I was just wondering if the spark plug gap influenced the idle quality.

Bigger gap = smoother idle or smaller gap = smoother idle?

I'm just curious because I just regapped my rapidfires to what i think its 0.044 and all of a sudden it feels 'rough' or something....weird.

 

no chance you cracked one , or didn't get a plug wire all the way on , is there?

 

My manual for my 88 says that they should be a 0.035" gap. I thought if you did any more or less you run the risk of fowling the plugs or bringing about a rough idle.

I'm still new at this so dont quote me. :D

j :flag

 

i don't think so....i'm pretty sure i got all of the boots on and that none of them are cracked.

the thing is, i'm certain it doesn't miss while driving, and it feels very strong. i don't know if it even misses at idle, but it feels more rough than before. ill check the boots again, and maybe i will do the minimum idle setting in the tech tips.

 

i know that the plug gap for the stock plugs on the 90 vette is 0.035", but ac delco says that rapidfire #5's for the 90 vette should be gapped at 0.045", and that is what is currently in there.

 

For a given charge denisty it takes a certain minimum amount of voltage to jump a gap. With a stock HEI and stock plugs you should stick with stock gap. If you install a hotter HEI you can open the gap and take advantage of a more thorough charge burn; cut-back ground electrodes also help increase burn efficiency.

The rapidfire plugs have sharp points on the center electrode which tend to allow a wider gap (for a given voltage, electric field strength is greater at points than on a plane surface). However, if these sharp edges round off with usage the gap should be reduced toward the stock setting.

 

65Z01, i've read your site. there's some good info on it.

anyway, my ignition is stock, with fresh cap, rotor, wires, and plugs, so i thought i should have it at 0.045 for my rapidfires. however, it still feels 'rough' at idle. do you feel that i should reduce the gap to try to smooth the idle? or, should i widen it since i don't have any misses at WOT. hopefully i did not round the electrode tips at all while gapping them.

i have had these rapidfires in for about a 1000 miles, and i pulled them to check the gap. some of them were as big as 0.048ish, so they must've not been set right out of the box.

if i pull the plugs again, my knuckles are going to kill me :D

 

I have rapid fires in my '85 and on the box it said that plugs come preset at .045 and should not be reset.

 

Generaly speaking, a wider plug gap will give a better idle BUT may reduce your WOT and top end if you run out of voltage.

 

Since I have a '96 that means I've got OBDII and a quite sensitive crank sensor. The PCM is also programmed to detect (using the crank sensor) misfires for emission purposes.

The reasoning behind this is that a misfire condition will increase unwanted exhaust emissions and therfore **** off the government. Unfortunately, this "misfire detection" is extremely sensitive. A mild cam upgrade, a smaller torque converter (in the case of A4's), or rough road conditions can set up a situation to set off the "misfire detection" in an OBDII PCM. I've done a ton of data logging on this feature and am intimately familiar with it.

My particular combination is an LT1 to LT4/Hotcam conversion (that means I'm running an auto) with a 9.5" 2800rpm stall converter.

In my case, when OBDII senses a "misfire condition" it will command the torque converter to unlock. This feature gives me fits under light load and idle conditions. The hotter cam causes a slightly uneven light load/idle running condition which the PCM perceives as a "misfire" (in both manual and auto trannys). This problem is exacerbated in my auto tranny due to the lower rotating mass of a 9.5" torque converter.

Now that I've given you a bit of background I can proceed to my experiences.

When I added a nitrous system I changed plugs to a copper version (versus the stock platinum style) in a step colder range (NGK TR6's). These new plugs are gapped at .038. The stock plugs are gapped at ~.050 (Note: I've run fresh stock AC plugs and Bosch +4's... both worked very well for me.. the only difference I can tell is through data logging... there is no physically perceptible difference between any of the plugs I describe).

According to data logging (...plus I can easily tell through torque converter application times)... that the .038 gapped NGK TR6's produce less than half the "misfires" compared to the ~.050 gapped platinum plugs.

I can't explain the true reason for this, I can only relay my results. But according to the computer on my car, the .038 gapped plugs run smoother than the stock ~.050 plugs. I imagine the .038 plugs aren't quite as emissions friendly as our government would like, though.

 

*** plugs in a nice SB Chevy? Naughty, naughty!

BIG JIM

 

In my 95 changing the gap even made the exhaust resonance change!