I'm considering rebuilding my motor. It's a 300 hp automatic car, but I have always been a little concerned about my car heating up in really hot and humid weather. What do you think about putting a lower compression piston in the motor in order to reduce the heat. I'm not after horse power, I just want a car that runs good. Will a low compression piston allow the car to run that much cooler? Does anyone have a facts that can substantiate this thought?

Thanks
Jack

 

That much cooler? How much is "that much"?

Why don't you just fix what is really wrong?

 

Lower compression means higher temperatures and more HP converted to heat.

 

????.......

 

The higher the compression, the higher the thermal efficiency and the more fuel converted to power rather than heat. Raise the compression ratio and raise the horsepower. Thermodynamics says so.

One reason diesels get better mileage(less fuel to heat) than gasoline engines is higher compression ratios.

 

Nothing beats a Dewitts aluminum radiator and aluminum water pump to beat the heat issue. A working fan clutch and fan shroud too.
Theres absolutely NO reason to rebuild with lower compression pistons in a 300hp car.
Better off putting a crate motor in, unless its the original engine and you want to keep it original. Personally, Id go with a 383.

 

Thanks.....I'm rebuilding the original motor, so a 383 is not an option. I have an aluminum radiator, but not an aluminum water pump. (Not sure if that helps reduce heat problems.)

My clutch fan and shroud is good. The purpose for my question was to just ask if lower compression pistons reduce heat.

Thanks to all

 

That sounds better but seems to be in conflict with your earlier post?????.

 

No lower compression increases heat.

 

Diesel engines have 20:1 compression ratio or higher.

 

?????????????????????????
You left me dazed and confused!



Would not it also follow that if you "used" the extra horsepower generated by the higher compression ratio (or by any other means), you would throw more heat into the cooling system?

 

Silly me. I thought it was the heat energy that drove the pistons downward.

To the OP, if your cooling system is operating properly your car will not have overheating issues.

 

Although Thermal efficiency may work to burn the fuel efficiently there are other factors that play into a cool running car. Assuming your cooling system is up to par and we are only comparing engines and compression ratio's.
Typically a high compression engine will make far more heat because it makes far more POWER. Any time you upgrade horsepower you will also need a increase in cooling capacity. In the Engine rebuilding business we bumped into this for years. Regardless of what an engineering book may say in generalities. In cars when you increase Horsepower you will increase heat which needs to be controlled by improved cooling systems.
The other wild card is when you increase Compression you will have far more detonation issues because the fuel we have access to will start to burn at the wrong time.
( Detonation ) once detonation starts your power goes down while heat goes up.
One of my former co workers was known to be one of the top Piston Design engineers in the world. His designs were tested in Race engines from every venue and in Heavy Duty work engines as well as passenger cars, he was well rounded. He told me using current available pump gas, not E85 but traditional fuel he can make more Horsepower with a dished piston even if he had to give up a half point of compression. IE: 9.5 vs 10.0 because the dished piston helped the flame front form a ball that efficiently burned the fuel and allowed more timing to be used increasing the efficiency. Those are words from someone that had to back up his words with happy customers.
Many modern engines use a dished piston with a small head chamber to maintain the high compression and help the flame front.
That was a diversion but you need to avoid detonation or you will increase heat as well as causing potential engine damage.
If our OP is running a 10:1 engine that is always trying to detonate so his timing is retarded back a few degrees to avoid detonation he might run cooler with less compression and more timing.
I do know that running very tight deck clearance can and will help avoid detonation but this post is about changing pistons to run cooler.
High compression is great and desired right up to the point that hot spots start to appear in your head chambers and they start working like an extra spark plug but in this case an uncontrolled spark plug that can cause big damage.
This OP might be very happy with a low compression engine that runs on 87 octane and does not overheat while most on this forum would opt for original compression and parts and run octane booster on top of 91 octane to keep it running well. Each to their own on this. The OP might find that a good distributor tune could eliminate his hot running if his cooling system is good. Seems like many have eliminated cooling issues by fixing the distributor and vacuum advance.

 

Don't over-think this. What makes a car run and cool at factory specs is components doing their job and a proper tune-up. Your car didn't overheat when it was new (and all engines, 250hp through 375hp, used exactly the same cooling system and radiator), and it won't now if it's assembled and configured correctly. Don't even THINK about changing pistons to make it run cooler - that won't work, and it's not the root cause of your overheating (?) problem.

