Hot Rat Monte
Roman Rzasa, Ortonville, MI: I am the second owner of a '75 Chevy Monte Carlo with a 454 and a Turbo 400 trans. When I bought this car in March 2004 it was all original with approximately 85,000 miles and all the original emissions equipment including the catalytic converter. I drove it as it was for about three months with no problems, and since it came with factory gauges, all the readings including the temperature always looked good.
The first thing I changed was the 2.73 open rearend to a 3.42 limited slip. The next day I had the converter removed, and a dual-hump crossmember and true dual 211/42 exhaust with DynoMax mufflers installed from the original exhaust manifolds back. About a week later I noticed the radiator leaking, so I replaced it with a larger one, a new fan clutch, and a 180-degree thermostat.
After these changes I noticed that the car seemed to run hotter than before, sometimes too hot. And I could never figure out why, since all I had was a new cooling system, 8.1:1 compression, no cam, and a no-power 454. Recently I decided to remove all the emissions equipment and I replaced the original exhaust manifolds with Hedman Elite shorty headers. I decided then to get serious about finding out why it runs hot. I started looking up all of what I thought would be the pertinent information regarding Chevy big-blocks.
The one thing that seemed to stand out was ignition timing, considering the big jump in initial timing from 10 to 16 degrees BTDC. Looking at the timing information, it seems to indicate that initial timing increased as emission controls increased and power decreased, and a large jump from 10 to 16 degrees in 1975 could only have been due to the addition of the catalytic converter. This information came mostly from Haynes and Chilton manuals and some from other GM books. My first question is, could the removal of the converter with no other accompanying adjustments in timing or carburetor be the cause of my running-hot problem?
My second question is, where do all the magazine and Web writers get their timing information? From all the magazine articles I've read and hours I've spent on the Web, the majority opinion on big-block Chevys seems to be to have the initial timing as high as possible, up to 14 or even 16 degrees BTDC, and to have the total timing from 34 to 36 degrees and all in by 2,800 rpm (including your answer to "Stumble In" in the Apr. '07 issue). Yet looking at all the data I've collected, it appears from past and current GM recommendations, including those on the latest 454 and 502 crate engines, that initial timing should be from 4 to 8 degrees BTDC and total timing should be between 26 and 30 degrees.
The reason for tuning with different total ignition timing numbers is to determine what timing the engine wants to make best power. While 36 degrees is a popular number with most current performance engines, each engine is different and may need more or less timing to make best overall power.
Jeff Smith: The first thing to do in this situation, Roman, is to look at the temperature itself. Unfortunately, you did not mention what "too hot" is exactly. Many enthusiasts get nervous with an engine above 200 degrees F when that's not really hot at all. Most factory engines run at that temperature normally. Consider that water boils at 212 degrees at sea level, but 15 psi of pressure raises the boiling point of straight water by 30-plus degrees. This means the coolant won't boil until around 245 degrees! When the coolant begins to boil, it creates steam pockets that prevent the transfer of heat away from the cylinders, especially the combustion chamber. That's when your problems begin. A good indication that the engine is in trouble is when it starts detonating at part-throttle. When that happens, shut the engine off immediately before this causes more damage. Our point here is that an engine running at 200 to 210 degrees is not in trouble, and as long as it's not pushing coolant out of the radiator, it's not hurting the engine to run if it's not detonating. The other side of this is that running an engine at 165 degrees all the time is not good for it either, since the oil temperature may not achieve at least 212 degrees so that the water in the oil can evaporate out. If this does not happen (such as in winter climates), acids begin to form in the water remaining in the oil, damaging engine bearings if allowed to sit for long periods of time.
To get back to your temperature question, let's first address the accuracy of your gauge. It's possible that the gauge may not be indicating the actual temperature of the engine. Or the sending unit could have gone bad, or perhaps a weak ground between your sending unit and the gauge itself could be affecting the temperature reading. The best way to check would be to calibrate your factory gauge with a separate thermometer or gauge that you trust to get an actual temperature reading, as opposed to using only the classic factory gauge that just indicates a range between hot and cold. Assuming everything is functioning normally, it's possible, although unlikely, that your new fan clutch may not be working properly. We've had experience with budget fan clutches that only work correctly for a short time and then get weaker as the miles pile up. A quick check for fan clutch operation is to allow the engine to achieve a minimum of 200 degrees F of coolant temperature (by placing a piece of cardboard in front of the radiator) and then listening for the fan clutch to engage, which will make more noise when it attempts to pull more air through the radiator. If there is no noticeable change in airflow, the fan clutch would be our top suspect. Another possibility is that the radiator you purchased has a low-density core while the original radiator used a high-density core. As we mentioned in our cooling system story in the July issue, budget radiators are still being made with low-density cores where the space between the cooling tubes is wider, generally around 11/42 inch, while high-density cores place more cooling tubes within the same area so that the width between tubes is reduced to around 31/48 inch or less. If you purchased a low-density radiator, that could contribute to the higher engine-operating temperatures.
As for ignition timing, the typical base ignition setting of 10 to as much as 16 degrees is established to generate the best overall power, especially at lower engine speeds. Most enthusiasts know that when an engine gets hot, the chance for engine detonation is more likely, and thus you don't want to lean on your engine hard when the coolant temperature is higher than 210 degrees F. Unfortunately, the new-car companies have to assume that the engine may be asked to perform these kinds of high-load duties while operating at high engine temperatures, so they purposefully reduce maximum ignition timing to minimize the chance for detonation-induced engine damage. They pull back the initial timing to a more conservative 5 to 8 degrees and also are very conservative with a total ignition timing of around 32 degrees.
CC Quickies
We were checking out this ornate hood ornament from a barn-find Plymouth at the Mopars at the Strip show. Plymouth used sailing ships as mascots to represent the Mayflower, the ship that carried the Pilgrims to Plymouth Rock-get it?
Because we are car crafters, we want to make the most power possible from our engines, so we optimize the ignition timing for every rpm point since this is an area that helps make power and also costs nothing. Generally, increasing base ignition timing will improve a potential overheating problem because starting the combustion process sooner means the engine will be closer to oxidizing most of the fuel by the time the exhaust valve opens. The exhaust gas temperature is reduced if most of that heat energy was expended pushing the piston down. If the onset of ignition occurs later, the engine can run warmer since the tail end of the combustion process could still be occurring when the exhaust valve opens, causing the exhaust valve and port to be subjected to a dramatic increase in temperature. This places a greater burden on the cooling system.
Most street engines these days seem to prefer a total ignition timing of around 32 to 36 degrees for best power, but each engine is different. Some may need more, some less. For example, the GM Gen III engines are often tuned from the factory with less than 30 degrees of total ignition lead because that's all those engines need to create maximum cylinder pressure. This is a strong indication of a well-designed combustion chamber.
A lower total ignition-timing requirement indicates that the combustion chamber is designed as a fast burn that does not require as much time (as measured in ignition timing) to complete the combustion process. So if the engine makes the same or more power with 32 degrees instead of 40 degrees of timing, that's a good thing.