There could be a number of reasons for the lower figure.

Both engines share a lot, including - space available - exhaust system - radiator - etc.

So -

More power equals more heat

the engine compartment has more stuff in it

The turbo restricts the exhaust and probably keeps exhaust heat in the engine longer

The intercooler blocks some of the flow through the radiator.

The turbo engine probably has more "hot spots" in it, requiring a better flow of water and heat.

Etc.


The thermostat may not just open when the engine warms up - it may cycle under some conditions - for instance, if the thermostat fails open, the temperature will drop on the highway on a cold day. This indicates to me that full flow may be too much for some conditions

BMW has used a by-pass system with two thermostats that by-passes the radiator under high speed conditions in really cold situations.

Your temperature gauge is not a good way to judge what is going on as far as temperature goes because it has circuitry to compensate the reading so that it stays at normal. If the gauge moves around, the temperature is changing quite a bit.

I do agree with other posters that say that a cold engine probably uses more fuel - and that the small difference in thermostat values does not make much difference in comfort.

I am for using Volvo's recommendation.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks