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, Thu, 12 Jul 2001 10:20:27
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The Volvo 850 is a good example of non-turbo vs. low pressure turbo (LPT) vs. normal turbo vs. high(er) pressure turbo.
In 1997 there were 4 engines available for the 850, all inline-5, ~2.4L, DOHC.
1) 2435cc, Non-Turbo, CR = 10.3
2) 2435cc, LPT, CR = 9.0 (same displacement, larger clearance volume)
3) 2319cc, Turbo, CR = 8.5 (smaller bore than Non-Turbo and LPT)
4) 2319cc, über-Turbo, CR = 8.5 (same engine as #3 with different wastegate)
#4 isn't really a true HPT (like, for example, Per Eklund's Saab 9-3). It just has a different wastegate to allow higher boost for a short period of time at full revs.
So when you turbocharge, you design your engine with lower CR to prevent knocking, among other considerations. The more boost you want, generally the more reduction in CR, as illustrated above. I don't know the actual boost pressures off the top of my head, but I would think the 850 LPT is around 6-7psi, whereas the normal turbo is around 10-11psi, and the "HPT" in the 850R maybe 12psi for that short duration. People of course tweak their engines to generate much more boost, upwards of 15psi or so on the 850. There are a number of limitations to this, like knocking, strength of connecting rods, etc.
LPTs generally have less turbo-lag then their higher pressure siblings. This makes them feel more responsive. Some say the LPT 850 is quicker in most cases than the 850 turbo because of this reduced lag.
When people talk about bolting LPTs onto a 900S, I would think they're talking even lower pressure than 6-7psi. Maybe more on the order of 4psi? Anyone? That's because they keep the CR as is (~10, versus 9 for the factory turbo 900). I would think the added LPT also neccessitates a switch to higher octane gas to prevent knock.
An interesting idea. I have a feeling that as the stock turbos get older and more rare, we'll see more people experimenting with the after-market LPT 900.
, Thu, 12 Jul 2001 10:13:21, Thu, 12 Jul 2001 10:20:27
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