my exhaust shop offered to put true duals on my 86 transam. i have a 350 with melling cam in and its also chipped with a custom one from TPIS.

He said he will put 2.5" pipes running back, two 1 chamber flowmasters where the car used to be and then go straight back and dump it before the rear axle. How will this affect my gas mileage?

Thanks

Gary

 

(Tongue in cheek) It will definately lower your chances of passing smog tests. I assume the Flowmasters will go where the "cat" used to be.

Typically, better exhaust flow will have no effect to some improvement in gas mileage, depending upon the tuning that is done to adjust to it. Since it will run better, you'll be more tempted to use the extra performance, so that will usually lower gas mileage.

But, a good 3" high flow cat and 3" cat back will probably flow as well as what he's proposing, without the negative effects mentioned in the first paragraph. Likely cheaper, too.

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82 Berlinetta, orig V-6 car. Rescued w/86 LG4/TH700R with all harnesses, sensors, ECM, etc. 2.73 open. Cat-back from '91 GTA, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LB9 w/ZZ3 cam, exhaust, paint, etc.).
57 Bel Air, my 1st car. Currently 396 .030 over, Weiand Action+, Edelbrock 1901 Q-Jet, Jacobs Omnipack, 1-3/4" headers, TH400 w/TCI Sat Night Special conv & shift kit, 3.08 10-bolt, AMSOIL syn lubes bumper-to-bumper. Best 15.1 @ 5800' Bandimere. Daily driver while Camaro was being put together.