This is a customer's '72 Cutlass with a 455. I refreshed the engine (which had been rebuilt before) with new rings, bearings and gaskets after the crankshaft was replaced.
I'll list the engine specs, then describe the conditions.

.060 overbore
9.0:1 compression ratio
Edelbrock 2152 cam (214/ 224, .472/ .496" lift)
Holley single plane intake
Holley 750 double pumper
Holley Black fuel pump with bypass regulator, set at 8 PSI
Base timing set at 10 degrees, 36 degrees total. Big-cap HEI distributor.
FlowKooler water pump
160 degree thermostat
4-row core copper radiator, disassembled and checked by reputable radiator shop.
C5 Corvette dual fan setup, set to come on whenever there is oil pressure.
Longtube headers of unknown origin, 3" dual exhaust.
TH400, unknown converter stall speed. No additional cooler.
Unknown rear axle ratio, but somewhere in the 4._'s

Here's the deal: The engine temperature gradually climbs higher and higher as vehicle/ engine speed increases.
If you drive it around town, there's no problem.
Drive it on the highway at 65 mph or less, it's under 3K rpm, and it won't get over 220 (which I still think is entirely too high for this setup, but it's not overheating.).
But once you take it onto the Interstate and hit 75 mph (about 3,500 rpm), in less than five minutes it gets to 240 degrees, and you have to pull over, or get on the frontage road.
Once traveling at a lower speed/ rpm, the engine temperature gradually lowers.

We have tried three different thermostats, with no luck. The fans are getting full voltage. There is nothing blocking the radiator.
I have checked and reset the timing several times. Coolant is a 50/ 50 mix of distilled water and Ethylene Glycol.

At this point, the customer has put a lot of money into the car, and he's getting so frustrated that he wants to sell it.
He's been my customer for years, and I'd hate to see that happen.

If anybody has ANY questions about the combination, I'll gladly answer it. I just want to get to the bottom of this.

Thank you all.

 

My car does the same thing at highway speeds. If I keep it under 3200 RPM it can go all day long in 110+ temps with the AC on, but above that and the temp climbs. Basically not enough cooling system capacity for the heat being generated. Need a larger or more efficient radiator.

 

What distributor are you using and at what timing settings?

 

No worries, I believe I have found the fly in the ointment.

The C5 is an incredible machine but its fans are weak. We are surprisingly talking 1600 CFM for both fans combined

For comparison the legendary go to mod for hot rodders when wanting to go with an electric fan is a Lincoln Mark VIIIs. Its singular fan moves 2700 CFM.


P.S. Thank you for such a thorough post. Makes it so much easier to try and diagnose an issue.

 

Additional info.

https://www.corvetteforum.com/forums...iator-fan.html

https://www.corvetteforum.com/forums...pal-duals.html

 

And this should do the trick.

https://www.lmperformance.com/113407...SABEgKfE_D_BwE

 

One more for good measure.

https://www.corvetteforum.com/forums...fan-cfm-s.html

Better safe than sorry want to push that CFM a little harder how bout twice the factory specs. Here you go.

https://www.dewitts.com/collections/...products/sp015

 

I had something similar in the summer heat. Winter was never a problem. After trying everything I could think of, the only relief I could get was to remove the thermostat and plug the bypass. That did something.

 

Since it stays cool at low speeds, but overheats on the highway, either the airflow is restricted, or you have insufficient cooling capacity.

Is there a spring in the lower radiator hose? Is possible for the hose to collapse and restrict coolant.

Is all the proper shrouds and baffles in place? The shrouding is designed to funnel air thru the radiator. Without the shrouds, are can go around the radiator. Is the lower air baffle (some people call it a chin spoiler) installed? It’s purpose is to create a low pressure area behind the radiator. Kinda like cowl Induction for your cooling system. It’s common for the lower shroud to get damage from curbs, some people don’t like the look and take them off. They do serve a purpose.

Try putting the fans on a switch. The fans really aren’t needed on the highway. If the fans run all the time, it’s possible for them to actually become a airflow restriction. If the fans exhaust the air behind the radiator faster than it’s trying to enter the front, you will cut cooling capacity. Next time your out on the highway, try switching off the fans.

I use to run a factory 4 core copper radiator in my car. It would usually run around 190-200, and in hot stop and go weather might creep up to 220. I replaced the radiator probably 20 years ago with a BeCool. That some of the best money I have spent on the car. It doesn’t matter the ambient temp, highway, stop and go, running the air, pulling a small trailer, round robin at the track, the hottest I have ever seen the engine go is about 205ish. As I mentioned in a different thread, I installed programmable Autometer gauges in my car last winter. I have the warning lamp set to come on at 215. Not that is too hot, it’s just because if the engine does ever get that warm, something is wrong because it’s so abnormal.

