I just had the drive belts replace about a month ago. Initially the tension was a bit loose and the belts would squeal at start up especially during cold weather. I then took the car back to have the tension tightened. But now I felt like the engine doesn't rev quite freely as before. The engine feels a little bit high strung.

So do you think if the drive belts (and ac belts) tension a bit tight, it will rob the engine some power?

 

they need some slack...

 

if the belts are too tight they'll tear or crack a lot faster

 

No, not enough that you could ever notice.

 

Belts that are too tight could also cause bearings in some of the accessories to fail sooner due to the added stress places on them by side loading.

 

Belts need some slack; too tight could cause premature failure.

 

Just curious, how do we know if the tension is about right or too tight or too loose? Is there a way to find out?

 

There is usually a spec measured with a belt tension tool. The tension tool pulls the belt slightly with a preset load and the deflection of the belt is measured.

I don't know what the spec is for the G35 however. Someone else will need to look it up.

IIRC, Autozone rents one

They look similar to this

 

there's a belt tension gauge, but it's like 100 bucks

 

You can find them cheaper than that