
Regardless, we should always 'mind the gap'
In my experience, pre-ignition is mainly caused by a hot plug, sharp edges in the chamber, excessive carbon deposits, localized cooling problems, retarded timing, etc... Plug gap, while interesting, wouldn't be my first choice in the trouble shooting list. I would be leaning towards a spark plug that's too hot (in terms of heat dissipation). I run a high compression 330, and I'm running the coldest plugs possible. My gaps are 32 thousands with an HEI.
This is an interesting conversation, maybe someone needs to school me on this, and I need to widen my gap?
Regardless, we should always 'mind the gap'
March 23rd, 2015, 01:10 PM
#6
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Join Date: Jun 2010
Location: Rogues Island, USA
Posts: 3,613
I think you're referring to 'run on' and not pre-ignition. That can be solved by turning down your high idle you stated you have. It's still running on a high idle step.
Secondly, trying to adjust the idle mixture screws while not in idle is a lesson in futility. At that point you've uncovered the transfer slots and shouldn't notice a difference with the idle mixture screws. As you've already noticed.
Lastly, there is no way in hell you're hurting an engine that has an HEI by running smaller gaps. If anything it's easier on the whole ignition system to jump that smaller gap. Especially the secondary side of the ignition.
You're spewing bad info here.
Secondly, trying to adjust the idle mixture screws while not in idle is a lesson in futility. At that point you've uncovered the transfer slots and shouldn't notice a difference with the idle mixture screws. As you've already noticed.
Lastly, there is no way in hell you're hurting an engine that has an HEI by running smaller gaps. If anything it's easier on the whole ignition system to jump that smaller gap. Especially the secondary side of the ignition.
You're spewing bad info here.
Last edited by TripDeuces; March 24th, 2015 at 04:12 PM.
March 24th, 2015, 12:52 PM
#7
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Join Date: Feb 2013
Location: Phoenix, AZ
Posts: 13,888
A few observations I have:
The following statements are contradictory:
Electricity will jump to ground as soon as it can. The smaller gap reduces the voltage in the spark to no better than the points setup.
So what was obviously (now) happening was that, due to the large 50,000 volts delivered by the HEI a spark was created that was so powerful in the small gap
The first statement is correct.
As the field in the primary side of the coil collapses, the voltage on the secondary (output) side increases until the potential overcomes the resistance of the air gap (ionizes the air between the spark plug terminals) and current flows.
The second statement is incorrect.
The resistance of the air gap between the spark plug terminals has not changed, so a coil with the capability of generating a higher voltage will still make the spark jump the gap at the same voltage (refer to the second sentence of the first statement).
The "50,000 Volt coil" has the capability of generating a higher spark voltage, but for that to happen the resistance between the spark plug terminals has to increase. The coil is not "dumping" a pre-set voltage into the plug gap (as a capacitive discharge system does); the voltage in the coil rises until the resistance of the plug gap is overcome and the spark jumps the gap.
The following statements are contradictory:
Electricity will jump to ground as soon as it can. The smaller gap reduces the voltage in the spark to no better than the points setup.
So what was obviously (now) happening was that, due to the large 50,000 volts delivered by the HEI a spark was created that was so powerful in the small gap
The first statement is correct.
As the field in the primary side of the coil collapses, the voltage on the secondary (output) side increases until the potential overcomes the resistance of the air gap (ionizes the air between the spark plug terminals) and current flows.
The second statement is incorrect.
The resistance of the air gap between the spark plug terminals has not changed, so a coil with the capability of generating a higher voltage will still make the spark jump the gap at the same voltage (refer to the second sentence of the first statement).
The "50,000 Volt coil" has the capability of generating a higher spark voltage, but for that to happen the resistance between the spark plug terminals has to increase. The coil is not "dumping" a pre-set voltage into the plug gap (as a capacitive discharge system does); the voltage in the coil rises until the resistance of the plug gap is overcome and the spark jumps the gap.
Last edited by Fun71; March 24th, 2015 at 01:01 PM.
March 24th, 2015, 01:08 PM
#8
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Location: The Hudson Valley
Posts: 21,183
+1. You are barking up the wrong tree.
As you observed, your spark plug gap with HEI makes little to any difference in operation, until you start to get too big (around 0.060"), at which point you begin to have leakage problems from the wires because of the unnecessarily high voltage. 0.035-0.045" is fine.
The HEI will not do anything differently at those gaps, other than operate at a lower overall voltage.
Your electrodes may be getting hot, but if so, it is not because of the gap or the HEI, it is because you are running plugs that are too hot for your engine. Switch to a set of plugs that is one gradation colder and see what happens - generally, you want to run plugs about 1 gradation higher than the range that fouls, whatever that is.
I agree that it sounds like your run-on problem is being caused by your throttle plates being too far open (I had a '68 Colony Park wagon with a HC390 once that I could switch off and then literally drive all the way down the block - what a smell!).
