Cooling Question

[XfireZ51]

Ok, last year I installed a Fluidyne rad to keep temps down particularly when in traffic w A/C on. Worked great have however, with ambient temps below 60, it was difficult to maintain operating temps on motor. At cruise, coolant temps would drop into the 150-160 range. Which is not desired if you want Closed Loop operation w Learn Control. So I finally bought a stock therm from Jerry's and had it installed. turns out the thermo coming OUT of the motor had 5 holes drilled in it. The stock thermo was installed w 0 holes.
The result is that coolant temps increase now unless motor is turning above 12-1300 rpm. That includes at highway cruise. So I know I don't need 5 holes in thermo, but I'd like to avoid doing this again. How many holes should I put in stock thermo so that fans can maintain steady operating temps?
I should add that I have under drive pulleys and temps creep up they don't shoot up.

[secondchance]

Ok, last year I installed a Fluidyne rad to keep temps down particularly when in traffic w A/C on. Worked great have however, with ambient temps below 60, it was difficult to maintain operating temps on motor. At cruise, coolant temps would drop into the 150-160 range. Which is not desired if you want Closed Loop operation w Learn Control. So I finally bought a stock therm from Jerry's and had it installed. turns out the thermo coming OUT of the motor had 5 holes drilled in it. The stock thermo was installed w 0 holes.
The result is that coolant temps increase now unless motor is turning above 12-1300 rpm. That includes at highway cruise. So I know I don't need 5 holes in thermo, but I'd like to avoid doing this again. How many holes should I put in stock thermo so that fans can maintain steady operating temps?
I should add that I have under drive pulleys and temps creep up they don't shoot up.

I'm running dual core Dewitt so may behave a bit different. Having clarified, I drilled one 1/8" hole on the thermostat. During 90 to 95 degree day, long idle with AC on takes coolant temp to 213-215. During the last winter, steady state 60-70 mph cruise stabilized coolant temp was 185-187. Hot day, 195-200 degrees at 60-70 degrees.

[XfireZ51]

Do u know what temps ur fans come on at?

[GOLDCYLON]

I have the Dewitt's radiator with a Ron woods 160 thermostat and one drilled hole. I have the fans on early mod and the Dewitt's spal fans. In wintertime the car randomly exceeds 1/2 band on the gauge. I would say no more than 2 Dom as 5 is almost a bypass

[secondchance]

Do u know what temps ur fans come on at?

My motor is a mini stroker - 4" bore x 3.75" stroke - 378, with .425 intake cams and secondaries removed. I'm running Marc's chip with secondary fan coming on at 205, I think.

[XfireZ51]

Ok, one other question. What diameter for the holes?

[secondchance]

Ok, one other question. What diameter for the holes?

One 1/8".

[Dynomite]

Ok guys.......just a little experience with thermostats (180 deg), Dewitts, Fluidyne, Ron Davis ALuminum Radiators, Fans, and Water Pumps for ZR-1s

1. Fans turn on at 205 deg F and OFF at 200 deg F (Haibeck Chip)
2. I use 180 deg Thermostats.
3. Ron Davis, Dewitt, and Fluidyne multi core Aluminum Radiators.

My experience in all ambient temperatures.

A. In sixth gear running 65 mph (less than 2,000 rpm) the Coolant always gets a bit over 200 deg F. On Hot days the coolant will get a bit over 213 deg F.
The key is the Water Pump is not pushing enough flow through the fully open thermostat to cool the engine to a Temperature where the Thermostat takes over (180 deg F).

B. If I shift to 5th gear at 65 mph (more than 2,000 rpm) the coolant temperature drops to Thermostat control (180 deg F) on cool days and drops to near 200 deg F on Hot days.

The Summary is simple.....

The Thermostat is fully open at all coolant temperatures over 180 deg F (no need to use cooler thermostat unless you want to run at temperatures below 180 deg F).