Define "heating up"

 

from Wiki:
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature. It may be more helpful to think of it as an "expansion ratio", since more expansion reduces the temperature of the exhaust gases, and therefore the energy wasted to the atmosphere. Diesel engines actually have a higher peak combustion temperature than petrol engines, but the greater expansion means they reject less heat in their cooler exhaust.

 

I agree. Let me give a personal example. I put a "high" compression head on my 1928 Model A Ford. Stock is 4.2:1. "High" is 6.0:1. With the high compression ratio my engine runs cooler.

With stock head, 1928/29 Model As ran hot from the factory. In 1930/31 Ford put in a bigger radiator.

 

While your experience may have applied directly to the Model A with 4.2 moving to 6.0:1 and your horsepower went from 28 to 33? I think it is misguided for our more modern engines. In these you might be moving from 8.5:1 up to 10.5:1, horsepower could go from 250 to 350 or 400 HP. In these engines more power makes more heat. 50 years of history backs this up.
In Diesels we have the same issue, modern Diesel's are computer controlled for injection and total turbo boost. Many guys will add aftermarket chips to gain more power and some gain more MPG. When they add more fuel and more Boost, ( higher compression ) they also add an exhaust temp gauge because when they add more boost and more fuel the exhaust temp climbs to a point they can start melting pistons. They watch the exhaust temp gauge so they know when to back off the throttle to save the engine.
This is true if racing or towing a load. Modern Turbo charged Diesel engines have lowered the compression to the 16 to 16.5:1, they did this to make room for more Air/Fuel that is now packed in with up to 40 pounds of turbo boost, you will find this in big hard working Diesels today, like a D8 Caterpillar as well as over the road trucks. You seem to have a solid background in engineering but some of this is old seat of the pants, what works, what does not experience.
If your cruising 60 mph on the freeway with a 327/250 or the same car with a 327/350 the heat production would probably be very similar but if you used the power available by flooring it pulling a hill I would guess your 350 HP will make more heat while making 100 more HP. I think we have beat this till most lost interest. Sorry

 

One last post. Everything else being equal, raising the compression ratio raises the efficiency of an engine. The fuel burned in the engine, the input, is equal to the output of the engine: heat to the cooling water and horsepower to the wheel and heat to the exhaust.

Raising the efficiency means more work out of the engine(horsepower) and less heat to the cooling water and exhaust.

Read about thermal efficiency at wikipedia.org

"energy conversion efficiency is the ratio between the useful output of a device and the input", useful output is work or horsepower

"the efficiency increases with the compression ratio"

Energy input = energy output

Energy output is work(horsepower) + heat. If work increases, heat decreases.

 

a word of caution about quoting 'wiki'.... wikipedia is only 'someone's' opinion and isn't necessarily 'the truth, the whole truth, and nothing but the truth'. essentially it is an open forum encyclopedia on a grander scale, like our forum...

Bill

 

I agree with all you mentioned above and I am sure textbooks will verify your facts.
In engines there is a limit to these facts.
I had a kid wanting warranty on my pistons, he changed from 76cc heads on a 400 sbc to 305 heads to increase his power. I think they were 58cc heads. He certainly increased his compression, I think we estimated he went from 9.7:1 to 12:1, this was several years back.
The engine lasted about one day before it detonated the pistons to death. Broken top rings and top ring lands. Major detonation. His timing was OK at 36 degree's total but not OK for the high compression. Thermal efficiency is limited. If this kid had run 104 octane it would probably have realized the benefits of compression but he had regular fuel, maybe 87 octane and it blew up. Just food for thought. When detonation is started your cylinder pressures jump, Standard 327 at wide open throttle properly tuned will make about 600 PSI in a cylinder. A wild 327 maybe 450 HP might reach 900 psi at wide open throttle. Mild detonation will jump cylinder pressures to 3,500 PSI, major detonation is cyl pressure of 5,000 PSI or more. This is why head bolts stretch allowing the cyl fire to burn the head gasket out between cylinders, the head actually lifts off the block and the fire blows out like a furnace flame melting out the head gasket and sometimes the block. It also breaks pistons, rings etc. In minor prolonged detonation it can pound out the rod bearings. All bad stuff.
I gave the kid warranty and an education on compression and fuel needs so the repaired engine would last. We pulled his compression back to 10:1 and a warning to always run premium fuel.
Modern engines now have direct injection so there is no fuel available to detonate, a huge advantage to more power and efficiency but even these engines limit compression. I think the highest I have seen is 11.5:1 with direct injection. The OEM's know how to balance Power/efficiency and long life in these engines. I do not know why they don't take Direct injected engines up to 15:1 but I know they have a good reason.