BBC temp creeps at idle; how did factory do it?

[jumpsoffrock]

How'd these trucks stay cool from the factory with AC?

My factory 402 truck does not have AC from factory. I took it upon myself to find a 4 core brass radiator out of a GM vehicle, this rad is almost identical to the factory 4-core for these trucks. Radiator shop said it was in "fantastic condition", they were rather surprised. Didn't need any work.

Thermostat is 180º

I use a mechanical gauge with sender placed at thermo housing.


Fan was upgraded to a giant 5-blade 20.5" metal fan out of a 1972 Buick riviera that had AC. Everything looks factory.(According to internet non-AC '72 Riviera's had a four blade fan). This fan fits in the factory shroud perfect, partially in/out, it's like they were made for eachother.

I also grabbed a 7-blade steel fan from a guy who has these trucks all over his yard.

Both fans have a nice pitch to the blades--these things were made to move air.


After installing and testing both fans on near 100º days, they had similar results and one issue:
They will NOT keep engine sustained at any temp at idle(7-800rpm). It always creeps up, haven't let it go too high because the creep is slow and it would take forever(good thing).
If at a stand still I bump the gas pedal to 1500 rpm it cools right back down in less than a minute at that engine speed.

After extended 3000rpm driving today(98º) I pulled over immediately and the engine was at 190º. I sat at idle for a solid 10 minutes and it crept up to 210. At this point I put it in granny gear and drove 5mph at 1500rpm, in a minute it was down to 190º and stayed there without issue.

As for as I'm concerned this is simply an issue of lack of air movement through the radiator at idle.


If my truck creeps 30º or more on a 98º day and it doesn't even have AC, how did these trucks with this engine and factory AC keep cool? Did GM just not expect anyone to sit at idle for extended periods?

I see guys in work trucks sitting at idle with AC blasting for half an hour today, I can't imagine people didn't do this 50 years ago. I can't imagine GM simply "overlooked" sitting at idle with AC on like nobody does that.



Did these trucks slowly overheat from the factory?

[dmjlambert]

My truck is close to stock with a 350 and I can idle it or drive it down the highway, winter or summer, and the temperature stays rock steady all the time. So I think if you have a working system it should idle fine.
You didn't mention, do you have a thermo fan clutch? Is the water pump old? Is the radiator cap new or the original old one that came with the used radiator? Is your distributor vacuum advance connected to manifold vacuum or ported vacuum?

[Steeveedee]

I went round and round with my truck running hot, something it has done since I met my wife 41 years ago, when it still belonged to her parents. Bigger fan, custom shroud (because the fan clutch is about 1/2" from the radiator, which is a different problem) severe duty fan clutch, blah blah blah. It wasn't until I put in a high flow thermostat that it was tamed. The AC compressor clutch shoe drags, so I have the belt off until I get that replaced. It doesn't creep up any more, and starts better hot- since it isn't hot enough to sear steak on the intake manifold, anymore. We'll see what happens when I get to that shoe and can run the AC again.

[jumpsoffrock]

Yes I have a Hayden 2747 HD fan clutch.
Water pump says Made in USA and is painted engine color, so I'd say it was replaced when they rebuilt the engine an "unknown long time ago."(what I was told)

Rad cap was given to me, looks newer, nice condition 16 lbs
Vacuum advance is hooked to bottom of carb, so Ported vacuum.
I believe I reset timing to 10-12º BTDC

Thanks for your input Steve.....although it sounds to me this running hot thing might be normal....

[dmjlambert]

You might try switching to manifold vacuum for the distributor and see if your engine runs cooler at idle. I've heard it makes a difference. Changing between ported and manifold vacuum affects idle speed, so you'll need to adjust your idle speed screw. And pull off the water pump and see if the impeller blades are rusted away would cost you just some time and a new gasket.

[Stocker]

Originally Posted by jumpsoffrock

Yes I have a Hayden 2747 HD fan clutch.

2747 is an excellent heavy duty fan clutch for most circumstances. In your case, though, you *might* benefit from a severe-duty unit.
Differences are explained here: https://www.haydenauto.com/en/produc...-types/thermal

I am curious about your 5-blade 20.5" Riviera fan. Small-block trucks use an 18" fan. Mine is 7-blade and works well. I was thinking big-block trucks use a 19.5" fan.
At any rate, since you have a 7-blade, if it fits your shroud, maybe it's worth trying that before considering a severe-duty fan clutch.

[jimijam00]

210 isn't that big of a deal.

I don't know anyone that sits at idle for 10 minutes other than taxi drivers at the airport.

Is your shroud sealed to the radiator?

How far is the clutch from the rad?

Have you run the fan w/o the clutch?

[jumpsoffrock]

Originally Posted by Stocker

At any rate, since you have a 7-blade, if it fits your shroud, maybe it's worth trying that before considering a severe-duty fan clutch.

I have run these idle tests with both fans at this point. I'll look into the severe duty clutch

Originally Posted by jimijam00

210 isn't that big of a deal.

