Alfa Romeo/Alfa Romeo Digest Archive
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
re: Still mo' Superchargers, EFI, Twin Sparks
Now that Biba has clarified the needs of his theoretical client (and in the process brought the discussion back down to earth), a few of these questions can be easily answered. I have some experience of running a supercharged car (not Alfa, that damn '94 Chevy Camaro again!). SOme years back I added a Vortech kit to my car, which came with pretty much all that is needed for the conversion. Based on my (albeit limited) experience I can make these points:
Since Biba wants only a 30%-50% HP increase, he can get by with a relatively low boost level. 6 PSI should be enough. My kit is a 6 PSI kit, and runs fine bolted on to a high compression motor (yes, GM does make them - it's about 10.25:1 I think). This is definitely not how you'd build a blown motor from scratch. The "big boys" in the late-model Camaro world use about 8.5:1 on their rebuilt motors, from which they extract up to 700 HP with at least some remaining streetability (I have reached about the 450 HP limit on the stock motor). High compression limits the max boost, and increases the risk of detonation (for which read holed pistons real fast!). If the client will rebuild the motor, more is possible, both in terms of power and reliability.
No fancy EFI was needed in my kit. Instead, it has a FMU - fuel management unit - and an auxilliary fuel pump, which, incidentally is more annoying in sound than the blower. The FMU is just a boost-controlled fuel pressure regulator, so the pressure is increased from the usual 45 to 60 or more proportional to boost. The ECU doesn't even know it's happening. Somewhat crude, yes, but in fact this only happens under full throttle or close to it, when precise mixture control is not critical (as long as you don't run lean, or it goes boom!).
Ignition was taken care of by a Crane HI6-TRC, included in the kit. This boosts the spark so that the extra mixture density under boost does not extinguish it (otherwise, again - boom!). In addition, it has a boost sensor, and it retards the spark progressively as boost increases, reducing the risk of detonation, but also reducing power. The rate of retard is adjustable for fine tuning. Back off when you get knock, or it's boom time again - don't get greedy! (For purposes of a guarantee, you might want to hide the dial!). I strongly recommend a boost gauge and a good (ideally electronic) fuel pressure gauge in a prominent, easily seen location.
The supercharger itself is a Vortech S-trim centrifugal unit. These are much more efficient than all but the best (and most expensive) lobe designs, and generally better suited to modern EFI motors. They mount low and out of site, and are belt driven. For 6 PSI, a cogged belt is not required. The main drawback of these is they don't provide instant low-end grunt. Some revs are needed before the effect is felt. OTOH, there is no lag, because it is driven from the crank and there is no spool-up delay. They kick in more smoothly than a turbo, although the best factory turbo designs are pretty good. They are also quieter than the lobed (or "positive displacement") types. Mine does whine, but just enough to sound cool without being excessive. And when cruising, at low rpm, it is not audible.
I would have to disagree with Greg Hermann (digest #1036) about the boost characteristics. Yes, centrifugal boost increases with rpm squared. But the turbo turbines are very similar and do the same. Then there is a second set of turbine blades on the exhaust (driven) side, also dependent on rpm squared, so the result is rpm to the power 4. So why is a turbo less peaky? Because it is possible to control the exhaust flow to spool it up at lower engine rpm, but limit the drive force at redline. This is done using a wastegate valve, which dumps excess exhaust gas post-turbo. Modern turbo cars use sophisticated electronic control of the wastegate to widen the power curve and maximise safe boost. The result is boost over a wider range than a centrifugal supercharger, but at the cost of lag. Supercharged throttle response is superior to turbo!
As for the idea of increasing the drive wheel so the blower revs up quicker, and then limiting boost with a pop-off valve, it can be done. However, there are 2 limitations. Firstly, the blower itself has a max safe speed, which you must not exceed at redline (see earlier Bonneville cars to see what happens when the turbine lets go, it ain't pretty). Also there is a "sweet spot" for max efficiency, which you want to match to the motor. So this limits how much you can overdrive it. Secondly, there is the practical problem of how to account for the air that is dumped by the valve. If the blower is after the air flow meter, the lost air has already been measured and the ECU will add fuel accordingly, so you'll end up running very rich. It is not usually a good idea to move the meter to after the blower, as it tends not to like the turbulent air coming out of the blower (even hot-wire designs don't work well in this location). I'm sure that a way around this problem could b!
!
e found, but it complicates things a lot.
Emmisions wise, my kit is 50-state legal, and comes with a CARB E.O. number. An eagle-eyed CA inspector should accept this, although I gather they sometimes take some convincing.
The point of all this is that most of Biba's requirements can be met without the expensive gadgets like custom EFI, ignition mapping, wideband O2 sensors, and so forth, because he is not trying to push the limits like the Bonneville motor. A kit of parts like mine can be purchased for under $4000, which is not pocket change but is certainly affordable to many. The real problem here is that such a kit does not apparently exist for the Spider, and it is the development and tuning which will cost serious dough. Unless one of the European suppliers has something? Of course you can forget about a CARB exemption from them.
Time for bed...
Dave J.
1994 Chevy Camaro Z28 (with lots of go faster goodies!)
1987 Milano Verde
1964 2600 Sprint
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided
Home |
Archive |
Main Index |
Thread Index