Q My question regards the direction. I have read that some early direct injected engines have had problems with oil cooking up on the top of the intake valves due to the lack of washing from the fuel. Jimmy, a wonderful performance machinist at Napa Auto Part in Vacaville, California, suggests that there may be enough air velocity to keep the intake valves clean. Is this further evidence for my theory that they put the red mark on the tach so we can know where the needle should be pointed? Is there anything other than keeping the needle pointed at the fun mark to keep the valves clean?
And how about the fuel? Chevy suggests that the car will be happy on regular gas even with 11.5:1 compression ratio. Will this engine respond to premium fuel like my big-block crew cab? I have been a big believer in the name-brand additives like Chevron to keep by injectors and valves clean. However, with the direction, do these have any advantage anymore? If not, can I save a few bucks by just burning the base stock fuel from Costco?
A As we all know, direct injection is the wave of the future. We really like your recommendation of keeping the engine at its red line to keep the intake valve free from deposits. It's amazing what the OEs have done to control the oil in our engines. It wasn't too long ago when you would regularly check the engine oil in our street-driven vehicles. Both of our family vehicles, equipped with four-valve, four-cylinder engines, go easily 5,000 miles between oil changes without burning a measurable drop of motor oil. Even Daniel's 2003 LS6-powered Z06 goes 5,000 miles without needing to add oil. That's saying something for an aluminum LS engine.
Basically, some of the deposits we've seen built up on the backside of the intake valves have been reduced dramatically from the improvements in oil control. The only oil that is getting to the valves is the small amount leaking past the valve seals, and the mist of oil vapor being introduced into the induction system by way of the positive crankcase ventilation. The harder you run your engine, the more blow-by and oil vapor can be introduced into the induction system. You're spot on about detergents in our motor fuels help to prevent deposit buildup. These detergents have aided the cleansing of the carbon buildup on the backsides of the valves in standard port injection. With DI, you don't have the fuel to carry the detergents to the backside of the valves. GM released a technical service bulletin on August 19, 2013, number PIP5029D, "Engine misfires due to Major Carbon Deposits on the intake and or exhaust valves." This covers all DI engines that are in the GM arsenal, from the 2.0L Eco-Tech up to your 3.6L V-6 in your Camaro. Illustrated in the bulletin is the effect of the carbon buildup, but GM did not give a specific reason for the problem. Currently, its only solution is to treat the engine with Upper Engine and Fuel Injector Cleaner. The application of this cleaner is a little different procedure, in that this cleaner must be introduced through the throttle body since it can't be introduced with the fuel, and it requires some very specific equipment. It's a "dealer only" procedure, and it could be that this should be a preventive maintenance procedure to keep the stems of the valves carbon-free.
Now, some manufacturers have adopted the strategy of injecting a slight amount of fuel during the overlap event to wash the backside of the intake valves with fuel. The amount of fuel injected is not nearly enough fuel to give you a burnable mixture, so it doesn't affect the combustion process. Diesel engines have been dealing with this problem for a long time. The secret is to control the amount of oil that is getting back into the induction system via the crankcase vent. If we had a DI-equipped vehicle we would add an auxiliary oil separator in the crank vent line between the crankcase and intake manifold. It could be something as simple as a catch-can positioned in the engine bay away from the engine, which is lower than the valve cover and manifold. This would give the oil time to fall out of suspension and collect in the can for drainage. The only issues are that when we do things like this, the Smog Police get unhappy, and it may void your vehicle warranty. Check with your local Chevy dealership and see what they have to say about adding such a device.
We also followed up with Tony Knight at Cylinder Head Exchange; as he touches more cylinder heads in a day than most of us do in a year. He said the Mazda DI engines are the worst, and he didn't think crankcase vapor is the main culprit, that because of the small-displacement engines, and turbocharging, the combustion heat is way up there. Without the cooling affect of the fuel going across the intake valves, the combustion heat is traveling up the valve stem and killing the valve guides in the cylinder heads. Once this has occurred, the valve seals don't have a chance and next comes the oil. From the photos in the GM bulletin, you see that the major carbon buildup is right up where the stem mates with the valve guide. This is why the carbon buildup is causing misfire by preventing the valves from closing. Back to your engine, the 3.6L V-6, he did say that he sees it the most in four-cylinder engines, and the V-6s are not as prone. This is most likely due to how hard the engines are taxed as the little four-cylinders are expected to pull around 3,000-pound platforms.
These same detergents that help keep your high-pressure injectors are being introduced to the flame front of the combustion process every other piston stroke. Also, down here in SoCal, our local Costco station has just upgraded its vapor recovery system and was advertising that it was about to release an additive package to its fuels. Time will tell.
As for using 91-octane fuels in your 11.5:1 high-strung six-cylinder and seeing big-block Suburban, we believe big power gains are things of the past. The DI really helps as you're able to properly time the injection of the fuel into the combustion space preventing pre-ignition. Also, if the engine has been calibrated from the factory to perform properly on 87-octane, you're just throwing money away. Back in the day, when we were doing EFI tuning for GM project cars, we found that there was tons of room to pick up power by adding timing and leaning out the fuel mixture. This is where the aftermarket tuner market came from. These days it's not uncommon to find that the engines are right at best power timing, and lean best torque from the factory and the manufacturers rely of the knock sensors and O2 sensors to keep the thing safe. Again, it's all about what the factory has originally tuned your vehicle to run on.