The Water Pump does not provide enough flow to cool the engine at rpms less than 2,000 rpm. This is not an issue at ambient environmental temperatures on cool days. This is a problem on HOT days. The Aluminum Multi Core Radiators DO provide better Heat Dissipation and offer cooler Coolant at ALL Temperatures for which the 180 deg F Thermostat Controls the Flow (above 180 deg F Coolant temperatures the Thermostat is fully open). The Cooling effects then being dependent on Water Pump Flow Rate and Air Flow Rate through the Radiator.

Having the Fans come on at 205 deg F DOES provide for COOLER Coolant in the radiator once sufficient coolant flow rate is provided above 2,000 rpm. Using Aluminum Multi Core Radiators ALSO provides for COOLER Coolant Temperatures in conjunction with the Air Flow provided by Fans and Vehicle Speed. The Heat Removal Rate from the Radiator is greater as the Air Flow Increases and as the Aluminum exposed to that Air Movement increases.

It DOES make a difference if you are moving at speed in conjunction with Fans PULLING Air Flow as the additional Air Pressure Up Front does add to the TOTAL AIR FLOW through the Radiator at any ambient environmental temperatures.

Using 180 deg Thermostats insures that the engine will not run cooler than that temperature since the Thermostat is in control and controls the Coolant flow rate through the engine at all Coolant Temperatures less than 180 deg F.

Thermostats, Fans, Water Pump, and Radiators (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp.html#post1580070558)

[Paul Workman]

Ok guys.......just a little experience with thermostats (180 deg), Dewitts, Fluidyne, Ron Davis ALuminum Radiators, Fans, and Water Pumps for ZR-1s

1. Fans turn on at 205 deg F and OFF at 200 deg F (Haibeck Chip)
2. I use 180 deg Thermostats.
3. Ron Davis, Dewitt, and Fluidyne multi core Aluminum Radiators.

My experience in all ambient temperatures.

A. In sixth gear running 65 mph (less than 2,000 rpm) the Coolant always gets a bit over 200 deg F. On Hot days the coolant will get a bit over 213 deg F.
The key is the Water Pump is not pushing enough flow through the fully open thermostat to cool the engine to a Temperature where the Thermostat takes over (180 deg F).

B. If I shift to 5th gear at 65 mph (more than 2,000 rpm) the coolant temperature drops to Thermostat control (180 deg F) on cool days and drops to near 200 deg F on Hot days.

The Summary is simple.....

The Thermostat is fully open at all coolant temperatures over 180 deg F (no need to use cooler thermostat unless you want to run at temperatures below 180 deg F).


The Water Pump does not provide enough flow to cool the engine at rpms less than 2,000 rpm. This is not an issue at ambient environmental temperatures on cool days. This is a problem on HOT days. The Aluminum Multi Core Radiators DO provide better Heat Dissipation and offer cooler Coolant at ALL Temperatures for which the 180 deg F Thermostat Controls the Flow (above 180 deg F Coolant temperatures the Thermostat is fully open). The Cooling effects then being dependent on Water Pump Flow Rate and Air Flow Rate through the Radiator.

Having the Fans come on at 205 deg F DOES provide for COOLER Coolant in the radiator once sufficient coolant flow rate is provided above 2,000 rpm. Using Aluminum Multi Core Radiators ALSO provides for COOLER Coolant Temperatures in conjunction with the Air Flow provided by Fans and Vehicle Speed. The Heat Removal Rate from the Radiator is greater as the Air Flow Increases and as the Aluminum exposed to that Air Movement increases.

It DOES make a difference if you are moving at speed in conjunction with Fans PULLING Air Flow as the additional Air Pressure Up Front does add to the TOTAL AIR FLOW through the Radiator at any ambient environmental temperatures.

Using 180 deg Thermostats insures that the engine will not run cooler than that temperature since the Thermostat is in control and controls the Coolant flow rate through the engine at all Coolant Temperatures less than 180 deg F.

