"BUT as you increase horsepower (cyl pressure) the more energy is required to jump that gap so you might have to bring the gap back down." - Pablo

Based on this statement about spark plug gap, I am assuming that if a 305 is replaced with a 383, and the 305 spark gap is supposed to be .035......then the gap should be smaller for the the 383 so that the spark can ignite all the fuel. Any thoughts anyone?

 

Speed,

It's more a function of compression pressure than cubic inches. Spark plug tip design is a factor as well, which is one of the reasons I'm not sold on the Bosch +4 ans other open electrode designs. You can maintain a wider gap under higher cylinder pressures if you have an ignition system capable of producing a higher energy spark.

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Later,
Vader
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but he does bring up an excellent question.. does more CI automatically mean more imep pressure? I mean take two engines of equal efficiency but one being larger than the other.. would that then mean the larger engine produced more cyl pressure? im no scientist but i wonder if it could be the same and its just more piston area to absorb the push force or something much like a stroker gives up power but gets it back in leverage vs the same displacement but with a shorter stroke

any of you smart folk have an answer to this? i was allready told i was the dumbest person alive on here so I defer to ya'lls

 

yeah the more i think about it.. i think the pressures would be the same and that they are increased only when you increase the efficiency of that given displacement

I could be wrong though

 

No your right on. Adding a turbo or sc will require you to 'possibly' tighten the gap.

Even with the best wires and cd igntion, the spark will find a better (easier) path to follow causing a miss.

This holds true to the same motor with the same ignition and wires running a higher effective cr, but there are many variables.

Seeing streaks down the white surface of the plug means you need to tighten the gap, but better wires will let you slide. This is not the only indicator since with excellent insulation and a cd ignition, the spark may favor a different cylinder (with no pressure) and jump to that location. You could 'match' a distributer and rotor to tighten up the clearence (most have way to much gep imo) but then something else will limit you (spark jumping between the coil and the coil body, ect., plug type has alot to do with it also)

Running the maximum gap also will increase the service life of components as resistance build from carbon on the plug, age of wires, oxidation on the cap&rotor...

On a street motor these gains are so small that a good 'in between' setup is the best bet.

 

I can't believe it. The two masters replied on the same post! I plan on beefing up the ignition but right now, the stock 305 ignition is in there. One thing that makes me think the ignition system is not putting out enough power is because detonation begins at 14* BTDC. That seems very high to me. It used to knock at just a couple of degrees advanced when the 305 ws in there. It seems like a smaller gap would quicken the spark's jump to where the timing doesn't have to be set so high.

Or does it hinge more on spark voltage rather than spark speed.

 

Wouldn't a few extra cubes (say a 350 vs. 383) increase the compression by such an amount as not to effect spark, but going 13:1, blown or turbo'd would cause problems? Even then, the gap is not as much the issue as the power to jump it is?? If you have a 383, you could get away with a stock HEI, but a 13:1, blown or turbo'd, you woul need MSD or something. Just some thoughs...

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1992 Pontiac Firebird 350/Six Speed
1987 Toyota Pickup 383/500+ HP

 

Would Rapidfire plugs be included in the open electrode variety? Also, I didn't realize that the spark had a choice of which cylinder it jumped to. It kind of makes sense though. And I can see how maximum gap would increase component life. It still seems like an increase in terms of spark voltage or spark speed would be needed to move a larger piston in a bigger engine. 305 to 383 is a 20% increase in area.

[This message has been edited by speedlvr (edited April 27, 2001).]