2 in / 2 out exhaust revisited

[Kevin G.]

Just curious about the low end losses when using a 2 In / 2 Out muffler on an early MFI car. What would cause this in a supposed performance upgrade? Is it particular to MFI cars only? Am I correct in assuming that the 2x2 lessens needed back pressure at low rpms, but the car doesn't need the same at higher rpms?

I have a 2.2S motor with the 2X2 muffler, and it stumbles (sometimes badly) at lower rpms, but loves to run at 4K+. I don't believe that the muffler is the cause - at least entirely.

Since the car came with this setup, would I be better off with an original muffler since the car is a daily driver? If so, what would I be losing at the high end?

Thanks for the info!!!

[tobluforu]

I have the 2 in, 2 out on my 72 w\rs pistons and e cams. With the stock setup, my car has more snap off the line. With the 2in,out, it has greater top end with the low end losing some umph. I like the trade off since I don't have s cams. Mine doesn't stumble though-runs great!

[stuttgart]

So if I were to cut another outlet on my back dated exhaust and i'm running SSI's.. this is on a 78SC.. i'd lose low end grunt?? i've herad that you can remove the baffles in a back dated exhaust and increaset he low end a bit....

Eric Mckenna
78SC

[NYSCAR]

I think a big part of the problem is AF mixture when you change the exhaust on an MFI engine. The MFI has no airflow meter, it figures out fuel mixture from throttle position and RPM, and it's hard coded in a shaped cam in the MFI pump. So if you change the dynamics of an MFI engine, whether it be exhaust restriction, intake restriction, displacement, etc etc you in theory have to change the shape of the lump of metal in the MFI pump (which is not easy to do, versus changing the chip in the new cars with the fuel map in the computer). You may lose low end because changing the exhaust either makes the low rpm range mixture too rich, or too lean (i would figure it would lean out but not sure), both will kill power quickly (but at high rpms the dynamics may not change so much and the cars tend to run rich at high rpm anyway).

Brad

[Joe Bob]

At least on MY SC....the SSI and dual out increased my power band....it used to be dog doodie until 4,000 rpms, now it comes on around 2800 revs strong to red line......

[stuttgart]

MikeZ,
What type of exhaust are ya running?

Eric,
78SC

[Joe Bob]

Dansk......got it from Pelican.....

[stuttgart]

You say you for sure feel some increase in the low end grunt huh... wel then,, i already have the SSI's.. so maybe i should check in to the D exhaust.
thanks,

Eric mckenna
78SC

[Joe Bob]

Yup.....the late 70s and early 80s Porsche 911s suffered from restrictive exhaust due to smog regs.....anything that allows them to flow without restriction is a help.