Thermostats, Fans, Water Pump, and Radiators (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp.html#post1580070558)

YUP!:thumbsup:

[KFoster]

Dynamite is correct. All those extra holes in the thermo could delay opening a little. Prob wouldn't notice. A good thermo has a bleed hole. All you need is one hole and install it at the top to bleed any trapped air. 180 is the way to go. And it will help some to have a fan come on at 185-190 but coolant flow is more important. With ac the fans should be running anyway, but idle in hotter temps is still going to stress the system. Too cool a thermo will result in the engine staying in "choke mode" in colder weather.


Sent from my iPhone using ZR-1 Net Registry (https://siteowners.tapatalk.com/byo/displayAndDownloadByoApp?rid=90383)

[secondchance]

Dynamite is correct. All those extra holes in the thermo could delay opening a little. Prob wouldn't notice. A good thermo has a bleed hole. All you need is one hole and install it at the top to bleed any trapped air. 180 is the way to go. And it will help some to have a fan come on at 185-190 but coolant flow is more important. With ac the fans should be running anyway, but idle in hotter temps is still going to stress the system. Too cool a thermo will result in the engine staying in "choke mode" in colder weather.


Sent from my iPhone using ZR-1 Net Registry (https://siteowners.tapatalk.com/byo/displayAndDownloadByoApp?rid=90383)

Also, possibility of building up water in the oil pan due to inability to boil off condensation in cold weather.

[Hog]

Remember the purpose of the thermostat, to delay cooling until its required. An air bleed is fine, bypassing the thermostat isn't.

The a major concern of an over cooled engine is cylinder wear. Not sure how a Nikasil cylinder bore would work into this.

[XfireZ51]

Frankly, I thought a stock thermo w a Fluidyne rad would have been just fine, but it appears to me that the under drive pulley setup may require some help at low rpms i.e. < 1500. The coolant temps were not an issue w the previous thermo until ambient temps got below 60F. Then I couldn't keep any heat in the motor at cruise.
However, now cruise rpm below say 1300 or sitting at a 900rpm idle, the temps creep up, albeit not rapidly. Once rpms reach above 1300 or so, temps drop fairly quickly and stabilize at about 187-192F.

[Hog]

Frankly, I thought a stock thermo w a Fluidyne rad would have been just fine, but it appears to me that the under drive pulley setup may require some help at low rpms i.e. < 1500. The coolant temps were not an issue w the previous thermo until ambient temps got below 60F. Then I couldn't keep any heat in the motor at cruise.
However, now cruise rpm below say 1300 or sitting at a 900rpm idle, the temps creep up, albeit not rapidly. Once rpms reach above 1300 or so, temps drop fairly quickly and stabilize at about 187-192F.
Your old setup was a stock therm. with 5 holes and caused difficulty with keeping heat in the engine at low ambient temps.

Now a new stock therm. with zero holes was installed and now you are getting some temp increase creep at very low rpms

I'm guessing that since the LT5 system was designed for high rpm usage that if waterpump rpm is affected by the aftermarket underdrive pulley setup, the combined effects are causing low coolant flow, which is then causing poor heat rejection from the system.

The old therm wasn't allowing the thermostat to do its job and allow the engine coolant to build heat, it was like driving with an always partially open thermostat. Which would be most problematic the cooler the ambient air was.

Speeding up the waterpump should solve your issue. EDIT:A small hole in the therm would help as well.

I was having trouble understanding the pre and post thermostat configurations at first. Sorry.

[Dynomite]

I see no difference between a 160 deg F thermostat and a 180 deg thermostat (both with the same Full Open Head Loss) once the coolant temperature has reached 190 deg F? I know the 190 deg F will be reached slower with a 160 deg F Thermostat but that is the only difference in that you reach the same Coolant Temperature at different rates of temperature increase. Kind of like using light weight pulleys to save HP which HP is only realized on acceleration as the rotational velocity of the pulley changes. Nothing is gained in a constant velocity (constant speed) situation.

http://i287.photobucket.com/albums/ll142/dynomite007/Dynomite12/23d8abf6-05e1-436c-8049-d58b3ad16c03.png

My findings are simply that the stock water pump is a bit low on coolant flow rate at rpms under 2,000 rpm. As you can see there is a big jump in Coolant Pump Flow rate between 1,000 rpm and 2,000 rpm and it is in this area or engine RPM that the Coolant Flow Rate is not sufficient in HOT Climates.

