Muffler?

[art leong]

We used a 3" Summit Racing muffler to chassis dyno the car this weekend. The reason for the muffler was to get a better wide band O2 reading at lower rpms, the O2 sensor is in the collector about 8" from the end. So it doesn't read well below 4500 or so.
This is a chambered muffler not a straight thru design.
We have a loss of HP and torque starting at 7100 rpms. and continuing downward at about a 45 degree angle.
We adjusted timing and fuel mixture. Was able to take power away but not add any.
This happened at the same rpms using 2 different intake manifold setups and tunes.
Thought it could be my hydraulic lash adjusters pumping up. But the cam manufacturer says that if that were the problem the car would misfire when backing off the throttle.
Noticed that when we first dynoed the car (inexperienced operator) a year ago there seems to be the same problem. But due to the circumstances never paid any attention to the sheets.
The first motor I built (less compression, less cam) made increasing power to 8100.
Other than cam and compression the only big difference was the first motor had an pretty intricate crank scraper. While the new one only has a windage tray.
Try to narrow down ideas.
Thanks.

[Daran Summerton]

So it ran better with the other cam and no muffler. What's confusing you?

[art leong]

Originally Posted by Daran Summerton

So it ran better with the other cam and no muffler. What's confusing you?

No I didn't say that. I said it made power to 8000. 218 hp to be exact. That motor had smaller cams and less compression.
This motor (bigger cams and more compression) makes 233 hp but at 7100 it noses down. If it could make power to 8500 it should be close to the 240 mark.
Just wondering if a muffler could be the cause of the down turn? I realize the muffler might hurt perfomance some though the range. But to just go to a given rpm and lose power. It still turned 7700. No breaking up, just down on power.

[greg fulk]

You don't have the cam advanced to much do ya?

[art leong]

Originally Posted by greg fulk

You don't have the cam advanced to much do ya?

We played with cam timing on the dyno last year. And I retarded the intake cam at the track in a test session one day, To try to get the cylinder pressure down (compression test =300pounds) It slowed up a bunch.

[smracer2002]

Art,I would try a straight thru design such as
a Max-Flow,MagnaFlow or a Borla.The cars we dyno
have usually picked up power over a chambered muffler
setup with the same id size.
Any reason you didnt use the crank scraper on the new
unit?