Part 3 complete and utter exhaustion!
My old exhaust, consisting of modified single-carb downpipes, assorted bits of small-bore exhaust tubing and topped off with a VW Type 4 silencer pack (the tips are exactly the right spacing to fit under the exhaust cut-outs on the Nova rear panel) would not fit the revised engine position without modification, and it was rapidly rusting away anyway. So I decided to fabricate a new, more efficient system which would stand me in good stead for further engine improvements.
I took some advice on primary and secondary pipe lengths, as well as minimum diameters, and ordered a load of preformed bends from Demon Tweeks. While I was waiting for the order to arrive, I got busy with the grinding wheel again and cut off the exhaust flanges from the old system, as well as the chrome tips from the EMPI silencers with a diameter of 3 they would finish off my new system nicely. I knocked out the internal reducers and stored them away for future use.
I bought a tungsten-carbide burr bit and started work on the exhaust flanges. Alfa Romeo put a weld bead around the inside of the flange which reduces the internal diameter to 1 inch! I reasoned that this would not help gas flow at all, and with 1.25 starter tubes already in place around the original downpipe I painstakingly milled away the internal weld and removed the original downpipe from the tube. Now my starter tubes were 1.25 ID straight from the exhaust port!
...Which makes a noticeable difference as you can see. Meanwhile the first of my preformed bends began to arrive. I ordered all of my tubing from the Jetex range, which start at 1.75 ID. I therefore had to flare the 1.25 starter tubes out to 1.75. I used a gentle taper for this, as opposed to a step, as the optimum placement of the latter needs a lot more analysis than I was prepared to do!
First downpipe done three more to do!
Then it was time make up the primary pipes. These had to be equal length, while at the same time staying within a confined envelope so as not to foul the suspension or compromise ground clearance - not an easy task. I began with the shortest manifold first (the LH one), which turned out to be a mistake as we shall see later. But for a first effort, I was quite pleased with the results...
Both the primary pipes merge into a 2 ID collector, and each of the secondary pipes in turn required merging into a 2.5 ID collector somewhere round the back of the car. This is where things started to go wrong for me, but we are getting a little ahead of ourselves here is the RH exhaust manifold...
The RH cylinder bank sits further forward than the LH bank. Because the 2.5 ID secondary collector would have seriously compromised ground clearance under the chassis, I fitted the collector vertically and threaded the 2.5 bore tubing alongside the gearbox. Because the gearchange rod is also on the RH side of the gearbox, the secondary collector had to go on the left, making the secondary pipe lengths asymmetric. As I understand it, this means that the resulting engine torque will be confined to a narrower peak at the primary resonant frequency. So, not quite as good as equal length primary and secondary pipes, but far better than the system I had before (dakdakdak)...
Laid out on the drive...
And trial fitted on the car. What I should have done, in retrospect, is build up the RH manifold and secondary collector first, then designed a more compact LH manifold, with both downpipes using convoluted tubing. I guess I could still do this at some point in the future.
I also made a mistake with the silencers thinking I would make a V8-style crossover system (which of course is designed to work with dual-plane, rather than flat-plane crankshafts) I ordered a pair of 25 long by 2 ID Jetex silencers. My reasoning was that I wanted to retain the twin tailpipes of the original Nova, even though my exhaust merged into a single 2.5 pipe. I therefore had to split the 2.5 collector back into a pair of 2 ID pipes, which I did with a second tee-piece, this time reversed and mounted horizontally.
The plan (which by this time was being revised on a daily basis) was to mount the silencers in parallel, with inlets at opposite ends and support the whole assembly from the body support frame constructed previously. With some bits of string, and some welded up exhaust clamps, the final configuration slowly emerged...
Once the assembly was tacked-up and trial fitted, it was time to seam-weld all of the joins...
...Which with a fully functioning welder was actually quite enjoyable, so much so that I quickly ran out of welding wire (had to get a new 5kg reel back home on the train) and shielding gas, which I had to get delivered in the end (££££'s).
Nice as it looks when polished up, I did the whole system in mild, rather than stainless steel, so it needed paint if it was going to last. I used a proprietary brush-on heat curing paint to do this, although the glossy finish of the silencer pipes suggests that this part of the system has not achieved the obligatory 200°C curing temperature yet!
Those last vestiges of the EMPI exhaust silencer, the chrome tips, add a professional touch to a what is otherwise a completely home-made system!
Next time a change of gear (change)...