Gapping Plugs
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Gage
If I have a petronix ign with their higher voltage coil, can I gap the plugs more than .035? What should I gap them?
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,402
From: At my Bar drinking and wrenching in Lafayette Colorado
You can get away with a wide plug gap (.060) on a low compression engine running low cylinder pressures.
However, on a high compression engine or an engine wth good volumetric efficiency (with resulting higher cylinder pressures), the increased pressure will increase the resistance across the plug gap. With increased resistance across the gap, the spark wants to find a lower-resistance path, and is more apt to arc across wires, inside the cap, and anyplace except at the plugs. So for high performance use, run a .035 to .045 gap, but not more. This will give you good performance with good reliability.
However, on a high compression engine or an engine wth good volumetric efficiency (with resulting higher cylinder pressures), the increased pressure will increase the resistance across the plug gap. With increased resistance across the gap, the spark wants to find a lower-resistance path, and is more apt to arc across wires, inside the cap, and anyplace except at the plugs. So for high performance use, run a .035 to .045 gap, but not more. This will give you good performance with good reliability.
Tech Contributor
Joined: Aug 1999
Posts: 14,382
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From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by Jeff 6T9
I'm running an ignitor II with a flame thrower II coil. I left the plugs at .035. I've noticed a little pinging - will regapping the plugs help, or is this solely a timing issue?
Race Director
Joined: Jul 2002
Posts: 10,154
Likes: 21
From: Texas
Originally Posted by lars
Changing plug gap won't help the detonation. You need to either retard the timing a tad, slow down your advance curve, reduce the amount of vacuum advance, or slightly richen your mixture.
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Le Mans Master
Joined: Mar 2002
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From: Coming home from Luckenbach Texas
If Lars doesn't come back, I know his response. Adjustable vac advance cans belong on the shelf, or in the garbage.
You need to fiddle with your mechanical advance springs to change your curve. And is overall timing at 36°?
You need to fiddle with your mechanical advance springs to change your curve. And is overall timing at 36°?
Race Director
Joined: Jul 2002
Posts: 10,154
Likes: 21
From: Texas
Originally Posted by page62
If Lars doesn't come back, I know his response. Adjustable vac advance cans belong on the shelf, or in the garbage.
You need to fiddle with your mechanical advance springs to change your curve. And is overall timing at 36°?
You need to fiddle with your mechanical advance springs to change your curve. And is overall timing at 36°?
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,402
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by page62
If Lars doesn't come back, I know his response. Adjustable vac advance cans belong on the shelf, or in the garbage.
The adjustable units have little merit. There are 2 problems with those adjustable units:
First, a vacuum advance control system has 2 parameters: The length of the vacuum advance and the suction required to pull it in. The adjustable units are only adjustable for one of these parameters.
Secondly, most people adjust the units, and never really know what spec they're ending up with. Few people have the capability to check the spec they end up with (for start-in vacuum point and end point).
For these reasons, you're much better off determining what your spec needs are and buying a fixed unit that meets these specs. For a performance application, you want a unit that pulls in 14 to 16 degrees of advance (crankshaft degrees) and pulls it all in at your idle vacuum setting with some margin.
For a detailed discussion on this, along with all the part numbers and specs for the vacuum "cans," drop me an e-mail request for my Vacuum Advance Facts and Specs document and I'll send it out to you.
V8FastCars@msn.com
Pro
Joined: Mar 2004
Posts: 584
Likes: 0
From: Okemos MI
Originally Posted by lars
Page has me all figured out...
The adjustable units have little merit. There are 2 problems with those adjustable units:
First, a vacuum advance control system has 2 parameters: The length of the vacuum advance and the suction required to pull it in. The adjustable units are only adjustable for one of these parameters.
Secondly, most people adjust the units, and never really know what spec they're ending up with. Few people have the capability to check the spec they end up with (for start-in vacuum point and end point).
For these reasons, you're much better off determining what your spec needs are and buying a fixed unit that meets these specs. For a performance application, you want a unit that pulls in 14 to 16 degrees of advance (crankshaft degrees) and pulls it all in at your idle vacuum setting with some margin.
For a detailed discussion on this, along with all the part numbers and specs for the vacuum "cans," drop me an e-mail request for my Vacuum Advance Facts and Specs document and I'll send it out to you.
V8FastCars@msn.com
The adjustable units have little merit. There are 2 problems with those adjustable units:
First, a vacuum advance control system has 2 parameters: The length of the vacuum advance and the suction required to pull it in. The adjustable units are only adjustable for one of these parameters.
Secondly, most people adjust the units, and never really know what spec they're ending up with. Few people have the capability to check the spec they end up with (for start-in vacuum point and end point).
For these reasons, you're much better off determining what your spec needs are and buying a fixed unit that meets these specs. For a performance application, you want a unit that pulls in 14 to 16 degrees of advance (crankshaft degrees) and pulls it all in at your idle vacuum setting with some margin.
For a detailed discussion on this, along with all the part numbers and specs for the vacuum "cans," drop me an e-mail request for my Vacuum Advance Facts and Specs document and I'll send it out to you.
V8FastCars@msn.com
When you say For a performance application, you want a unit that pulls in 14 to 16 degrees of advance (crankshaft degrees) and pulls it all in at your idle vacuum setting with some margin.
Should I not be using the ported vacuum for my vacuum advance?
I would appreciate it if you e mailed me your Vacuum Advance Facts and Specs document
lotusj12@aol.com
Thanks,
John
