Hey guys, I'm having the wierdest overheating problem and I could sure use some second, third, etc. opinions.

I have a 409 SBC in my '76. Here's the combination:

• GM 509 iron block (fresh)
• 10.56:1 compression
• .041" quench (dish pistons though)
• 93 octane gasoline
• GM Performance 19168406 aluminum water pump (new)
• "heavy duty" fan clutch (new)
• Stock 7-blade rigid fan
• Stock fan shroud
• A/C condenser removed
• 4-core brass radiator (new)
• Gates hoses (new), spring is present in lower hose
• HEI ignition, new distributor
• 180 degree brass high-flow (new, and replaced again with new)
• Quadrajet carburetor 17058504, set up as per Cliff Ruggles' recipe #2
• Temperature gauge is AutoMeter 4455 electric (new)
• Temperature sending unit is mounted in left cylinder head
• Walker/Summit 1-5/8" full length headers, free flowing exhaust
• Stock "chin" spoiler is still there and in good condition
• 50/50 water/ethylene glycol antifreeze mix
• 3.50:1 rear end gears, TH350 transmission

Here's the wierd thing- in town, it stays cool and drives fine. On the highway at any speed over 60 MPH, the temperature slowly creeps up endlessly. A/F ratio at cruise is just under 15:1 (using new Innovate Wideband LC-1 wideband O2 sensor) and I have tried total cruise timing (initial + vacuum + mechanical) everywhere from 50 degrees BTDC to 32 degrees BTDC (yeah, all over the map, in 2 degree increments).

It never seems to boil over and leak, it just gets HOT. By HOT, I mean I just can't stand it anymore and slow it back down at 230 degrees Farenheit. I have confirmed the temperature gauge is in the right ballpark with an infrared Raytek temperature measurement.

I can't seem to get this problem to disappear.

Speculation:
Since the temperature seems to be OK in town, but overheating on the highway, I don't think the clutch fan is inadequate- but I'm wondering if it's actually too stiff on the highway- not allowing air to flow freely from the nose through the radiator but actually being blocked by the fan. This heavy duty clutch fan might be slowing down the airflow (not overspeeding as a lighter clutch might allow). It just "seems" like an airflow issue to me.

Once I get a chance, there are three experiments I want to try:

1. Re-install stock fan clutch in place of heavy duty one
2. Try opening the hood to see if trapped air can escape (not sure if the high pressure air in front of the windshield or draft out of the sides of the hood will dominate this test)
3. Flip headlights up so that they're not right behind the nose grilles

Am I overlooking anything? Who else has run into this problem? I'm used to fixing cars that overheat in town but are fine on the highway- but this is the first time I've dealt with this in a C3. The original engine never did anything like this, and at steady highway speeds this engine wouldn't be making any more horsepower than the stock L48 did. The rear end gears are a little lower (3.50 now vs. 3.08 stock), but it's not that big of a difference- a lot of us are running 3.73 gears routinely.

 

Is your front air dam missing? If it is, the air is going under your radiator not thru it.

Just noticed you said it was.

Did you get all the air out of the radiator?

 

It's not missing, it's still there (not very big, but it's there) and it's in good condition.

I think so. I even replaced it (I first went through this with the original radiator) because I figured the original radiator was just somewhat plugged after 30+ years, but the brand new one acts exactly like the old one did.

 

New guy here but not to the 400 small block, you may have air in the cooling system. Did you lift the front of the car when you added your coolent, this is so the air can come out to the top of the cap on the radiator. You may have some steam pockets in the block after the radiator change

 

I know these are default questions, but you have the shroud and seals in place, right?


after reading, i see the shroud is... and the fan pitch is going the right way, right?

 

Could be air in the system but it would overheat running around at low speeds too you would think. My cools down when it is moving and the electric fans will not come on if I am driving on the highway on a hot day. It won't go over 175degs or so.

You have to make sure your air dam is deflecting the air properly across the rad at speed. You are lucky in a sense that you are not overheating at low speed with a 400 block

Also I would richen it up if nothing else works, take it down to the 13-14:1 range our old gen 1 motors usually don't like it that lean and I am surprised you don't have any lean misses or surges

 

You need a better radiator, like an aluminum one!

