Classic/Antique Car Repair: Follow up to thermostat question, cam xe, aluminum pistons

Question
Dick,

Thanks for the reply to:

Just wondering if an AMC V-8, 390 ci, supposed to have a 195 deg. thermostat, was replaced with a thermostat of 180 deg. Would I notice a significant difference in gas mileage, say more than 3-5 mpg drop? If I live in a warm climate, would the 180 therm. compensate for the warmer climate, in terms of gas mileage?

The reason I'm asking this is because my engine was rebuilt recently and the machinist decided to run a 180 instead of the 195. My mileage has dropped 2-3 mpg. since the rebuild, but I understand that the rebuild may also contribute to this situation.

FOLLOW UP QUESTION:
I just talked with the machinist and he said that he wouldn't change the 180 thermostat because, on the AMC 390 hotter temp's are harder on the gaskets and block. Plus, living in southern cal. it doesn't get too cold, so making the engine run hotter isn't necessary. He feels that the engine will last longer if it runs just a little cooler. BTW, the engine was removed from the vehicle, taken apart, and new main bearings, a good used AMC crankshaft, Comp cam XE 262 cam, new forged aluminum pistons (9.5 to 1), valve job, new guides and seals installed (hardened seats already there), good used AMC rods installed, hi flow AMC water pump installed. Also, the carb. was overhauled by a competent technician, I believe

So, I'm just wondering, based on this info., if I'm getting a reasonable msg. from the machinist.

Thanks!


Answer
Well, his advice is contrary to what I was taught (and common sense), but since the engine has some modifications done to it, this person might have had bad experience with this particular modification; he might have learned the hard way that it has to be kept cooler, perhaps because of the higher performance cam and the compression ratio, for example pre-ignition might burn holes in the piston tops if it is allowed to run at the original design temperature.  "Harder on the block and gaskets" sounds like pure bull, to me - I never heard that one before!  

As for where you live, the engine operating temperature is set by the thermostat, regardless of the ambient air temp, if the cooling system is in good condition, so that argument makes no sense either, in my opinion.

The bottom line for you is: If you want him to stand behind his work, you are forced to go with his recommendation.  Once his warranty on the work is expired though, I'd go back to the 195 'stat, unless you get severe "pinging" with the current settings, because, regardless of what he says, it will keep the innards of the engine cleaner.  In the mean time, change the oil frequently, and use only Diesel rated oil in it - 15W40 is the best choice, but make sure it is rated for Diesel use -as the wear reduction additives in modern gas engine oils are not sufficient for use in our older engines.  The "SM" and "SL" rated oils are particularly bad for old engines.

The thermostat has very little to do with gas mileage.   The various performance mods certainly do, however!  Once the engine is thoroughly broken in, I recommend you take it to a shop which has an engine analyser and have a tune-up performed for maximum economy - and that will be the best MPG you can get out of it.

Since you are in Southern California, I can recommend a shop to do this in Temecula (where I live) if that is convenient for you.

As I say in my intro on this site, I don't have any experience or knowledge about modified vehicles - I don't do it, I don't recommend it, and I certainly don't know how to advise how to tune them for economy - you're on your own here, I'm sorry to say!

Good luck with it - and save the original parts, just in case!

Dick