Corrected 1/4 mile times are meaningless guys, take it from an old NHRA guy.

 

Heya Dog, so in theory of course less back pressure would be fantastic for any forced induction car however in reality (tried it already) on the 996s what happens. For some reason they are ultra-sensitive and the CEL comes on instantly. Once that happens it ends up running super rich and like poo poo (that's a good technical term btw). You'd be better served trying higher flow cats (which also can and occasionally do set the CEL off). The last time I tried it I got about 2 pulls before it went into the "safe" mode state. The turbo is a little different. It still runs a-ok and takes a long time for the CEL to come on before it goes into a "lesser" mode without cats. I assume you have a boost gauge right? If so you can try and compare boost levels but if I remember right we got 1.25 PSI more without cats and then with a 100 cell cat .75...

 

Wait a minute, you guys got 1.2PSI MORE on your supercharged 996 without cats or on a TT. It would makes sense on a TT but I would think your boost numbers wouldn't change if not go DOWN slightly on the supercharged car by bypassing the cats..."increasing the motor's VE."

---Kevin

 

11.8's uncorrected still isn't too shabby.

 

I am using hi flow cats from Evo (Dansk) on my Supercharged C4S and there is a noticable difference in performance. I have not yet done a dyno run (I will in about 3 weeks) but my butt dyno says WOW. No CEL's yet. And the sound...I love it.

 

A compressor is a compressor and the compression ratio of a belt driven compressor is not effected by exhaust flow. Im other words, if the compressor will compress at 5 psi, it will always compress at 5 psi. What changes is the amount of air that enters the compresoor that is to be compressed. If you increase the amount of air that can enter the compressor, you will be increasing the available air that will going to the cylinders...even tho you remain at 5 psi.

You lose compression from the moment the compressed air leaves the blower and passes through the duct work, aftercooler and throttlebody. Like a water pipe, every twist and turn of the pipe reduces pressure. If you have a long intake tube and an aftercooler, you may have to compress the air to 15+ PSI in order to get 5 psi in the cyinders.

It is for this reason, having a low static compression ratio and higher boost can have more performance benefit than a higher static compression and lower boost, even thought the effective compression ratio is the same. The compression ratio in the cylinders does nothing to effect the amount of O2 avalable to burn, but simply makes it smaller in size. For example, if, on the down stroke, the cylinder can hold a quart of air, there will still be a quart of air avalable to mix with fuel whether the cylinder compresses the air to 10 psi or 20 psi.

However, if you can somehow increase the capacity of the cylinder, such as compressing the air before it gets to the cylinder, you can increase the amount of fuel you add, making more power.

I have raced cars with compression ratios below 7:1 and boosting the engine with 30+psi. The performance is crazy. But there is a balance that has to be maintained on a street car. Compression increases heat which will require more efficient cooling, which increases line restruiction which decreases boost getting to the engine. Increasing boost increases mechanical wear on the blower and drastically increases noise. And, as I mention in a previous post, you now have to find new ways to increase the amount of fuel that can get to the cylinders to match the increase in air.

If you want a sub 10 second Blown Porsche (while maintaining safety), you would have to run a boost level of probably 12 - 14 psi and have a static compression ratio of about 9.5:1. In order to have the car as a daily driver, and maintain continuos low intake temps, you would have to find a blower that could provide a boost of about 30+ psi in order to get the 12 - 14 psi into the cylinders. This is hardly and easy task.


Increasing exhaust flow may effect your EFFECTIVE compression ratio as would increasing valve overlap, but does nothing to a mechnical compressor ratio.

 

I see your point about the heads but I made a thread about using X51 cylinder heads with the EVO supercharger and got no responses. Isn't the head already there? The x51 cylinder head should be a nice power adder with the supercharger.

If it was only possible to increase the boost from 4 psi to 7 psi on the factory compression. Has anyone looked into water/alcohal injection on the 997/996 supercharger platform? I know the E46 M3 and E36 turbo guys use it to great success.

 

Even just sport cats seem to make a big difference but if there is no way to get rid of the CEL with a cat bypass I don't think it is worth it. Has no one solved the issue of CEL's with a cat bypass yet?

 

Stick I don't believe so no

 

Its 5PSI on the factory compression now....8PSI would be easy and safe on the type of HG RUF uses on their kit.(9.7-to-1;EC) However, the Vortech V2 flows a lot more and is a much larger unit (that is obvious just by looking at the two)...it would be quite easy for the standard vortech V2 blower to pump out three more PSI into the M96 motor. And with the appropriate fuel & intercooling mods will make upwards of 460rwhp. Thats a no-brainer.
...I feel it can be made & offered in a very cost effective package by a company like EVO too as an upgrade to their stadard kit. Forget the meth or water injection. Never liked filling bottles, Porsche or RUF would never do it that way either.

---Kevin

 

Remember, that you do not get a 1 for 1 improvement by inceasing boost even if you appropriately increase fuel. You also increase the heat which degrades performance and increases the risk of damage to the engine. With increased engine heat, timing has to be retarded which reduces performance. In other words, if a 5 psi boost increased your HP by 150, increasing boost by 40% to 7 psi, will NOT increase your HP by another 40% of the initial 150 increase. (60 additional HP).

Todd has spent alot of time and money researching and developing his product and IMO, the SC offers a great perfromance increase for the NA Porsche while maintaining a safe level of operation and dependability. EVO's product also maintains the normal feel of the engine that the Porsche is known for. No reputable company is going to offer a product to the general puplic that they feel would not maintain this level of safety and dependability.

 

I don't think there is any way to do 8 psi on stock compression without water or alcohol safely.

Adding fuel and intercooling wouldn't be a problem, I just don't think the motor would hold together safely at 8 psi on 11.8 or 11.3 : 1 compression.

It seems as if the stock compression and a little bit of boost is the best way to do it as big power gains won't come from adding more boost and dropping the compression with a head gasket and will make make off boost driving or low rpm driving less enjoyable.

 

I understand these concepts well, and I agree the power for each PSI increase wouldn't be 1-to-1....however, the drop off point for the V2's efficiency isn't even CLOSE to being met and 8PSI can be had safely under 9.7-to-1 comp. ratio. A bump in intercooling ability and fuel system tuning + timing should keep a safe tune and a 50rwhp gain would be a conservative and realistic goal. (20BHP gain per PSI...whereas they're getting 30BHP gain per PSI now in the standard set-up). Also, you have to remember the torque curve here too* ...with the centri set-up spinning to blow the higher # boost its coming online earlier in the powerband for a NICE gain in the "area under the curve" to boot....making it a significant increase in overall power/acceleration in total.

 

That is simply not true. It is actually the most common way techs/racers get the power of a high compression F/I'ed motor without having to do a $$$$ motor build up. More boost = more CFM's...while manipulating the compression ratio's to maintain the safe static compression ratio's for pump fuel. CFM + Added fuel = POWA*

 

I know you say you "understand these concepts well", but it is obvious that your do not. The EVO Blower that is delivering 5 psi of boost to the engine, is not running at a 5 psi compression. It is more like 15+ psi. So, in fact, the compressor is running much closer to it's maximum effeciency than you state. Maybe you should try modifying your own blower system and see how all these factors come into play. It would be a good learning expereience for you. No better way to learn, than to do. It can be humbling, too.