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1968 corvette with overheating problem




I have had conversations with Jack fro Stewart Water Pupms. I am looking for an help or suggestions.


I have a 1968 vette with a 350 SBC that has a 144 B&M supercharger.

I have replaced the radiator with a new factory size aluminum radiator (from Dewitts) . I also using the stewart stage 3 water pump. I have also mounted a 16" electric fan to the radiator, and yes the fan works. No shroud and no mechanical fan.

Now the problem I have is the the car is running hot when at speed.

I have repeatedley burped the system to remove trap air.

The one thing I have noticed is that when I drive over the mountains and start heading down hill the water temp drops down to 180, (when on level or up hill it is 210-225). This tells me that the system can remove the heat but it seems that I do not have enough collent in the radiator?

I previously had an over heating problem at low speed ie stuck in trafic. I have tried a heavy duty 7 blade flex fan along with the shourd. At freeway speed the car was fine but once in trafic it would start to run very hot.

I am using the stewart 160 deg thermostat, the one modified with holes.

I am using a set of billet pulleys that are 1:1 ratio. I know they are the same size as the OEM, since I measured them.

I have been fighting this overheating problem for a long time. At this point I am think of taking the car to a performance engine shop and ask them.

One thing I have not tried yet is buying a new water temp gauge such as an autometer and use that to verify my temp. Also the only other thing I can try is to test the coolant for gases in the in the coolant.

In case you are not aware of it the 68 corvette has the radiator mounted such that it is tilted back something like 20 to 30 degrees.

I am starting to consider buying the biggest radiator that I can fit into the car, but from reading your forms and a few other places on the net this might not work. And also there might be something else that is wrong with the car that I am not aware of.

I was hoping that using you water pump and mounting an 16 inch electric fan (which pulls ~ 2000 cfm) would do the trick but in actuality it is worse then before. Previously at least I was able to drive at speed and the temp was normal. At this point I am starting to look at the factory shroud and see how and what I need to modify it so that I can re-install it into the car along with a mechanical fan.

I know that this kind of problem is tough to resolve via the net or the phone since you are not able to actually look at the car.

So any advice, ideas…….prayers……would be helpful.


Matt Mason
image 3

I have posted 3 images of the setup so people can get an idea of what I did

Does the electric fan pull enough air? As I recall most of the CFM ratings I have seen are in the 2900 and up range for the small block. It seems to me that the big block would require more.

Is the car lowered?

The fan pulls ~2000 cfm I do not know of a 16 inch fan that can pull 2900. I have looked at spall fans the dual 11" fan and it pulls 2780.

The engine is a small block making about 450 hp.


No the car is not lowered.
For some reason I thought you had the big block. Looking at your post it's apparent that you don't. Sorry for the confusion.

I had been looking at the Permacool, Summit part prm19115, which pulls 2950cfm. I was under the impression that this is a 16 inch fan, though it allegedly is one of the highest flow 16 inch fans.

I read that the dual Spals that pulls 2780 cfm is deceptive because they are rated with the resistance of a radiator where some of the others are not, simply the movement of air.

Of course I can't find the post now, but somewhere I had read that higher HP cars require a bigger CFM. I've also read that not only fan draw, but coverage over the full size of the radiator can affect cooling.

Permacool posts this info. http://www.permacool.com/Catalog/Cat_page16.html
I think we can fix this.

I have three thoughts, and I will list them in order of most critical.

1. Front air dam????? Is it there, is it in tact, is it mounted solid? In conjunction with that, how is the seal between the radiator and core support?

2. Wrong water pump/pulley combo. IN other words, your water pump might be backwards---turning the wrong way. There are forward and reverse direction pumps.

3. Finally, the drive ratio on your pump/pulley being two fast.

Now, I say this because when running at speed two things are happening. One, you can only suck air that is diverted up under the nose just in front of the radiator. That is why a proper air dam is crucial. Two, you are turning more RPM and if the wrong water pump is on there, or if it is turning too fast because of your pulley size, it will over pump and create a cavitation.

One quick thing you can try. Get some aluminum, plastic, or fiberglass paneling. Build a air dam and attach it to the CORE support and go take it for a spin. If it runs real cool, then you have a air flow problem. I had this problem on my V8 Fiero exactly as you describe. I had some panel fiberglass off a semi truck cab. I trimmed, cut, and mounted it under the core support. It dropped the operating temp over 30 degrees at 70 mph!

