Hey all. Recently an old timer radiator shop guy told me a 7 pound radiator cap would drastically lower the engine temp from the stock 16 pound version. Supposedly because the coolant is flowing more often into the overflow bottle and giving the fluid more chance to cool by moving around more often. He said about 2 degrees less every pound. Its a stock 305 TPI / radiator setup on a 88 GTA. Any opinions? It sounds great if it works but I don't want to screw up my engine either!

 

Obviously someone who didn't do well in high school physics...

Won't make A DAMN BIT of difference to the temp; that's determined by the equilibrium between how much heat the engine puts into the coolant, and how much the radiator lets out.

All that will happen is, the boiling point of the coolant will be lowered about 2° for every psi. So, whereas a 60/40 water/AF mixture under 15psi has a boiling point around 265°, lowering the pressure that much will lower the boiling point to around 250°. Meaning, the cooling system will begin to spew at about 15° cooler than it should.

Hardly a benefit.

There's usually about 1 - 2 quarts of fluid in the overflow bottle. The engine & radiator hold about 4½ gallons. The fluid sitting in that bottle isn't significantly impacting cooling in the slightest, one way or the other.

Nod politely, thank him graciously and kindly for sharing with you, ignore the "advice".

 

I knew it sounded to good to be true or else everyone would be doing this. Back to the original cap. Thanks for the advice!!

 

I can also add that reducing the coolant pressure will drastically increase the thermal expansion of the coolant volume as it heats up. You may find you leak out of the overflow tank and have to keep adding more when it cools off.

This happened to me and then I did some research on thermal expansion of liquids under pressure and figured out that I wasn't possibly acheiving 16 psi with the amount of overflow that I had. A new cap fixed it right up. Once in a while being a mechanical engineer helps in every day life . . .

 

Thanks for the tip! I've already gone back to the regulation 16# cap since reading what far more knowing people have adviced. Thanks all!