Originally Posted by WALTSTAR

Originally Posted by WALTSTAR

Oh, and as a last thought on this, the early 928 engineers could not tell the future. They had no idea what would be in the fuels we use today.

Originally Posted by docmirror

If the car in question has an O2 sensor, and it is running in closed loop mode, the sensor will work in concert with the rest of the LH control system(timing, RPM, air flow, etc) to maintain correct stoichiometry at a given temp surrounding 160-230F. I believe, but do not know for certain that once the engine reached ~145F, there is no longer any cold enrichment. As engine parts approach 450F, the metalurgy of both steel and more important alloyed Al begin to change. If the coolant temp is ~180F, there will be hot spots in the engine that are well over 400F, and approaching 500F under stress conditions. Young's modulus is the tool used to determine the strength and ductility of metals(alloys) and is affected greatly by thermal stress. Adding to this, a hot engine is taking in less air by volume than a relatively cool engine. Intake temps can validate this, but simply knowing that an intercooler raises the HP of an engine is a quick validation. Finally, modern oils are pretty resilient to temps, but since the dawn of civilization the fact of viscosity and temp has stood by us. A hot engine runs lower oil pressure, and lower viscosity than a cool engine. The hotter oil is also more prone to shear damage of those long-chain molecules that are used as viscosity stabilizers, and once those begin to break down, the oil suspension system can fail catastrophically. Ask me how I know...

The good folks in the 996 world suffer from various M96 engine failure modes. Universally, the low temp 160F thermostat is recommended for cars although of course, Porsche cannot recommend it because it results in different NOX generation. It's a matter of operational limits, but keeping the engine cooler, and suffering a slightly higher NOX production is fine with me, which is why I put in the 160F thermostat. Intake air is cooler, and my fuel economy actually improved. I got ~1/2 bar better oil pressure(indicating better viscosity control) in summer, and the AC was more robust. So - notwithstanding the increase in NOX production, I'm a big fan of cooler rather than warmer, and certainly better than overheating. The same results were discovered on the 968 when I changed that from 182F to 160F. It had a black int, and the AC finally could keep up with the outside heat, and I no longer needed to pull over in traffic on a hot day.

The original block of the 928 was designed for thermal stress of ~200HP, or so in the beginning. By the S4, the block dimensions had not changed hardly at all, the cooling system was basically unchanged(maybe a bigger rad, I don't know), the water jacket got larger, but the cylinder walls got smaller, and the passages in the heads were possibly slightly larger, but not significantly. However, there was a 50% increase in HP(heat) with the S4/GT at over 310HP. All that heat needs to be dissipated from the same basic mass as in the 1978 model. I'm sure various things were improved a bit along the way, but it's my opinion that the S4/GT suffers the most from thermal stress, and a full cooling system will still not dissipate the work of the S4/GT engine sufficiently from it's beginnings of around 200HP(I have one data point on this, but it was a very costly data point).

A proper running S4 with a low temp thermo shouldn't run richer, and I'm pretty sure will have similar fuel economy as stock. I know it will have better oil viscosity/pressure, and there will be safer margin from operating, to failure limits. The downside is higher NOX, although I have no idea how much higher, but possibly high enough that it won't pass a state smog check, which is a bummer.

Originally Posted by docmirror

The one mentioned in your original post: " As a backup, I have a 16" electric fan in front of it as a puller. "

Having a fan in front of the rad, and using it as a puller, will force air movement forward, against the natural flow of moving air. I would propose to reverse the polarity of the wires serving the front mounted fan so that it pushes(along with the natural flow) air from the nose of the car though the rad and exits into the engine bay.

Originally Posted by docmirror

IMNSHO, the ASTM fuel standards for motor fuels have actually improved since 1970s. If you are talking about the addition of Ethanol, and how it affects temps, as I recall from fluid dynamics, Alcohols have one of the lowest vapor pressures, and certainly lower than unleaded motor fuel. They also have a lower specific gravity, and the evaporation coef should be better than gas. I believe, that's why drag cars like to use nitrates of Methane, to help control detonation by increasing the Octane rating. Ethanol(Alcohols in gen) have many net negatives, but thermal exposure, or thermal heat soak isn't one of them.