Exhaust tuning

[Tom B]

Hank's comment, in a thread on Avanti Exhausts brought this to mind.

I drive regularly a 60 Lark VI flathead with a single exhaust. At 60 miles (57 to 65, maybe) an hour there was a strong, hard 'Whap Whap Whap' from the exhaust. I counted them up to 60 in 15 seconds, this means four times a second. If I had been on a long trip I would have ended with a major headache. I sawed off the (square) end of the tail pipe on a long diagonal and it moderated to about 1/4 of what it had been. Still there, but not nearly so annoying. Obviously there is a major harmonic at work. Windows in all states of open and closed, it's still there.

The system looks stock, except.... about 8 inches from the manifold, the old pipe is stuck inside the new pipe and held together with a U bolt clamp.

What can I do to eliminate this harmonic?

I'm looking for an answer that doesn't include a new exhaust system, perhaps just the front pipe, but I will welcome all suggestions.

Tom Bredehoft
'53 Commander Coupe
'60 Lark VI

[curt]

I get that type of noise with back windows open (on many makes of cars).

[hank63]

How's your system mounted? Firm or loose (to chassis)?
If it really is harmonics, you might experiment with stiffer / softer mountings. At least, that won't cost an arm and a leg and, who knows, it just might help.
/ H

[jcstude]

Tom,
There has been much study recently over the merits of 'tuning' your exhaust but most of it has been devoted to V-8s with dual exhausts. What might help on a single exhaust is reducing the diameter of the pipe after it exits the muffler. The objective of tuning is to keep the gasses flowing as rapidly and efficiently as possible. That can be accomplished by reducing the pipe diameter as it gets farther from the headers. It doesn't look cool but it is more efficient. The other trick is the 'resonator' route which adds expense and weight. On the other hand, ear plugs are only pennies...........

Lew in Escondido, CA

[Tom Bredehoft]

I'll keep playing with it 'til I get it right. Before I added the Over Drive transmission, 2500 rpm was in the middle of the shift range, now it's at 60 MPH.

Oh, By the Way, one two hundred mile stretch of pure InterState driving got me 29.2 MPG. Keeping up with traffic.

Tom Bredehoft
'53 Commander Coupe
'60 Lark VI