air cooled engines

[kachok25]

I don't know what to think of the air cooled engines on one hand they are looked down on as being low tech out dated, but back in the 80s porsche was building some awsome air cooled engines for their racecars, what was the downside and upside of air cooled vs water cooled?

[Moppie]

The upsides are light weight, and reduced complexity. They are used extensivly in the avation industry for these reasons.

The downsides?
They require huge amounts of cool air flowing around them, which means locating them in areas of high air flow, which means compromises in chassis design. You also have to create the airflow when the car isn't moving, which means useing very large fans which are noisy and again cause design compromises because of thier size.

You will notice that as soon as Porsche went to water cooled engines in the 911 they also moved the engine forward so its now over the rear axle, rather than behind it. Its not something they could have done that with an air cooled engine, as there would not have been enough air flow to cool the sort of heat the HP they now produce generates.

[kachok25]

Aprox how much weight difference is there? In the case of a low-mid RPM non boosted engine would it be benifical to use an air cooled engine block if noise/chassies was not a concern? Would an air cooled engine have any aditional problems with emissions or pre-ignition than a water cooled engine?

[GreyGoose006]

well if the ambient air is 100*, it will have a hard time keeping cool, but a properly built air cooled engine should be fine in most circumstances.
are you thinking of CONVERTING a water block to an air block, or BUYING an air block and building it.
as far as i know, you cant convert an engine to air cool because it wouldnt have the passageways and such to cool off.
if you are trying to convert, you will only save the weight of the coolant, and have serious problems with overheating. in otherwords it cant be done.

if you are buying a block, it should be fine, just make sure it is from a company that knows about how to build them.

[curtis73]

Typically air cooled blocks have higher oil temps, which as long as they stay below about 235* its a good thing. High oil temps burn off or evaporate off bad contaminants. Above 235* and you start cooking the oil itself.

I think the water cooled setup is worth the complexity for consistent temps, higher power levels, and consistent oil performance, but there is nothing wrong with a good air-cooled engine as long as you're not trying to get really long life or really high power levels from it.

[Black Lotus]

Originally Posted by kachok25

Aprox how much weight difference is there? In the case of a low-mid RPM non boosted engine would it be benifical to use an air cooled engine block if noise/chassies was not a concern? Would an air cooled engine have any aditional problems with emissions or pre-ignition than a water cooled engine?

The weight question is basically unanswerable with the info you've given. Generally they are a bit lighter, but......Yes air cooled engines have more problems with emissions and preignition than water cooled. That's some of the reasons why you don't see them on new cars anymore. The temperature of a water cooled engine is kept more stable and even, particularly at the cylinder head.
Some of the better built (Porsche) air cooled engines have been scabbed into small sports racer chassis in the past, and they have done well. The low CG of the "flat" cylinder configuration can be a nice bonus. For long racing events you'll may need a very large oil cooler.
For some guidance you might look into the Porsche sports racing cars of the 60s and early 70s (Types 904 thru 917).

[beef_bourito]

just wondering, how much power does a water pump usually take? wouldn't an air cooled engine have a bit more power as well for the same displacement, etc.

[UncleBob]

Originally Posted by kachok25

Would an air cooled engine have any aditional problems with emissions or pre-ignition than a water cooled engine?

any vehicle that is driven in a wide variety of loads and ambiant temps, would have less pinging with a water cooled engine. Air cooled engines, due to their lack of cooling capacity, are usually backed off a bit on the timing from optimal for this reason. In hotter temps/extended high loads, they would get hot and cause pinging, where as a water cooled engine wouldn't.

There is also the drivability issues. Air cooled engines are more finicky in cold weather and/or when the engine is cold.

There is too many advantages to water cooled engines, to make air cooled engines attractive IMO. Even if you're talking a very limited, specialized situation. For example, many drag race vehicles (cars and bikes) will take a watercooled engine and fill the water jackets with a metal compound to add strength to the "block", but these engines are never ran longer than 60 seconds, so they can get away with it. Thats about the only situation where I can think of that it really doesn't matter.

[curtis73]

Agreed... even dirt bikes which (for years) all were air cooled are all going to water cooled. The added complexity and weight even in the lightest of vehicles is sooo worth the trade off to get the added benefits of water cooling.