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The coolant return is often called a ‘water neck’ in catalogs. The thermostat brings the engine up to operating temperature sooner than if not present. This controls the operating mechanical clearances which are not the same cold static clearances you measured when putting it together at room temperature. Getting the engine up to temp evens out the cylinder to cylinder power because the front pair run cooler than the middle and certainly the rear cylinders. The bypass is used for several purposes but reducing the chill on the front cylinders is a big part of its function while the thermostat maintains the average coolant temp.

The hottest parts of an SBC is the center adjacent exhaust valve seats. GM normally takes he’d coolant off the drivers side head between cylinders 1 and 3 as this is just a bit off the hottest spot in the engine and the first to see trouble if it’s brewing. This is the hottest spot not only because of the adjacent exhaust seats being near by but also with a clockwise turning coolant pump it’s output slightly favors the passenger side, an out come of the direction of pump rotation. There is an internal bypass on the passenger side used to balance the flow slightly, it was eliminated on the counterclockwise rotation serpentine belt drives starting on the 96 Vortec.

Bogie
But let me ask you this. By this rationale, wouldn't then the engine reach operating temp much quicker with the heater hoses disconnected than when they are connected, given the extra distance the coolant has to travel? I am now wondering why some folks disconnect their heater hoses.
 

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But let me ask you this. By this rationale, wouldn't then the engine reach operating temp much quicker with the heater hoses disconnected than when they are connected, given the extra distance the coolant has to travel? I am now wondering why some folks disconnect their heater hoses.
Often unhooked because the heater core springs a leak and they don't want to fix it either due to difficulty of removal or cost to pay someone to do it.

For a fair weather hot rod, a heater core is mostly just sitting there unused anyway.
 

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But let me ask you this. By this rationale, wouldn't then the engine reach operating temp much quicker with the heater hoses disconnected than when they are connected, given the extra distance the coolant has to travel? I am now wondering why some folks disconnect their heater hoses.
Perhaps but there are important and multiple considerations. For one is cabin heat in cold climates so circulation is limited to the block and head’s and heater core to get heat into the cabin quickly. The other huge one especially with engines that use adajacent exhaust valves which was pretty common for any engine designs the SBC classic generations being one of many. Here circulation is needed to prevent local overheating which can happen even when the bulk coolant temp is cold without pump circulation. The critical areas of all engines is the exhaust seats and spark plug bosses. It’s just more critical where exhaust valves are adjacent simply because there is twice the heat in a small area. But this is important on any valve arrangement because of the amount of heat being put into the seat and the port compared to the cold metal all around that area. So like most everything compromises have to be made to achieve overall functionality.

A big thing the OEMs did was put an engine oil heat exchanger in the radiator to get the oil up to coolant temperature much faster. This cuts warm up time about in half. It is strictly an emissions and mileage feature it is not an engine oil cooler in the sense that you’re moving heavy loads up a mountain side in Death Valley as this it isn’t going to offer much engine oil cooling. The unfortunates of this system, however, is GM can’t seem to get the system oil tight. Maybe to their credit something else is going on as I’ve rebuilt several to my own design using AN components more and less hard line to flex combinations in testing to see which works better and even they get leaking after a while. So there’s some dynamic going on I still haven’t figured out.

Bogie
 
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