not the typical 308 cooling question

Some interesting ideas here and I will leave the details to someone more familiar in this field than me. What I will say is that replacing the thermostat on a 308 is a little more involved than on a small block Chevy so swapping out restrictors may not be reasonably in the cards.

 

whats your coolant temp at the block? the 20psi 328 motor here is routinely seeing about 220~230*F while the tank gauge reads 175~195. running with water & redline water wetter. the coolant system on this car is in top notch shape, even upgraded the pump to one of nicks units, it won't climb past 195* even idling on the dyno.

what about a colder plug since your keeping the revs high?

 

The stat is a two stage unit, as it opens the passage to the radiator it blocks the re circulation port going back to the engine. If you run a restrictor you must tap and plug the recirc port. This is what the one inch disc on the bottom of the stat does. I recommend against it as it will take a long time for warm up and once the stat is open there is no benefit from that to a restrictor plate other than a little baffling that the stat body creates over a plate.

Impeller clearance to the body is critical at sustained high RPM. Few folks check this clearance when throwing a pump together but for everything to work properly it must be around .7 to .8mm, adjust this with the gasket thickness. Most cars require custom gaskets cut out to obtain the correct specs.

Foam all around the radiator and along the top and bottom is essential to duct all of the ram air through the core. Is this all in place? Check the temps of the aluminum coolant tubes going in and out of the radiator and see if there is a substantial drop from one to the other. The infrared guns can give misleading data here due to the reflective nature of the tube, use a touch type pyrometer for this reading. This will tell you if the radiator is suspect. Aluminum is nice but I have not seen a good brass core that wouldn't do about the same for heat dissipation with a reasonably built engine. That said there is a lot of weight to get rid of hanging way in front of the wheels...

The last thing I do for the high RPM race cars is get rid of the 19th century paddle wheel impeller on the pump. These paddles bat at the coolant and at high RPM with generate cavitation. I machine aluminum scroll type impellers for these cars that cut through the coolant and moving it effectively without whipping up a froth. When using this type impeller I run the clearance to the body quite tight, usually under .5mm which often requires I use no gasket at all and oring the body to the housing.

Dave

 

Colder/hotter spark plugs have nothing to do with engine temperature, only to spark plug electrode temp and the ability of the electrode to shed/retain heat and, therefore, to keep the tip properly clean. Changing spark plug range in itself has no effect on engine operating temperature.

 

Paying attention to this detail made an incredible difference in the running temperatures on my BB. Although the water pump design is a bit different than on a 308, the end result, by reducing the impeller to pump body clearance, was over a 20 dF drop in engine temperatures...

Regards,
David

 

A higher pressure cap wont matter unless your overheating to the point it boils over. A 17 pound cap (about 1.2 bar) will allow coolant to reach about 260F (3 degree increase in boiling point per pound of pressure increase) before it can boil. However, localised temps around the combustion chambers within the cylinder head can often exceed average temps. You could be boiling in the heads and not in the tank. But the 308 head seems to have a lot of coolant volume so that may not be a factor provided there is enough coolant flow. One way to know is to shut the car off hot and immediately listen to the engine. If you hear what sounds like thumping and can feel it in the hoses, its boiling. You either need to look at the pump or find other ways to lower temps. 195 wouldnt bother me at all as long as I knew the system was holding and oil temps were normal.

All of the racing 308 cars I have seen have modified front hoods that duct the entire backside of the radiator out the top. As long as your tracking the car why not rig up an aluminum hood to accomplish full venting. If you search around you should be able to see what I am mentioning.

The research I did looking at aluminum vs brass radiators parellel Dave's experience. Aluminum is weaker so the tubes are round and thick. So to equal a brass radiators heat loss capacity, an aluminum radiator has to be larger and thicker. While the aluminum radiator is certainly much lighter compared to a brass radiator when both are empty, the greater internal coolant volume of an aluminum radiator can add a great deal more weight in coolant, offsetting the weight savings you thought you had when you compared them empty. IOW its usually a wash by the time their filled. They also corrode much faster and are more expensive and sometimes impossible to repair. Stuff as big of a brass radiator in the car with as many rows of core you can find.

Dave, what are your feeling of the stock pump impellor for normal use, would your scroll design help or is it only when rpm is high that you need to do something?? Interesting insight about impellor clearance, thank you.

 

I think the stock unit is just fine for every day use, sprited or mundane. The problems only come in with sustained high RPM use such as a dedicated track vehicle. Putting one on a street car is as my father calls it, "belt and suspenders", no real need for both. Next time one has their pump off the housing look for the pitting in the aluminum taper seat area adjacent to the impellor... I have seen a few that were so bad I mounted them on the rotary table in the mill and recut the tapered seat and then the gasket surface the same amount. Once the pitting gets real bad even a properly fitted clearance wont do the trick.

Dave

 

I've got coolant sensors mounted in the valley in place of the old thermoswitch, it'll read 230+*F before the tank temp sender gauge reads 195*F. seems a bit hot to me. on my mondial i've been able to keep the gauge temps down to 130~145 but the engine temps are 185~195, and I've noticed the car runs better. once it hits the 195 on the gauge on hot summer days, power is down.

 

brain fart, the union. but let me know what you find on the other sender, I've got a std GM one in there. could be the resistance map but it reads good cold, hmmmm.

 

My 77 still has the stock unit.

Is measuring the inlet/outlet temp with an infrared or contact type meter going to give me a realistic read of the radiator's efficiency???

If i do the test before the fans come on, what amount of spread should i be looking for???

(Good foam around the radiator and new thermostat recently, water wetter and 40% glycol based coolant. I have installed a shouded pair of 12 inch spall fans but the car still hits 240 on the track when ambient gets close to or over 100F.) Oil temps ride a bit higher and i am assuming that if i can get the water down to 210-215, the oil will follow suit??

Steaming,
chris

 

If anybody has a power problem at 220F, you need either to retune your carbs or reprogram your EFI, or make sure your intake air is cold.

The NASCAR boys run about 220F in their motors, and it isn't because they have cooling problems.

Hotter coolant means less heat and power loss to the cooling jacket, and thus greater motor efficiency, power, and fuel economy.

NOTE: Hot temps and carbs don't mix well for street use due to carb boiling at shut off.

Doug

 

The twin cooling fans can loose performance over time too - the bushings wear and there can be a build up of corrosion in various areas. It's really easy to rebuilt the fans, and make a few improvements too - you'll be impressed with how much better they run after a proper rebuild. Do search for the fans and you'll find a couple threads with step by step instructs including pics.