[Healeys] radiator thermostat

Thu Sep 18 11:38:24 MDT 2025

Hi All,

Thermostats and Healey cooling always generate interesting discussions, and
I'd like to clarify my previous comments on what controls (or often doesn't
control) the coolant temperature.

To summarize some of the points made: Coolant flows down the radiator, out
the bottom hose, and into the water pump. It's then pumped into the engine
block's cooling jacket. The goal is to direct the coolest fluid to the
hottest part of the engine for optimal heat transfer (i.e., the largest
delta T). The coolant, having absorbed heat, then moves up to the cylinder
head, through the thermostat, and back to the radiator. This process is
known as thermosiphoning, where heated coolant rises through the engine and
falls as it cools in the radiator.

The heat generated by the engine is a function of the combustion process
(timing, carburation, etc.) and horsepower efficiency loss. More horsepower
output means more heat transferred to the coolant. The radiator removes
this heat, but its effectiveness is proportional to the difference between
the air and coolant temperatures.

Assuming the radiator has sufficient capacity to remove heat, if the
coolant temperature the thermostat "sees" is less than its opening
temperature plus 20 degrees, the thermostat will slightly close to restrict
flow to the radiator. In this scenario, the thermostat is actively
controlling the coolant temperature. However, if the coolant temperature
exceeds 20 degrees above the thermostat's opening temperature, then the
coolant temperature is primarily a function of the incoming air
temperature, and the thermostat is no longer a factor. In such cases, the
coolant temperature is effectively uncontrolled by the thermostat.

To add to the discussion, the engine fan has minimal effect at speeds
greater than 15 miles per hour.

Harold

On Thu, Sep 18, 2025 at 8:46 AM David Nock via Healeys <
healeys at autox.team.net> wrote:

> The thermostat does not control the high end temperature of the engine.
> Its job is to get the engine up to an operating temperature as soon as
> possible. Then in maintains the low end temperature of the engine. Once the
> thermostat is open the engine temperature is controlled by the radiator,
> radiator fan and air flow.
>
> On the 6 cylinder engine there is a sleeve attached to the thermostat,
> that blocks off a bypass in the cylinder head. The bypass is there to allow
> the coolant to flow when the thermostat is closed. Once the thermostat is
> open the sleeve is there to block the bypass allowing ALL the coolant to
> flow thru the radiator.
>
> David Nock
>
> British Car Specialists
>
>
> On 9/17/2025 12:54 PM, Richard Mayor wrote:
>
> 165 is too cold for good running.  Our engines run better in the 180 to
> 190 range.
>
>
> Sent from my iPad
>
> On Sep 17, 2025, at 12:13 PM, Robert Markovich via Healeys
> <healeys at autox.team.net> <healeys at autox.team.net> wrote:
>
>  165 is the one I use
>
>
> Sent from the all new AOL app for iOS
> <https://aolapp.onelink.me/eG2g?pid=NativePlacement&c=US_Acquisition_YMktg_320_EmailSignature_AttributionDL&af_sub1=Acquisition&af_sub2=US_YMktg&af_sub3=&af_sub4=100002473&af_sub5=SentFromNewAOLApp__Interstitial_&af_ios_store_cpp=ce85ce34-ad0f-4811-a92b-a172743b064e&af_android_url=https%3A%2F%2Fplay.google.com%2Fstore%2Fapps%2Fdetails%3Fid%3Dcom.aol.mobile.aolapp%26listing%3Demail_signature_attribution>
>
> On Wednesday, September 17, 2025, 2:50 PM, Richard Antal via Healeys
> <healeys at autox.team.net> <healeys at autox.team.net> wrote:
>
> Gents,
>      What is the desired coolant temperature for optimal engine
> performance? I've searched the tech articles and can find nothing that
> specifically answers that question. This summer my BJ8 seemed to run fine
> with a 160 thermostat which, according to the gauge which I've checked and
> found to be accurate, kept the coolant at around 165. Thanks to all who
> respond.
> rich antal
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> --
> David Nock
> British Car Specialists
> Stockton Ca
> 209 948 8767www.britishcarspecialists.com
>
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