- Eric
As you observed, your spark plug gap with HEI makes little to any difference in operation, until you start to get too big (around 0.060"), at which point you begin to have leakage problems from the wires because of the unnecessarily high voltage. 0.035-0.045" is fine.
The HEI will not do anything differently at those gaps, other than operate at a lower overall voltage.
Your electrodes may be getting hot, but if so, it is not because of the gap or the HEI, it is because you are running plugs that are too hot for your engine. Switch to a set of plugs that is one gradation colder and see what happens - generally, you want to run plugs about 1 gradation higher than the range that fouls, whatever that is.
I agree that it sounds like your run-on problem is being caused by your throttle plates being too far open (I had a '68 Colony Park wagon with a HC390 once that I could switch off and then literally drive all the way down the block - what a smell!).
- Eric
March 26th, 2015, 07:50 PM
#9
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Location: Matamata, Waikato, New Zealand
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We need way more info about the car. Model year to start, and then a list of what has been done to it (ignition and carb changes specifically).
Does the engine "run on" after the ignition is turned off? If so, that takes a completely different troubleshooting path vs. pinging while being driven.
Does the engine "run on" after the ignition is turned off? If so, that takes a completely different troubleshooting path vs. pinging while being driven.
March 26th, 2015, 07:58 PM
#10
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Location: Matamata, Waikato, New Zealand
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March 26th, 2015, 08:27 PM
#12
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I would think that running a smaller gap would allow the plug tip to run cooler, correct? That's my understanding, but I may be wrong.
In my experience, pre-ignition is mainly caused by a hot plug, sharp edges in the chamber, excessive carbon deposits, localized cooling problems, retarded timing, etc... Plug gap, while interesting, wouldn't be my first choice in the trouble shooting list. I would be leaning towards a spark plug that's too hot (in terms of heat dissipation). I run a high compression 330, and I'm running the coldest plugs possible. My gaps are 32 thousands with an HEI.
This is an interesting conversation, maybe someone needs to school me on this, and I need to widen my gap?
Regardless, we should always 'mind the gap'
In my experience, pre-ignition is mainly caused by a hot plug, sharp edges in the chamber, excessive carbon deposits, localized cooling problems, retarded timing, etc... Plug gap, while interesting, wouldn't be my first choice in the trouble shooting list. I would be leaning towards a spark plug that's too hot (in terms of heat dissipation). I run a high compression 330, and I'm running the coldest plugs possible. My gaps are 32 thousands with an HEI.
This is an interesting conversation, maybe someone needs to school me on this, and I need to widen my gap?
Regardless, we should always 'mind the gap'
Use of cold plugs depends upon the type of driving the engine will experience.Some years ago I had a Holden Torana SLR5000 - small car with 308 V8 etc and it would start missing and farting if I drove it sedately.I used to drop down a gear or two and plant it to get the revs up and engine happy.I changed the plugs to a hotter version and all was good.
Ijust found this about cold plugs on the Denso plug site.
"cold plug" has a shorter insulator nose, which minimizes the amount of surface area exposed to the combustion gases. Cold plugs are typically used in racing conditions because of its ability to transfer heat out of the cylinder chamber quickly. Most turbo, supercharged and nitrous oxide applications use this type of plug because of the tremendous heat they generate. A colder plug can also minimize the risk of pre-ignition and detonation, however if the plug is to cold there is risk of fouling during extended periods of idling and low-speed operation.
See how they mention using a cooler plug to minimize pre-ignition.
March 26th, 2015, 08:51 PM
#13
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Location: Rogues Island, USA
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They mention a colder plug in racing situations not everyday driving.
There is a simple test for a plug. Buy a new set of plugs in the heat range you think you should have. I suggest something stock. Warm up your car on an old set of plugs and then switch to the brand new set and make a pass down the 1/4 mile or whatever you have for testing. Shut the car off at the end of the run, don't drive back to the pits. In fact shut of the ignition and coast to a stop. Check the plugs. Whatever they read is the bible. It's that simple.
You can't always hear pre-ignition and that is the most ignorant way of testing a plug. Complete combustion has always been the name of the game not testing to failure, like pre-ignition. A light tan insulator, LIGHT tan, is what you want.
There is a simple test for a plug. Buy a new set of plugs in the heat range you think you should have. I suggest something stock. Warm up your car on an old set of plugs and then switch to the brand new set and make a pass down the 1/4 mile or whatever you have for testing. Shut the car off at the end of the run, don't drive back to the pits. In fact shut of the ignition and coast to a stop. Check the plugs. Whatever they read is the bible. It's that simple.
You can't always hear pre-ignition and that is the most ignorant way of testing a plug. Complete combustion has always been the name of the game not testing to failure, like pre-ignition. A light tan insulator, LIGHT tan, is what you want.
March 26th, 2015, 09:14 PM
#14
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Absolutely correct.If you look back at what I wrote I said I had thought the car may be running rich until I saw the spark plugs condition (spot on) and because I had sooty pipes and could smell fuel at the rear when the engine was running I then looked at the electric choke and idle mixture as possible answers.
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