To Alan in OZ & ALL INTERESTED PARTIES-

Some exhaust information.

1. Lets get the terminology right - or at least consistant for discussion. The exhaust "manifolds" are the cast iron pieces that bolt to the engine. The "downpipes" or "header pipes" are the exhaust pipes that connect the manifolds to the "Y", where the two downpipes connect into a single pipe.
2. If your Rover has a pair small looking pipes (about 1-1/2" - one from each bank) with or without cats, you have a mondo restrictive setup such as the '87-88 NAS system came with. The '87-88 NAS systems generate 5-8psi backpressure with their triple cats and small headpipes.
3. If your Rover has two pipes on each side, you have the beginnings of a good system. This appeared on non-cat EFI V8s starting about '86.
4. The factory EFI manifolds are adequate for off-road service and operation in power levels up to about 25% more than stock (figure about 200hp).
5. The '89-later exhaust systems are decent, generating about 2-4psi backpressure. Thy use 2" headpipes and 2-1/2" pipes. Muffler changes will produce some slight increases in power.
6. The Rover EFI system limits the amount of power you can get with modifications. Dyno tests I personally performed barely yielded a 5% imnprovement with a very well designed performance exhaust. This was not the fault of the exhaust, but due to the limitations in the fuel injection system.
7. The small headpipes (one on each side) are the main impediment to power on the rigs so equipped.
8. You can install an '89-later exhaust system (including cats) onto a '87-88 for an improvement in power but you will need to change the square trans. crossmember with a later round one, plus relocate some hangers. A 3.9L control unit (the whole thing) will further enhance performance but the 3.5L NAS "low-lift" cam will also slow you down. Consider a 3.9L or mild aftermarket cam.
9. Here's a very cost effective blueprint that I know works (10-12 built with good results). If you alter it much, I make no claims to how it will work.
   1. If emissions regulations aren't an issue, get the aforementioned dual-on-each-side downpipes and open their collectors up to match a 2" pipe. Otherwise, use an entire '89 or later assy complete with cats.
   2. Get hold of the "Y" pipe assy from an '89-later 3.9L RR. Used is ok. It's got 2" pipes. Open the collectors (where the dual downpipes on each side meet) to fit the 2" pipe and section in the 3.9L Y pipes. Alternately, you can have something like this made but the OE LR "Double D" collector flows well.
   3. From the second "Y" use 2-1/2" pipe in a short section to a front muffler. Use a free flow design.
   4. From out of the first muffler, use 2-1/4" pipe. Make the pipe follow the original path and if you want a quiet system, add a second, smaller free flow muffler or resonator.
   5. Add an exhaust tip of your choosing.
   6. Not sure how this system would do with the "big block" 4.5 & bigger engines. Should be OK for normal use, off-road, etc. If high revs and power were desired, probably not a good choice but then a Tri-Y system isn't a good choice where power & rpm capability is desired over torque and economy anyhow.

Why "Tri-Y" this works is that inside your manifolds, the exhaust runners are paired giving two outlets. On the left bank, cyl 1 & 5 are paired and 3 & 7. The dual downpipes connect them about 2 feet down from the manifold. The last Y connects the two banks into a single pipe. A total of 3 "Ys" from head to main pipe. Ever hear of the "Tri-Y" concept?

A "Tri-Y" scavanges better at low speeds than a standard exhaust system. Lets work on the left bank again. Cyl. #1 fires and exhausts and the hot gasses flow out the manifold and down the pipes. As it passes the runner from #5 cyl, the flow creates a partial negative pressure in that runner. When #5 ex. valve opens, the exhast is effectively "sucked" out and the cylinder is cleared of exhaust just that much better. The same thing happens at the next Y. The pulses alternate and "draw" the flow from the opposite pipe. The same thing happens at the third Y. In this case, the runner-to-Y lengths are not optimised but the effect produces a noticable increase in scavanging and is a cost effective modification. It tends to enhance low end to about 2500rpm and as the rpms increase, the effect gradually lessens and by 4000 rpm, is essentially nullified. At that point, the free flow mufflers and larger pipes are just outgassing large amounts with little finesse. I used to scavange 3.9L Y pipes and cut sections out to do this on customers cars. Had to buy the double downpipes, though. Five years ago, the '86 dual downpipes were still in the parts books.

The reason for the single pipe and the larger pipe at the front of the system is temperature.At the front of the system, the exhaust is hot and expanded. By the time is gets though lolligagging in the first muffler, it has cooled considereably. What happens when you take a water hose of 1/2" diameter and neck it out to 3/4"? What flowed quickly in the 1/2" slows considerably as it goes into the 3/4". A similar thing happens to exhaust flow. If it exits the muffler into a large pipe, the flow slows and tends to back up. A slightly smaller pipe tends to maintain the flow rate.

Jim Allen