Have a 1994 Volvo 940 non-turbo with 287,000 miles on it. Put new stainless exhaust system with magnaflow resonator and muffler along with 2 & 1/4" pipe and low restriction cat about 2 years ago and the car ran great until very recently.

The resonator, a straight through piece with little baffles showing from the outside diameter of thepipe towards the center, came loose internally and started rattling. Had exhaust shop put on new resonator and the carhad no power. The change was absolutely instant after installation of this new, smaller resonator.

The resonator he installed is about 4 " shorter than the one that was replaced and it's also very small diameter on the inside....guessing about 1 & 1/4" and doesn't go straight through either like the original one. Since the old one was thrown away, I have no idea what size it was I.D.

So the first thought was that the exhaust is now too restricted and the back pressure is too great due to the decreased I.D. of the replacement resonator.This new resonator was replaced again only this time with a magnaflow straight through resonator that is about 2" in diameter. Now it still has no power on low end but is a bit more lively above 3500 rpm's. Still no where near as strong as it was prior to changing the exhaust two years ago.

Rather than go through a complete trial and error of swapping pipes and different size resonators, would like to find the exhaust specifications for proper back pressure for the 940 non-turbo.

The engine has had the head ported and polished, the throttle body is 2 mm larger than stock, it has a K&N filter and runs a Volvo 3/4 race came. The plugs are new Iridiums. I tried swapping plugs back to the NGK and there is no changeat all, with theloss of power still quite obvious. Also put it on a tester and checked the cat and it's precisely within specs and working fine.

I'm an absolute fanatic about maintenance. Plugs are always tip-top, always run premium fuel, fuel filter is new, air cleaner is clean, run synthetic oil in both the engine and tranny, and run magnacore thick spark plug wiring.The car has 287,000 on it and the compression is within 1% of factory specs andall cylinders are within 1% of each other as well.

I cannot find any specs on exhaust back pressure in the volvo manuals, the dealers don't have a clue and surely someone out there has played with their exhaust before.

So, the questions are these:

1. What is the normal, stock size of the exhaust pipe? Is it 1 & 7/8"??

2. Does it matter where the back pressure is created in the exhaust system? Should it be closer to the front just after the cat. convertor or further back near the axle.

3. Does positioning the resonator either forward or further back have any beneficial effect on exhaust scavenging pulses?

4. Does anyone out there know of a shop that actually knows how to measure exhaust flow instead of just throwing parts on till it seems to run?

5. Does anyone know the actual back pressure these things are supposed to have?

Any help would be appreciated. Thanks much.