The Coolant Pump Flow at 800 rpm is 15 gpm. The Coolant Pump actually gets more efficient as the rpm increases from idle to 2,000 rpm.

As Per Marc Haibeck graph provided to the ZR-1 Net email list by Graham Behan about ten years ago, the Coolant Pump flow rate is:
15 gpm at 800 rpm
18 gpm at 1,000 rpm,
44 gpm at 2,000 rpm,
65 gpm at 3,000 rpm,
90 gpm at 4.000 rpm,
120 gpm at 5,000 rpm at which time cavitation is starting.

1. The Dual Thermostat Bypass pressure is apparently 5 psi and block resistance at 100 gpm is approximately 20 psi. I am not sure what the radiator Head Loss is at various flow rates but definitely depends on the radiator type.
2. It would seem that the Coolant Pressure Relief Cap on top of the Coolant Reservoir in front of passenger side set at 15 psi would assure the radiator maximum pressure would be 15 psi plus the Bypass Pressure of 5 psi or 20 psi.

[Hog]

Dyno:When you say "head loss" what are you describing?

[Dynomite]

Dyno:When you say "head loss" what are you describing?

That is the pressure required to push the Coolant through the engine block and Radiator (and associated hoses including the Thermostat). That is a Hydraulic term describing the pressure required to push fluid through a pipe. The more pressure you apply the greater the fluid flow.

Or if you google Head Loss To move a given volume of liquid through a pipe requires a certain amount of energy. An energy or pressure difference must exist to cause the liquid to move. A portion of that energy is lost to the resistance to flow. This resistance to flow is called head loss due to friction.

The First set of Calculations is for The Pressure Drop in Oil Lines and is a bit complex but the same calcluations can be done with Coolant Flow. In the Second set of Calculations below we already know the flow characteristics of the Coolant Pump and Marc suggested the block resistance at 100 gpm is approximately 20 psi (which is the Head Loss at 100 gpm Coolant Flow) ...so a lot simpler as we really do not need the pressure drop unless we want to calculate flow rates (which we already have by measurements). In Marc's suggestion above the Head Loss is 20 psi.

Tech Info - LT5 Pressure Drop In Oil Lines Calculations (http://www.corvetteforum.com/forums/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-11.html#post1590759146)
Tech Info - LT5 Coolant Flow Calculations (http://www.corvetteforum.com/forums/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-11.html#post1590759152)

[XfireZ51]

Big jump in flow from 1000-2000rpm. Which is what I am experiencing.

I'm thinking 1-2 small holes to stabilize temps at low flow.

[Hib Halverson]

LT5s at low highway speeds in sixth, will run warm because of marginal flow speed through the cooling system. Underdrive pulleys will make that worse.

While there might be an advantage in a drag racing application, 160-deg 'stats run the LT5 too cool for good durabilty on the street. You need to be at least 170 and prefarably 180.

When I owned Barney, the "Purple Thing", I ran a 170 'stat with no extra holes, 0-10% antofreeze, Red Line Water Wetter, Fluidyne Radiator and lower fan on temps. Sixth gear cruse at 60-70 was usually 180-185. If I sped up or downshifted to fifth, ECT was in the mid 170s. In traffic I might see 205. In aggressive driving on an uphill mountain road with A/C on, the highest observed ECT was 216.

[Dynomite]

Big jump in flow from 1000-2000rpm. Which is what I am experiencing.

I'm thinking 1-2 small holes to stabilize temps at low flow.

