How many you guys out there do you leave the bypass on your intake to water pump

Why would you not do so? I always do.

I do. I have one on my stroked 360. HP Silicone

If you are running a thermostat you need the bypass, if you are running an electric pump ditch the bypass and thermostat.

The bypass allows the coolant in the engine to circulate while the thermostat is closed. The bypass lets the engine coolant heat up uniformly so no hot spots are created. Once the thermostat opens, it does bypass some water that could be going through the radiator and cooled. I've sometimes wondered how big the bypass really needs to be in order to work the way it was designed to. Could a restrictor be added that would maintain the bypass function yet allow more coolant to be cooled by the radiator? Has anyone done it?

Dunno, but the earlier motors (pre 70) used a smaller hose. No clue as to why the change, and never really thought about it until now. Must have been a reason.

I think if you get the bypass hose too small you can create back-pressure and cavitation in the pump, which will tear it up in short order.

I also have Milodon HD pump with anti cavitation plate.

I found this out the hard way. When I built my 434 the Indy intake wasn't drilled for the bypass nipple. The engine would get hot quickly then the thermostat would open and it would cool off then repeat the process. I drilled and tapped the intake for the nipple then installed the bypass hose and it's been fine since.

After I switched to Evans Cooling NPG instead of antifreeze on a 1995 Magnum 5.9 V8 in 1998, I experimented with partially blocking the always-open bypass hose with a 3/8ths drive socket, 17 mm if my fuzzy memory is correct. The slight restriction did not seem to have any effect that I could notice.

Evans Cooling sold a special aftermarket thermostat housing that plumbed into the main coolant outlet hose and housed a thermostat similar to what was used on the 2003 5.7 Hemi, which was that type of thermostat that had a post that moved down and blocked off the bypass when operating temperature was reached.

I have since read that a bypass reduces hotspots in the cylinder heads that can cause warping or cracking during the warmup period.

NASCAR cooling guru Howard Stewart recommended drilling extra 1/8 th diameter “air bubble holes” in thermostats even if they had a single factory hole.

I have also used an old 1/2 drive 9/16 socket in my bypass hose on a few 318s and have not seen any ill effects.

I also always drill 2 each 1/16" hole in my thermostats, 180 degrees apart.

While more done for Circle Track racing, some of the mods are streetable and I have done some of the 'Sanborn' water restrictions on the two holes in the back of the block to keep the water from rushing into the head. This will require making sure the smaller holes in the block and the heads are aligned well and then opening up the gasket holes to match. This directs more water around the exhaust valves. I used 2 core plugs bit drilled a 1/16" air bleed hole in them so no trapped air.

The other mod I am going to try on my 410 stroke 5.9 is the timing covers water ports tapped for a 7/16 or 1/2 inch AN line ran over to the center of the blocks core/freeze plug.

The other cooling mod is done more for oiling to the main bearings but it allows more oil to flow around hot areas. That is the McCandles small block mod of drilling the mains feed from 1/4 inch to 9/32 or 5/16's inch and all larger feed oil ports to 1/2 inch.

But on my 65 B-cuda I was running the standard open element air cleaner. My Barracuda was always over heating and pirc'ing the fuel. The biggest cooling fix was to use a sealed dual snorkel air cleaner housing from a Cop Car and run two 4 inch drier hoses under the bumper. Not ram air but a 'Cold Air Intake' system. This single mod cured 95% of my over heating. Let the Chubby and Dorf drivers run open air cleaners and suck up HOT air from under the hood....

Not much to add other than that for size big blocks use a 7/16-1/2" bypass internal to the water pump housing. There is no or minimal cooling circulation with the stat closed, and I always thought the bypass was to protect the heater core from excessive pressure.