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C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

L98 Spark Plugs

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Old 08-04-2018, 11:25 PM
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Default L98 Spark Plugs

Hi, I have a 1985 and was wondering if the spark plugs I bought are good. I got the autolite double platinum at the parts store a couple months ago because it was the higher quality ones and havenít gotten to changing them yet. Will these give it the most power possible? I just donít want it to be holding back the engine at all because of putting the wrong spark plugs in. Also how do you know if the wires are still the ones from the factory? I donít think mine are because they are red and say high heat resistance on them. Thanks

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Old 08-04-2018, 11:36 PM
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The best thing that you can do your your L98 engine is give it a very good tune up-- including plugs. Unless you had a spark plug related issue, just swapping plug wont give you a huge SOTP improvement.
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Old 08-04-2018, 11:57 PM
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I know it wonít change the feel of it or nothing I just wanted it running the best and wanted to make sure these plugs werenít made wrong for the car or something. What do you suggest I should do for a very good tune up? I was thinking about changing the wires but wouldnít want to change them if they werenít the same old factory ones because they are red and say high heat resistance on them so I figured they were aftermarket and are good the way they are now and they look good being red too. I just know the guy who owned it before said it hasnít been given a tune up in a long time so I figured I would get new plugs for it.
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Old 08-05-2018, 01:07 AM
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Originally Posted by 85 CRVET View Post
I know it wonít change the feel of it or nothing I just wanted it running the best and wanted to make sure these plugs werenít made wrong for the car or something. What do you suggest I should do for a very good tune up? I was thinking about changing the wires but wouldnít want to change them if they werenít the same old factory ones because they are red and say high heat resistance on them so I figured they were aftermarket and are good the way they are now and they look good being red too. I just know the guy who owned it before said it hasnít been given a tune up in a long time so I figured I would get new plugs for it.
Any good plug that is the right heat range and mechanical fit will be fine as there is little to find there. Aftermrket wires in my experience are good for 5 years or so. What it has probably not seen in ages is cap and rotor fuel filter air filter that sort of thing.
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Old 08-05-2018, 02:37 AM
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Ok thanks I probably wouldnt want to do the fuel filter myself as that involves fuel lines but I will probably be getting the other stuff. I already got a new air filter and cut the lid. I also got the red air filter and then painted the air box red when I cut it because I found out that you canít see the red filter at all even when the lid is cut. I have the autolite double platinums and probably going to get some jegs red spark plug wires and an msd rotor and cap. This would be a good tune up then? It runs fine now but I just want to make sure everything is going perfectly since the last owner said it probably needs a tune up.
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Old 08-06-2018, 12:20 AM
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Ok so I found the wires and the cap and rotor I want but was wondering about the ninth wire on top of the distributor. I thought there is always an extra wire than how many cylinders you have and was wondering if I should replace that too or if that wire, I think itís called the coil wire, is fine and doesnít need changed often.
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Old 08-06-2018, 12:23 PM
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Originally Posted by 85 CRVET View Post
Ok so I found the wires and the cap and rotor I want but was wondering about the ninth wire on top of the distributor. I thought there is always an extra wire than how many cylinders you have and was wondering if I should replace that too or if that wire, I think itís called the coil wire, is fine and doesnít need changed often.
the extra wire is for the ignition coil. That car has an HEI distributor so the coil is built into the distributor cap. There will only be 8 wires to replace.

Thar car came from the factory with copper plugs. Putting in double platinum will not make any difference. Honestly your just over paying for a plug in this case. Make sure you gap the plug properly. It should be .045. You will slightly hurt performance if you dont do that.

the fuel filter is easy to do if you can lift the car high enough to get under it. One bolt and unscrewing the lines the go in each end of the filter.
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Old 08-06-2018, 04:17 PM
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It doesnt really matter what plug you run, I don't run the super expensive ones, just regular Autolites. The car came with Rapidfires. Just make sure you gap them right.
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Old 08-06-2018, 04:30 PM
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One nice simple upgrade that is easy to do is when you replace your distributor cap change the coil while you are at it. I have a D.U.I (Davis Unified Ignition) coil on my car and that makes one heck of a hot spark. I also replaced the spark control box inside of the distributor's body with an aftermarket unit. Before I finished I put new Champion plugs and new MSD "Super-Conductor" plug wires. There was no difference in performance measured by my Gtech pro when I was finished. I thought it would make a small difference but it did not. I guess my car was not running that badly before.

If you were able to cut the hole in you air filter box then you should be able to handle the fuel filter, it is easy and not too messy. I would however not have a cigarette while changing it out....

I have a test rig setup using an older MSD ignition box and coil hooked up to a battery. I love testing the various new "cool" Spark plugs, my favorite so far is the BOSCH FUSION plugs, wow, what a deep dark blue spark. The E3 plugs look like they might be a contender in the automobile market, they make a great spark and have multiple ground pathways. I go through plugs in my other Corvette, a 1968 L-71 car that has been rebuilt to use a 12.25-1 compression ratio. The Champion plugs started transferring metal from electrode to ground in less than 2 hours of driving.

