What effect does changing the spark plug gap have?
Road Warrior
Joined: Aug 2010
Posts: 1,788
Likes: 5
From: Tennessee
My indexing/gapping experience. Me and a buddy were scheduled to do some bike maintenance and do some drinkin in the process. I was late gettin there due to an emergency service call so he had the service done and was drinkin without me. A few days later we were riding together and I commented about how much better it ran and couldnt figure out why. All he did was change oil and filter! He laughed and told me he had gaped and indexed the plugs! I'm not saying it had more hp but it just ran smoother, especially at idle.
Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
Likes: 31
From: earth
Indexing is a complete waste of time. Think about this. The intake valve is open on the down stroke of the piston. At the bottom of the stroke the intake valve closes. As the piston comes up the entire fuel/air charge is swirling around in the cylinder. As the piston comes to the top, the charge is compressed into a small area and completely surrounding the spark plug. Then the spark plug fires. It will not make a bit of difference to have the gap pointed at the intake valve at this point of the cycle when the charge is no longer entering the cylinder. On these engines, it will make no difference in performance to have the plug gapped at either end of the tolerances.
Not being a wrench on much more than maintenance and adding the occasional chrome goody, can anyone point to the direction the plugs need to face to be indexed? I have no idea which side the intake valves are located on the Twin Cam 96. I look forward to trying this indexing and seeing what happens.
As for those that notice a seat of the pants change, I would like to see if they would notice the same change if a buddy did the indexing of the plugs correctly and then incorrectly with a 10 pass test ride and see if the rider could accurately determine which of the 10 passes were indexed correctly and which were reversed?
As for those that notice a seat of the pants change, I would like to see if they would notice the same change if a buddy did the indexing of the plugs correctly and then incorrectly with a 10 pass test ride and see if the rider could accurately determine which of the 10 passes were indexed correctly and which were reversed?
Outstanding HDF Member
Joined: Jan 2010
Posts: 2,916
Likes: 31
From: earth
Not being a wrench on much more than maintenance and adding the occasional chrome goody, can anyone point to the direction the plugs need to face to be indexed? I have no idea which side the intake valves are located on the Twin Cam 96. I look forward to trying this indexing and seeing what happens.
As for those that notice a seat of the pants change, I would like to see if they would notice the same change if a buddy did the indexing of the plugs correctly and then incorrectly with a 10 pass test ride and see if the rider could accurately determine which of the 10 passes were indexed correctly and which were reversed?
As for those that notice a seat of the pants change, I would like to see if they would notice the same change if a buddy did the indexing of the plugs correctly and then incorrectly with a 10 pass test ride and see if the rider could accurately determine which of the 10 passes were indexed correctly and which were reversed?
Road Warrior
Joined: May 2008
Posts: 1,802
Likes: 2
From: Salt Lake City, UT. Dushore, Pa.
Here is an old test when you could play with the system on 80 cub. Well I see it didn't attach. It was in HOT Bike 07/1995.
Stock Plugs out performed Platinum and SplitFire Plugs. Stock Coil gap .045 = 55 hp, gap .060 51.5 hp With hotter coil .045 gap = 59.5 hp and .060 gap = 63 hp. Gas mileage also went from 46 to 52 mpg with hotter coil and wider gap. There also have been articles of upgrading the plug wires to improve spark.
That being said the new systems have upgraded coils I think since you don't see hotter coils being offered. I changed my plug wires to the Thundervolts and gap them .045 to stay within the range for the spark sensor or whatever it's called on the EFI bikes. I also believe in indexing the plugs. I use Blue Lightning plugs that have proven dyno runs at SLC HD to improve hp.
Stock Plugs out performed Platinum and SplitFire Plugs. Stock Coil gap .045 = 55 hp, gap .060 51.5 hp With hotter coil .045 gap = 59.5 hp and .060 gap = 63 hp. Gas mileage also went from 46 to 52 mpg with hotter coil and wider gap. There also have been articles of upgrading the plug wires to improve spark.
That being said the new systems have upgraded coils I think since you don't see hotter coils being offered. I changed my plug wires to the Thundervolts and gap them .045 to stay within the range for the spark sensor or whatever it's called on the EFI bikes. I also believe in indexing the plugs. I use Blue Lightning plugs that have proven dyno runs at SLC HD to improve hp.
Last edited by gmc; Jan 25, 2012 at 10:24 AM.
Cool, I think I will give this a try this afternoon. It's raining outside today, so will have to see how it idles in the garage, but should be able to see if it smooths out at all. Thanks for the info.
Road Warrior
Joined: Jul 2009
Posts: 1,579
Likes: 144
From: SC
Waxing a clean bike is worth more speed than indexing plugs... no dirt to weigh it down and the wax eliminates wind drag... it's also dyno proven... LOL!!!! Indexing will help efficiency but telling it would be virtually impossible...
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Grand HDF Member
Joined: Jan 2012
Posts: 4,459
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From: Oklahoma
Indexing is a complete waste of time. Think about this. The intake valve is open on the down stroke of the piston. At the bottom of the stroke the intake valve closes. As the piston comes up the entire fuel/air charge is swirling around in the cylinder. As the piston comes to the top, the charge is compressed into a small area and completely surrounding the spark plug. Then the spark plug fires. It will not make a bit of difference to have the gap pointed at the intake valve at this point of the cycle when the charge is no longer entering the cylinder. On these engines, it will make no difference in performance to have the plug gapped at either end of the tolerances.
Road Master
Joined: Dec 2007
Posts: 885
Likes: 14
From: SoCal
A tighter gap is typically used in a higher compression motor, I think the tighter gap makes a smaller but more powerful spark to resist flame out, or whatever the tech term is. I would say experiment with it, try the .043 and .038, see if you feel any difference, either way you're not going to hurt anything. I've been messing with indexing, side gapping, and colder heat ranges, My opinion the side gap does feel like it makes a stronger running motor, but it does shorten the life of the plugs. I've always been an advocate of indexing, it positions the ground strap so it doesn't shield the spark from the air/fuel charge.