No life (a.k.a. DattoMaster)  
Joined:
2008/10/10 22:02
From Melbourne Australia (and likely under the car)
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In general - I'd avoid the Saab turbos (unless they've started doing something drastically different in the last decade) because they 'look' good initially, but the disproportionate exhaust side tends to _really_ negate full rpm power. I'm not just saying 'it'll fall off a little up top' - I'm fully aware that average power across the rpm range and mid range in particular are _far_ more important (and useful) on a streeter - but the Saab units are generally pretty low boost, adn whilst it comes on quick, it just doesn't 'go anywhere'.
Didn't you have an SC12 at one stage? Do you still have it? I know you mentioned issues with the belt drive setup, but I reckon they are probably able to be solved. Reason I mention it is that whilst the toyota superchargers aren't 'massively' efficient at higher boost levels, $ for $ they are hard to beat for the rpm range over which they'll boost power.
I suspect I could make a pretty good case for an sc12 or 14 (from a perspective of 'very low budget, bang for buck, value for money' - even if the individual efficiencies weren't fantastic - and I make that as a serious point - who the hell cares if a particular setup is not what you'd run in an ultra tightly regulated racing class where efficiency is paramount to getting the most out of a specific combo on specific fuel, with specific restrictions etc - who cares - on a streeter it's about whatever gets the job done. If you can get the same (or even better) 'actual performance' from a combo that is much cheaper but not technically as optimal, it's gotta be a win). OK so to continue on from this - where the hell am I going? Simple enough - you could consider twin charging (or whatever the current name is - compound charging?? I dunno) - run a supercharger like that, which is cheap, and spin it _relatively_ slowly so it might only make peak boost of perhaps 5-7psi. That way it won't be running ridiculously hot, and (aside from risk from detonation issues being avoided, or minimised) also it'd mean the supercharger itself would last a long time (whereas it might not at much higher blower rpm levels and heat levels etc) and then get a turbo - again it can now (thanks to the supercharger effectively making the engine 'breathe' in and out (out of course is important as it spools the turbo) be a VERY cheap turbo option - and one for something like a 1.8 or even 2 litre factory car. It will spool just fine on supercharged 1.3-1.4 litre engine (whereas with the turbo all by itself it'd be a pig on the street slow spooling on a smaller engine). And you'd get a compressor side to match that factory 2 litre. If you blow that into the supercharger, with the turbo outputting only perhaps 5-7pdi, then it'd be fed into the blower as denser air, then further compressed and your overall boost would end up being in the low to mid teens (taking into account some small losses, and so forth). But it'd not behave like a (slightly) laggy 'straight' turbo setup pushing 15psi boost, it'd act like a 2 litre (give or take) running more modest boost. It'll have a wide power band, good boost response, all around a very good street option.
Now of course adding both turbo and supercharger will add some weight but (imo) not so much more than just a single s/c or single turbo that it'd be a no go.
Having said all of this - I'd go back to the start and say that if it's only (and 'only' is a relative term, I'm not criticising your power goals ) required to produce 100rwkw (probably around 150 flywheel bhp/112 flywheel kw - and I also must clarify that I attribute far lower drivetrain losses than most people would, and I've detailed the reasons for it in other posts, so won't rehash it here) - anyway if that is the power goal, then I reckon a supercharger would give you the power you want, and over a wider more useful powerband, and basically have an advantage over a turbo for this sort of boost/power level. In other words - in this specific 'mild' boost scenario, if the turbo and supercharged engines (hypothetical ones) produced the same peak power, the supercharged one should be noticeably quicker on road.
With the respect to the dizzy - absolutely locking the mech advance is a very good option (esp as it's so simple) but it _might_ make starting a little bit difficult (still will start, just stresses the battery/starter a bit more. There is actually a way around that - that I was shown probably more than 20 years ago now. And that is simply to repace the advance springs with very very soft springs. Then it will have (for example) perhaps 5-6 degrees initial timing for easy starting, but the springs are so light, that the full mech advance comes in at perhaps 1000-1500rpm. And from then on it acts as if it were a locked dizzy (you can also have your cake and eat it too, by reducing the slot so as to reduce the mech advance, AND use light springs,. Then you can run (for arguments sake) perhaps 10 degrees initial timing (easy starts and a little more timing to make it work better just off idle, esp if you have lower than std compression ratio).
DEFINITELY a big thumbs up for the suggestion of boost retard. You can get the diaphragms off later model (approx 1980s) dizzys (there'll be one out that that'd fit your current dizzy - there would almost have to be!) with 2 pipes coming out the vac cannister - one on each side. What the deal is there is that the pipe on the 'inside' of the cannister will see vac (possibly at idle) and REDUCE timing - purely an emissions thing. And vac to the outside hose fitting on the cannister works just like 'regular' vac advance. It doesn't take a genius to work the next bit out (thank goodness, as I wouldn't have grasped it myself!) - and this is simply that you 'disregard a vac/hose fitting to the inside the cannister, but run your regular one to the vac advance. THere's no vac suction on the inside pulling timing out at idle, but 'boost pressure' on the outer cannister has the same effect as vacuum on the inside cannister - so you get your vacuum advance AND boost retard. The big reason this is so good (apart from better economy and throttle response off boost with part throttle vacuum cruise conditions) - is simple. If you lock the mech advance, then it'll be 'safe' at full boost. No biggie, and the vac advance would work for high vac cruise conditions, BUt in situations where there's low manifold vac, but no boost (say up a modest incline, or even sometimes at highway cruise speed towing something) - well in that situation - no vac to give more advance, but no boost to require the locked total timing - it'd have less advance than is ideal. But if you run a mech advance amount that is perfect for this zero boost low vac situation it'll be optimal, and you then use the boost retard so it is still safe to run boost. That might sound like much ,but you'd be surprised how often you end up in cruise/light uphill or other situations where you actually have that low vac zero boost condition.
David - I'm now working full time day shifts (first time in a long time) and as such have my weekends free. If you have any time up your sleeve would it be possible to catch up and see what might and might not fit your engine bay and so forth, might be interesting.
Posted on: 2011/9/25 10:57
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