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I often drive from Pittsburgh, PA down to DC/Baltimore. Lately (ever since I had a coolant leak!) I've been keeping a close eye on engine temps. On my most recent trip I noticed -

1. Cruising on the highway in 6th put my car around 195 degrees, but
2. if I dropped the car into 4th gear, between 4k-5k rpm, the coolant temp went down to 180 deg.

Is keeping the car near 5k rpm actually better for the engine? It was almost certainly producing less heat, I don't know if that matters? Of course if I smashed the throttle to blast through traffic, the temps would go back up. This seemed to be pretty consistent, I tried it over and over again to make sure the hills were not affecting my results.
 

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An NA engine will almost always be the most thermally efficient the wider the throttle opening. Downshifting like that will take much less throttle, so I would be very surprised if it is actually rejecting less heat. I've driven from Pittsburgh to DC dozens of times, and the hills are extremely inconsistent, so that's a massive variable you just can't ignore.

EDIT: After trying 4th gear, did you try 6th again to see if it warmed right back up? I wonder if the extra buzz in fourth may have helped bleed the system a tiny bit and bumped up the heat transfer.

EDIT #2: Water pump is driven by the engine, so you just pumped the water faster. I'd bet that's all it was.
 

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I would agree this is mainly because you increased the water flow rate through the system. Engines do operate more efficiently at higher RPM but they also use more fuel. I would bet that you actually increased the heat load on the cooling system by doing this but the increased flow rate over compensated for the slight added heat load.
 

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1. Cruising on the highway in 6th put my car around 195 degrees, but
2. if I dropped the car into 4th gear, between 4k-5k rpm, the coolant temp went down to 180 deg.

Is keeping the car near 5k rpm actually better for the engine?
The water flow factor has been mentioned - I'll expand on that a little and just point out that centrifugal impeller water pumps (like almost all cars use for coolant pumps) have an exponential delivery curve. Simply put - spinning the engine 20% faster usually results in more than 20% increase in coolant flow. So it doesn't take much of a change to make a big difference. That's one of several reasons car engines have had thermostats for most of a century.

A point that hasn't been directly mentioned (@cyow5 alluded to it) is that combustion efficiency is always greater at a higher throttle opening. A side effect of this that isn't obvious is that you're getting more work out of each combustion event, which means that you actually reject more heat to the cooling system near the sweet spot of minimum BSFC, max VE, and max torque (usually all the same place if you're doing things right) versus the heat being dumped overboard in the exhaust without doing work first even though you're actually burning less fuel to do the same work. So more heat goes into the cooling system because less is being dumped into the exhaust as waste than at higher RPM and lower loads. This is also the point at which oil temperatures are the highest because more heat is being carried off the pistons that way.

An engineer would make an apples-to-apples comparison by loading the engine at the same percentage of available torque output at the two different crank speeds and then comparing the proportionate heat rejection of the engine in the two modes. You can't really do that in the car on the road.

One last note: Higher coolant outlet temperatures in the allowable range are actually more efficient and better for the (notoriously overcooled) 2ZZ-GE in a Lotus. What's the allowable range? Well, the cooling fans don't even turn on until 207 F. Anything below 200F is fine for a NA street car (street car being a key point here). I worry more about coolant temps as low as 180F on a hot day. Your thermostat might not be doing its job.
 

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combustion efficiency is always greater at a higher throttle opening.
My wording was carefully chosen since "always" is not accurate. I said NA because boosted cars tend to run overly rich at WOT, reducing efficiency in the name of peak power. Additionally, the throttle can be used as a turning vane and sometimes produces greater power and efficiency at a less-than-100% opening. I worked with the Viper program a few years back, and they wouldn't let me see their throttle calibration in order to protect that IP. This was the race shop though; the factory ran a beyond optimal angle.

EDIT: to add, NA cars also tend to go rich at WOT, just not as bad as boosted cars, so when you look at BSFC islands, peak tends to be slightly below WOT at the rpm of best torque. Obviously this is outside the operating range in question by OP, just trying to be as accurate as possible.
 

