Ask Away! with Jeff Smith: Engine Knock and Boost with E85

It's all engines this month in Jeff Smith's Ask Away! column at our OnAllCylinders blog.

Rod Knock?
The first question comes from D.L., who has an kicking issue with the small block in his Camaro:

"I recently bought a really awesome street Camaro with a 355 cubic inch small block, a Super T-10 4-speed, and a 12-bolt with 4.10 gears. The engine runs really great and pulls hard all the way to 6,500 RPM, but it has this really bad knock when the engine’s cold. After it warms up, it’s not nearly as loud. It  sounds like the rods are going to come right out of the pan when the engine is cold. I called the guy who sold me the car and he said this was normal because it has race pistons in it and that the engine was fine. He says not to worry about it because the engine has always done that and the engine has lots of street miles on it without a problem. Did he just pawn off a broken motor on me?"

Jeff explains that the cause could be rod bearings going bad, but it could also be those 'race' pistons. Those pistons are most likely forged, which require additional piston-to-cylinder wall clearance to compensate for piston expansion once the engine warms up. You probably know it as piston slap, and it can make for a noisy engine depending on how much clearance the engine builder dialed in during assembly. That would explain why the noise goes away once the engine is warm. If it were bad rod bearings, you would hear the noise regardless of engine temperature.

Boost and Compression for a Supercharged LS3
T.B.'s question is short, but as you'll see the answer isn't:

"I am building a 427 cubic inch Dart motor with LS3 heads and a Kenne Bell 4.2L blower. I am looking for maximum horsepower out of this motor. What compression would you recommend? The blower is capable of 32 pounds of boost and it will run on E85."

Jeff explains that while E85 is rated around 100 octane, that doesn't mean the sky's the limit for engine compression with a supercharged engine. In fact, he advised to keep compression around 10:1 for durability reasons and to keep decent throttle response when the engine is not under boost.

Jeff goes on to talk about fuel system requirements (an engine on E85 requires about 25% more fuel than the same engine on gasoline) and explains why 32 PSI of boost is just crazy talk for this engine combination.