If the radiator cap on your car has this seal directly underneath the cap, it is designed
Cool is Cool?
Willie Pretorius, Germiston, South Africa: My son and I have replica Cobras that we built ourselves. Mine is equipped with a Ford 351W motor and speed equipment, while my son's is a Chevy 350 powered with equivalent Chevy speed parts. The cooling system works fine with the temperatures on the open road at 197 degrees F and 221 F in town. We are trying to lower the temperatures by correcting the timing and air/fuel settings with information obtained in your magazines. However, I have found some discrepancies:
Hot Rod, Jan. '00 ("Boiled Rat"): "Lean mixtures and too much advance can increase running temperatures." Car Craft, Jan. '00 ("Too Hot"): "More timing improves low-speed cooling, and a rich mixture can make an engine run hot." Car Craft, Jan. '07 ("Corrosive Nature"): "When the engine cools, the coolant is drawn back into the radiator." Chevy High Performance, Apr. '06 ("Pressure-Type Radiator Caps"): "As the engine cools down, the vacuum serves to permit air into the radiator." Car Craft, Nov. '03 ("Radiator Caps"): "Coolant is purged into the overflow tank, though it will not be drawn back when the system cools."
What adjustments must be used if the engine runs hot? Advance or retard ignition? Lean out or increase mixture on Holley 600-cfm carb (lean-counterclockwise, richer-clockwise)? Must air or liquid be sucked back into the radiator when the engine is stopped and cools?
Finally, we bought two Stant radiator caps rated 16 pounds with red pressure relieve levers. One has vented printed on top, and the relieve valve hangs loose at the bottom. The other relieve valve needs force to pull it down, and it doesn't have vented printed on it. What is the difference?
Jeff Smith: To make this more confusing, Willie, most of the above statements are true-but more information is necessary to make them totally accurate. Let's take them one at a time. The statement "lean mixtures and too much advance can cause an engine to run hot" is only partly true. In my experience, a lean mixture with very light load (highway cruising in your car) will not make an engine run hot. Because the engine is only making 20 hp, there are really no problems here. If the engine runs too lean, it will usually surge and exhibit poor light-throttle acceleration, but it shouldn't necessarily run hot. However, retarded timing is guaranteed to make an engine run hot. This is because the late start of combustion exposes more of the cylinder wall to heat that transfers to the cooling system. Excessive vacuum advance could contribute to an engine running hotter (although this is less likely) since the engine is working against cylinder pressure. Usually, overadvanced timing will be felt as a surge as the engine is struggling to overcome the start of combustion before the piston gets to top dead center (TDC). This not only kills mileage but can contribute to making the engine work harder. More often, the engine may run hot because of insufficient timing at part-throttle.
A good place to start with timing for most performance engines would be 12 to 14 degrees initial timing with 20 to 22 degrees of mechanical advance all in by 2,600 to 2,800 rpm. Then it is important to also use vacuum advance that will kick up the timing at part-throttle another 10 to 14 degrees. This may put the part-throttle timing at cruise rpm at 44 to 46 degrees BTDC-and the engine may want that much timing to optimize part-throttle performance. It's also possible that the engine won't require nearly that much timing. That tends to be the case with later-model cylinder heads with good combustion chambers, which generally don't need as much timing as older cylinder heads. The only way to know is to experiment and determine the best overall timing.
As for tuning the idle mixture screws, unless carburetors operate differently in the Southern Hemisphere, your adjustments are backward. The majority (but not all-we'll deal with that in a moment) of Holley carburetor idle mixture screws are adjusted the same way. To lean the circuit, turn the idle mixture screws clockwise (as if tightening). Conversely, to richen the mixture, turn the idle mixture screws counterclockwise. The exceptions are a few emissions-style Holley carbs whose idle circuits are backward (which means the idle mixture screws are turned clockwise to richen the mixture), but these carburetors are relatively rare.
Before you start the engine, fully seat both idle mixture screws then turn them counterclockwise 11/2 turns out. Then start the engine and normalize the temperature. Tuning feedback is always good, so connect a low-speed tachometer or a vacuum gauge (or both). Turn both idle mixture screws in about an eighth turn and watch the engine speed or vacuum. If either or both increase, continue in this direction until idle speed or vacuum tops out or drops off. Always adjust both screws the same amount and then return the screws to the best position. If the initial turn-in hurts the idle speed, reverse the direction until you achieve maximum idle speed and/or vacuum.
As for movement of coolant after the engine is stopped, both statements are correct if applied to the proper installation. If you have a radiator with a pressure cap that dumps to the ground, then when the engine is turned off, the radiator may push a little coolant out the overflow onto the ground. When the engine cools, pressure drops and any liquid that was pushed out of the system will create a void that will be replaced with outside air pulled in past the cap. If a tube is plumbed to either an overflow or purge tank, any coolant pushed out of the radiator when the engine is hot will collect in the tank. After the engine cools and the system pressure drops, that coolant is generally pulled back into the radiator-assuming the overflow tank line is plumbed into the bottom of the tank or there's a tube that extends to the bottom of the tank. This is the proper way to build a cooling system, and all new cars are set up this way. A purge tank is necessary if the radiator cap is lower than some portion of the engine. A low radiator cap makes it difficult to fill the system, requiring a purge tank located at the highest point of the cooling system. This allows the system to eventually purge the air into the purge tank. The reference in the Nov. '03 Car Craft discussed a bad radiator cap that would not allow the coolant back in. We're assuming here that the cap works correctly. One thing mentioned in that story that is not correct is the suggestion to use distilled water. The distillation process strips electrons from the water. When used alone, distilled water will immediately pull electrons from the softest metals in the cooling system-zinc, magnesium, or aluminum-which is not what you want. The best water to use is either softened or reverse osmosis filtered.
As for your question on the Stant caps, the difference is probably between an overflow system and the older-style vented system. Look for two rubber sealing gaskets on the nonvented cap. The normal seal on the very bottom is what seals the radiator pressure and is relieved only when the pressure exceeds the rating on the cap. The second seal will be located directly underneath the cap and is designed to seal the cap for use with overflow systems. The vented cap probably does not have this second seal under the cap and allows overflow out the tube where it is dumped on the ground.
For what it's worth, many car crafters become upset by coolant temperatures that approach 220 degrees F at low speeds. While high, this is not an excessive temperature as long as the system is not venting steam, pushing coolant out the overflow, or audibly detonating. This high temperature isn't necessarily harmful to the engine, although you would not want to run the engine at maximum power and rpm at this temperature since it could easily detonate. But at low engine speeds with mild throttle openings, 220 degrees F is not an excessive temperature.
Ted Toki showed us the ballpoint pen test to quickly identify good big-blocks in the junkyard or at the swap meet. If the pen doesn't fit between the bores, it is a 454 or a 396 that can be made into a 454.