Misfiring under heavy acceleration, fine under normal acceleration

[steeda763]

Hey there...need some suggestions. I just upped my boost to 10psi on a T25-going-bad, and I installed new spark plugs ('91 SE-R Sentra). One day I was really on it at about 6k rpm and had a real bad misfire (backfire?) and a large plume of black smoke came out. I checked compression and it's all equal, and the idle is fine. Vacuum is dead even at 20 while cruising. I know it's not a bad block. The spark plugs seem fine, white-to-light-tan looking.

My car is still misfiring like I have a Mitsubishi Evo rev limiter only when I accelerate heavily on full boost, but there is no more black smoke. When I ease into the boost it's fine all the way to 6500 rpm. Any suggestions??

[afracer]

Do you have a wideband O2 sensor? Check your a/f ratio under full boost...sounds like detonation/running lean.

[kickflipdogsled]

I had this problem on my s14 motor only when it was cold out, I guess it was over boosting because when the weather/car would worm up or I would turn down the boost controller It would run fine. Upgrade your injectors, thats what Im going to do.

[ILikeMy240sx]

Sounds like your spark is blowing out under heavy load.

Black smoke out the back, feels like a spark-cut rev limiter... typical spark blow out problem.

Do you remember what type of plug it was? i.e, what manufacturer, what material, what heat range, and what gap?

Generally, for SR engines, I've had fantastic results with NGK BKR7E-IX gapped at .028"

[steeda763]

Do you have a wideband O2 sensor? Check your a/f ratio under full boost...sounds like detonation/running lean.

No I don't have a wideband...yet. But every once in a while I get a pretty good, embarrassing backfire, so I'm pretty sure I'm not leaning out.

Do you remember what type of plug it was? i.e, what manufacturer, what material, what heat range, and what gap?

I'm pretty sure they're NGK Iridium, but the gap is whatever the manufacturer set them for the '91 SE-R. I never had a problem until now with that...

Is .028" the ideal gap for the SR?

[ILikeMy240sx]

.028 is what most people including me recommend. .030 is the absolute max in my opinion. I know for fact that SE-R plugs do not come gapped at .028. Try it out first and see if it changes.

What heat range were they? SE-R plugs are 5 heat range normally and I think thats just too hot for an SR. For stock turbo setup Id go with 6. For a bigger turbo 7(more so)~8 is usually recommended

[steeda763]

Wow I know nothing of spark plugs! I thought the higher the heat range the hotter it was.

I'm going to give this a shot today or tomorrow and see what happens.

Thanks!

[steeda763]

Ahem...what do I ask for at the auto parts store? Obviously it's not going to be SE-R plugs...

[steeda763]

Sounds like your spark is blowing out under heavy load.

Black smoke out the back, feels like a spark-cut rev limiter... typical spark blow out problem.

Do you remember what type of plug it was? i.e, what manufacturer, what material, what heat range, and what gap?

Generally, for SR engines, I've had fantastic results with NGK BKR7E-IX gapped at .028"

I just installed the BKR7E gapped at .028" and the problem cleared right up. Thanks!