Lower Temperature vs. Higher
 

I was just thinking and people usually say the colder the more hp is made when talking about the air. Makes a lot of sense, the air is denser thus allowing more air into the engine. Now i see people changing out their thermostats, fan adjustments or switches, as well as other cooling modifications and I wonder.

If the temperature of the engine is lower, wouldn't it slow the combustion of fuel and air? The slower combustion would mean less complete burn of the mixture and thus less power.

Now it doesn't make sense to me that people generally try to get the engine running as cool as possible with these modifications when it could affect the combustion. To what extent, I'm not sure but I know it would.

Now is this true that there should be a certain best temperature? How do you find what is the best engine temperature?

What do you say?

Re: Lower Temperature vs. Higher
 

The higher you can get the engine temp without running into detonation, the more power you can make. That's because our engines don't run on fuel, they run on heat. The fuel is just there to CREATE the heat.

Do a google search on Smokey Yunick and his Adiabatic engine for more info.

However, it is easier to run the compression up, advance the timing and simply control detonation with colder temps, which is why people do it.

Re: Lower Temperature vs. Higher
 

Do a search in this section for post by Mr Horsepower on the subject.

Re: Lower Temperature vs. Higher
 

I think the problem is that heat radiates. If you could keep good heat in the motor without detonation, and keep the intake cool that would probably be ideal.

Re: Lower Temperature vs. Higher
 

On my 87 GN over the years I found it ran best between 140 and 160 deg at the start of a pass, it would be about 180 at the end of a pass. This is not taking into account of the outside temp. HP would improve with colder outside temps but for my given combination that is the range it ran the best and most consistant. I would think any given combination will have a sweet spot where it likes to run. I have not owned my Z28 (69 with 470 Reher Morrison) long enough to collect enough data to see where it likes to run at.

Take Care

Re: Lower Temperature vs. Higher
 

Yep that's the best deal.

Re: Lower Temperature vs. Higher
 

Dyno testing is the best way to determine what temperature an engine creates the most power at, because different variations will affect the temp at which this occurs, of course. For a street engine, however, 160 degrees is about the best for an iron block, and 180 for an aluminum block- lower temperatures might make more power but at the expense of lowered gas mileage, more frequent oil changes, and increased wear on piston rings and cylinder walls. I hear most race only engines run around 140 degrees (or at least they try to, cooling system size will increase exponentially the closer you try to get to ambient temp- or in other words, going from 220 to 200 is a lot easier than 160 to 140).

To sum it up, pressure is what pushes the piston down and creates power. This pressure is created in two ways: one, where the piston travels up and compresses the mixture (a negative, but neccessary drag), and two, the mixture igniting and the pressure increasing dramatically- this would of course be where the power actually comes from. The more compression you can run here, the more power you can create. The problem is that gasoline vapor will self-ignite at a certain temperature without a spark, so too much compression will be bad unless you can find a way to get around it. One way is to lower the operating temp of the engine so that the mixture is cooler up until the point at which it ignites. Most street engines don't use this method for reasons mentioned above, later model engines run on the verge of detonation all the time to maximize power and efficiency. If you lower the operating temp the engine will automatically advance the timing slightly to take advantage of this- keep in mind that more power out means more fuel spent- for best results tuning is usually best.

As far as ambient air temp, about a 50-60 degree intake air temp is usually best, so a cold air intake will benefit about anything out there. The cooler air coming in is more dense so it needs more fuel, which will boost dynamic compression slightly, which means an increase in power. One would think that with more power comes lowered mileage, but only at wot situations. The rest of the time, the engine's pumping losses are decreased because it doesn't have to work as hard to draw in the same amount of air as before. Most claims of increased mileage from a cold air intake are true, at least from the likes of K&N, etc. (although they can manipulate the test results to exagerate what you'll actually get) - keep in mind that a shiny metal intake is not a cold air intake- more like a hot air intake since metal is a great conductor and takes the heat from the engine bay and feeds it to the intake air. (take that, ricers)

Another new method that has come along that allows higher compression without adverse effects is direct injection. It's pretty simple, since the fuel is sprayed directly into the cylinder it has a cooling effect on the air, so it doesn't heat up until right before combustion. An ideal combustion event would be one where the mixture stays cool until right before it ignites, where it would heat up until it was just under the point of detonating, and then it would be ignited by the spark plug. This is of course an impossibility, and the whole point is to find a compromise between cooling (poor combustion) and heating (detonation) to find peak power and efficiency. Another thing we may see down the road would be compression ignition for gasoline engines, but that's probably got more obstacles to overcome to it than direct injection did, so we'll see- it may end up being impossible or impractical overall.

Anyway, that's just the tip of the iceberg, I'm no expert but I'm fairly knowledgable on the subject having taken a few classes on it. If you're really interested in why modern engines run the way that they do, I would suggest getting some technical books- you can find quite a few good ones at online book stores, I've bought a couple from amazon.

Re: Lower Temperature vs. Higher
 

We just played around with a Spec Miata on the dyno. The owner's lap times were falling off measurably after the first 2 and he suspected the power was falling off as the engine warmed up.

We did several dyno runs playing with timing and spark plugs, with the first run being 106 and the last being 109 hp with the engine at 160 - 180 deg. We then turned off all the fans and let it heat up to 235 deg and made another pull. To all our surprise, it made 108 hp. That leads me to believe that most V-8 pcm's soften up the tune when the engine gets hot and that's why we do the things we do to keep them cool. Dyno testing with the LS7 in the ZO6 showed significant losses in power (up to 20 rwhp) when the engine was mildly warmed up. But the Miata, with 9.5/1 compression and a superior pent-roof anti-detonation chamber doesn't change the tune with heat.

Mike

Re: Lower Temperature vs. Higher
 

I run a 13:1 alcohol engine. If I'm lucky, I can get it up to 160* on the starting line. Temperature reaches about 190* as I cross the finish line then drops below 140 by the time I get back to the pits. I'm only using a rad from a Firefly. The alcohol has a cooling effect on the engine because of how much is being dumped in.

Below 160*, the engine can't burn off contaminants in the oil such as condensation (the white goo you find under the rocker covers). Above 195* and detonation starts to become a risk. Factory EFI cars all run at 195* because a hot engine produces less emissions. Newer computer controller diesel engines don't even turn the engine fans on until 205*.

Re: Lower Temperature vs. Higher
 

The stock LT1 ECM doesn't turn on the first fan until 225 deg F.