I Dare Not Use the C-60 When it is Hot Outside

[Larry S.]

Re: I Dare Not Use the C-60 When it is Hot Outside

The sum total of my knowledge on this subject is that my SB (327/300) runs hot with A/C on and my BB (427/400) does not.

[Patrick H.]

Re: I Dare Not Use the C-60 When it is Hot Outside

Patrick, the car is NOT overheating, just giving the appearance that it WILL at some point if things continue. I will never allow the temperature gauge to peg all the way over to the right side. So, I have NOT experienced a boil-over for the same reason. I shut off the A/C long before that can happen.

I had an "expert" IR gun the various places and he determined that the car was running hotter than it should with a stock 180 degree thermostat. He also determined that the gauge was accurate. His report caused me to buy a new correct DeWitt radiator.

I am using a reproduction RC-26 cap (15psi) for an A/C car. The car is a RARE L76 (327/365) with C-60 (factory A/C). -Clark

Clark,

"I think it might" is a long way from "it is."

My first thought is that I've seen repro radiator caps that did not function correctly. New RC-26s are still available from GM if you wish, and it's a cheap fix. Incorrect system pressure will affect cooling.

Second, what did the "expert" report? Numbers? Conditions? Had it been run on the road for a while, or just sitting in the shop? Until you either a) move the needle above 230 if not more, or b) boil over, you have not yet convinced me that there's an issue. Well, not with the car anyway.

Patrick

[Joe C.]

Re: I Dare Not Use the C-60 When it is Hot Outside

I dare not use my factory A/C when the car is in traffic or if the air temperature is over 90 degrees. I have been told by numerous sources that cars like mine will never keep their coolant/engine cool (under 230 degrees) when running the A/C under those conditions. Some have told me that Chevrolet made a big mistake putting A/C on the L76 engines. I find this statement incredible since A/C was an option for the big block engines.

I have had experts set and reset the ignition timing until I am sick of paying for it. I have changed to richer carb jets, replaced the fan clutch, checked my 7-bladed fan, changed to a new DeWitt's aluminum radiator ($900), changed thermostats, checked for collapsed hoses, IR'd the various spots, etc. to no avail. I have done everything that this forum has suggested except change out the water pump or dismantle the engine.

I want to keep my car "NCRS Top Flight" original to do well in upcoming regional and national flight judging. So, I am reluctant to do anything at this time that would make my car appear less correct. So, electric fans and a larger radiator is not in the picture.

But, I noticed in the Long Island catalog that for a 1966 small block with A/C, there are upper and lower radiator "seals" listed (sec. #26). Other than these seals being incorrect for a 1965 (according to the catalog), would the installation of these '66 upper and lower radiator seals help my car run cooler with the A/C on, in hot weather? What say you experts out there?

Clark,

This has been hashed and rehashed ad nauseum................and I say the same thing over and over, each time. Just like Duke has a favorite rejoinder "it's the timing, stupid", mine is:
"these cooling systems were designed with VERY little margin, or tolerance. They are operating on the ragged edge of their capacity, and so ANY upset of any kind will cause the engine to overtemp. The gauge, with its poor calibration and hypersensitivity to temp changes in some parts of the scale, and hyposensitivity to temp changes in other parts of the scale, serves to exacerbate the design flaws inherent in the cooling system. You MUST have an IR gun to verify the temp indicated on the gauge. The entire design is antiquated, and by today's standards, WOEFULLY INADEQUATE.........................."

The usual "band aids" will alleviate the condition, but the most important cause is INADEQUATE AIRFLOW through the radiator. As alluded to above, the engineers were hard pressed to find a solution, and so they added periphery seals to BBC and SBC with C60 in 1966. It would behoove you to retrofit your 1965 with these seals upon completion of its NCRS campaign. Until then, use the a/c sparingly.

More spark advance at idle, richer mixtures, jammed open heat riser valves, removed/modified splash shields can be investigated also.

If you add a replacement fan clutch, it should help, also. The only "problem" with this, will be the annoying and constant howling of the fan since it will be pushing more air than originally intended and at lower temps, too. The sealing of the radiator air leaks along with perfect fore-and-aft placement of the fan (half in/half out) of the shroud should alleviate the need for a replacement or clutch with re-clocked spring which is not only annoyingly NOISY, but robs engine power as well as fuel mileage.

Joe

[Duke W.]

Re: I Dare Not Use the C-60 When it is Hot Outside

All that you noted has already been done by two different shops. I do not remember the specs but nothing was found wrong except a loose plug wire and a pinched coil wire, which were immediately and permanently remedied.

I have no idea who worked on your car or whether they know anything about the spark advance map for an L-76. If past experience is any guide, his name may be "bubba".

Owners of vintage cars need to take control of their own situation. Start by getting all the factory service manuals and AMA specs so you know what the factory specs are. Then get a dial back timing light, a vacuum gage, and accurate test tach and dwell meter and learn how to tune your own car. I've been doing this since I was a teenager.

These cars are finnicky and require periodic checks and adjustments (what was traditionally known as a "tuneup") unlike modern cars that can run virtually forever with nothing but oil changes and a set of spark plugs every 100K miles. Plus, regardless of how good many old Corvettes look or how well that score in Flight Judging, many have been cobbled up inside and have all kinds of problems.

I don't know if your configuration has an "idle-up solenoid". If not, when the compressor engages the engine will lose 2-300 revs (including fan speed), which will loose manifold vacuum, which will loose total idle timing due to loss of vacuum advance and all of the above can conspire to promote overheating.

Can you report this data from the in-car tach (which may not be that accurate at idle speed)?

In any event, if there is no idle-up solenoid, you want to maintain at least 900 revs/no less than 10" manifold vacuum with the compressor engaged. This will require setting the normal hot idle speed (compressor not engaged) a few hundred RPM higher.

GM had pretty rigorous high temperature test schedules - even back in the sixties, and I refuse to believe that they would have released L-76 with C-60 unless it passed their tests, which would be worst than most ever see in service.

The key is to understand the setup specs, have a basic understanding of the engine's required spark advance map (which is a whole bunch more than "initial timing") and have the test equipment to verify that everything is to OE spec. Then you have a baseline to work from.

Without the above there's really nothing anyone on this Board can do other than just guess!

Duke