The Thermostat is fully open at those temperatures which are in the range of 200-210 deg F even on 50 deg days (moving 65 mph in sixth gear at less than 2,000 rpm). Hib Halverson describes that situation very well :thumbsup:

I run this unmodified 180 deg F Thermostat on ALL ZR-1s. You will notice there is BUILT IN BYPASS on the lower brass flange (two slots) to make sure the Coolant reaches constant and even temperatures as quickly as possible as this allows coolant to circulate in and out of the block (Not Through The Radiator) before it reaches a bit less than 180 deg F . It IS Important to have those two slots on the Recirculating Flow end of the Thermostat for the LT5.

You want to INSTALL The Thermostat squarely in the Housing making sure the rubber gasket is not pinched or otherwise corrupted as the Slotted End of the Thermostat could end up bound from opening or closing if the Thermostat is not squarely seated. One event could blow your Stock Radiator at High RPMS and the other event could cause Over Heating as the flow through the radiator would not be maximized.

http://i287.photobucket.com/albums/ll142/dynomite007/91%20ZR1/A-THERM1-1.jpghttp://i287.photobucket.com/albums/ll142/dynomite007/Dynomite12/23d8abf6-05e1-436c-8049-d58b3ad16c03.png

I doubt a couple holes in the thermostat will help increase the Coolant Flow through the Block from maybe 30 gpm to over 44 gpm as it does when you shift to fifth gear. It is the Head loss through the block not just the thermostat the Coolant Pump is creating pressure against to increase the Coolant Flow. Your two small holes in the Thermostat is negligible compared to that total restrictions within the Block Coolant Passages.

As the Coolant Temperature moves downward towards the Thermostat operational range the Thermostat will close but will now have to close just a tiny bit further to compensate for your two small holes to keep coolant at 180 deg F Temperatures. Recognizing the Thermostat may start opening at slightly less than 180 deg F and be fully open at say 190 deg F.

The 180 deg versus the 160 deg thermostat or no thermostat (http://forums.corvetteforum.com/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-6.html#post1584987828)

[Hog]

That is the pressure required to push the Coolant through the engine block and Radiator (and associated hoses including the Thermostat). That is a Hydraulic term describing the pressure required to push fluid through a pipe. The more pressure you apply the greater the fluid flow.

Or if you google Head Loss To move a given volume of liquid through a pipe requires a certain amount of energy. An energy or pressure difference must exist to cause the liquid to move. A portion of that energy is lost to the resistance to flow. This resistance to flow is called head loss due to friction.

The First set of Calculations is for The Pressure Drop in Oil Lines and is a bit complex but the same calcluations can be done with Coolant Flow. In the Second set of Calculations below we already know the flow characteristics of the Coolant Pump and Marc suggested the block resistance at 100 gpm is approximately 20 psi (which is the Head Loss at 100 gpm Coolant Flow) ...so a lot simpler as we really do not need the pressure drop unless we want to calculate flow rates (which we already have by measurements). In Marc's suggestion above the Head Loss is 20 psi.

Tech Info - LT5 Pressure Drop In Oil Lines Calculations (http://www.corvetteforum.com/forums/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-11.html#post1590759146)
Tech Info - LT5 Coolant Flow Calculations (http://www.corvetteforum.com/forums/c4-zr-1-discussion/3005470-tech-info-lt5-modifications-rebuild-tricks-500-hp-11.html#post1590759152)

Thanks, I didn't want to appear critical in case it was a typo.

Looks like increasing w/p speed is the "correct" way to tackle this. The 1st world issues that we have. The ZF trans combined with high rear gears, coupled with a high rpm LT5 all combine to give us non-optimal off idle cooling.

[KFoster]

I think I've installed over 50 sets of underdrive pulleys in my life, maybe many more than that. The percentage reduction has little effect with cooling at high RPM, but at lower RPM and idle it is more noticeable. Alternator and cooling can be greatly impacted at idle. So, if you are underdriven by 25%, at the 15 gpm flow rate listed by Dynomite at 800 rpm, your waterpump is seeing only 600 rpm equivalent. That would be around 11.25 gpm. Underdrive pulleys are great for a small increase in HP as long as cooling and charging is not impacted.

