question on spark plug gap.

rnln · 06-05-2009, 02:59 PM

I know this topic has been discussed many times but I do have additional questions. Base on reading here, there are 2 things I understand:

1- enlarging the gap to 0.045 (with some modifications) may benefit you some on performance and mpg. There won't be any disadvantage.

2- enlarging the gap close to 0.04 (without modification) will not benefit you much (maybe a little) but won't have any disadvantage.

My question is on (2). If there is no disadvantage, then why Manafacture didn't do so?

911pcars · 06-05-2009, 03:27 PM

Larger gap requires/creates higher voltage to fire the A/F mixture. Usually an advantage.

It also:
a. places more stress on the ignition system to maintain the desired current path, i.e. insulation must be adequate.

b. can produce a misfire if ignition system isn't capable at all loads and engine speeds.

In most cases, the factory specs are more than adequate for the factory engine. For example, excess voltage, if not needed, doesn't provide more power. However, example "a" and/or "b" can result.

Sherwood

88-diamondblue · 06-05-2009, 03:36 PM

Motronic systems do not like larger plug gaps. I tried to find the thread that Steve Weiner talked about this. As I understood ignition systems that can deliver more energy to the spark plug seem to benefit from larger plug gaps the most.

Steve@Rennsport · 06-05-2009, 07:57 PM

Sherwood (as usual), is spot-on,...

Big plug gaps have been adopted by most OEM's for years as an aid to emissions compliance and idle quality, but one must have a robust ignition system to support this. Big gaps also make more HP with better drivability if the ignition system is up to the task.

Most Motronic systems will not reliably fire plug gaps larger than .035 and its a strain on the coil drivers at those gaps. MSD's will fire huge gaps but we've found best power and drivability in the .040-.050 range.

Short story,.....Several years ago, we were experimenting with some misfire issues on a 935 engine running on the engine dyno. We connected a pair of MSD 6AL's and matching MSD coils and the misfire instantly disappeared. Good power to 8500 RPM (735+HP). We installed a set of old fouled race plugs that I had completely snipped off the side electrode leaving a plug gap around .130.

It started up instantly and idled like a CIS engine (very smooth).

Naturally, those were too wide to reliably operate at 1.4 bar so we installed a set of plugs gapped at .040 but the engine's manners were unchanged. Boy, what a difference when you can use big gaps and the appropriate ignition system to take advantage of such things.

rnln · 05:49 AM

wow, talking about 735+hp. sherwood, steve, guys, does anyone know what is the best gap for good condition stock 911 carrera system? Is it still .032 or should I enlarge it a little bigger?
Thanks.

SteveinTO · 06-05-2009, 08:30 PM

If a spark is just a spark, (and now I'll go a little Freudian), why does a bigger one work better? Intuitively it would seem all you're really doing is igniting the flame front, and the size of the spark wouldn't make a lot of difference.

911pcars · 06-06-2009, 12:15 AM

Quote:

Originally Posted by SteveinTO

If a spark is just a spark, (and now I'll go a little Freudian), why does a bigger one work better? Intuitively it would seem all you're really doing is igniting the flame front, and the size of the spark wouldn't make a lot of difference.

Not necessarily big or size. A "normal-size" spark is sufficient, Spark voltage is important if it means the difference between lighting the A/F mixture and not, usually under load. Big gaps increase the electrical resistance to ground and the system responds (or tries to) by providing increased electrical pressure (voltage) to jump the gap to ground.

Sherwood

stlrj · 06:09 AM

[QUOTE=Steve@Rennsport;4705505]Most Motronic systems will not reliably fire plug gaps larger than .035 and its a strain on the coil drivers at those gaps. MSD's will fire huge gaps but we've found best power and drivability in the .040-.050 range.QUOTE]

From personal experience, I have been gapping my plugs at 0.060" on my 86 3.2 and have been reaping the benefits as mentioned. However, I doubt the limiting factor has to do with the adequacy of the Motronic coil driveres, otherwise my system would have failed by now, after more than 10 years of abuse.

The limiting factor is, however, due to the use of stock ignition components that are not high energy ignition (HEI) rated. In order to use a larger gap, you must replace all your secondary ignition components: cap, rotor, and ignition cables to ones that are HEI rated, just as the OEMs have sucessfully done for years.

Cheers,

Joe
74 911 w/ 86 3.2

psalt · 06-06-2009, 04:49 AM

If a spark is just a spark, (and now I'll go a little Freudian), why does a bigger one work better? Intuitively it would seem all you're really doing is igniting the flame front, and the size of the spark wouldn't make a lot of difference.

A spark is not just a spark, Sam. The spark plug may look simple, but it is a little like gravity, no one knows exactly how it works. There are at least three different theories on how the spark actually ignites the fuel. The mixture inside the cylinder does not instantaneously explode, it is ignited in stages, and it burns in stages. To understand the "why" of larger gaps, you need to learn about the stages of ignition, transport lag, voltage lag, ignition lag and growth lag. The flame rate really doesn't get going until it reaches 0.100". There are benefits to wider gaps and all OEM's have increased their gaps toward the experimentally ideal 0.100". Most of the benefits of larger gaps for OEM's is in reducing the misfire rate in lean and EGR diluted mixtures for mpg and emissions.

stlrj · 06-06-2009, 11:27 AM

"Given the field's significance to motorsport, it's surprising how little attention is given to the ignition side of engine management systems. Graham Western of EFI lamented to this end in RaceCar Engineering, v7n3: "It constantly amazes me how the importance of electrical energy and a strong spark doesn't seem to be appreciated."

http://www.atlasf1.com/99/oct06/burckmyer.html

911pcars · 06-06-2009, 11:39 AM

Perhaps there's relevance to mere street vehicles, but maybe the author was referring more toward the state of F1 ignition systems than 30 year old engine spark needs.

Do you really think a really large spark will produce significant or measureable power increases in a 8-9:1 compression engine? Maybe or maybe ......

Sherwood

stlrj · 06-06-2009, 12:12 PM

Quote:

Originally Posted by 911pcars

Perhaps there's relevance to mere street vehicles, but maybe the author was referring more toward the state of F1 ignition systems than 30 year old engine spark needs.

Do you really think a really large spark will produce significant or measureable power increases in a 8-9:1 compression engine? Maybe or maybe ......

Sherwood

By itself, a large high energy spark kernal would not produce any significant or measurable power increase but would make it easier to ignite a marginal air fuel mixture to produce power regardless of load.

Steve@Rennsport · 06-06-2009, 01:26 PM

Quote:

Originally Posted by stlrj

By itself, a large high energy spark kernal would not produce any significant or measurable power increase but would make it easier to ignite a marginal air fuel mixture to produce power regardless of load.

Now THAT, dear friends, is a boatload of wisdom,...

Its the reason why MSD's work so well on cars with imperfect AFR's such as carbureted and MFI-equipped ones.

T77911S · 06-09-2009, 04:04 AM

Quote:

Originally Posted by stlrj

By itself, a large high energy spark kernal would not produce any significant or measurable power increase but would make it easier to ignite a marginal air fuel mixture to produce power regardless of load.

great subject.
well put. i was trying to think of how to say/ask this as i was reading. could not have said it better.

steves story about the 935 and MSD, by the way, how lucky to get paid to play with toys like that. anyway, this may be a good referal to the never ending bosch/MSD/permadoom debate, no?