I am thinking about getting these spark plugs, NGK Racing Spark Plugs, and a K&N Air filter. What would I need to gap the plugs to for a 390 engine and do you think the K&N filter would boost horsepower? Let me know what you guys think. Thanks.

 

I have no idea what you should gap them at. But letting someone know if the engine has electronic or point ignition would help.

In regards to the K&N. Go for a ride without an air filter. It will not boost horsepower any more than that.

Good luck
Drew

 

The plugs are a waste of money, so don't bother. The K&N will boost power as much as 5 to 7 horsepower but that is at WOT and you spend almost zero timing at WOT. But it will improve gas mileage slightly, so if you drive it a lot it will pay for itself .

 

I almost said the same thing, but in the back of my mind I remembered something about airflow, etc., went and had a look in one of my books and found the following pic. As you can see, there is an advantage to airflow over running without a fliter if you shape the carb opening correctly.



Number 11 is the way to go and K&N makes a Stub-Stack for that purpose, although you need to mill the choke horn off.



As Bear mentions, peak HP is obtained at high RPMS and that's not much of a concern for street driving as you're so seldom doing that.

Low end torque is what you want (well, it's what a lot of us want...maybe you too), and that's mostly dictated by the cam. It maybe possible to gain a little low end torque by advancing the cam a few degrees (I've heard 4 degrees) but someone else will have to advise on that.

I'm a firm believer in improving exhaust flow, so you might want to look at headers vs those cast iron manifolds. Add a cam change, and you're in business.

On the plugs, as long as they fire, they're doing their job. Any new plug, regardless of manufacturer should light off the mixture the same. Brand, heat range, and other design criteria help the plug last through the type of driving you want to do.

DeepRoots makes a good point in that you didn't mention what type of ignition system you use. I gap my plugs at 0.035", although with electronic igntion, I understand you can increase that. 0.040 to 0.045" is not uncommon.

 

Didn't think about that with a filter Argress.... I was kinda just being a smart ***.
For me, I don't care for K&N type filters, we use them at work on the big Cat's but I'm always leary that a screw up could dust an engine pretty fast.

With the duraspark I'm using, they say you can gap .045. I run Autolite 46 plugs and don't have any problems.
I didn't even think about exhaust tho.... Yeah the logs suck, it's standard for me to recycle them as soon as I can.
Dynomax headers are 1 3/4 primary, 3inch collector, fit pretty well with no excessive grinding and are cheap.

I got this pair new for $75:


Jacking up the engine to install them is required and you need to bust them loose to change the starter tho.

 

NGK work great but heres the catch, you need to run an MSD or Mallory box if you run NGK's. Otherwise Autolite 124's work great if your not running a box. JMHO

 

Didn't know you still frequented these parts Wes. Good to see not all of the old school guys have left completely.

 

I pop in every now and then! LOL