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, Fri, 22 Dec 2000 15:30:22
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I find it hard to believe that a given turbocharger would yield the exact same volumetric/thermal efficiency numbers, regardless of what engine it is bolted to.
Yes, all the air is going though the same hole, so that _would_ determine a given amount of airflow -translating to a hp number- being within or outside of the compressor efficiency map for a given compressor wheel.
BUT:
Say I put the same turbocharger on a 2.3 litre engine instead of one of 2.0 litres capacity. All else being equal, you are not nearly working the turbo as hard to reach a certain torque/hp level on the 2.3 vs. the 2.0. I'd say the boost pressure, which might be 17 psi on the first and 20 psi on the second to reach 230 hp @5,500 rpm, would have something to do with turbo efficiency, not?
Secondly, there should be a huge difference between cars with intercooler upgrades, ported/polished heads, higer or lower static compression ratios, improved ductworks, balanced/blueprinted bottom ends/etc. A 900T16 with an Abbott Racing chargecooler and exhaust makes 220 hp _without_ changing anything to the stock APC boost map. Will it still be on the rough edge of compressor efficiency, just becuse Swedish Dynamics tells you the stock turbo is only good for, like, 200 hp?
Look at the turbo's Saab put in early 90's 2.3 litre versions vs. the old 2.0 16 valvers. It's significantly smaller on both ends. Which, would you say, would have the best hp potential on the stock turbo? I think a look at the info from Abbott, Nordic and Hirsch pretty much answers the question for you... ;-)
So my educated guess would be that a given turbo configuration can be measured to have a compressor efficiency of 65% @ 275 hp and 55% @ 300 hp, _on a given engine in a given specification_.
There are more ways leading to Rome. I, also, feel, that a lot of USA tuners revert to simply bolting on a bigger turbo to avoid having to do more detailed work to engine ancillaries and management systems. It might be 'theoretically' more right to choose a turbo big enoug to put your chosen peak boost/(+rpm =) hp level well into its peak efficiency range, but on the road there are massive disavantages. I think it has something to do with trdition. Saabs are not the best drag strip (or stop light) racers you can imagine, they come into their own on spirited 'real world' driving with sweeping bends and lots of overtaking. Having to wait what seems like ages for the boost to come on, and then having to deal with a steering wheel ferociously fighting in your hands because you get 20 psi on 'the flick of a switch' will grind you down rather quickly and won't be as fast anyway. Have you ever driven a Saab that's been converted by Abbott, Trent Saab or Hirsch? They achieve not only big hp numbers but also a very enjoyable overall experience and good reliability without 'upgrading' the turbo. All this at hp levels that American tuners say are not feasible to achieve without their 'super red series stage III T3/T4 hyrid yadda yadda..." upgrade. Remember the stock turbo achager will only run out of airflow at high boost levels AND high rpm's. Unless you are on a race track or an empty stretch of Autobahn you're not going to be at redline for more than a fraction of a second before you shift up, so if you are determined to get max boost up 'til redline, why not rig up a water injection system that deals with the excess intake air temps AND peak combustion temps occurring at the same time (and which are NOT cured by a bigger turbo - intake air temps actually have little to do with combustion chamber temperatures; see the ERL/Aquamist website for details).
My take is, that there is a lot of things you can do that will improve the performance of a saab turbo engine in EVERY aspect: better intercooling, water injection, reducing turbo lag with intake/exhaust mods, fueling/ignition, increasing engine capacity, etc. If you have done all these things and STILL want more power, only then you will have trade low rpm power/response for even more grunt at high rpms (bigger turbo's, cams, big valve heads). But at this level, ther will always be a trade off so you might not get a faster or more enjoyable ar for real-world road driving conditions. I decided to stop just before that and will be getting 260+ hp on a 2,1l engine with a stock 9k T03, with utter smoothness and the torque curve of a steam train, in a C900.
, Fri, 22 Dec 2000 12:21:12, Fri, 22 Dec 2000 15:30:22, Sun, 24 Dec 2000 14:12:43, Fri, 22 Dec 2000 17:32:40, Fri, 22 Dec 2000 17:42:09
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