Over the years one of the hottest topics (excuse the pun) in street rodding has been engine-operating temperature. Take an old car with a relatively small radiator, stuff in a big engine, cover it with a smooth hood, add A/C then go cruise around a fairgrounds and the results are often an automotive version of Old Faithful. Then there are those cars that cool fine when cruising but get hot on the highway, and we all know how annoying it is to drive at speed with one eye on the temperature gauge.

Regardless of when and where it happens, there are consequences to overheating an engine. The first sign is usually pinging and detonation or running on after the ignition is shut off. In severe cases the abnormal combustion that takes place in a hot engine under a load can blow holes in pistons, and damage rings and rod bearings. Overheating can cause pistons to scuff or even seize, exhaust valves may stick in their guides, and warped heads and blown head gaskets are common catastrophes. On the other hand, running an engine too cool has its own set of problems; among them, reduced performance and corrosive condensation collecting internally.

To look at engine operating temperatures from a technical standpoint we decided to pose a series of questions to experts in their particular fields--Bill Mitchell of World Products, manufacturer and supplier of blocks, heads, and complete engines; Scott Leon and Neal Matheson, both former GM proving ground technicians specializing in hot weather performance and reliability; Ed Newton of Amsoil, one of the most established names in synthetic lubricants; and Jeff Cannul of Stewart Components, manufacturers of high-performance water pumps. All took time from their schedules to respond--here's what they had to say:

Bill Mitchell
SR: How is the coolant path through an engine determined?
Bill: In earlier ('90s) engines the water went into the block, then up into the heads and back into the radiator. In later-style engines the water goes into the heads (while still cool) and then down into the block and back to radiator. This later method allows the cooler water to control ping and allows higher compression ratio by keeping the chamber cooler before firing. This is why our water jacket design of more water, fewer air pockets and less solid metal can take more compression, more timing and makes more power.

SR: How is coolant flow through the block and heads controlled?
Bill:The water holes in the block have another job long before it gets to be a block. Those holes in the foundry are called core prints. They hold the entire core package together and after the block is poured and cooled down the holes are the only ways to get all the sand out of the block. Therefore water flow and volume is controlled by the size of the holes in the head gasket.

SR: Are some engines harder to cool than others? Why?
Bill: It's all about real estate inside the engine. We need sand cores to make the water jackets. Close-bore centers, big bores and thick cylinder walls all take real estate away for the sand (water). If you look at our parts we hunt for real estate everywhere--bulges around cylinders in the valley, outside the block, everywhere. We connect all cores and take into account the opposition of the part installed to make sure there are no air pockets.

SR: Is there an ideal engine operating temperature?
Bill: There will be lots of opinions on this question and they're all right if it works for them. Generally 200 degrees, give or take 20 degrees is in ballpark.

SR: What's too hot? What's too cold?
Bill: All I can tell you is cold engines don't make power on dyno because cold pistons, oil, etc. rob all the heat you're trying to make in-chamber. Hot engines usually will ping and everything you do to get rid of the ping will make less power.

SR: What are the differences in cooling-aluminum versus cast-iron heads?
Bill: Aluminum robs the heat (like welding something in a vise). The consumer loves aluminum and we all need to give the consumer what they want, but cast iron usually makes more power.

SR: What are the differences in cooling-aluminum versus cast-iron blocks?
Bill: Same answer as with the heads, however we have found that using ductile-iron sleeves (exclusive in all our aluminum blocks) actually seals rings so much better that our aluminum blocks usually make slightly more horsepower (usually 10 to 15) more than cast-iron blocks. Only downside is ductile sleeves are two to three times cost of cast-iron sleeves.

SR: What causes steam pockets and are they a common problem?
Bill: Steam pockets form as a result of a water jacket that you can't get water into because there is no way to get the air out. Our new Warhawk LS1/LS7 heads have been designed with unimpeded water flow to eliminate dead ends and the steam pockets that result.

SR: Are there cooling problems with Siamese bores (cylinder walls touch at their widest point with no water between them)?
Bill: Again, it's all about the real estate. Siamese bores are a way of getting more bore size in the same space. Fortunately the head gasket manufacturers have a good handle on controlling flow with hole sizes and it doesn't seem to be a problem.

SR: How does compression ratio affect engine temperature?
Bill: Simple, more compression makes more power, and power is made from heat. At some point the engine will ping, then all hell brakes loose, resulting in overheating, broken parts--you know the drill.

SR: Explain the theory behind using higher compression with aluminum heads.
Bill: If you cc the chamber in an aluminum head and then heat the head, the hotter you make it the bigger the chamber gets and the lower the compression ratio becomes--so just start it out a little higher. Besides all that, the aluminum is going to rob heat and make less power, so get it closer to the ping level right from the beginning by going with more compression.

Neal MathesonScott Leon
SR: Is there an ideal engine operating temperature?
Neal: 190 degrees is probably ideal, with a quick warm-up. That's hot enough to boil off moisture and prevent sludging, and low enough to provide a cushion for heat spikes.

Scott: 190-degrees to avoid sludge buildup and to remove moisture from the oil.
SR: What's too hot? Too cold?
Neal : Lower temperatures results in rapid sludging, can prevent closed-loop fuel injection systems from properly regulating fuel mixture, and results in decreased fuel economy.

Lower operating temperatures provide most people a false sense of security, but lower temperatures, either by thermostat or thermostat removal, do not result in increased cooling capacity. All it results in is a different starting point. Too cold results in unnecessary sludging of the oil, and may prevent engine/fuel/spark management systems from operating at optimum efficiency or power. Engine clearances are designed for normal operating temperatures. Thicker oil causes unnecessary drag.

Too hot is when coolant is lost. An operating, full, pressurized system (13-psi), with 50/50 glycol can safely go to 260 without engine damage from heat alone. The key is not loosing coolant, which results in hot spots, vapor pockets, warped heads, etc. However, 230 degrees and above can change the spark map required to prevent engine damage from pre-ignition/detonation. Most factory ECM spark-controlled engines automatically reduce spark advance as coolant temp increases. Mechanical distributors or non-ECM-controlled engines have problems with detonation at higher temperatures, and that can cause damage.