Spark Plug Gap? - Ford Muscle Forums : Ford Muscle Cars Tech Forum
FordMuscleForums.com is the premier Ford Forum on the internet. Registered Users do not see the above ads.
Reply
 
LinkBack Thread Tools
Old 07-13-2011, 07:38 PM   #1 (permalink)
Registered User
 
Join Date: Feb 2001
Posts: 237
Spark Plug Gap?

Finally got my 347 in and running in my 65. It's time for the fine tuning, can't wait to feel the full potential. I changed form my old mallory dual point distributor to a HEI and a CD box. The instructions say to set the plug gap for the most HP, Duh! It didn't even call out a starting point. I was wondering where you guys have set your plugs? When I tested the cd box it pulled a spark across a 1/2" gap. .034 has got to be way too small.
Thanks
__________________
64 Galaxie 500XL 428CJ Tri Power C6
65 Mustang coupe 347 4spd 4wdb, road racer
67 Mustang GT Scode coupe, project
Brett is offline   Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Old 07-13-2011, 08:28 PM   #2 (permalink)
Registered User
 
Join Date: Dec 2009
Location: Excelsior Springs,Mo.
Posts: 179
Re: Spark Plug Gap?

With my 351C after changing from points to a MSD trigger in the dizzy and adding a 6A box I increased my spark plug gap to .045. No problems so far, starts quick with plenty of response and no raw fuel smell at tailpipes....Jim
__________________
1970 Mustang Mach1,Chestnut Metallic,351C,AOD,3.50:1 Locker.
70leadfoot is offline   Reply With Quote Quick reply to this message
Old 07-13-2011, 11:47 PM   #3 (permalink)
Subscriber
 
Join Date: Jan 2001
Location: Texas
Posts: 2,216
Re: Spark Plug Gap?

That 1/2 inch was at 1 atmosphere. The spark gap will vary depending on your compression ratio, what coil and wires you're running. I personally stay in the range of .035 to .045 with a blaster 3 coil, 8mm wires and a 6AL. My engine is somewhere around 10.5 compression.
__________________
1965 289 mustang fastback
Hottarod is offline   Reply With Quote Quick reply to this message
Old 07-15-2011, 11:57 AM   #4 (permalink)
Registered User
 
Join Date: Aug 2006
Location: Greater Puget Sound Area
Posts: 4,003
Re: Spark Plug Gap?

Whether you are racing or driving on the street the best gap is the smallest gap you can run without getting miss fires at idle. The smaller gaps put lower loads on wires and plugs but also have more power to light off the fuel. Larger gaps require more voltage from the ignition systems. That tends to break-down plugs and wires more quickly. Higher voltages mean that you have less amperage (heat) in the spark to light the fires. I run .028" gap in my current engine and have run gaps as low as .020" in some of my engines - like the 7200rpm 289 that I built. Contrary to popular beliefs large gaps are not good (better than smaller gaps) for high compression or high rpm engines. Running smaller gaps also makes your tune last longer.
Run them the way you want to but you should at least try the smaller gap too - just to see how your engine responds to it.
__________________
66 Mustang Coupe. 365hp, 4spd Toploader, sub-frame connectors
Shelby drop, suspension tuned for slalom and hill climb
body mods and weight reduced to 2000lbs; name: Muskrat
PaulS1950 is offline   Reply With Quote Quick reply to this message
Old 07-15-2011, 06:54 PM   #5 (permalink)
Registered User
 
Join Date: Feb 2001
Posts: 237
Re: Spark Plug Gap?

That is very interesting, I never thought about pressure limiting a arc. I am running 10.5:1, a Mallory Promaster coil, 8mm wires and a Summit CD box. The car is used on the street and open track events on road courses. I push the car pretty hard at the open track events, sessions are 20min, 4-6 sessions each day. It is a great time to check for heat range, so I will also try some different gaps. I did notice the exaust note changed at idle. Kind of like a flutter. I thought I had a leak. Has this happened to any of your guys?
__________________
64 Galaxie 500XL 428CJ Tri Power C6
65 Mustang coupe 347 4spd 4wdb, road racer
67 Mustang GT Scode coupe, project
Brett is offline   Reply With Quote Quick reply to this message
Old 07-16-2011, 06:18 AM   #6 (permalink)
Registered User
 
Join Date: Dec 2009
Location: Excelsior Springs,Mo.
Posts: 179
Re: Spark Plug Gap?