I don't know anyone that sits at idle for 10 minutes other than taxi drivers at the airport. Every water service truck, Utility service truck, delivery guys routing their next move, internet service guys--all these people sit in their trucks for ages. Can't imagine this is a new thing

Is your shroud sealed to the radiator?
Very well, made sure of that when first fitting everything to the new rad
How far is the clutch from the rad?
I haven't measured but it can't be much different than factory
Have you run the fan w/o the clutch?

The original setup was its original 3 core rad with a 5 blade fan and no clutch. It did everything I've stated in this thread, only worse. It would take much longer to cool back down and would heat up at idle much faster.

[72c20customcamper]

210 isn’t hot for a big block . Where is the probe installed? If it’s in the head and not on the manifold 210 is normal as the head temp will always be hotter than the rest of the water. My 496 with a mechanical sensor in the head will creep up past 210 to 215 range before the fans kick in . The fan sensor is in the radiator and is set to come on at 195 and off at 180 . So the actual water temp in the system is cooler than the small amount in the head .

Has your truck actually overheated?

[Mike C]

djmlambert is on the right track. You can Google a search for Lars white paper on big block Corvette ignition timing and engine cooling.

[jumpsoffrock]

Originally Posted by 72c20customcamper

210 isn’t hot for a big block . Where is the probe installed? If it’s in the head and not on the manifold 210 is normal as the head temp will always be hotter than the rest of the water. My 496 with a mechanical sensor in the head will creep up past 210 to 215 range before the fans kick in . The fan sensor is in the radiator and is set to come on at 195 and off at 180 . So the actual water temp in the system is cooler than the small amount in the head .

Has your truck actually overheated?

Sensor is in the thermo housing.

You make an interesting point about BBC temperatures, because when I decided to change radiators a few weeks ago I did LOTS of reading and there was about a million threads all over the internet saying two things: "Why does my BBC runs hot?", and "BBC's tend to run hot".

No it has not actually overheated. Some of the threads had guys who regularly ran their engines at 220-230 and made no issue.


My problem with all this is, I am expecting to make this truck a prolonged daily driver. I live in a hot climate, and expect to make many trips in deserts and mountains.

What if there is some crazy traffic jam on my way to vegas in the summer and I'm left sitting still for 10-15+ minutes in 90-100º weather? I need to know this truck isn't gonna let me down. My modern car doesn't cry and moan when I sit still with AC--but that cars on it's way out the door, so this truck has to behave well for me to be able to rely on it. (maybe I'm the one crying and moaning and not my truck)

[weq92f]

.

My 402 with the A/C running on a hot day will creep at idle to about 215-220. This with two 14" fans pulling tons of air through the rad and the condenser.

As mentioned, confirm your timing then let the thing creep. I suspect it will stabilize at a temp around 200 with no A/C. If you add A/C that temp will be higher.

Before I upgraded to electric fans, my fan was running straight off the pump, no clutch. I was afraid of the temps at idle and would often do as you; increase the RPM to cool it down. My rad is the old school 4 row.

Hth,

-klb

[Davidf]

My bet is that the used radiator is not as good as you think/hoped. Get a new radiator.

I had similar issues with my '71 350 sbc (with A/C). Tried every trick in the book to solve the problem. New radiator dropped engine temps drastically and temp creep at idle was a thing of the past.

[Carl Spangler]

Get a new 4 core copper radiator! Run no more than 50:50 glycol/water and make sure the block is purged of all trapped air: run heater, burp hoses, etc.

[jimijam00]

Originally Posted by jumpsoffrock

My problem with all this is, I am expecting to make this truck a prolonged daily driver. I live in a hot climate, and expect to make many trips in deserts and mountains.

What if there is some crazy traffic jam on my way to vegas in the summer and I'm left sitting still for 10-15+ minutes in 90-100º weather? I-need to know this truck isn't gonna let me down.

Nothing to worry about. Don't sit at idle and hit 210 if you are afraid of it, though it isn't a big deal as I and others with big blocks have stated. Just turn it off for that 10+ minutes. In that wicked summer heat you have, drop your ratio of water to antifreeze down to 10-15% green stuff and the rest distilled water. Your truck has not failed you or overheated yet, so drive on and enjoy.

[jimijam00]

Originally Posted by Davidf

My bet is that the used radiator is not as good as you think/hoped. Get a new radiator.

Agreed. An alum with 2 1"+ cores will almost certainly out perform that 50 y/o oem stuff.

[HO455]

Originally Posted by jumpsoffrock

How'd these trucks stay cool from the factory with AC?

Did GM just not expect anyone to sit at idle for extended periods?

I see guys in work trucks sitting at idle with AC blasting for half an hour today, I can't imagine people didn't do this 50 years ago. I can't imagine GM simply "overlooked" sitting at idle with AC on like nobody does that.



Did these trucks slowly overheat from the factory?