I got this idea from the third gen F bodies. In fact, I read a great article in Car Craft about installing the 3rd gen F body Air Dam on any high out put muscle car to help keep them cool on the road.
I agree with Chris - ordinarily, heating at highway speed says you either have a) Not enough radiator capacity, or b) Not enough airflow. I don't think your radiator is the issue - I'd focus on airflow:

1. The stock air dam is absolutely essential - this car was designed as a "bottom-breather", and without the air dam, ram airflow to the radiator is reduced by over 50%.

2. Make sure your radiator is sealed completely to the back of the radiator support on all four sides, to force all incoming airflow through the radiator core, and not let any pass around it. This also means sealing from the top of the radiator to the hood, as is done on A/C cars.

3. The electric fan doesn't help highway speed airflow at all, and actually reduces airflow through the radiator; that's why the stock fan/shroud setup is much better - doesn't interfere at all with highway airflow.

The above assumes you have the correct rotation water pump for pulley rotation, and it's driven at the stock ratio from the crank pulley.

Yes I have the stock front air dam.

The water pump is the correct type and it is turning the correct way. I know this since the upper hose has pressure and does not colapse whent he throtle is hit. The pulley ratio is 1:1 and the are the same size as the stock pulleys.


Yes I do have the air dam.

The radiator is sealed almost completly I would say it is sealed approx. 95% or more. At the top of the radiator there is a small gap where there is no seal. I can easily seal that area off.

I have to agree with you that the electric fan does not help at freeway speeds and that it might even be slightly restrictive. But I still have the overheating problem when I am in trafic where the fan is on and the car is NOT at freeway speeds.

I originaly had the stock shroud and a 7 blade fan. This setup when at freeway speeds the car stayed cool, but when in trafic it would over heat. Hence the reason I am using a electric fan.

I am running out of ideas here.

I will do what 69myway suggested and add an aditional air dam at the bottom. and see what happens.

I had heating problems.

I have electric fans hanging up on my garage walls. I have tried the dual 11" electric fans with my aluminum radiator. Nothing performed better than the OEM fan shroud, 18" fan and heavy duty clutch.

Provided your motor is tuned properly: Carb and timing and you are having these problems in warm weather, where the cooling system is under the most stress, then here are my beliefs:

Neither flex fans or electric fans will pull/push enough air at speed or idle. Now, I live in a hot, humid climate, so those of you who live in predominately cooler climates, don't run highly modified or blown motors, please bear with me. All cars should run pretty cool in the cold weather and if not, then the cooling system might be only part of the problem.

PULLING air through the radiator, creates a low pressure condition on the motor side of the radiator, so the incoming air from the front will effeciently pass through.

Incoming air is turblant and will seek low pressure areas. (Path of least resistance) A strong fan, with heavy duty clutch and properly sealed shroud/radiator will better create a low pressure area and just as importantly push the hot air out of the engine compartment. Modified motors with thin tube headers, cams and items such as blowers create more heat in an already small engine compartment. It needs to move.

At idle, movement is minimal both in front and back of the radiator. High CFM electric fans might be able to push or pull air through a radiator, but can't move a significantly large amount of air out of the compartment and they also create turblance or restriction through the radiator at speed. A high performance motor produces more heat than a stock motor and will require more from the cooling system. Also, motors that are bored out and/or using aluminum heads and blowers will create more heat for the water to carry out of the motor.

Now, you most likely have clearance issues with the blower drive. You can get a steel fixed fan (not flex), good up to 10K RPM, and opt not to use the clutch, but you will need adapters to clear the belt on the blower. You need to make sure the fan is the largest you can fit in the shroud and the shroud covers at least 1/2 the blade. The closer the blade to the shroud the more pull you will get at idle and at speed. Make sure you allow clearance for engine movement. Mine is about 3/16" and I use the polyurethane motor mounts.

If it's mandatory to use electric fans, you will need to get the absolute highest rated CFM fan/s you can get and make sure they point toward the block and not down toward the cross member (will require a shroud of some sort). You need to get the hot air flowing out of the engine compartment. I think a lot of people would be surprised at the CFM flow of a properly operating OEM fan system on our year cars, where we like to keep the temperatur down to around 200 degrees or less.
GWHITE75 said:
I think a lot of people would be surprised at the CFM flow of a properly operating OEM fan system on our year cars, where we like to keep the temperatur down to around 200 degrees or less.

