69_olds

Looking for suggestions for colder spark plug with HEI. Currently running Autolite 86 and hear slight pinging at part throttle after about 20 minutes of driving.

350 .030 over TRW pistons
Compression ratio about 10.5:1
#5 iron heads mild porting
Lunati Cam 224/234 duration @ .050" .496/.520" lift
DUI HEI distributor 36 degrees timing
93 octane gas

I'm looking for an HEI plug with gap around .050" and cooler than the Autolite 86 or ACDelco R45S

Any suggestions would be appreciated.

oldcutlass

Try disconnecting your vacuum advance and see if it pings.

cutlassefi

X2. Carb? Did you rejet it for your new combination?

69_olds

If disconnecting the vacuum advance resolves the ping, I have too much advance too early and need to recurve the distributor?

The stock Quadrajet was swapped out for a Holley Street Avenger 770 CFM

Thanks for the responses!

droldsmorland

Yes...I run 86s without any problems. I have spent time super tunning the ignition and fuel curves. Do this before replacing parts.

69_olds

Got it... thanks for all the feedback!

oldcutlass

No, you need to install a stop to limit your vacuum advance to 10*. Also run your plug gap at .045, I doubt its your plugs causing the issue. You can pull them and see how they look.
https://www.summitracing.com/parts/CRN-99619-1/

cutlassefi

You still didn’t totally answer my question, did you rejet the carburetor for your combination? There’s no guarantee that carburetor was Jetted correctly when you installed it.

69_olds

I have not rejetted the carb. Plugs look good so I don’t feel like it’s running lean.

Followed advice and disconnected the vacuum advance and pinging is completely gone. I will try the 10 degree stop and see how that works.

Thanks!

coppercutlass

I would ditch the vacuum advance all together. IM running a 10.25 to 1 compression 355 with #6 heads and 93 octane. With that much compression on the street specially if you are running pump gas I would install and AFR gauge and jet the carb by what the AFR is showing you (big reason I finally went to one with my new engine). a box stock 750 HP holley was too lean for my engine combo had to go 2 jet sizes up for it to be happy and I think the jetting in the HP 750 is a bit more than the 770 street avenger Possibly . Also when is your timing coming in ? IM running an HEI im stuck running 14 initial and the distributor has 22 mechanical . Its all in by 2500 RPM.

FWIW im a fan of the NGK spark plugs.

69_olds

I will get an AFR gauge and see where the carb is running.

Thanks for all the support!

Schurkey

Why throw away part-throttle fuel economy and throttle response?

coppercutlass

If you want throttle response lock the timing. If you have your timing all in by 2500 which most decently gear perfomance cars even for the street are with a decent timing curve., throttle response won't be an issue as you will be already crusing well into the advance with a fast ramp in timing. I havent ran a vacuum advance in a long time . imo thw vacuum advance only helps on a car the cruises at 1500 RPM and is bearly into the timing curve so the added timing from the vacuum advance helps in that situation. But on a car with say a 3.90 and a 3500 stall you are usually cruising well over 2500 rpm most of your timing will be in already and adding more timing to a set up like that would only cause issues imo.

Schurkey

Locked timing is going to provide on the order of 32--35 degrees of timing. Centrifugal-only, all-in at 2500 rpm will also provide about 32--35 degrees of advance once the engine is running 2500 rpm.

The engine would run more efficiently at PART THROTTLE if there were another ten or fifteen degrees of advance. This assumes that the PART-THROTTLE fuel mix is properly lean instead of pig-rich. The physics of combustion are that a leaner mix needs a bit more advance, and providing that advance can drop exhaust gas temperature. The heat of combustion is pushing the piston, not going out the exhaust valve. Part of this equation is whether your power valve is adding fuel at part-throttle cruise. I hope not!

Properly tuned, you get better power AND better economy, potentially less plug fouling and less cylinder-wall washdown, and less heat-rejection to the cooling system with vacuum advance on any engine that's primarily run at part throttle.

coppercutlass

Im not disputing what the vacuum advance does. For certain combinations. I certainly would not run one. If it was a stock application yes or even a mild application but for something more aggressive I would not. That's all.

matt69olds

In my opinion there are no downside to running vacuum advance on a street car. The old rule of thumb about 32-34 degrees of timing is a good suggestion,,,,for maximum power at WOT. You can run much more timing, and a much leaner mixture at part throttle.

The way timing curves were explained to me goes something like this: at part throttle, you have a incomplete full of air/fuel mixture. This is because the throttle blades and engine speed limit the TIME for the cylinder to fill. In order for combustion to burn everything, the ignition needs to start sooner. That’s because it takes more TIME for the flame front to jump from one fuel molecule to the next, with all the empty space.

As as far as part throttle fuel mixture, when I had the Q-Jet I kept leaning out the fuel mixture with the APT stop. When it bottomed out and had no more adjustment, I dropped jet size and ran the screw fuel rich, and started again. I kept doing this until I got a lean miss at part throttle. I thin richened the mixture with the APT screw just enough that the miss went away. Fast forward a couple years later, I got a LM-1 aft gauge from a friend. After I installed it, imagine my surprise to find the part throttle cruise AFR were about 15.3. At WOT, I was about 12.9. I drove the car for years with zero issues. Most people would think an engine would melt running that lean, and obviously it would at WOT. How much power does it really take to push. 3900lbs car 70mph on the interstate? Maybe 80hp? How much fuel would a well tuned engine need to make 80hp?