Now our Mach1 with a mildly built 351C is a mostly street driven / weekend cruiser with 9.5 compression, MSD 6a box and magnetic trigger installed in stock dizzy, Blaster coil with MSD 8mm wires and like I stated we have the plugs gapped at .045 which is what MSD tech advised as the hotter spark could melt the electroid on the spark plug if I left it at .035. No I have not had any strange motor or exhaust noises. I understand the point being made that each person drives their car differently and motor specs can widely vary so like was said try different gaps and see what works best for your application.... Jim
__________________
1970 Mustang Mach1,Chestnut Metallic,351C,AOD,3.50:1 Locker.
70leadfoot is offline   Reply With Quote Quick reply to this message
Old 07-16-2011, 10:07 AM   #7 (permalink)
Registered User
 
Join Date: Feb 2001
Posts: 237
Re: Spark Plug Gap?

EEEEHHHHHAAAAA, Overtime was canceled today and it isn't going to be over a 100 out. I will set the plugs today to .045 and take it out for a run. Thanks for your reply Jim. I also want ot install a 3/8 fuel line to the stock pickup/sending unit. Have any of you guys done this? What did you use to open up the stock pickup?
__________________
64 Galaxie 500XL 428CJ Tri Power C6
65 Mustang coupe 347 4spd 4wdb, road racer
67 Mustang GT Scode coupe, project
Brett is offline   Reply With Quote Quick reply to this message
Old 07-16-2011, 02:39 PM   #8 (permalink)
Subscriber
 
Join Date: Jan 2001
Location: Texas
Posts: 2,216
Re: Spark Plug Gap?

Somebody may sell a 3/8th's plate now but they didn't when I needed to expand my fuel flow. I took the stock plate, drilled a hole for a 3/8th's tube and JBwelded the tube into the plate. I recall having to do something to the pickup sock on the end too. I clamped a plug over the stock 5/16th's tube. This has held up for me for some 8 years.
__________________
1965 289 mustang fastback
Hottarod is offline   Reply With Quote Quick reply to this message
Old 07-16-2011, 04:00 PM   #9 (permalink)
Registered User
 
Join Date: Feb 2001
Posts: 237
Re: Spark Plug Gap?

Cool, I was kinda thinking along the same line. I am glad to hear it has lasted soo long.
Thanks
I was even kickin around putting a tee in the tank so I can get a pickup on each side. I am going to check with the local hydraulic shop to see if they have a lite duty apring loaded check valve to put in each side so the pump won't pickup air from hard cornering.
Attached Thumbnails
Spark Plug Gap?-img_0072.jpg  
__________________
64 Galaxie 500XL 428CJ Tri Power C6
65 Mustang coupe 347 4spd 4wdb, road racer
67 Mustang GT Scode coupe, project

Last edited by Brett; 07-16-2011 at 04:12 PM.
Brett is offline   Reply With Quote Quick reply to this message
Old 07-21-2011, 02:47 PM   #10 (permalink)
Subscriber
 
Join Date: Jan 2001
Location: Texas
Posts: 2,216
Re: Spark Plug Gap?

I've seen road race cars that run pickup's through each side of the tank from the top. I think they angle them toward the back of the tank but on each side so they are still getting good fuel flow through hard corners. The T comes together on top of the tank. I only have to worry about the backward slosh on mine.
__________________
1965 289 mustang fastback
Hottarod is offline   Reply With Quote Quick reply to this message
Old 07-23-2011, 09:19 AM   #11 (permalink)
Registered User
 
Join Date: Jul 2011
Location: Jacksonville, FL
Posts: 52
Re: Spark Plug Gap?

My recommendation is: Old points style ignition or and 60's early 70's style coils limit your gap to .035 as these style ignition systems only put out about 30k volts and the spark will have a hard time jumping the gap between electrode and ground strap at any gap wider than that.

Modern and aftermarket performance ignitions usually put out more than twice the voltage of the older points style ignitions and are making around 60k-80k volts. That means that the arc is stronger and can jump a larger gap. The higher voltage the arc is, the bigger the spark in the cylinder which equals more power, more complete fuel burn, and less chance of fouling a plug (as long as the plug is the correct hear range)

So if your have gotten rid of your points and upgraded to a performance coil, you can run gaps from .045-.055.
Maxlt is offline   Reply With Quote Quick reply to this message
Old 07-23-2011, 12:03 PM   #12 (permalink)
Registered User
 
Join Date: Jan 2008
Location: Seattle, WA area
Posts: 8,996
Re: Spark Plug Gap?

