Muffler Mod...too lean?

[Wil Ferch]

The PO had modified the stock muffler on my 85 Carrera, in adding a right side duplicate exhaust tip. It appears, however, that the point of connection to the muffler is "under" the horizontal seam of the muffler, whereas the original left outlet connects right "on" the seam. As a result,I get a lot more flow ( and noise) from the right side..a bit "booming" in the garage, as evidenced when I cover the right tip with my shoe sole..it goes away.
Questions:
1.) Is it possible to have "too little" back pressure with this set-up, and might it make the car run too lean? How much compensation can the stock DME / O2 sensor system tolerate? How can I tell if I am running too lean? CO test?
2.) If I install an orifice plug on the right side tip..what size should I shoot for? Maybe a ne inch diameter hole plug perhaps?..., within the 2 1/2" diameter exit tip? Looking to balance the flow between the two exits.
--Wil Ferch
85 Carrera

That's a great question Wil. Since I installed a Flowmaster 1 in 2 out, my SC has been running a little funky. Wonder if is too lean now?
Mark

[Joe Bob]

Do plug readings and/or put it a gas exhaust monitor.

[Wil Ferch]

mikeZ:
-Can you be a bit more specific on the gas exhaust monitor procedure?

-Wil

[Joe Bob]

Your local p-car mechanic or smog shop has one.....the best/easiest way to measure a car's performance is to see how it is burning at the tail pipe. If it is too lean the readings will tell you.

The poorman's way is to read the plugs....black=rich, brown is good, white= not so good.....

[911nut]

According to the engine experts that I work with the right amount of backpressure is present when you have equal length headers and collectors. Mufflers add unwanted backpressure and Porsche engines are no different than any other in that they don't need more backpressure than, say, a V-8.
Your mixture is controlled by the DME on your Carrera, from a voltage signal it's recieving from the O2 sensor, plus other data like temperature, speed, throttle position, ect. Assuming that you now have less backpressure than a stock set-up, the mass flow sensor will compensate for the increased flow rate through your engine so you wouldn't be too lean. If your car had Webers then you would have to re-jet the carburators to handle the increased flow.
I'm not knowledgeable on the fault detection of the DME but on other cars with engine management computers of the '85 vintage you get a "Check Engine" light if something like a bad sensor makes the engine run in default mode.
I think the only problem you have at this moment is an annoying reasonance problem with your "custom" exhaust. Cut off the added outlet and plug it and you should be ok.
PB

[Wil Ferch]

911Nut , and others:

Thanks for the reply. I need to clarify a few things...
First...the resonance isn't too bad. In fact, it's nearly gone now that I'm running a Porsche pre-muffler ( with O2 sensor port added), instead of cat.

The issue I have is "too loud" ,... and too much flow through the right side. Jury-rigging a smaller diameter on the right still gives me the advantage of another opening...but not as obnoxious. Wanted to gauge an idea of how much to restrict on that side as part of a more permanent fix.

As to the DME...yes, I have closed-loop control via DME for adjusting the mixture, but the back of the DME has a small rubber plug you can insert a small screwdriver in...and alter the mixture and advance ( either one or both). Would you exercise this option ONLY after you know the basic controls can't compensate ENOUGH? If so, how do you tell? That's where I was going with the exhaust gas test procedure question.
--Wil Ferch

[surflvr911sc]

Don’t worry about too much flow w/ your muffler. Your heat exchangers are more restrictive than your muffler even if it wasn’t modified. I would take it to a muffler shop & explain the problem of more exhaust out of one pipe. To get them equal, I believe you need to have work done to the baffles inside the muffler. It probably wasn’t done or was done incorrectly. You could also just have them restrict that side for you and throw it back on. Or tear out the baffles and really piss off the neighbors. Either way, take it to a muffler shop, they’re cheap and you’ll be happy. Good luck!

[Wil Ferch]

Guys:
Thanks... !
So in summary for the important point....no worries about too lean, ...correct?

I will take "no response" as being correct...to save bandwidth.
---Wil Ferch