You can easily get 400+ rwhp out of a high compression engine when tuned properly on pump gas. Tuning is key. If you are trying to set some new ulimate horsepower record then yes you will need lower compression but you can get very high horsepower numbers from high compression rotors. You will eventually come to a point in the horsepower game where the higher compression engine makes less power than the low compression engine since it has to have it's timing retarded so far as to not detonate. If this threshold is higher than the desired power level, there is no point in using lower compression. Higher compression will give you faster spool times and better off boost power. Only use lower compression if you have to.

 

Oh and I do know that with an SAFC-II I will not get the best part-throttle driveability. The problem is that the SAFC-II doens't read boost and can't calculate that into cruising. It turns out that a mechanic I know at a local tune shop is the sole US dealer for Wolf 3D ECUs, and he's having a sale for $950, $1050 with the hand controller. The question is how bad is the driveability with the SAFC-II, and how much time am I willing to spend on tuning the standalone.

 

You are wrong.

The temperature gain from compression is represented by the following equation:

Absolute Temperature Gain = (((Compression Ratio^0.28) - 1) * Manifold Air Absolute Temperature

As you can see, the higher the compression ratio, the higher the temperature gain. Therefore, with everything else being equal, a higher compression engine would be closer to its detonation threshold even prior to adding boost. When air is pressurized by the supercharger, it is heated, which increases the Manifold Air Absolute Temperature in the above equation.

Calculated Combustion Chamber Temp @ 90F (550R) Outside Air Temperature:

13B with S4 NA rotors =
((((9.4^0.28) - 1) * 550R) + 550R) = 1030R

13B with S4 TII rotors =
((((8.5^0.28) - 1) * 550R) + 550R) = 1001R

13B with S4 NA rotors @ 10psi boost assuming 55% Ec for a roots type blower=
((((9.4^0.28) - 1) * 706R) + 706R) = 1322R

13B with S4 TII rotors @ 10psi boost assuming 55% Ec for a roots type blower=
((((8.5^0.28) - 1) * 706R) + 706R) = 1285R

 

If only there were empirical data representing the power potential of each compression ratio rotary engine just below their detonation thresholds using equal boost creation mechanisms . . that would be great.

Sonicrat, if I were u, I would get a six rib pulley and give it a try. If it does not work out, at least there was a chance that it might have . .

My millenia s compressor is eight rib but the stock serpentine belt on the engine runs a couple more accessories along with the supercharger . . the supercoupe might be the same way and not need all eight ribs to achieve the same slip threshold of the belt if used alone vs. with the other accessories.

 

Evil Aviator: Thanks for all the info, I believe though that we have a different idea on tuning, where-as you were stating temperature from compression increases, such to pose a higher risk of detonation, I was including the idea of tuning is to keep the temps withstandable (via air-water/etc intercoolers/etc), however I completely understand and agree with what you said, I was just trying to prove the point that when setup correctly, the N/A block can hold just as much boost as the TII block, as long as that threshold isn't surpassed, and albeit the setups will definatly be different. Thanks for the info though!

 

If you are trying to say that good tuning and charge cooling techniques would allow an NA block to run as much boost as a stock TII, then that is true. However, you could use those SAME tuning and charge cooling techniques to run MORE boost on a TII block.

BTW, more boost does not necessarily mean more power. Many people on this forum tend to run a lot of boost, but then retard the timing and enrichen the air/fuel mixture to the point that they would have been better off with less boost and running the engine more efficiently. Additionally, peak hp does not tell the whole story, and there is some off-boost advantage to the higher-compression rotors.

 

Yes, that's exactly what I meant, as in, by tuning, you make sure you make up for the charge, and so forth. It is a good point that most people simply crank up the boost to 14 because that's what everyone else dynos on, ignoring the fact that timing/air temp/etc has everything to do with it. Boost as a # is useless, it's all about CFM and volume of air, not pressure! Thanks for the info!