 

A while ago I needed to replace a headlight and when I took off my headlight covers I thought with the headlights popped up and with the covers off , the rad would have 2 nice little air scoops rolling along the highway. I don't have a heating problem so I don't know 100% if it would work , but hey give it a try. Its not like its going to cost you anything.
My own guess is that you are running just a little lean though .

 

i say take your fan off and go for a highway drive but i dont think its your problem as you wrote its a clutch fan . i think if it was a fixed fan then it could have beeen your problem.

 

a copper/brass 4 core can cool to 160 just as an aluminium one can.

 

Great suggestions guys, please keep 'em coming.

Air bubble
I understand what you're saying northern77, and it's a good guess, but I really don't think I could have exactly duplicated this every time it was refilled- with the old water pump, the new water pump, the old radiator, and the new radiator. I've filled it so many times you'd think I got it right at least once. I have had this 400 since 1986, although I did freshen it up before putting it in the '76 vette in October 2008. Maybe I'm wrong, so here's my procedure in case you see something I'm overlooking:

I usually fill the block by keeping the front of the car elevated, removing the water neck and thermostat, and filling the block through the radiator until the water fills the water neck hole. I carefully keep filling until the intake is almost filled to the brim, carefully put the thermostat in place, reinstall the (iron) water neck with a new gasket, and top off the radiator until it's just below the radiator cap while milking/burping the upper radiator hose. This has always been a very effective way to make sure there was always an uphill escape route for air and I've never had to add more coolant to the puke tank after the block was filled this way. It would have been hard to fill it just good enough to cool in town but not on the highway.

Radiator
I don't have cooling data for copper/brass vs. aluminum radiators, and I do see that the aluminum radiators do have the benefit of outstanding marketing. However, I do notice that copper has a much higher coefficient of thermal conductivity than aluminum, and dramatically more than aluminum oxide-

Aluminum- 255 W/(m *c K)
Aluminum oxide- 30 W/(m *c K)
Copper- 400 W/(m *c K)

Source: http://www.engineeringtoolbox.com/th...ity-d_429.html

Based on these material properties, if everything else remains equal (construction, capacity, etc), a copper radiator is superior to an aluminum one at a ratio of approximately 2:1 as far as being an effective heat transfer device. This doesn't help us shed weight, but it does help keep the engine cool. On paper, I'm with gingerbreadman1977. All that said, a lot of people say that a kick-butt super duper pure inobtanium coated aluminum radiator was a key element in fixing their overheating problems, so maybe there's a secret in how they're made. I hope I didn't make a mistake in ordering my new 4-core copper one.

Fan direction
It's drawing the air through the radiator blowing the air towards the motor.

Air dam
Got any pictures of a prototype or example of a well done one? Making a more effective one might be the way to go. The one I have is the stock one, and I've never done anything to/with it.

Other
I'm real tempted to pull off the fan and clutch and take it out on the open highway. Unfortunately, there are several stoplights and a few miles of road between my driveway and the interstate, so I'm a little concerned it wouldn't make it back from the highway to my house after a good flogging on the interstate. It's very tempting though. Maybe I could reinstall the fan on the side of the road, but that can certainly be dangerous.

I haven't tried any kind of "water wetter" or anything similar. Are these snake oil or are they really effective?

I'm really puzzled by this. I didn't even know I had a problem until I took it on a road trip. In town it's fine. I've never seen overheating issues that only crop up on the highway. Maybe I do need to richen it up a bit at cruise. What AFR are you guys getting at cruise?

 

I think the 400 sbc has steam holes that are supposed to be drilled into the heads and will run hot if those holes aren't there. What heads are you using? If they're aftermarket, do you know if the steam holes were drilled in them?

Just a thought.

 

Copper/brass radiators work fine when the outside temp is under 70 degrees. Over 70 degrees aluminum is the only way to go. Dewitt's

 

Oops, yes, I should have mentioned that.

I'm running Edelbrock Performer RPM aluminum aftermarket cylinder heads on this engine. I drilled the six steam holes (three per head, only the lower/outer ones as per Edelbrock's instructions). The part number for these heads is 60899.