What pulleys are you using? Maybe you have a pulley mismatch and are over/under driving the water pump?

Last thought, have you actually verified the engine temp with a different gauge? Many years ago I swapped in a 455 to replace a junk 307 in a 87 cutlass. The car had factory gauges, the original engine didn’t overheat and the factory rally pac was accurate. The 455 would get warm on the highway. I did pretty much everything you have done trying to fix it. Turns out that the replacement temp se ding unit I installed to replace the factory part I broke during removal didn’t have the same resistance scale as the original part. I spent a bunch of money and time trying to fix a overheating issue that wasn’t there. The temp on the gauge matched within a few degrees of the thermometer in the radiator, I assumed it was accurate. Once the coolant temp went above 200 degrees, the resistance of the factory sender and replacement part started to really change, screwing up the temp gauge.

 

My car has similar behavior. If its above 80ish and i get on the highway, after 15 or so minutes I get off as the temps are climbing (220 ish) and i dont want to wait n see where it stops. Around town idling or cooler weather there are noissues

i agree its cooling capacity, my car still has my sbo radiator even tho it now has a bbo. In your case tho you have a 4 row radiator. As matt pointed out there should be baffles on both sides of the rad and under the rad to direct airflow. If the ac condenser is still in front of the rad and is clogged w bugs n leaves that isnt helping. If the e fans have a flat type shroud vs the factory cone like shroud that can limit flow. 69shc pointed out the fans arent great either.

so if your 100% sure the rads good id start looking at all the little things that add up.

 

Matt69Olds, Fun71, Retro, I hear you guys with regards to insufficient cooling capacity. Makes sense, but his checklist indicates it is sufficient.

What's a screaming eyesore to me is the insufficient fans. He has a 60 over big block that's turning 4200 RPM or worse at 75 MPH. I know he mentions 3500 RPM at that speed but that doesn't compute.... Unless he has a ridiculously tall tire...

Maybe it is the air dam effect that you mention Matt, and maybe its what SuperChevy mentions here. "We once saw a Pro Street '66 Nova electric fan that was wired backwards. It somehow ran OK in town but it overheated on the highway. Its puller fan was pushing air, causing almost no air to pass through the radiator." http://www.superchevy.com/how-to/eng...g-system-tips/

One way or the other it comes out as fan related...

 

Fans aren't a factor on the highway. They're only a factor at low speeds. For reference, at 75 mph (6600 ft/min) and a 28"x18" radiator - you're getting a theoretical 24,102 CFM. Now, due to aerodynamics you're not going to actually get that through the radiator, but it still stands that fans will come nowhere near capable of matching highway speed airflows.

Yes, IF you somehow manage to install the fan wrong you'd interrupt that flow IF you left them on when on the highway...but that's pretty easy to check - is it blowing the right direction in the driveway? Then its fine.

I do agree his RPM or gear ratio estimate seems off.

 

Is this one of the older flowkooler pumps with the riveted plate on the back of a stamped impeller, or one of the new flowkooler pumps with the CNC machined aluminum impeller?

A big block with 4.xx gears on a highway rejects a ton of heat. You want to make sure all the radiator ducting and flaps are in place. A new aluminum radiator will greatly exceed the heat transfer performance of the original brass/copper. The original 4 row has 4 rows of 3/8" tubes - that's 1.5" of total tube depth contacting the fins. A good aluminum radiator has 2 rows of 1" tubes - that's 2" of total tube depth contacting the fins, a 33% increase in heat transfer area (and they have options for 2 rows of 1.25" tubes as an upgrade).

DeWitt's makes some of the best stock-appearing radiators out there, you won't regret spending the money:
https://www.dewitts.com/collections/...adiator-manual

I would upgrade the fans too, but that's not likely the root cause of the issue on the highway.

 

Good stuff 83hurstguy. DeWitts is new to me, glad to learn about them and glad to hear about there solid reputation.

I agree with you guys, cant argue with what you are saying. I guess the 1600 CFM thing sent me for a loop. Honestly I thought it was a misprint, surprised it can cool that engine at low speeds.

Lets hope the OP checks in on this thread, would love to know what the issue ends up being.

 

Thanks for all of the responses, guys!

It's an HEI with the coil in the cap, not original for this car. Base timing is 10 degrees, 36 degrees total. Timing pointer and balancer mark are accurate; confirmed during the build.

No spring in the lower hose, but I had my son rev the engine up to almost 4K (at operating temperature) and tried to squeeze/ collapse the lower hose, and it was still sufficiently firm, so I don't think that's the problem, but I won't rule it out.
It's using factory stamped pulleys, the chin spoiler is in place.
I have verified temperature with two Autometer mechanical gauges, and a cheap part-store mechanical gauge. They were all within about 5 degrees of each other.