Quote:
Originally Posted by Maxlt View Post
... the bigger the spark in the cylinder which equals more power, more complete fuel burn, and less chance of fouling a plug (as long as the plug is the correct hear range)
Class in session - have a seat.
I'll back that up. Small gaps are not where it's at for performance. Quick outline - there are two ends of the spectrum on gaps, small and large. Small gaps used to be the mechanic's trick to help a tune and set of plugs last longer, starting back when all you could get was 'standard' plugs that eroded relatively quickly, and plug changes were recommended at 15-20,000 mile intervals or less and plug wires broke-down more quickly. The other end is the performance view, which assumes you are changing plugs and other ignition system parts whenever necessary to maintain the largest spark you can for power and efficiency. Factory settings are in-between these two.

For best performance and efficiency, you want the largest, hottest, and longest duration spark you can get. That means the biggest gap you can run reliably. How do you figure-out what gap that is? Simple testing. There are two primary limits to the gap you can run - compressive resistance and coil energy.

Dynamic cylinder compression increases the resistance of the gap the spark must arc. Air is an insulator, and if you pack more of it into the same space by compressing it, the insulation level rises and becomes harder for the spark to jump. Coil energy in a single coil system reduces as RPMs rise due to reducing coil dwell, the time the coil has to charge between firings. There simply isn't enough time for a single coil to fully saturate (charge) on a V8 at high RPMs, and so coil energy reduces. This is why multi-coil distributor-less ignition has become popular, as each coil fires much less often at the same RPM and has plenty of time to charge between firings (coil dwell).

So, misfires due to resistance and lower energy begin to appear at WOT low RPM through peak torque where dynamic compression is greatest, or higher RPM where DC is falling but coil energy is falling-off quickly—or both. To find your best gap, start big and reduce it until you have no more misfires anywhere in your WOT RPM range. This job is easy and accurate on a dyno, as you can see the misfires on the traces. A bit more effort and less accurate on the street or track, but obviously simple and do-able. Remember—as the gap gets harder to jump, and if the rest of your parts (wires, cap, and rotor) are not in good shape, the spark will go the easiest path, and cause misfires when they jump to a close wire or cap terminal (a cylinder with less compressive resistance) or a convenient ground like the block, valve covers, or headers.

Wait - why did Paul say the smallest at idle then? Actually, that is an old tuner's track-side trick. When you don't have the opportunity to test for best gap, and you need to get the car on the track with the best chance that you won't have compressive or dwell misfires, you can do it that way until you get the chance to do it right - which takes time. Most hobby folks don't want to pay a mechanic extra for thorough testing, and they want to avoid a tune-up as long as possible, and so they get the 'little gap' trick. Yes, the tune will last longer, but no, the power and efficiency won't be peak. It's your choice.

David
__________________
-=≡ If it was easy everyone would do it ≡=-
[size=1]HotShotsJava.com Hot Shots Java Coffeehouse, Poulsbo ,WA fair trade and 100% organic. Voted best espresso and coffee![/size]

Last edited by PSIG; 07-23-2011 at 12:05 PM. Reason: Mis-spelling
PSIG is offline   Reply With Quote Quick reply to this message
Old 07-24-2011, 07:32 PM   #13 (permalink)
Registered User
 
Join Date: Aug 2006
Location: Greater Puget Sound Area
Posts: 4,003
Re: Spark Plug Gap?

Um, there are a couple of errors in your description. The air fuel mixture ionizes more easily as the density goes up - that makes it easier for the spark to jump

BUT

turbulence in the chamber make the ionization more difficult to achieve.

Why do so many people believe that big gaps are better then?
When Detroit went to lean burn engines they needed bigger gaps to ignite the lean mixture - something that doesn't, or shouldn't, exist in a performance engine. To do that they went to electronic ignitions that use capacitors fired through the coil to avoid the limited dwell problem. With electronic ignitions (specifically anything with a capacitive discharge circuit) that use an amplifier there is no problem as long as the RPM don't exceed the maximum speed that the discharge can take place. (approx. 30000 rpm)
Can a spark actually melt the electrodes? SURE! all you have to do is get the electrodes to the melting point of steel, inspite of the fact that the plug is designed to handle 5000+F combustion temps without getting hot enough to ignite the gas without the spark.
OK, that wasn't fair, the high amperage in a AA fueler will actually get enough heat localized on the plug that the electrodes will erode - very quickly. On an engine with 10 - 11:1 compression running gas from the gas station on the street and weekend racing your plugs will not have the problem of melting despite the claims from the manufacturers. You will get some erossion over the span of a year but not so much that you could distinguish it from normal plug wear.

