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Does a cold engine make more power? #5391326 04/01/20 08:48 PM
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Avery4 Offline OP
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Hello everyone, I am wondering if a typical gasoline engine will make more power when the coolant is cold or hot. I know that cold air and fuel is denser and will make more power, but what about the coolant? I have seen 2 theories on this and not much actual testing.

Theory 1- Hot engine makes more power. Heat is energy, keeping as much heat in the combustion chambers as possible will produce the most powerful explosion and therefore the most power. A cold combustion chamber will absorb thermal energy from the explosion, therefore decreasing pressure. Piston rings seal best when at normal temp, increasing compression. Fully warmed up engine has less friction due to proper clearances as well as thinner oil. Hot engine atomizes fuel more efficiently, so more efficient combustion. ECU may limit power with a cold engine for safety .

Theory 2- Cold engine makes more power. Cold head means cooler intake manifold, ports, and valves, therefore a denser charge. A cooler combustion chamber will accept more air and fuel, therefore more power. Ignition timing can be advanced more, making more power.

To make things even more confusing, different people have reported different experiences with this. For example, some people report that they get their best time at the drag strip with an ice cold engine while others say that their ET is better with a hot engine. One person even said that they blew the fuse for their electric water pump at the staging line and their engine was up to 250 by the time they got to the finish line and they broke their record and got their fastest time ever with an overheated engine! Same with the few hot vs cold dyno comparisons I found online.

So what's the reality on the power potential of a hot vs a cold engine? I'm thinking that most engines are probably going to make the most power when running at the temp that they are designed to (fully warmed up but not overheated), but please correct me if I am wrong. Thanks in advance!

Last edited by Avery4; 04/01/20 08:49 PM. Reason: Spelling
Re: Does a cold engine make more power? [Re: Avery4] #5391340 04/01/20 09:08 PM
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You can make more horsepower with a cold engine as evidenced by Nascar qualifying. This is a moot point on modern engines with fuel injection and closed loop oxygen monitoring computations. The computer controlled stoichiometric ratio insures the best efficiency will also pretty much insure the most horsepower at the same time without harming the engine.


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Re: Does a cold engine make more power? [Re: sloinker] #5391348 04/01/20 09:14 PM
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Avery4 Offline OP
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Originally Posted by sloinker
You can make more horsepower with a cold engine as evidenced by Nascar qualifying. This is a moot point on modern engines with fuel injection and closed loop oxygen monitoring computations. The computer controlled stoichiometric ratio insures the best efficiency will also pretty much insure the most horsepower at the same time without harming the engine.

Thank you. So could the performance of a modern computer controlled fuel injected engine benefit from running at a lower temp as well, such as if someone installs a lower temp thermostat? Or would other things like ignition timing need to be adjusted to take advantage of the lower operating temp?

Re: Does a cold engine make more power? [Re: Avery4] #5391360 04/01/20 09:25 PM
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A warmed up engine with cold air coming in is what you want. Some engines heat up the intake tract with sustained running which hurts power, heck some engines used to run coolant around the intake manifold to boost intake temps for mileage efficiency, but I think most now have a plastic intake with reduces heat soak quite a bit if the engine is pulling some air through. The scangauge in my Focus shows air intake temps at ambient when running on hwy. The intake temp will creep up quite a bit if the car is just sitting idling though as the intake is right behind the rad.


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Re: Does a cold engine make more power? [Re: IndyIan] #5391379 04/01/20 09:47 PM
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Originally Posted by IndyIan
A warmed up engine with cold air coming in is what you want. Some engines heat up the intake tract with sustained running which hurts power, heck some engines used to run coolant around the intake manifold to boost intake temps for mileage efficiency, but I think most now have a plastic intake with reduces heat soak quite a bit if the engine is pulling some air through. The scangauge in my Focus shows air intake temps at ambient when running on hwy. The intake temp will creep up quite a bit if the car is just sitting idling though as the intake is right behind the rad.

The coolant through the intake manifold was for keeping the carb warm so the gas would atomize and not build up ice and operating the choke before electric chokes.

As hot as possible is what you want the combustion chamber to be but as cold as possible on the intake tract.


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Re: Does a cold engine make more power? [Re: jhellwig] #5391380 04/01/20 09:51 PM
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Avery4 Offline OP
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Originally Posted by jhellwig
Originally Posted by IndyIan
A warmed up engine with cold air coming in is what you want. Some engines heat up the intake tract with sustained running which hurts power, heck some engines used to run coolant around the intake manifold to boost intake temps for mileage efficiency, but I think most now have a plastic intake with reduces heat soak quite a bit if the engine is pulling some air through. The scangauge in my Focus shows air intake temps at ambient when running on hwy. The intake temp will creep up quite a bit if the car is just sitting idling though as the intake is right behind the rad.

The coolant through the intake manifold was for keeping the carb warm so the gas would atomize and not build up ice and operating the choke before electric chokes.

As hot as possible is what you want the combustion chamber to be but as cold as possible on the intake tract.

Thanks. My car had coolant running through the throttle body too, from what I read to keep it from freezing up in cold weather. I bypassed it because I live in the south and adding more heat to the intake air is certainly not what I want to do LOL.

