[Eric_Oz_S2]

Here's an interesting question for the boosted 968 engine.



MBT (max brake torque) timing is where the timing is set to produce more power. This is where ignition timing (advance) is where the flame front / pressure wave produce most power - ie coincides where the piston is at the top of its stroke so the pressure wave is not working against the upward travel of the piston or after the piston has started moving down.



In high CR engines, it is often not possible to get MBT under high loads as these engines have a tendency to knock before MBT timing is reached. You can, however, still get a little more power if you advance timing beyond the threshold of the start of knocking (small - not large - spikes in pressure wave), but obviously this is a delicate balance between power and longevity.



As the tendency to knock is dependant on the density of the air/fuel mixture in the cylinder, higher boost will result in the flame front moving faster and therefore less advance is required. This is why we retard ignition as boost increases. Now, if we drop the CR for a given boost we can add more timing back in and get closer to MBT. Also, we can fit more air/fuel in the cylinder to get more power. Hence, boosted engines always get more max power with lower CR (obviously at the expense off less low down torque).



So if we drop the CR of the 968 engine to say 10:1 from 11:11, we should get more power for a supercharged engine. There should be an optimum CR boost relationship, eg 11psi boost; CR 9:1; 5psi CR 11:1; 8 psi maybe 10:1?



I'm curious as to what the torque and power curves would do if we did this for a supercharged 968 engine with moderate 7-8 psi boost. This question is more aimed at a track car than street car.



Interested in people's thoughts.

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[MB968]

Eric, here's one more complication. If I recall correctly, it is the derivitive of the torque value over the rpm range (the area under the torque curve) that gives maximum performance (maximum work done which relates to max. accel.). And for a track car, you would only be interested in a more limited area, say from 3.5k to redline, since the car should always be in this rpm range.



I've seen/used formulas to calc. the hp from various builds. But, I don't know how you'd factor in the difference in tendency to knock for various CRs and boost psi to find an ideal. And, does the volumetric efficiency change with boost on different builds. If so, this would also have to be factored into the calculation as it would effect hp acheivable.



I know this doesn't help you, just a couple of thoughts on other factors in the equations that come to mind.

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[Eric_Oz_S2]

Yes, area under the curve is important. And my curve is from about 4500-6800. That is a disbenefit of supercharging - you actually want more boost in the midrange (where the engine can use it) and less in the upper rpm (where too much boost leads to detonation). That's why turbo's are so good - you can get a very flat torque curve across the rev range.



I know Steve's car (Dubai944) gets around 400hp with 11psi boost and 9:1CR. There is also a limitation of course of the C30-94 unit - it is already flowing max air.

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[flash]

lowering the compression ratio can allow you to run more boost, and reduce knock potential, but then there is the tradeoff of higher temps, which can put you right back where you were, as each pound of boost has a raise of temp. going from 5 to 8psi will raise the charge temps about 40 degrees F, though you would gain about 35hp. you may or may not be able to work with that. adding an intercooler to peel off that temp will cost 1.5 to 2 lbs of that boost, leaving you very little in gain, and with a lower compression ratio, proportionately less power to begin with.



some quick and dirty math says that an 11:1 968 engine, starting with 5lbs of non-intercooled boost, going to a 10:1 8lbs intercooled engine will result in a 15hp loss, but should have the benefit of being safer to run at higher temps

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