Running a turbo on a Normally asperated 12a engine is GREAT if you have all the nessessary modifications. The 12a fitted with a S4 turbo is a excellent power inprovement and seems to work pretty well with streetporting as far as matching to each other. The actual type of setup this is called is a "Blowthrough carberator setup". The system in this setup is a blowthrough carb setup. Compressed air being forced through a sealed carb + a fuel regulator able to match the added air pressure + base fuel pressure necessary with no boost. Base fuel pressure is 3-4psi, and can go up to 20psi fuel pressure. So about 16PSI turbo boost is possible, but not necessarily a good idea. These systems work VERY well other then more engine stress on the dowels- cracking at the rear dowel by oil filter and side movement of the rotor housing and sideplates. Detonation is a concern(and happened), its important to keep boost in check, reasonable timing, good fuel mixture. This is not considered a long lasting setup in my opinion - and should be driven with this in mind and be consider rebuilds often if you plan to abuse it.
Some measures can be taken to improve this such as additional stragically placed dowels, larger tension bolts, etc. These are relatively expensive upgrades that are necessary with the engine being pushed too far into the design limit's too often. Poor tuning with these will still result in a broken engine. In reality, the engine life with minor/cheap upgrades can be pretty good depending how often boost is being applied. A completely stock 12a with fresh apex and other seals and in spec housings/tolerences can safely handle 6-7PSI just fine or a little more as long as the tuning is also correct. Issues- quicker transmission aging, quicker brake pad/disc aging, and most importantly lower fuel economy. Positives: fast acceration, higher top speed, lower noise from exhuast. Exhuast system GREATLY effects maximum power. But for city use, a somewhat restrictive muffler makes it very quiet but still reasonably quick. Make a alternate muffler setup for city driving and another for track. Respect local laws and follow regulations as directed.
Technically you should be running a adjustable EFI such as HALTECH. But not everyone is interested in spending $3,000+ for all the needed parts. Thus a blowthrough is a economical 2nd choice on a 1st gen.
Thanks to Greg for donating a S4 turbo/manifold in decent shape, welding the $20 in aluminum stock into a carb bonnet I cut out, and TII waterpump/housing I modified to work on a 12a(lighter weight then stock 12a waterpump housing as a bonus). Thanks to Kevin for some of the silcon bends I put to good use, a downpipe, free "exhuast ricy cannon", and other minor things.
The goods: Weber carburator 50DCO, custom carb plemonem bonnet, S4 13b turbo+modified S4 manifold to fit with 12a with sidedraft intake. Turbo manifold without dynamic flapper, twin scoll ported to equal size, ported wastegate and larger wastegate cover to reduce boost creep. TII Waterpump/pump housing modified to work on 12a engine, modified front cover to supply a return for oil from the turbo. Rear banjo bolt fitted with a fitting for oil to the turbo(very cheap to make). 80psi oil regulator. Coolant feed and return lines connect directly to the waterpump housing(TII has the connections). Turbo 2-1/2" downpipe and 2-1/2 inch connecting pipe to midpipe gaspak, then to a powerpulse muffler or a straight through pipe depending on what I feel like any particular day. Several stock TII silcon bends and a fairly nice effective blow off valve(unknown brand and somewhat worn out), 2inch aluminized pipe for IC piping out to a TII intercooler front mounted ahead of the radiator, re-routed coolant overflow bottle. Modified gas delievery-using a GSL-SE sender unit with a 1/2inch aluminum return line to the tank, 3/8inch feed line. Aeromotive street rod 280gph fuel pump, Aeromotive rising rate fuel regulator, locked distrubuter at max advance. 2nd gen directfire conversion-very easy to do for stronger spark. Lightweight steel flywheel, race clutch cover. (updated- S4 transmission for MPG improvment). Engine: streetported intake/exhaust by me, all stock components except corner seal springs from 93+engines. A pic of the custom S4 exhuast manifold.
The results: 7psi boost is fun(great increase over mildport/stockport alone). 0-60 with good shifts around 5-6seconds at that boost level when traction is avaliable. 10psi boost is more fun yet. My limit is 14-15psi because of the fuel pump limitation, n/a rotors, small inefficent intercooler. A trip to the drag strip in Rock Falls near Eau Claire shows a best time of 14.13seconds@98mph with only 7psi. This is with bad street tires and unexperienced starts. Lots of wheelspin or not enough causing a bogging effect. I can safely say it would be a 13.5second car if the tires were better at 7psi and didn't have the muffler on at the time. I was all over the place with my times. Its about controling wheelspin as good street tires are not good for drag racing. But my goal isn't the best 1/4mile possible otherwise I would have a lot less reliable car on the street.
I want to retain stock tranny/rear LSD differencial for a while which will hold up to 5-8psi boost without breaking for a long while...and its plenty impressive at that level. Top speed, thats another story. But its able to reach 140mph easily on a long road course, which is too much for a FB Rx-7(pre 1986 RX-7). Rx-7's('86-92) and ('93-95) are A LOT more stable at those speeds. Also warrenty on this info expires while you read it!!!
