|08-15-2012 10:55 AM|
Under-drive pulleys are used with competition engines to reduce the power extraction the pump takes from the crankshaft. The power curve on the pump is such that power used is a geometric function of RPM (goes up in powers like doubling the RPM squares, or quads the power extracted) so at 6500 RPM you can be looking at 15 to 20 horses just to spin the pump. Additionally the pump output, while not growing quite so much as the power extraction, does start to output more coolant through the system than is needed at high RPMs to keep the engine temp stable. So without a thermostat or suitably sized restrictor it over cools the engine in addition to taking a lot of power from the crankshaft.
Taking these two events, it then becomes advantageous to slow the pump down which reduces the power extraction from the crank and does not force a situation where more coolant is being pumped than is necessary to maintain proper temperature. A win-win, how often do you see that?
This, however, does not work on the street in-so-far as moderate highway cruise speed and RPMs in the range of 2000-3000 are concerned. Under-drive pulleys will lead to insufficient cooling in these street settings, and like I said above, except at very high RPMs the power extraction of the pump is almost incidental. Of course one alternate solution is to use a high flow pump along with under-drive pulleys, but since very few aftermarket makers and sellers of these pumps provide numerical data compared to the OEM pump output and drive speeds, buying this stuff can lead you down a merry path of spending a lot of money to get unknown to incorrect results. Which is why for a street engine; I always recommend staying with what the factory puts on the engine unless you have numeric data, not making engineering decisions on advertising superlatives.
P.S. Back in the good old days of NASCAR level stockcar racing when much of the equipment was in fact stock and modified for the peculiar needs of racing. It was common to control power extraction and coolant delivery of the pump at high RPMs by not only slowing it down with under-drive pulleys but, also, grinding the impeller blades down. Needless to say when a caution yellow or stop red flag came out these engine immediately puked coolant and overheated.
|08-15-2012 09:29 AM|
|08-15-2012 08:51 AM|
"Not under diving the pump"
|08-14-2012 07:27 PM|
|08-14-2012 07:11 PM|
I learn something....
|08-14-2012 03:56 PM|
|bigdog7373||Bogie always has the best answers|
|08-14-2012 03:54 PM|
|S10 Racer||Bogie I always love your replies, they are like reading a short book. You take your time to go indepth so people understand better. Very good!!!!!|
|08-14-2012 01:48 PM|
Everybody's engine does this in some manner of design. Chevrolet's system routes from the return passages at the front of the intake to the heater core if not equipped with air-conditioning and through a 3 way valve that shunts coolant around the heater core when the AC is on so it doesn't have to cool the heated heater core. The return is from either the heater core for non AC models or the three way valve if it has AC to the suction side of the pump. The return is classically into a fitting on the top of the pump that feeds to the impeller inlet but there some other arrangements where the return hose feeds into the pump inlet side of the radiator tank or into a tee of the pump inlet hose. Everybody uses some configuration of a system similar to this. Some isolate the heater circuit from the bypass running the bypass hose directly from the intake return sourced behind the thermostat into the pump, Ford and Chrysler are examples of this arrangement.
The purpose besides providing faster cabin heat on cold days is to prevent hot spots from forming inside the engine and eliminate pump cavitation before general circulation starts with opening the thermostat. Hot spots mostly form around the exhaust seats and spark plugs; the metal immediately around these parts can become so hot they boil the coolant while everything else is cold. When this happens the hot metal tries to expand with the increasing temperature but is trapped by the colder metal surrounding the hot area, this can and does lead to cracking the casting. Keeping a circulation prevents the formation of local hot spots while the general warming of the circulating coolant tends to level the temperature throughout the castings so hot spots aren't trapped when surrounded by the cooler metal further away from the concentrated heat source.
In terms of cavitation; when the pump is dead headed, it beats the coolant into froth. Bubbles do not offer any cooling to hot engine parts, they only block flow and encourage further localized overheating. Additionally, the froth within the impeller beats on it shock loading the pump shaft, its bearings and seal all of which reduces the life span of these components.
There are those who argue that the constant flow of hot coolant into the mainstream after the thermostat opens contributes to overheating. The engineers that designed this system took that into account. Actually the LT1/4, the LS engine series and most other current production engines not to mention those of heavy trucks and equipment have for many years if not decades used a two part thermostat to feed hot coolant from the engine into cold coolant from the radiator to develop a tightly controlled temperature within the engine for best power and maximum efficiency. The modern Mustang V6 couldn't get to 300 horsepower and 30 miles to the gallon with many tricks this being one of them. I would suggest that overheating problems are better solved with not under diving the pump on street engines, and using a clean and properly sized radiator along with adequate fans and shroud to pull air over the entire core surface.
For straight up competition engines the thermostat is often eliminated as it can become a potential failure item that will take you out of the race. So a simple restrictor is often used to manage the full power operating temperature, which usually proves to be more than necessary cooling at slower speeds. This also eliminates the external plumbing of the bypass system without incurring the problems of no circulation as would happen with no bypass and having a thermostat that is closed when the engine is cold. The restrictor will cause the engine to be slow in initially warming up and the temp will tend over a wide range between idle and screaming around the track. This is less of a problem for racers as they are in the engine for maintenance a lot and the big time racers are using after-market castings which whether iron or aluminum are much more robust than is OEM production hardware.
|08-14-2012 11:09 AM|
|68NovaSS||The radiator inlet is blocked by the thermostat being closed. It's a closed system, no air, so nothing is being replaced or moving in or out of the radiator.|
|08-14-2012 11:02 AM|
Water Pump Functionality
Hypothetically, if the thermostat does not open until 185 degrees, and the water pump works continuously prior to the thermostat opening,what prevents the water in the radiator to flow out and empty into the engine prior to the engine warming up?