Tony Orlando said:
Actually, the boost in HP in the STS-V was due to intake changes made before the HP was validated. If it were solely the result of the new test, the XLR-V would have seen a similar jump.
There is very little difference between the "intake" on the STS-V and XLR-V... Other than the different bend in the induction tube front half the rest of the system on the engine itself is identical.
The power difference between the two is due to the more restrictive exhaust manifolds on the XLR-V (tighter packaging in that vehicle) and the shorter oil pan on the XLR-V causing extra windage loss (once again, tighter packaging and the engine is lower in the XLR-V). It has nothing to do with the SAE power test procedures and "intake" differences. Using the old test, there would still be a significant difference between the STS-V and XLR-V power ratings. The intake restriction (from the air cleaner and ducting) is very similar and VERY low on both cars. Same for the exhaust system...within the constraints of the passby noise requirements the exhaust backpressure is very low.
The change in the SAE power test procedures affects different engines in different ways. The comparisons being used are not really valid because normally aspirated engines are affected differently than supercharged engines and differently than intercooled supercharged engines.... You have to understand the specifics of the test procedures for any given engine and application before and after the SAE test procedure was changed to understand whether it will increase or decrease power. There is no single explaination that covers all cases and all cases are NOT comparible.
In general, however, the new procedure will cause a decrease in the advertised power if the manufacturer was "cutting corners" to get a high number....or it will cause an increase in the power if the original power tests were somewhat conservative in relation to what the new SAE test allows.
In the case of supercharged engines with intercoolers the old test allowed the manufacturer to pump ice water thru the intercooler to get a good number...not exactly representative of consumer use. The early power tests with the Northstar were done with hot intercooler circuit coolant to simulate worst case, continuous operation under maximum boost in a hot climate. Correcting for transient acceleration conditions on an 80 degree F day that the new test specifies actually allowed much cooler intercooler coolant and raised the power level significantly for the test which accounts for the change in the power rating from the preliminary numbers leaked out earlier.
There was a lot of "interpretation" in the test requirements previously that has been eliminated and specified in the new test. In addition, an SAE representative witnesses the actual power tests to certify it was conducted per the new procedure and is representative of what the consumer will experience. This takes a lot of the monkey business out of the rating. European power ratings have required a "witness" for many years. Now the same is required if the manufacturer wants to claim the "certified" power rating.
For what it's worth....consider the fact that the STS-V engine is making well over 100 HP/liter as it comes from the factory. It is NOT going to be easy to get more power out of it as all the "low hanging fruit" has already been picked. The drive ratio for the supercharger is already pretty well max'd out (it already has the "smaller" pulley..) so be careful.... If I was going to go after something for power I would consider a lower backpressure exhaust system as the engine loves to breath......
Consider the fact that the STS-V engine is actually making about 540 HP. It takes about 70 Hp to drive the supercharger at peak output so the engine "only" outputs 469 as measured at the crankshaft. But it is still flowing 540 HP worth of air, fuel and exhaust thru the engine at peak output so as to maintain the 469 at the crank AND drive the supercharger. So....the intake system, the fuel system and the exhaust system is actually being taxed significantly MORE than the 505 HP LS7 engine. Think an exhaust might help....???
