RPM and Temp

Q: RPM and Temp

I’ve got a 1968 Firebird, built 350, 3:73 rear end and a 3 speed AT. When I’m on the interstate, what is a reasonable rpm to be at (ie., what is too high). Also, I think my thermostat is blown, the engine is a 73 Pontiac, what is a good temp?

A: The higher RMP your engine runs, the greater the wear, especially for sustained periods. Not to mention the penalty in fuel economy. I’d try to keep the RPMs below 3300 or 3500 for highway cruising speeds. Depend- ing on tire size, that might be 55 or 60 MPH?

You can try a taller tire, change the rear ratio, or get an overdrivetransmission.

Regarding the temp question, I feel that 180 to 205 is within a normal range, depending on load, speed, and air temp. Even 210 is OK if climbing a steep grade on a hot day, or sitting in traffic in August. Modern cars are designed to keep pretty close to 200 for max. efficiency. A frozen thermostat in the open position will not allow the car to warm up, or be slow to warm up, and will result in premature wear in the cylinders. Heat is required for good operation, but not too much heat. Frozen shut will cause overheating pretty quick. I run a 160 or 180 degree thermostat in the summer, and a 190 in winter.

If you are relying on the factory temperature gauge to determine operat- ing temps, be warned that they were never too accurate when new, and with age, they are completely suspect. Add an aftermarket guage, even if only temporary, to determine how hot you’re engine is getting. Mech- anical guages are more accurate than electrical.

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