Loud exhaust sound under heavy throttle

[samwfine]

I have a 1977 911S with about 70,000 miles on the engine. Recently it began to make a loud sound under heavy throttle. It is not loud if I ease into the throttle. My mechanic has checked for vacuum leaks, inspected the exhaust manifolds for leaks and replaced the muffler. Still makes loud noise under heavy throttle. Engine runs fine and has plenty of power just noisy. Does exhaust run through the heat ex changers? Any ideas what this could be?

[john walker's workshop]

Loose heads. Drain oil and remove the valve covers. Retorque the head nuts to 23lb. Typical with a 2.7. Hopefully no head studs are pulled out of the case.

[Peter Zimmermann]

I second John's comment. I'd like to mention that it might be wise to sneak up on that 23 lb/ft torque number. Do 15 lb/ft first, then 18 lb/ft, then 21 lb/ft, finally finishing with 23. Do this with a cold engine. After doing this you'll know if you're in for an expensive job, or if you'll be OK for a while. If you have a "pulled" stud(s) the engine will probably require a tear down, and case savers installed, to secure the heads. If the nuts are loose, but torque, move your major service interval from every 15K miles to 10K miles, and be sure to have your technician torque those heads as part of your valve adjustment.

FYI; you might want to consider a different mechanic, because usually a quick test drive is sufficient to make this diagnosis. It can sound "loud," as you describe, or it can sound similar to when you were a kid and put playing cards in your bicycle wheels. It can also sound like any number of old VW Beetles that have a similar issue. Because the car is a '77 it might also have one or more broken cylinder head retaining studs, which is very possible.

[HaroldMHedge]

I think the 77 has an air injector pump. On my 911SC with the smog air pump removed. I had a similar loud exhaust when one of the plugs had come loose.

Harold
82 911SC Coupe, Ferry Porsche Edition
87 911 Carrera Coupe