The coil is a transformer, it has a pulse of DC current put into it and the output pulse cannot be more power (amps x volts) than approx 90% of the power input. The higher the voltage needed (bigger gaps, more resistance, etc.) the less amperage you have. (Amps = heat) What is the gap size used on a AA fueler? It has been a while since I was involved in drags but I doubt they are running more than .030" gaps on their plugs. (in the 70s it was around .024") I think I still have an old adjuster for the side-gap plugs somewhere.
__________________
66 Mustang Coupe. 365hp, 4spd Toploader, sub-frame connectors
Shelby drop, suspension tuned for slalom and hill climb
body mods and weight reduced to 2000lbs; name: Muskrat
PaulS1950 is offline   Reply With Quote Quick reply to this message
Old 07-25-2011, 05:12 PM   #14 (permalink)
Registered User
 
Join Date: Dec 2010
Location: Grand Prairie,TEXAS
Posts: 165
Cool Re: Spark Plug Gap?

I have tried all gaps from .030-.065 & found my car to run the best at .050. 66 mustang 302 h.o. roller motor with bumped up cam & cobra R roller rockers, heads, yadah, yadah, yadah. MSD dist. & MSD 6-al ignition & it seems to have the most power & best running condition there (.050). Just saying, that .050 works for me but maybe not for everyone. You just have to try something way up to way down & see what works well for you.(like has been said by others)There's alot of better knowledge on here than me, all I'm saying is, it works for me. My son has a 93 Ford Ranger with a 331 stroker & his runs well there too.
__________________
2011mustang//66 mustang not bad ass,but bad enough.
2stangman is offline   Reply With Quote Quick reply to this message
Old 07-25-2011, 07:30 PM   #15 (permalink)
Registered User
 
Join Date: Jul 2011
Location: Jacksonville, FL
Posts: 52
Cool Re: Spark Plug Gap?

Paul,
I hate to disagree but I think your post is only half right. Its true that if all other things remained constant, with a fixed level of voltage and current on a coil, wider gaps don't necessarily help. A smaller gap requiring a lower voltage would indeed produce more heat, but when you upgrade your ignition two things usually happen.

First the instructions will tell you to remove the old ballast resistor or 12v wiring to the old coil and ensure that you get a good 12v source. The ballast resistor or wiring harness with resistance built in was used as a current limiting device on most older ignition outputs. So as soon as you upgrade you've already increased available voltage and current.

So right away your new ignition is getting more voltage and amperage to the primary side of the coil... this results is a significantly higher output on the secondary side, and if your coil is now a higher voltage coil, this means that you have both more voltage and amperage available to fire the plug.

Secondly, since power = voltage x current (in a purely resistive circuit), as you add the effect of capacitors into this with a modern Capacitive discharge ignitions power goes up even more. The capacitors charge up during the period between each spark plug firing. This stored energy in the capacitors is then released as the plug fires resulting in even more power being released.

From what I've learned, as we look at the energy needed to quickly ignite the atomized compressed air/fuel mixture in the cylinder, it just makes sense that a hot, strong spark provided by a wider plug gap and the increased capacity of a high voltage ignition system will always ignite the air/fuel mixture faster with more complete combustion.

I think the reason most drag racing ignition systems (that aren't the magneto type which actually generate their own output without needing a battery source for power) is lot's of dragsters don't have alternators or have very small ones that don't rob a lot of hp that they really want to use to go faster. Relying solely on the the battery to power the ignition, you need to try and use as little energy as necessary in the ignition system in order to keep the battery from going flat during your run.
Maxlt is offline   Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Ford Muscle Forums : Ford Muscle Cars Tech Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
What spark plug ?? 1 Old Guy Drag Racing 3 08-31-2009 10:10 AM
'74 351 CJ Spark plug gap coolcat82 All Ford Techboard 4 05-16-2009 07:48 AM
spark plug gap R428Galaxie Galaxie Pages 1 10-15-2008 05:08 PM
spark plug gap? streetstang67 All Ford Techboard 8 06-13-2005 09:59 PM
what spark plug should I go with mikemustang289 All Ford Techboard 5 12-13-2003 12:40 AM


All times are GMT -7. The time now is 03:57 PM.



Powered by vBulletin® Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Friendly URLs by vBSEO 3.6.0
 

Garage Plus vBulletin Plugins by Drive Thru Online, Inc.