What plugs are you guys using on your mid or late 70's stockers?  I've been using AC Delco's, R45TS.  A mechanic (old-school non-computer cars) told me they're too cold and I should switch to a hotter plug.  He suggested Autolite 26's.  The internet parts houses recommend this plug as well.   Any experience with either plug?  Or any other recommendations?   I'm almost all stock, but with true dual magnaflow exhaust (no cats).  I'm getting some popping and occasional backfires during deceleration out both pipes when the car is warm.  Could the colder plugs be responsible for not igniting the mixture in this condition?   Turning the mixture screws out a turn helps - popping is not as bad.  The same mechanic also suggested putting a cross-over pipe in the exhaust.   Could that help with the popping? 

 

I've posted a few times here regarding a lean mis-fire condition along with the above issue.  I've completely eliminated the EGR with a blocking plate and it's running a lot better, but the popping/backfire issue is still there.  Thanks for all the help. 

I just changed my plugs in my '79 this past fall. I stuck with the AC Delcos that are the stock application. They worked just fine.

I did this thinking I was being cautious and not wanting to set up a chain reaction of tuning issues that may have resulted from the hotter plugs.

This is just my '2 cents' on this. I'm sure the more accomplished members here can help you with more in depth info on this subject.

Good luck with it!

 

P.S.  Also check out "X-Pipes on a C3" posted by Steves74 on Feb. 10th.

 

Larry

 

The heat range is really how deep the porcelain goes into the plug body.  The deeper the groove, the less heat is able to transfer into the plug body and dissapate into the engine via the head, and thus cool the plug.  A cooler plug has less depth to the porcelain contact allowing greater heat transfer and thus a cooler plug.

 

What fires the cylinder is not the heat range.  That is the spark temperature across the plug tip.  This temperature is a result of the spark intensity and resistance of the electrical flow through the ignitions system and spark plug.  You need to include the gap of the plug as part of the resistance.

 

If the spark is too cold, the plug will not light all of the fuel and the car won't run well.  If the spark temp is too high from too much resistance, the spark duration becomes very short due to the very high load required to make the spark jump in the first place.   To short of a spark duration will also not light fuel well due to the lack off exposure.  It' s like trying to start a long on fire with a bic lighter in only a few seconds.  Not enough heat transfer into the fuel.   Granted the low resistance will create a much longer spark duration time, but it's too cold to light the fuel well.

 

So the proper resistance keeps the proper temperature and duration for the spark itself.  That is critical to proper combustion.

 

BUT......  The heat range has almost nothing to do with that.    The heat range is still very critical.   Regardless of spark temp,  the plug temp has major effects.   A plug that is too cold will tend to build deposits and condense fuel.  Either one or both can cause the plugs to foul.  The fouling is a build up on the porcelain that will allow the spark to travel down the plug and short out without creating an arc to fire the fuel.  The short is an arc, but it is not fully exposed to the fuel.  In some cases the plug may light a small quanity of fuel, but often will just misfire completely.

 

By keeping the heat range up, the plug won't foul, and it will promote the spark across the tip and allow the energy to do it's job.

 

If the spark plug has too high of a heat range, the porcelain can burn and blister, causing the plug to fail.  Depending on the degree of damage the tips can also burn off the plug, creating a very large gap and failure.

 

More importantly, if the heat range is too high it will radiate heat into the cylinder and cause the combustion chamber to heat up.  Over 2500 degrees F. in the cylinder is where it starts to develop a lot of NOx emissions, which is worse than CO, and the engine starts to ping and knock.  The ping and knock when mild will simply create poor combustion and poor performance due to the fact you now have an exploding in the cylinder instead of a controlled burn.

 

The explosion can cause a lot of damage.  The worse it is, the more damage can be created.  This can go all of the way up to burned and or melted valves and pistions.  In other words major engine failure.

 

When you push an engine at high speeds or hard throttle, a lower heat range is needed.  Low speed, around town, and cold operation will require a higher heat range.   The manfactures install a good blend for proper operation for both conditions.

 

 

Now for the popping in the exhaust.

 

The reason for the pop is combustion in the pipes.  This is a result of unburned fuel and oxygen mixed together, and the heat in the exhaust system lights the mixture.

 

So how does it get there in that situation?  One is the operation of the exhaust system.  Something in motion tends to stay in motion.  So when exhaust is flowing down the pipe and the exhaust valve closes the gasses moving away from the valve creates a vacuum pulse in the pipes.   Headers take advantages of this applying vacuum to the next  valve that opens to help suck the exhaust out of the next cylinder to better clear it, and allow more fresh air/fuel mixture into the cylinder creating better combustion and more power.

 

But it also tends to draw fresh air into the tail pipes.  You can feel the puffing effect at the pipe outlet at some loads and rpms.   If there is too much fuel remaining in the pipe from a overly rich mixture blending with air can make the pop.  The longer the pipe the less likely due to the fact that the exhaust is cooling while traveling back through the pipe.  Larger diameter pipes create a better condition to make this happen.  Unless the pipes get too large and the gasses flow too slowly for the vacuum effect to happen.   This is why too large tube headers actually lose low end power, instead of helping.

 

But a small leak further up the pipes or at the manifolds really suck in a lot of air, and can make the pop a lot.   Keep in mind this usually happens on decelleration due to the fact a carb is running very rich with the high engine vacuum and lack of air into the engine.

 

Any lean condition can cause this to happen.  A vacuum leak can really set it off.  If the condition needs to be rich how can a lean condition cause this?  You get a partial misfire in the cylinder due to the lean condition.  The partial misfire allows unburned gas along with unused oxygen into the pipes, and the heat with the mixture can create the in pipe or manifold explosion, and you once again get the pop.

 

So any lean condition can cause the pop.   When does richening up carb mixture stop the pop?  If it was lean you corrected this.  If it was already okay,the rich mixture burns cooler than a lean mixture, and the cooler exhaust does not light.  No pop.

 

High heat range plugs can increase the cylinder temp resulting in hotter exhaust temp helping to promote the pop.

 

Now you don't have to search.

 

I've been worried all along about both potential vacuum leaks and exhaust leaks.  The problem started (or at least became noticeable) when I replaced the exhaust system.   What's the best way to check for exhaust leaks?  Is the manifold to exhaust pipe connection a likely place? 

 

I've spent quite a bit of time on the vacuum side.  I've checked all hoses coming off the carb and manifold, rebuilt the carb (throttle bushings are tight), checked for flatness on the carb base and manifold, new base plate gasket, sprayed carb cleaner around manifold to head interface - nothing.  Currently I'm running the stock manifold, but I've got an Edlebrock Performer in my garage that I'm thinking about putting on just to see if it improves the situation.  Is it possible that I have a leak on the underside of the manifold?