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God bless the front mount intercooler

From memory I think Air con takes around 5hp - 8hp from an engine when running. On a 100hp car that is quite a hit, on a 200hp engine then it is a small effect. 300hp it would hardly be noticeable. If you are travelling around at 2000 - 3000 RPM, then your engine is probably only producing about 1/3 the max HP. so a 5 - 8 hp drop would be quite noticeable. Fuel economy would probably be hit a little, but no where near as much as driving, say at 50pmh with the windows down. Air temp & altitude also can greatly effect the power of an engine. The latter we don't need to worry about too much in the UK, but the former will cause issues, hence many 'switched on' modifiers strive to get decent cold air feeds for their engines intake rather than just fitting a 'trick looking' cone filter but still pulling in air from a rather hot engine bay. Basically, our hot summers which would effect engine performance last about a couple of days, so not something to really worry about.

Also keep in mind that in these sort of exceptionally hot conditions the air coming into your engine is far hotter than normal.....and hot air intake equals power loss