[XfireZ51]

The previous thermo had 4 holes drilled. I'm going to drop the coolant and put 2 small holes in the stock 180. We'll see if that mitigates the effect of under drive pulleys at idle and slow cruise.

[XfireZ51]

Ok, coolant is back in the car. We put 2 holes in the thermo. Previous therm had 4. Haven't driven it yet, but I started it to get coolant back through the system and burp it. Temps got to 104C and then stabilized. I was able to then put another 3 quarts in. I'll let it sit overnight to burp it further, then maybe get a chance for a run later this week after the rain. Maybe a pizza run.

[Dynomite]

Ok, coolant is back in the car. We put 2 holes in the thermo. Previous therm had 4. Haven't driven it yet, but I started it to get coolant back through the system and burp it. Temps got to 104C and then stabilized. I was able to then put another 3 quarts in. I'll let it sit overnight to burp it further, then maybe get a chance for a run later this week after the rain. Maybe a pizza run.

Drive 55-60 mph in sixth for a while to see where the Temp goes....Use the HVAC display for Coolant Temp in deg Centigrade readout (Convert to Deg F for us as you like). Then shift to fifth (55-60 mph) and see where the Temperature goes. Wait until Fri (Close to 70 deg in Chicago) or Sat (in 60s) to take your run. Report back Speeds, Outside Air Temperature, Gearing (fifth or sixth), HVAC Display Temperature, and Vehicle Modifications. We are assuming Stock Differential.

Let us know what happened. My experience is just a bit over 200 deg F in sixth and drops to less than 190 deg F in fifth on relatively cool days such as you will have in Chicago on Fri or Sat but will run that experiment again on Thur and Fri here in South Dakota.

I will run an identical experiment in SD on Thur or Fri (will be very close to 70 deg F) and report back to you using a relatively stock 90' with Haibeck Chip, Multi Core Aluminum Radiator, and Stock DIfferential. I will report Speed, Outside Air Temperature, HVAC Coolant Temperature (Go to #16), Gear, and assuming no wind that may effect Air Flow one way or other. I wil also report the approximate time Coolant Temperature re-stabilizes after shifting from sixth to fifth gear. Again......this experiment will be done at 70 deg F Outside Air Temperature within a couple degrees.

In CA at 100 deg F outside Air Temperature it is quite obvious the differences in cooling running moderate speed (55-60 mph) in sixth gear vrs fifth gear except the Coolant Temperatures do run a bit over 200 deg F even in fifth Gear and approach 215-220 deg F in sixth gear. This using a 90' ZR-1 with Haibeck Chip and Multi Core Aluminum Radiator.

Oh.....I run with Debree Screen Clean Radiator, Clean Oil Cooler, and Clean Air Condensor and Green Antifreeze good to minus 20 degrees F in all vehicles. One 90' I am running is good to 0 deg F which I may add some antifreeze :p

[XfireZ51]

That's part of my plan. I'll do some idle testing today, then the FBI is having a Pizza Night Friday for our Corvette Challenge victory (again).
BTW, the 92 has a direct Coolant readout, along w other gauges, on the Speedo display.

[Dynomite]

That's part of my plan. I'll do some idle testing today, then the FBI is having a Pizza Night Friday for our Corvette Challenge victory (again).
BTW, the 92 has a direct Coolant readout, along w other gauges, on the Speedo display.

50 deg F - 70 deg F outside Air Temperature.
Ron Davis Radiator.
Haibeck Chip (fans ON 205 deg F, OFF 200 deg F).
1990 ZR-1 #72 (40,000 miles).
Stock Exhaust.
180 deg F Thermostat.
Green Antifreeze down to -10 deg F.

6th Gear
65 mph
1,500 rpm
207 deg F Engine Coolant Temperature

5th Gear
65 mph
2,200 rpm
190 deg F Engine Coolant Temperature

17 deg F cooler running 65 mph in 5th Gear rather than running 65 mph in 6th Gear.
Did the cycle several times with identical results. Coolant Temperatures moved from min to max and back in a minute or two after shifting Gears.