About the fans: We ended up going to electric fans after trying two different fan clutches AND a solid fan spacer (with the factory shroud in place).
We decided to go with the C5 fan assembly because it was an almost perfect fit, and we figured that if it could cool modded 'Vettes on the race track, it should work fine for a mild Big Block.
I'm well versed with the MK VIII/ Mustang fan; I have one in my '89 Trans Am. I don't like how much power it pulls, though.
I figured that two smaller fans (each with their own relay) would be better.
The fans' polarities are correct; I made absolutely certain of that when doing the wiring, because I have seen incorrectly wired fans cause all kinds of weird problems.
With that said, I don't think it's a fan issue, because this is only happening at highway speeds.


It's got the billet impeller.


One last thing to mention: radiator cap is new (16 psi), and the cap mount/ sealing surfaces are good, so no chance of leakage coming from there.

 

With your base timing at 10, you should not be getting to 36 total with the vacuum hose disconnected at the cannister without mods. Retarded timing will cause the engine to heat up. I'd bump the timing to 16/18 BTDC.

 

As all the usual stuff seems to be covered I’m trying to think of other possible culprits, even if a bit far fetched.
Questions that come to mind that haven’t been touched on-

1) Piston brand, model, type and clearance?

2) Actual rear end ratio?

3) Running vac advance? If so how much?

4) OAI Hood with no seal to air cleaner?

 

I got the 36 degrees by: Base (10 degrees), vacuum (with a hand vacuum pump on the vacuum diaphragm) + centrifugal (measured at about 3,500 rpm).
I forgot to mention that it's running ported vacuum, not full manifold vacuum.

 

It appears you are calculating the total advance with the vacuum advance added in. That is not correct, the total needs to be with out vacuum advance. Too little advance will cause the engine to run hotter.

 

I was wondering the same thing. With the initial, centrifugal, and vacuum advance I would expect around 45-50 of total timing. This may well be the root cause of his problem.

 

It does not matter whether your running ported or manifold although manifold is preferred for your vacuum advance. First you need to set your timing to 34-36 at the rpm at which the timing stops advancing. Then check and leave the initial at what ever it winds up to after setting your total. It should be around 16-18*, then add your vacuum advance which according to your numbers is around 8-10.

 

Have you considered changing pulley diameters. Maybe the pump is turning too fast or slow at highway speeds.

 

Wow ok there goes my idea. Sorry pal I thought I was onto something.

I agree with you I would've figured the C5s fans would be kick ***. I had no idea about there rating till I looked into it just before posting. For some reason it jumped off the page at me. Maybe its my bit of distrust towards electric fans. Have a friend with a built 454, spent plenty on the cooling system and also has complaints about the car running too warm.

In any case I was wrong as this is not the issue.

I'm thinking the gear ratio discrepancy is speedometer error. If he has 4.11s a TH400 and factory height tires 3500 RPM is around 65 MPH not 75 MPH.

(My speedometer is off 10 MPH at highway speeds...)

 

It sounds as if the coolant flow is to fast.

 

Oh boy.

 

Okay guys, bad news: He sold the car a couple of days ago.
Apparently he had actually been wanting to get rid of it for awhile, and someone made him an offer.
He mentioned a few other issues (body work, etc.) that he didn't feel like dealing with, so at least the heat issue wasn't the dealbreaker (which makes me feel a little better).

With that said, he gave me the guy's number, so maybe I can still do some research and check back with some answers.
I appreciate all of your help, and I hope to check back here soon.

 

Well then...

 

Glad to hear that, this is a little too late but I will post for good measure.


Spoke to my friend who I mentioned previously has a built 454 that also runs on the too warm side.

After I explained to him the issue at hand of overheating at 60 MPH and up. He suggested several possible causes, that matched what many of our fellow members have already suggested.

Its not a fan issue because of speed at time of occurrence.

Sounds like a flow issue.

Could be a timing problem.

Could be a carb adjustment problem.

Could be a lack of proper wind funneling issue between the grill and radiator.

Maybe a water pump issue.

 

Yeah, I can post a pic of my Flowkooler pump with the riveted back plate on the pump. There is a big gap between the impeller edge and pump body. The factory closed impeller or the new CNC impeller Flowkooler pump will work much better, it never helped my 403. At least 3 other places sell the stamped tank rads for cheaper. Autocity with 1 1/8" rows https://www.autocityclassic.com/chev...ore-sl-282-at/Cold Case with 1 1/4" tubes https://www.coldcaseradiators.com/pr...mance-radiator and Engineered Cooling Products http://www.speedcooling.com/1968-197...1-4-TUBES.html