When you say I want the combustion chamber as hot as possible but the intake tract as cold as possible does that mean that a higher coolant temp would make more power presuming the engine will survive it without pinging and the temp of the air entering the cylinder remains the same?

Last edited by Avery4; 04/01/20 09:53 PM.
Re: Does a cold engine make more power? [Re: Avery4] #5391391 04/01/20 10:09 PM
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The hotter the combustion chamber the less the heat of combustion looses energy to heating metal leaving more for the expansion of the gasses. Obviously this has to not cause other issues like precombustion and the melting of things so there are some trade offs. The colder the air means it is more dense and there is more oxygen in the amount of air drawn in. More oxygen makes for more complete combustion thus more power. There are also trade off in that area.


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Re: Does a cold engine make more power? [Re: jhellwig] #5391404 04/01/20 10:23 PM
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Avery4 Offline OP
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Originally Posted by jhellwig
The hotter the combustion chamber the less the heat of combustion looses energy to heating metal leaving more for the expansion of the gasses. Obviously this has to not cause other issues like precombustion and the melting of things so there are some trade offs. The colder the air means it is more dense and there is more oxygen in the amount of air drawn in. More oxygen makes for more complete combustion thus more power. There are also trade off in that area.

That's what I thought, thanks. So presuming everything else stays the same, a higher coolant and therefore cylinder temp will increase power presuming the higher temp doesn't cause any problems, right?

Years ago I read an article where someone dyno tested apparently otherwise identical iron and aluminium heads to see which made more power and with everything else the same the iron head made more power, which made sense to me since the iron head holds more heat in the combustion chamber and absorb heat slower. However, I saw an Engine Masters episode yesterday where they did the same test and with everything else the same the otherwise identical aluminium heads made more power because they kept the cylinders cooler. I'm so confused confused

Last edited by Avery4; 04/01/20 10:23 PM.
Re: Does a cold engine make more power? [Re: Avery4] #5391409 04/01/20 10:40 PM
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A fully warmed up engine will have better ring seal so that means more power. But you want cooler air entering the cylinders to get more dense air to make more power.

This is why muscle cars had ram air,cowl induction etc and diesels have an intercooler.


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Re: Does a cold engine make more power? [Re: Chris142] #5391412 04/01/20 10:43 PM
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Originally Posted by Chris142
A fully warmed up engine will have better ring seal so that means more power. But you want cooler air entering the cylinders to get more dense air to make more power.


My thoughts exactly. When pistons/rings/cylinders are at operating temp, that should give the best seal.


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Re: Does a cold engine make more power? [Re: Avery4] #5391454 04/02/20 12:20 AM
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Here's my empirical evidence gained from real world experience but it might not apply to new vehicles with exotic tech...
fuel injected cars made from the mid 80's up to at least 2004 will see a significant boost in performance at the dragstrip if allowed to cool off for an hour or so and then driven straight onto the track without being warmed up. We used to go as far as packing ice on the intake manifold while cooling the engine down for a denser intake charge and it does help.
Dedicated drag racers used to be commonly seen running an electric water pump on their drag car engine with the engine off and a fan blowing on the radiator between runs.
Kenny Duttweiler said you want hot oil and low coolant temps for drag racing engines to make the most power. But this I think he was referring to a forced induction engine. Low coolant temps allow for more timing...
That being said...
My street bikes liked to be fully warmed up for the best times at the dragstrip. 220+ degrees seemed to be the norm. ...I don't know why.
...I used to work for a dirt track (oval) race team that used 800-900+ hp Ford engines. Those engines ran in the 220+ range during the race.
...hope this gives you some ideas.

Last edited by j_mac; 04/02/20 12:24 AM. Reason: adding info
Re: Does a cold engine make more power? [Re: Avery4] #5391621 04/02/20 08:04 AM
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Moot Argument on your Modern F.I. passenger car with wideband lamda control and aluminum heads.

ABSOLUTELY DO NOT CHANGE THE thermostat to a lower value.

You wan the c. chamber and upper 1/3 of the cylinders as hot as material science allows.

You can make more power with a cooler , denser intake charge. But most cars have a cold air intake from the factory.


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Re: Does a cold engine make more power? [Re: Chris142] #5391634 04/02/20 08:24 AM
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Originally Posted by Chris142
A fully warmed up engine will have better ring seal so that means more power. But you want cooler air entering the cylinders to get more dense air to make more power.

This is why muscle cars had ram air,cowl induction etc and diesels have an intercooler.


Not to mention Mopar huffing AC’d air into the most radical Hemi engines

Re: Does a cold engine make more power? [Re: Avery4] #5391635 04/02/20 08:28 AM
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Yes in a basic sense.

BMW for example with the N55 engine would modulate coolant temperature based upon driving profile (Max efficiency on hwy vs max power). Keep in mind that coolant under max power wouldn't be "cold". It would just be comparatively cooler vs the max efficiency profile.

Last edited by BMWTurboDzl; 04/02/20 08:31 AM.

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Re: Does a cold engine make more power? [Re: Avery4] #5391646 04/02/20 08:39 AM
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The ideal coolant temp for peak power will vary from one engine to another. You can't really generalize it. For my drag car, I use a 160*F thermostat and let it fully warm to that temperature before pulling into the burnout box.


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