Idle at 850 rpm 203 deg F.

Got this Alternator diagnosed on this run and it turns out the Rectifier was burned/shorted....New Rectifier on Ebay for $68.
This Alternator will go into 1990 #72 later on (130 amp Alternator) ;)

[XfireZ51]

Ok so today, first try

Ambient Intake Temp 53F/12C
Idle condition 850-900rpm
Coolant Temps 86C Stable
Fan 1 operating

[Dynomite]

Ok so today, first try

Ambient Intake Temp 53F/12C
Idle condition 850-900rpm
Coolant Temps 86C Stable
Fan 1 operating

Do you have a Chip?
You have a fan running at 187 deg F???

When do your fans come on and off?
My fans with Haibeck Chip (fans ON 205 deg F, OFF 200 deg F).

What Radiator are you using?
I will do some idle tests tomorrow also at 70 deg F outside Air Temperatures :handshak:

I am assuming you have stock Differential and would run same RPMs at 65 mph as I noted.
I just observed my Speedometer and RPMs so could be off a bit but think I am pretty close to speed vrs rpms.

[XfireZ51]

I do my own calibrations. Fan 1 comes on at 83C, Fan 2 @ 89C
I am using a Fluidyne rad and 180 Thermo.
Diff is 4.10 BUT w 19" Z06 wheels/tires. So overall ratio is more
like a 3.55.

[Dynomite]

Today at 70 deg F Outside Air Temperature got same results as at 50 deg F Outside Air Temperature.

50 deg F - 70 deg F outside Air Temperature.
Ron Davis Radiator.
Haibeck Chip (fans ON 205 deg F, OFF 200 deg F).
1990 ZR-1 #72 (40,000 miles).
Stock Exhaust.
180 deg F Thermostat.
Green Antifreeze down to -10 deg F.

6th Gear
65 mph
1,500 rpm
207 deg F Engine Coolant Temperature

5th Gear
65 mph
2,200 rpm
190 deg F Engine Coolant Temperature

17 deg F cooler running 65 mph in 5th Gear rather than running 65 mph in 6th Gear.
Did the cycle several times with identical results. Coolant Temperatures moved from min to max and back in a minute or two after shifting Gears.

Idle at 850 rpm 203 deg F.

[XfireZ51]

Has the opportunity to try out new thermo both yesterday and today.
Yesterday, ambient driving temps started out at 48F and heated up to 56F by the time I ended the drive. In all driving situations, highway cruise, local street driving, driving under 65 and over 65 ( fan cutoff speed), the coolant temps
Varied between 85-89C.

Today, I got to drive in ambient temps of 73F. Again, in various driving situations. Hiway cruise, temps were practically glued at 86C. Idle would sit at 86C and climb a bit to 89/90. With fan turn on temps would begin to drop. Any increase in Speed would rapidly bring temps back down to 86C.
I changed calibrations to bring fans on later and drop out at a lower speed to make sure the fans were not assisting in the cooling.
With FAN 1 coming on at 88C for low speed driving (<12mph), temps circled 86-88C. High speed driving (>12 and past 55), temps were in the 86-88C range. So just air moving thru the rad is enough to maintain a very stable temp. Driving at 100mph raised temps +1.
Overall, I'd say I am pretty satisfied. The thermo w 2 bypass holes appears to keep enough heat in the motor w cool ambient temps. We'll see what happens next year w 80F+ weather, but so far it looks good.

[XfireZ51]

Quick update. Sunny New Years Day. 35F outside so took opportunity to roll the tires and basically heat the car up a bit. I was curious how the thermostat would work with now 2 bypass holes rather than 4. This is a stock thermo.
At cruising speeds 55-70mph, coolant temps stayed at ~ 185F/85C. Unlike before, driving around in 35F weather would have caused the coolant temps to drop down to 150 or so.
So overall, the combination of a Fluidyne rad w stock thermo (w 2 bypass holes) seems to have given the result I was looking for.