Watch Out For The Iceberg
Q: I need some help putting together a 406 small-block for my '73 Nova SS. Here's what I have: a rebuilt 400 short-block bored 0.030-inch over, stock rods, stock crank, and dished pistons with four valve reliefs. For heads, I have a pair of aluminum Procomp 190cc runners, 2.02/1.60-inch valves, and 64cc chambers. My math puts the compression around 9.8:1 and I'm going with an all roller setup; a Comp Cams 270HR with 0.500-inch max lift. I also have an Edelbrock RPM Air-Gap with a Holley 750-cfm carb up top. For the exhaust I have a set of Hedman 13/4-inch headers with Flowmaster mufflers. Would this be a well-balanced setup? The trans is a rebuilt TH350 with a shift kit and 3.08:1 gears out back. The plan is to drive the car nearly every day from spring to fall and I'm wondering if it'll get up and go on a chilly Maine morning with no exhaust crossovers and an Air-Gap manifold. Also, how fast can I safely spin the motor with a stock bottom end?
I apologize for the assault of questions, this is my first engine and I want to do it right. Thanks for any input!
A: We never experienced carburetor icing until we lived in Northern California, and it took quite a while to figure out what the problem was with early morning performance. The engine was equipped with an Air-Gap-style manifold and a 750-cfm Holley with no choke. The car would start and run fine until we got a couple of blocks from home; then the engine would turn very rich. After looking for all the normal rich-running Holley issues, we found it on the engine dyno at work. The Dyno cell temperature was in the mid-30s one day and after starting the engine and running it for a few minutes, it began to run very rich. We quickly went out and pulled the air turbine and found ice on the throttle blades plugging the transfer slots on the idle circuit. This caused the engine vacuum to suck fuel straight out of the idle circuits and go pig rich.
With extreme cases of carburetor icing, the throttle shaft can stick in an open position. We wouldn't be concerned about it if your engine will start and run in the cold conditions; instead our concern would be from the carburetor icing. Putting some type of lubricant into your gas helps prevent icing because the ice can't stick to the throttle blades. We had Red Line Oil blend up some additive to run on the engine dyno and snuck a gallon home for the El Camino. Carburetor icing is probably a regular occurrence in Maine and your local auto parts store should have some magical elixir for your situation. If not, check with Red Line Oil.
As for a max rpm for your stock short-block 400, stay conservative. Unless you just have to run it out, stick to a 5,500 rpm limit. The stock cast crank, cast pistons, and short 5.565-inch rods put a tremendous amount of side-load on the cylinder walls. The higher you spin it, the more frictional load you will see. Also, all those cast components flying around would make us nervous running higher than this. The 400 is going to make great mid-range torque, and shifting at 5,500 rpm will drop the engine speed right back into the sweet spot of the torque curve. Enjoy the fun of big torque!
Headers For You
Q: I'm having a really difficult time trying to find a set of headers for the GM 385 FastBurn. This engine is going into a '69 Firebird. It has quick-ratio power steering and no A/C. I believe this can be the same setup as a '69 Camaro. I have talked to a GM tech, Hedman, Hooker, and Doug's, and because of the raised roof they don't seem to have any suggestions on header. Hedman told me I could use its PN 68270 headers, but it has a 15/8-inch tubes and the corners would be very close with the D-port and may get blown out. One fellow told me I couldn't even use the stock manifolds. ANY help or suggestions? I'm desperate.