Absolutely true! NOTHING moves air through the radiator like a stock fan, shroud, and clutch; electric fans are a band-aid substitute for a properly-designed cooling system. It cooled fine when it was new, and will today; if you've added more motor since, you may have to add more cooling capacity (radiator), but keep the fan, shroud, and clutch in place. Almost every car brought to me with "cooling problems" has a butchered cooling system with no shroud, no fan clutch, and some Harry high-school anodized aluminum flex-fan or an electric fan that moves half the air the stock one did.
These ideas may have already been posted and I just didn't see them, but here are a couple more:

I know it's just too simple to believe, but is the fan running in the right direction? I've known two people that hooked them up backwards.

My son has used waterwetter in both of his Corvettes and it dropped the temperature quite a bit.

Ol Blue
JohnZ said:
Absolutely true! NOTHING moves air through the radiator like a stock fan, shroud, and clutch; electric fans are a band-aid substitute for a properly-designed cooling system. It cooled fine when it was new, and will today; :eyerole


My stock fan, etc does a good job cooling my Vette. I run 200 degrees on the hottest of Atlanta days as long as the car is moving. If I get in stop and go traffic then it gets up to around 225, which if not desirable is understandable.

I was looking at replacing the stock fan with electric, not so much for cooling as for the air conditioning. I have been playing with that for a year or so now and can get the air coming out of the vents down to 38 degrees with the car at speed. My problem is if you stop for any amount of time the temperature shoots up into the high 50's low 60's. Do you have any advice on this? Perhaps a pusher fan on the outside?

If you replace the stock fan/clutch/shroud system with an electric fan, I doubt if it will make any difference at all to your A/C, and your idle cooling (engine) will DEFINITELY suffer, as the electric fan won't pull anywhere near as much air through the radiator as the stock system will. C3 A/C systems were always marginal; there's a book titled something like "C3 Corvette A/C Strategies" that most of the resto parts houses sell that has lots of tips for getting all the performance possible out of the factory A/C system. I'd leave the engine cooling alone and focus on maximizing the performance of the A/C system.

I've got the book and have been following the guidelines in it with what appears to be good luck. At least I'm told by the a/c shops that 38 degree air on a C3 with an R134 conversion is good.

One of the areas that Davis discusses is inadequate air flow over the condensor when the car is not moving. I seem to have the seals, fan, clutch, shrouding and air spoiler that he discusses looking into and have been stumped on how to proceed further. The only other suggestion seemed to be an electric replacement primary fan or an auxillary fan to improve low speed air flow.

My knowledge is limited and would appreciate any other guidance you could give.

I think the source of the problem is the supercharger. It's packing in much denser air per cycle and the flame temperatures are getting much hotter, probably the cooling system just isn't efficient enough to keep up with it. This is certainly not the first time I have heard of a Chevy with a supercharger running hot.
mqms 68 I just checked to see if there were anymore repsones to your issues and I also tried to look at the pictures you had in the thread.

The pictures are a little small and hard to zoom, but it appears as though the radiator you have in the car is smaller than the OEM or the aluminum radiator I bought from Griffin. What are the approximate dimensions? Mine is about 26" wide and 19" high. In the first picture, it looks as though the radiator does not completely cover the radiator mounting bracket from left to right.

Also, to electric fan you have is definately blocking air passing through the radiator, which will hinder its ability to cool. Its very restrictive.

I had a 1980 Vette with a supplemental electric fan, but the cooling system was engineered to handle a greater heat load than the motor was producing without the A/C running, so the electric fan's physical presence did not hinder the system's ability to cool. The OEM electric fan moved a trememdous amount of air, minimized the restriction of air flowing past it, BUT it was not stand alone. It accompanied the manual non flex clutch fan.

Engines of our era were made to run around 185*, not like the newer, more effecient, fuel injected motors of today that are set up to run 200-225* under normal conditions. That heat is needed for better engine burn (emissions). Because of this, and the need to squeeze as much HP from an engine, electrical fans and composite manual fans are used, successfully.

Now, it's popular to think that one can achieve adequate cooling for the older style engines with electric fans and that is just not true, if you live in a hot climate, with a modified engine and you don't increase the cooling system's ability to take the heat out of the motor by more cooling capacity. You certainly cannot increase the stock system's ability by replacing a tried and true air moving system with one that moves less CFM of air, like the electric fan does.

I can e-mail you better pictures if you like.

The files are as large I can upload to this site.

so if you give me your e-mail I can send you better pics.


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