Thread: Spark Gap

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Hi Guys!
I'm having problems with my newly built 350 SBC. It's a 1983 but all engine parts are in mint condition, rebuilt a year ago.
Now I bought new spark plugs (Accel #576 as an upgrade for my old R45TS) and they have a stock plug gap of only .03in, but my engine calls for .045in. What difference does the plug gap make?
Do carb and intake manifold call for a change in plug gap? Please help me out on this, because I'm running 8mpg at the moment, with little power and irregularly, timing has to be set at about 34° BTDC so that it runs at all. CAN THAT BE THE PLUG GAP?
Thanks, folks!

Mad, this is opinion and opinion only...

Under normal driving conditions, I wouldn't expect much difference. However when in high R's, larger gap helps keep the timing more stable. If you have a powerfull enough ignition system that can thow some spark, gap 'em in the 40's. If you're using stock coil (or HEI), you may not see the R's because those coils are weak and do not saturate appropriately the higher you go (they're good to about 7000 max). And yes, changing your fuel/air intake does make a difference. You need to be able to burn that stuff. A weak spark may not efficiently burn all of the fuel, thus causing you to have to idle a little higher or run more advance...or hotter plugs.

So, about the timing issue, see above about pumping up that spark. Change out your coil if you suspect the plugs are weak. Some other things to consider is have you changed the cam? If so, the lumpy "poketa pokata poketa" type cams generally have a large lobe overlap. This will cause a lot of reversion (exhaust to intake backpressure), thus giving you a contaminated fuel charge. That leads to bad combustion at low RPM's and bad gas mileage when driving in the low R's on the street. This could also cause you to advance the distributor so it will idle.

Take it for what it's worth...

tj

Originally posted by MadMax
Lumpy Cam

Thanks a lot, but what do you mean by a "lumpy cam"? I have a normal cam, the one, that is in the engine when you buy it. It is not worn or anything, lobes are like new. Will an Edelbrock cam (that fits the edelbrock performaer carb and intake) give me better mileage? Or only more power and less mileage?
My ignition system doesn't seem to be all that weak, how dio you test if your coil is good?
THX a lot anyway!

I have a normal cam, the one, that is in the engine when you buy it.

Lumpy cam meaning large lift and/or duration. If yours is the factory cam, you're good with standard ignition and carb. You asked if changing the cam will help power or gas mileage. I hate to use this phrase, but it's the only one that fits: It depends...

Gazillions of cam profiles are designed for different reasons. However, there are two main categories...gas mileage or performance. You generally don't have the luxury of both...although some offer an acceptable balance. If you didn't go crazy with a wild cam, then you can assume you should have decent mileage. The only way to really tell is to determine the best cam profile for your driving needs. You can contact any cam mfg and tell them you need a cam for ???? reason. They will ask you a bunch of questions like street or race driving, type of induction, headers, gear ratio, vehicle weight, etc... They can pretty much hit dead on with what you need.

The question about what's a weak spark is subjective. If you pull a plug and crank the engine to watch the spark, you should see a "powerfull" blue spark (maybe about an 1/8 inch wide). If you see a little "tick" (maybe a small line less than 1/16 inch), then you've got a weak spark. Yip, I know, subjective huh?

Hope this helps...

tj

THX, tjpoc!
I don't have a lumpy cam, just the ordinary stock one. I just thought of one thing, my secret theorie (hehe):

If you increase the spark gap the spark ignites more fuel in the same time, because there is more fuel between the electrodes. (always assuming your spark is strong enough.) Then the mixture should actually burn quicker and give more power, using the same amount of fuel, so you don't need so much accelerator and use less fuel to get the same power, i.e. with a bigger gap you get better mileage so long as your spark stays strong enough (and the electrode doesn't hit the piston)

And now the question: Is my theorie correct?
I would appreciate all kinds of answers to this question.
And why would different engines require different gaps?

Generally speaking the narrower the gap the hotter the plug effectively becomes (the more intense heat between the electrode and ground).

Also in general terms the plug gap is determined by the ignition system used. When the car manufactures went electronic ignition in the early 70's with it's increased spark potential the plug gaps increased from about .035 to anywhere between .045 to .065. This combination also helped reduce emissions.

If your running a pretty much stock engine and ignition system, I would recommend starting with the manufactures sugested heat range and gap and then after driving for a while, pull the plugs and adjust the plug heat range accordingly.

One last thought, never assume the new plugs are correctly "factory gapped".