 

My issue was the fan was wrong, it did not have the correct # of blades, also was flattening out at high rpm's. I was always told the 400 was a bad combo in a corvette because the bores are closer together and create more heat. More than the stock airflow/cooling system will allow.

 

Are the big blocks easier to cool than the 400?

 

A tranny cooler might help.

On my 1st vette, the air dam was missing. I temporary put a piece of 3 inch wide tin across the bottom of the radiator. it worked fine.

 

Two things I disagree with...

*Your clutch fan theory...
The cooling fans definetly help at highway speeds, contrary to the theory that a ton more air is coming in then the fan creates. I believe the fan helps direct all that air THROUGH the radiator.

I just tested this two weekends ago on my own car that runs hot on the freeway. My car had been running 195-200* at highway temps in 88-90* ambient temps. I run the 7 blade clutch fan and used a new Dr Rebuild Delco replacement fan clutch($130)...the clutch tests good by all accounts...(less then 1 rotation after killing the engine when hot)....I had a local recommend a Hayden Severe duty 2799 fan clutch and said it helped his on the highway.....Skeptical, but I went and bought one and bolted it on. It was quite a bit stiffer then the Delco clutch and instantly dropped 5* off of my highway water temp. I tested the car twice, second time was at 92*....a solid 5* reduction in water temp on the highway. If you take your fan off and run it, I guarantee you it will run hotter on the highway. On a 30 mile highway run mine was stabilized at 190-192* in 92* temp. This with the original copper/brass radiator. This temp is still too hot for my liking so I ordered a Dewitts
radiator and will be installing/testing it tomorrow. I will let you know if it helps.

*My second disagreement is with your AFR at steady cruise. I'm my opinion running near 15.0 is very lean and is probably detonating the crap out of it...I can't believe it's not missing with it that lean. Richen your cruise AFR to mid 13s and it could very well lower your running temps. Mine is about 13.2-13.5 on average at freeway rpms(3000).

My motor is a 650hp BB with air conditioning......all temp testing I have done was with the AC ON.

Good luck. Keep us posted.

 

The total package of brass rads are a little less conductive than most alum rads because of the of solder joints. Fin spacing and tube id size also enter into it.

You can just add to the stock spoiler with some rubber or flex plastic and screws or bolts.
Doc Rebuild sells the GM add on extension that GM had available back then. It will help, both for air in and air out.

The clutch fan is designed for temperature operation. I know a lot say that you don't need it over 30 mph or so, but it's not that simple. It operates on radiator air heat which depends largely on ambient temp, so you may hear it at any speed.
Some times down here, I don't hear it at 60mph, other days it will roar, depending on the ambient temp. Bottom line, if it is still roaring at 70mph, you re getting plenty of air, no matter what the fan is doing (See above).

I use a Stewarts stage 2 pump and run at tstat temp all the time in S Florida and I didn't have a chin spoiler for 2 years, still don't have any of the foam.. You may try that or plug the bypass in the one you are using. Be sure to use the pressure balanced tstat with bypass holes if you do.

You can cool down considerably with your fuel mixture.
You can try all the "no cost" tests like the hood open and removing the splash guards etc.
What rpm is 70mph?

 

Fantastic information. The fan clutch thing was just a left-field theory based on the confusing bits of information I am/was working with. I have been pondering this all winter, and have just in the last couple of days taken the car out of storage.

I need to get it back out on the highway for more testing. I can certainly fatten up the cruise AFR and see how it behaves. I couldn't hear pinging with the T-tops off and the exhaust roaring (nor could I hear much of anything) but there is no question it could have been happening undetected.

PLEASE DO keep us posted on what you find with the new radiator. I might have to save up for one of those and try to return the new copper one I have now. Nothing sorts the wheat from the chaff like real data.

I'll give the adjusted cruise fuel mixture a try. What's a good target AFR for cruise?

I'll also check into the extended chin spoiler. Maybe I can fabricate something or retrofit something from a junkyard car for a test.

I don't remember my RPM at 70 MPH, but I can check that the next time I get it out.