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I often drive from Pittsburgh, PA down to DC/Baltimore. Lately (ever since I had a coolant leak!) I've been keeping a close eye on engine temps. On my most recent trip I noticed -

1. Cruising on the highway in 6th put my car around 195 degrees, but
2. if I dropped the car into 4th gear, between 4k-5k rpm, the coolant temp went down to 180 deg.

Is keeping the car near 5k rpm actually better for the engine? It was almost certainly producing less heat, I don't know if that matters? Of course if I smashed the throttle to blast through traffic, the temps would go back up. This seemed to be pretty consistent, I tried it over and over again to make sure the hills were not affecting my results.
Just curious what radiator are you running with now (Assuming your coolant leak was radiator related) ?
 

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Discussion Starter #7
Heh, I have a hard time with negatives - what does "rejecting less heat" mean?

Alas, I was mostly worried about premature engine wear. I thought the lower temps in 4th were indicative of the engine not needing to work as hard. After this conversation, that seems like a stretch. Is it? It would be a cool finding if 5k were safer for the internals than 2k :)

After trying 4th gear, did you try 6th again to see if it warmed right back up?
Yes, repeatedly, maybe 30-40 times over the trip? At ~80 mph, 4th gear was always cooler than 6th.

So it sounds like increased waterpump speed was increasing the cooling. My engine was doing the same work, but with more cooling. I do wonder though, if coolant flow were kept constant, for a given road speed, what the most (heat) efficient speed would be. I'm guessing it's somewhere near the powerband peak.

Just curious what radiator are you running with now (Assuming your coolant leak was radiator related) ?
It was a hose leak, so I'm still running the factory radiator. I definitely could use something more efficient though. Recent hot days at the track have lead to 200f+ coolant temps on track, and up to 215f while stopped!
 

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Heh, I have a hard time with negatives - what does "rejecting less heat" mean?
So with heat transfer, "positive" or "negative" are just ways of saying "towards" or "away". Think of it like this - you never "cool" something; you are instead using it to heat something else, and this causes it to lose thermal energy to whatever you heated up. In the case of a radiator, all the engine's heat is being transferred to the air.

Another heat lesson - efficiency is "what you get out relative to what you put in". "Fuel Efficiency" has completely muddled this definition. The most heat efficient condition for almost any naturally aspirated combustion engine is around the rpm of peak torque and about 90% load (look up BSFC islands). It isn't realistic to drive like that on the interstate though since the interstate only requires around 15-20hp to cruise, and this puts you around 50hp-ish. Your fuel economy would also tank because now you'd be speeding like crazy. "But you said this would be efficient??" Well, yes, you are putting in a bunch of fuel but also generating a bunch of power and wasting a bunch, just the ratio is a bit lower. If you simply were making less power less efficiently, you'd reject less overall heat and still get better fuel economy despite a lower thermal efficiency.

Think of it like this. A brand new Ferarri on a 75% off sale is a great deal but still costs more hard cash than a new Chevy econobox with no discounts. So a better deal can also be more expensive.
 

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I found myself having to explain what @cyow5 is talking about to my aeronautical engineer dad (with a powerplants specialization, no less) one day. I explained that while the Brake Specific Fuel Consumption on a particular engine was worse (higher) at 1800 RPM than it was at 2700 RPM (max VE / min BSFC / peak torque), the aerodynamic drag of the vehicle at 37 MPH was so much less than at the torque peak (something like 50 MPH) that the actual fuel consumed per mile traveled was less even though the engine's specific efficiency was worse.

This never happens on airplanes, because the airplane has a minimum flying speed below which the wings can't lift the airplane off the ground, so you're always somewhere on the fat part of the curve, which means that the throttle is always pretty far open when the airplane is in the air and you're always trying to run near the torque peak except at takeoff, where you're near the horsepower peak and don't care about fuel economy. That's why Dad had never thought about it that way.
 
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