Dear Car Talk:
I have a project truck. It's a 1979 Ford F-100 with a 300-cubic-inch inline six-cylinder engine. Before I had it bored out to 30/1000s, it didn't ping. Now, using the recommended spark plugs, it starts pinging under very light load as soon as it warms up. I am assuming that's because of the increased compression ratio, which is causing a higher cylinder temperature. Everything else -- timing, carbon buildup, fuel mixture -- seems fine. I haven't checked the exhaust gas recirculation (EGR) yet, though.
So, am I right about higher compression and higher temps leading to pinging? I have been unable to find cooler spark plugs that will fit. Could I run a cooler thermostat instead? Thanks! -- Don
Good thing this is a project truck, Don, because I project you'll by tinkering with this for a long time. Here's the problem: We all know that the original engine was durable, but underpowered. Or, as we say in the business, with no disrespect meant to our canine friends, that six-cylinder engine was a dog. Most people who bought the truck with that engine wished they had opted for the V-8.
But instead of making your "project" replacing this engine with a V-8, you tried to increase its power. So you increased the size of the cylinders, and then I'm guessing you decided to replace the pistons with some differently shaped Mount Kilimanjaro ones that would generate more power. In doing so, you increased the compression ratio. And that's probably why it's pinging.
You should check the EGR valve and make sure it's working properly. The job of the EGR is to inject some exhaust gas into the fresh charge to cool it off and reduce pinging. So, hope that the EGR is not working, because that would be your easiest fix. Unfortunately, there are no cooler spark plugs or thermostats that will resolve this.
If the EGR is working correctly, then you have two options. Since you appear to have money to burn on this truck, Don, one option is to spend an extra 50 cents a gallon on 93 octane fuel, and see how much that helps. Alternatively, you can try to retard the timing. But that's going to have to be done through trial and error. You'll need to tinker with it and look for some middle-ground timing setting where the pinging goes away but you still have sufficient power.
If you're lucky, you'll be able to set it so the pinging stops, and you'll have almost as much power as you had before you re-bored the engine and installed those stupid pistons. Or you could just let it ping. And when the pinging melts your new pistons, put in a V-8 instead. Good luck, Don.