The 1985.5-86 systems seem to respond best with 10 degrees base timing and approximately 16-18 PSIG for street use. This engine strategy has its worst pinging in the 3000-4000 rpm range but seems to be just fine above 4000 rpm if fuel quality is good.
Adding more boost and octane will show more performance, up to a point. It is very important to stay out of detonation. The 1985.5- 86 strategy has a faster response than the 1984 system.
If yours is there, move it to the rearward location (by runner for cylinder 4). The 1985.5 and newer intake valves have a small dish in the combustion chamber side and a 20 degree undercut on the port side which improves airflow throughout the whole range. The dish in the combustion chamber side also reduces the compression ratio which will make more on-boost power.
The jury is still out on whether the undercut on the intake valve has any effect on the carbon build-up on the port side of the valve. The only way to remove this material is to take the head apart and wire brush it off (then re-face the valve!).
Porsche developed a device (tool?) that attaches to the intake port on the cylinder heads of some of their engines that used crushed walnut shells and compressed air to blast the carbon deposits off of the valves.
The procedure was something like this:
Fortunately, removing the head on the SVO in not such a major job and is not done frequently enough to develop a tool for this purpose.
Bottom line: use a major brand of motor fuel with a good octane rating and beware of snake oil cleaners. The wrong combination or incorrect strengths of fuel system cleaners will increase deposit tendencies.
Retarding the factory camshaft by 6-8 degrees will move the torque peak higher in the rpm range and make more upper rpm power. The 1985.5 and later strategy shuts off fuel at 6500 rpm so there is some room to move the power up in the rpm range.
The smaller turbine A/R housing on the 1985.5 and later cars will bring the boost in pretty low, with a good response at low rpms so a large cam will not hurt performance too much.
If staying with the factory cam, retard it 6-8 degrees for starters and bring the boost up to about 16 psig with a good exhaust system. Then you can go out and give a lesson to a "v-ate" GT owner!
If you want to experiment with camshafts, start with the Crane CCH270 or the Engle 55. Both of these will significantly improve mid- and upper rpm power production while staying emissions legal and not requiring any machining. They are hydraulic and direct bolt-in sticks.
Always use new cam follower/rocker arms when replacing cams. Both of these sticks respond well to minor timing changes. Experiment between 4 degrees advanced and about 8 degrees retard to see what the effect is and decide where you like it.
Reducing the exhaust system back pressure will improve throttle response and increase power through the whole range. I recommend the Borla exhaust which previously was available in a dual muffler system for the 1985.5-86 SVO.
There has been some discussion recently on the www.mustangsvo.org Web site about getting enough interested parties together and seeing if Borla would make a complete 3" system (probably single muffler). If you have an interest, visit the Web board at www.mustangsvo.org.
Exhaust system, higher boost, cam timing changes, port matching, the later exhaust manifold, porting the exhaust manifold, head porting, larger valves, cam changes,... are the order to improve performance on the 85.5-86 strategy.
The 1985.5 and later SVOs came factory equipped with the REP-AG gearbox that has paper three-piece synchronizer rings and is rated to 250 ft-lbs.
The 1985.5 engine produce 240 ft-lbs. so there is a reasonable margin here. Use Mobil 1 5W/30, the engine oil in the gearbox. Try it, you'll like it.
Enlarge the outlet of the lower intake manifold about 0.10 inches on all sides being careful to not punch through the wall near the injector seats. Smooth out the interior of the lower manifold, 180 grit paper should be fine enough, while keeping all runners the same size and volume. Clean up the upper manifold also, and enlarge the outlet on the upper to match the lower.
To do a real accurate job requires adding alignment or dowel pins between the upper and lower intake manifold pieces. Before assembling the two manifold pieces together, apply Gasketcinch to the lower side of the gasket and the mating surface of the lower manifold and place the gasket onto the lower manifold.
Use an X-acto knife (or equivalent) and trim the gasket holes to match the port openings. The stock gasket going onto the stock manifold will need trimming if the upper manifold outlets are matched to the lower manifold inlets.
Another item that I check for is the location of the Knock Sensor on the 1985.5-86 strategy. In the early releases of the engine, the knock sensor was located in the front boss of the lower intake manifold, pointing straight at the left shock tower. This makes it pretty tight on the Mustang SVO for clearance between the shock tower and the Knock Sensor and, as a result, a lot of sensors were broken on installation of the engine assemblies at the factory.
If you get a chance to look at the clearance in this area on a Merkur XR4Ti, you will see about 4 more inches of dead space! So the lower manifold was drilled and tapped at the boss at the rear of the manifold to allow the Knock Sensor to face rearwards toward the firewall, where there is more room and it is protected from damage during installation. Some manifolds will have both locations drilled and tapped, so I recommend to use the ear-facing location.
The jury is out as to which one gives a better signature of the detonation-sound image in relation to time, phase angle delay, intensity, etc.
Oh yeah, if you hear it pinging for more than a couple of burning cycles, it's time to get OFF THE THROTTLE NOW! A quick rattle when shifting while at full throttle is OK but anything more than that is not a good thing and can get very expensive.