Having spent half of my youth in Nevada (Outside of Reno) I can say that A/C was not a common item 50 years ago. It wasn't installed in most buildings and very few homes of the time and was an expensive option on vehicles. Even when installed on vehicles it affected gas mileage so much many people only used it on the hottest of days. And idling the car for 30 minutes just to run the A/C is something that we never did in the one car we owned that had a dealer installed unit..
I remember the family going to the casinos in town on 100 degree days just to walk around in a cool place. Or standing in the door of a grocery store freezer to cool off as the store's swamp cooler would only bring the store temperature down to 75 degrees when it was 90 or more outside. That grocery store was only 10 or 15,000 square feet. I find it amazing that today the same store is 150 to 200,000 square feet and perfectly climate controlled. Is my gray hair showing?
When these trucks were designed in the mid-sixtys stop and go traffic wasn't really a problem and trucks were not used for commuting. So as far as GM was concerned if your big block stayed cool going down the road fully loaded they were happy.

[Stocker]

As mentioned above, 210* is far from overheating. I understand your concern about it creeping up very far above 210* in extreme conditions, but keep in mind that at zero pressure, water boils at 212 and a 50/50 mix boils at 226.
Just 8 psi raises water's boiling point to 233 and 50/50 mix to 248.

[Steeveedee]

FWIW, my truck has gotten as hot as 260F pulling a 6000 pound travel trailer up a steep grade without blowing a head gasket, and this rebuild has 100k+ miles on it. It didn't make me happy to see that, but it survived.

[jumpsoffrock]

I supposedly have a modern aluminum 2 1" core coming, back ordered till the end of June, we'll see. I bought it first, then saw this original radiator at a junkyard and figured they aren't that common to see so I bought it.

Yes it is true that AC is more prevalent and used more willingly today.
My coolant ratios are a bit mixed from removing the radiator a few times, so maybe that's an issue is well.
Steve that's nuts that it got that hot, but its pretty fun knowing your ole truck can still do it. What temp was the truck running when you were cruising on flat ground on that trip, just for reference? Had it ever gotten hot like that before?

[67ChevyRedneck]

What a lot of people forget, is these trucks were brand new at one point. Almost nothing on them is "factory set". We tinker with carbs, change air hats and filters, eliminate emissions stuff, change timing, etc. I don't know what affects it would have, but was also don't even run the same types of fuels and oils as when these trucks were new. I believe the factory timing setting back when leaded fuel was used was 6-8* at idle. Now, if you run HEI and modern fuel it's closer to 10-12*.

And as mentioned above, 210* is fine. I had to learn and accept that.

My truck has aftermarket A/C, 7 blade? clutch fan, and big 4 row. It runs about 195*-200 on a hot day if I have the air off. If will run 200-210 with the A/C on. At idle, it will cycle between 205-210. It will run 195-200 down the road. I think my T-stat is a 190. I've found it's better to run a hotter T-stat because in SC it's going to run that hot anyway. It's better to let the water have more time to cool in the radiator with a higher T-stat than to have it constantly running through the radiator trying to cool.

My 2015 Z71's operating temp is 210*.

[Davidf]

Just for informational purposes. As I mentioned, my sbc would creep way up in temp when sitting at idle, a/c on, in traffic. Start driving and temp would come down to about middle of gauge or slightly above. Then, took the truck off the road to do rust repair and paint. One year later, I put the truck back together with a new smaller (not heavy duty) 2 core aluminum radiator. Now, it seems my temp sender is broken. Temp gauge has never made it to half way. Stays in lower range of gauge and never goes above half way mark no mater the outside temp a/c on or off.

It was my old radiator causing my temp issues!

[Steeveedee]

I don't really remember, I was just worried about the high temps.

I worked for Chevrolet dealers on and off in the early '70s. I remember the '71 SB400 engines, even in the big wagons with AC, were getting 2-row radiators. We had a bunch of them in the service department with toasted engines, getting retrofitted with bigger radiators. I don't remember if they simply replaced the engines or rebuilt them, but some of them were pretty crispy inside. So GM built some marginal cooling systems on occasion.

[MikeB]

In your part of the country, I'd use 40/60 or even 30/70 (antifreeze/water). Water transfers heat better than antifreeze. Then check the chart to select a pressure cap with a boiling point above 230. I'd also consider a high-flow t-stat from Robertshaw or Flow Kooler. And if you don't have a recovery tank, get one.

But don't get caught up trying to keep the temp at 180 degrees. 190-200 is just fine, and does a better job of causing condensation in the crankcase to evaporate.

One more thing: Some folks will say aftermarket aluminum radiators are junk. However, aluminum does a better job of transferring heat. On my square body, I bought a 2-row aluminum radiator from O'Reilly Auto with a lifetime warranty. Even through five Texas summers it kept my 350 in the 190 range, and no higher than 200 at a stop light. The truck did have a new AC Delco water pump and a clutch fan with shroud.

There were two larger radiator sizes available, but I bought the smaller one after speaking to a guy who used it with a blown 383! And I already had brackets and shroud for the smaller one.

My truck's water temp gauge was a decent quality Autometer electric, but one of their "full-sweep" mechanical gauges would give you better resolution.

https://www.autometer.com/2-5-8-wate...80-f-mech.html

[Stocker]

Originally Posted by MikeB

And if you don't have a recovery tank, get one.

Yes, absolutely. And if you get a recovery tank, you need a radiator cap for a sealed system so the coolant can be drawn back into the radiator as it cools.