IF you really want to know that you made a difference, you need one of these to use before and after you make any changes. Today they are $300 but worth every penny to somebody that likes to make changes. You can determine whether or not the new carburetor is helping or hurting, does the tire pressure make any difference, whatever you want to do can be tested without breaking the law. Here is the info on the GTECH:

http://www.gtechpro.com/G-TECH-Pro%20SS

I have one that I have had for close to 15 to 20 years now. I am surprised more Corvette owners don't use these gadgets, it will show you how much horsepower or torque you make and even graph it for you. I find an empty parking lot or someplace they won't mind you doing a couple acceleration runs. The most important for me is usually the 0-60 mph and 40-70 mph times as they give me a real world view of any changes! The most important thing is to not lie about your car's weight. I took mine to a scale to get a precise number and I get a more precise answer when I test any of my toys that way. The landfill weighs your vehicle going in and out so I just bop out there and get weighed when I need to know the weight correctly. This is probably one of the most useful tools for anybody who makes modifications. Why go spend the money for a dyno run? Around here it is very expensive.

Good Luck with your Corvette!
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Old 08-06-2018, 09:10 PM
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Ok thank you. I guess I will just put in the autolite double platinums I have and get the jegs red hot power wires because I like how bright red they are and the msd distributor cap and rotor. I guess I wonít get the coil since they are pretty expensive and mine seems to run good right now but I just want to make sure itís running the best it can be. I will be getting the k&n fuel filter and put it in since it shouldnít be too hard I guess. So what is the gap supposed to be? I thought I read that they changed it to be supposed to be at .35 after they left the factory. Thanks for the help. Oh and how does that g tech thing work? I have always wanted to know my horsepower and will probably bring it to a dyno sometime just so it is tuned the best but that would be nice to be able to calculate how much horsepower you add after each thing you do to it. I donít get how it calculates horsepower without being connected with wires to the vehicle somehow.

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Old 08-10-2018, 03:21 PM
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You can't go wrong with double platinum spark plugs, they should last a while and are very good at "self-cleaning" themselves. I just bought a set of NGK Platinum plugs for my C3, I hope they last longer than the Champions I had before. My C3 has an MSD ignition system that makes one heck of a nasty hot spark.

Using the GTech that I have I am "dyno testing" each modification to see if there was any improvement. Inside the Gtech there lives a VERY sensitive accelerometer. When the device is turned on you simply come to a complete stop and then push a button on the screen, after that, the minute the car starts to move the accelerometer starts counting and calculating. It takes the weight of your car and then it can calculate the horsepower and torque needed to propel your car. I have four different carburetors for my C3, as I swapped out parts I got each one of them to give their best without ever setting foot into a dyno chamber. I went to a open area and did a couple runs from 0-60 mph and that alone told me the answer I needed using the Gtech. It will give you a graph of your runs and have peak torque and peak HP shown on it. I would strongly suggest that you check out their website as this device is wonderful, albeit a bit expensive. When I bought mine I paid less than the current price of $300 and $400 for the road racing version with it's abilities to do even more. The RR model lets you download the data for you to evaluate your car's performance even more. If you are an autocross person you would really like the lap function. It makes a map of the track and shows improvements as you drive. One of my favorite features was their built in shift light, it does not go off at any set rpm, it goes off when you start to fall off the torque curve and varies from car to car. If you want to really drive that car to the maximum then you need this function.

I like to play with my car and knowing right away what the change did to my performance is very valuable, it is way better than the seat of the pants method so many people use. Add a K&N air filter and you can see right away if it gave you more power or less. The GTECH is extremely accurate and for what it does I might have paid double for a device like this little tool.

If you really want to, the device will measure 1/4 miles times, I find that using my C4 that is okay but not with my C3, I don't think many people would be happy with you making 1/4 mile runs in public. Being a private Pilot I used to drive the Corvette to the little private airport and use the runway when there was no activity. 2600' of perfectly flat pavement, lots of room to slow back down. Just don't do this without permission or else paint a big "N" followed by four numbers and a letter on both sides of your Corvette.

I hope that this helps you a little with the information regarding the GTECH. I am glad I have one and it is a valuable testing tool. The only wire I use is the power cord to supply the 12 volts it needs. I helped people who thought they had a fast car find more speed and learn to drive their car faster than ever.

Enjoy Your Corvette and have a great weekend!

Chris
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Old 08-10-2018, 07:10 PM
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Ok, thank you for explaining that. That helps understand it a lot. I will probably end up getting one sometime. I have always wondered around what horsepower my car has. Right now I have an 85 with 2.5Ē dual exhaust with Allenís chambered mufflers and the air box lid cut out with a spectre performance air filter. Soon I will have the spark plugs and cap and rotor to get it running the best possible and a k&n fuel filter.

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Old 08-10-2018, 07:34 PM
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Originally Posted by 85 CRVET View Post
Hi, I have a 1985 and was wondering if the spark plugs I bought are good. I got the autolite double platinum at the parts store a couple months ago because it was the higher quality ones and havenít gotten to changing them yet. Will these give it the most power possible? I just donít want it to be holding back the engine at all because of putting the wrong spark plugs in. Also how do you know if the wires are still the ones from the factory? I donít think mine are because they are red and say high heat resistance on them. Thanks
If you are going to keep the plugs in for 100k miles then platinums make sense, if you are going to change them every two years regular nickel plated plugs are just as good.

As far as brand goes, I like autolite and delco plugs. I dislike champion because I have had bad lick with them. That is based on my personal experience and probably has no real relevance.
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Old 08-11-2018, 11:32 PM
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Ok, thatís what I wanted to hear since I already bought them months ago and just havenít put them in yet until I buy the wires and cap and rotor this week. I will change them before 100,000 miles because I donít drive it much at all and would make sure they are good. I was afraid at first I already paid over thirty dollars for the spark plugs and would need to get different ones because these arenít right but Iím glad theyíll work good.
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