Increasing the Spark Plug Gap?

[911st]

There we go.

I believe what is at work is a lean mix burns slower because there are less fuel than oxygen molecules packed together. Because it is burning slower it makes heat for a longer time inside the head and cylinder. Thus, more heat is retained in the motor.

Get to much heat and detonation can occure.

With a longer burn time timing dose have to be advanced to achieve peak pressure near the ideal which I think is about 12 to 14 deg atdc.

Thus, we have to start building pressure earlier on the piston's way to the power stroke. Not sure but I think this called increasing our 'pumping losses'. This pressure before TDC takes away from the power made by the pressure built up ATDC and thus reduces our HP.

Twin plugs do accelerate the burn rate and ensure a poor mix is ignited. Especially important with low compression where the mix is not as closely packed.

Thus, should not make for any higher peak cylinder pressure when ideally timed. It dose however reduce the among duration over which pressure starts building on its way to the power stroke. On a Porsche this can make for about a 3% increase in power.

I am still learning so take this with a grain of salt.

[WIL84911]

Originally Posted by Vincent Hill

??.2 What? Inches? MM?

I'm sorry... I threw out the wrong number there... I meant .035 inches as opposed to .028 (copper plugs for a 3.2 motor).

[T77911S]

Originally Posted by 911st

There we go.

I believe what is at work is a lean mix burns slower because there are less fuel than oxygen molecules packed together. Because it is burning slower it makes heat for a longer time inside the head and cylinder. Thus, more heat is retained in the motor.

Get to much heat and detonation can occure.

With a longer burn time timing dose have to be advanced to achieve peak pressure near the ideal which I think is about 12 to 14 deg atdc.

Thus, we have to start building pressure earlier on the piston's way to the power stroke. Not sure but I think this called increasing our 'pumping losses'. This pressure before TDC takes away from the power made by the pressure built up ATDC and thus reduces our HP.

Twin plugs do accelerate the burn rate and ensure a poor mix is ignited. Especially important with low compression where the mix is not as closely packed.

Thus, should not make for any higher peak cylinder pressure when ideally timed. It dose however reduce the among duration over which pressure starts building on its way to the power stroke. On a Porsche this can make for about a 3% increase in power.

I am still learning so take this with a grain of salt.

i wish i new how to multi quote.

too much heat and detonation will occur. yes

advancing timing to "fix" a lean condition, no.
first, the only way to properly set timing is on a dyno, anything other than that is guessing. timing is increased until no more power is made at a given RPM. any further and it reduces HP. once timing is set, you dont change it.
second, timing and fuel mixture go hand in hand. if you change one, you have to change the other.
look at it this way. lets say your mixture is 14.0 and timing is 15 BTDC. if you increase the timing to 20 BTDC, the fuel has longer to burn. more burn time will give the "appearance" that the mixture has gone leaner. a gas analyzer may now read 14.7. the mixture may look good now, but is the timing set for optimum? not if peak HP was created at 15 BTDC. you have "fixed" your mixture but your HP is down.
since most of us dont have dyno's, we have to either rely on what someone else has found out, or go by porsche specs for timing and tune around that.

twin plugs are not for low compression poor mix conditions. its for the hi compression engines. you want the combustion process to happen faster, but at a controlled rate.

study the article in that link. it has a lot of good info.

not sure what your last sentence means.

[kach22i]

A post I made back in 2006

what could cause this?

Originally Posted by kach22i

I just changed my stock plug gap from .028 inches to .035 inches. This change fixed the running rich at idle problem, the little back fires when I let off the gas, and the engine in general just runs smoother.

I should clarify that my spark plug gage has .025 and a .03 inch wire loop. What I did the time before last was make the gap a geneorus no touching .025, yet still to small to fit the .03 inch wire in. I thought this was ending up at .028 inches, perhaps it was not.

I think there are other issues with my car which were causing the buffeting under load at low speed/low rpm, running rich, and backfiring (such as the WUR). However increasing the gap slightly has the car running better than it ever has, including starting cold and starting when warm.

Warning: if you regap your plugs larger than what I did you may burn out your coil, burn out points if you have them (I have a Pertronix), cause your spark plug wires to arc, plugs to arc and ground out at hight rpm, and a host of other issues. I chose .035 inches after reading more than a dozen threads on the topic, I think I have the gap right on the edge of trouble, but not in trouble, time will tell. Until that time, I have a well running car that I'm still sorting out and seeking to return to factory spec in most regards.

I've since fixed a bunch of CIS vac leaks and leaned up my mixture. The plugs no longer carbon up after a few hundred miles. However I now gap slightly larger at .03 inch and not .028 inch.

I also use this spark plug tool, it helps get the gap level but is a bit shoddy.

Summit Racing SUM-900313 - Summit Racing® Spark Plug Gapping Pliers - Overview - SummitRacing.com


Our host may sell the same tool or something better, check around. With gaps these small you need all the help you can get.

[kach22i]

Originally Posted by Vincent Hill

Last, speaking of this, there are Kits now available to replace the Coils and Spark Plug wires on my Motorcycle. Has anyone made a "Stick Coil" conversion for the 911?

After seeing this sort of set up on a new Mini at a car show (stock factory) I got real interested in the topic. I researched the topic about six months ago and found nothing. However there may be something out there soon, it's just a matter of time (I hope).

[T77911S]

i believe permatune recomends a pretty wide gap. i have set mine pretty wide, but i was having other issues at the time. i set mine now between .040-.050.
i have an MSD setup.

[Vincent Hill]

Thanks Wil, that is right where I have mine now at 0.035 inches. I feel that 0.040 should not cause a problem and after reading the posts I now do not think it is a good idea to go as Big as I can and end up wearing out the Ignition Wires or carbon lines in the Dist cap.

I like that Plug Gapping tool and think I will get one. Like most of us, if the tool is neat I want it!

[WIL84911]

Originally Posted by Vincent Hill

Thanks Wil, that is right where I have mine now at 0.035 inches. I feel that 0.040 should not cause a problem and after reading the posts I now do not think it is a good idea to go as Big as I can and end up wearing out the Ignition Wires or carbon lines in the Dist cap.

I like that Plug Gapping tool and think I will get one. Like most of us, if the tool is neat I want it!

Glad to be of help even though not a techie but I know it works. I may try .040 next time.. can't wait..