Vishnu Stage Sub-Zero Vs. Cobb's CT Alpha

[ArthurS]

I have been interested in the Vishnu's Stage Sub-zero or 0, but just looked at Cobb's CT Alpha. For around the same price range...which do you think is better.

The Stage Sub-Zero comes with 321 Up-Pipe kit, Underdrive Pulley Kit, and EMI Adapter for $635 (744$ with Vishnu Plug-in EMI wiring harness).

The Cobb CT-Alpha comes with K & N High-flow air filter , Urethane Exhaust Hanger, and Stainless Steel Cat-Back Exhaust System for $625.

I know they are both very differnet...but for the price which is better. Seems like the vishnu will help the performance a little more. Just curious.

[MattR]

The stage sub zero would be way better. The two kits are very differnet, the Vishnu kit is a tuned out uppipe mod= good

that cobb kit is basically a cat back mod, not too much more horsepower

[ArthurS]

I was just curious. Cobb hadn't listed any performance results on their page. I knew that Vishnu's was a great power product, just wasn't sure of Cobb's. I am pretty new to the whole exhaust idea. I have made simple mods to my old cars. like air filters, intakes, ect....nothing like this though.

[MattR]

No worries dude, I'm still learning too, if you guys saw my ride before my WRX you'd laugh, and understand why I know so little.

Anyway, just a general rule of thumb with turbos, I've learned that aside from an Uppipe (pre-turbo cat), most of the horsepower and torque is made after the turbo. Exhaust, Downpipes, Headers, etc, Or with engine management systems. I haven't seen gains from intakes, or filters that really make it worth doing. I've heard over and over, to spend your money on these parts, and leave the intakes to the N/A cars.

good luck

[sperry]

Originally Posted by MattR

No worries dude, I'm still learning too, if you guys saw my ride before my WRX you'd laugh, and understand why I know so little.

Anyway, just a general rule of thumb with turbos, I've learned that aside from an Uppipe (pre-turbo cat), most of the horsepower and torque is made after the turbo. Exhaust, Downpipes, Headers, etc, Or with engine management systems. I haven't seen gains from intakes, or filters that really make it worth doing. I've heard over and over, to spend your money on these parts, and leave the intakes to the N/A cars.

good luck

That's true, 'cept the headers are *before* the turbo.

In all car tuning, the way to make more power is to get more oxygen into the combustion reaction. On normally asperated cars, this usuall involved advancing the timing, or raising the compression with heads or something. On a turbo car, it's all about raising the boost. Unfortunately, turning up the boost isn't as simple as just tossing a Manual Boost Controller on there (a $12 part)... not unless you plan to bomb the motor in about 100 miles.

To make boost *safely* there's a few things that can be done. IF you can get the exhaust to flow freely, then you can spin the turbo up faster, and therefore make boost throughout more of the power band. More boost in the power band doesn't mean higher boost, just more rmps where the boost is available. That's the idea behind the up-pipe and turbo-back exhausts. However, on the WRX, it looks like headers don't really make much of a difference. The only thing really bad about the stock headers is that they weight a lot.

Another method of making boost, is of course the MBC, which allows the car to run max boost in every gear, or to run higher than stock boost. But to make it safe, you need some sort of ECU adjustment, so the motor can be supplied with enough gas to prevent the lean condiditon that leads to knock that leads to detonation that leads to a fragged engine. Vishnu does this with the Unichip piggyback computer. Basically, they can throw a cheap $12 MBC on the car safely, because there's a $500 computer making sure there's enough fuel in there to avoid detonation.

The reason intakes don't help a lot, is because on a turbo, you're already able to compress the air (increasing oxygen) far more than a free flowing intake will help. In fact the only gain from an intake on a turbo comes from the intake charge being colder. However the WRX already has a decent cold air setup (esp. if you left that snorkus thing on there). Since the Vishnu system is tuned for the stock intake, an aftermarket one can be dangerous, since it will increse the O2 to levels the computer isn't expecting, and you risk detonation. So, on a WRX the only reason for a CAI is for the awesome sound they make.

[AtomicLabMonkey]

*Warning* Lecture Follows *Warning*:

Advancing the timing actually doesn't get any more air into the combustion chamber, all it does is maximize the efficiency of the combustion when the sparkplug lights off. Since the flame front/pressure wave of the ignited mixture takes a little bit of time to travel downwards from the cylinder head towards the approaching piston, if the plug fired exactly when the piston was at Top Dead Center then some power would be lost since the pressure wave wouldn't reach the piston until it was already withdrawing down into the cylinder.

Soooooo, Firing the plug a certain amount of time before the piston reaches TDC results in the combustion pressure wave reaching the piston exactly at the right moment (TDC) to produce the optimal force on the piston. Factory engines are usually set with the timing somewhere in between these two extremes, so when you play with "advancing" the timing, you're moving it Before Top Dead Center from zero towards the theoretical optimal point.

On most engines you will start to get predetonation (knocking, pinging, etc..) before you reach the optimal spark advance though. This happens since the peak combustion chamber pressure increases as you advance the timing - as the sparkplug fires and the A/F mixture lights off the chamber pressure goes up, but this pressure increase causes the mixture somewhere at the bottom to light off spontaneously. This results in the two flame fronts colliding with themselves and the piston, making the engine internals resonate which we hear as pinging. Enough of this for an extended period will blow a cylinder head gasket or actually superheat and weaken a piston enough to blow a hole right through it.

Going to a higher octane gas makes the A/F mixture less volatile, so it can withstand higher chamber pressures before spontaneously igniting. I'm not an engine designer, but this is all as far as I understand it with a reasonable amount of reading. :?

[sperry]

Good points, advancing the timing doesn't add o2 to the combustion, but on a N/A car, it's one of the primary ways to make additional power. On a turbo car, the primary way to make power is by increasing boost. Adding boost can result in detonation as described above because the additional o2 makes the fuel/air mix more volitile. As the Monkey stated, higher octane gas can help.

The Vishnu Stage 0 both advances the timing and adds boost. Timing is controlled by the ECU and the Unichip piggyback tricks the ECU into advancing the timing. The Manual Boost Controller sets a mechanical max for boost, and any attempts by the ECU to adjust the boost are lost because the actuator hose is plain old disconnected.

Stage Sub-Zero, tho it doesn't have a Unichip or MBC, does still trick the ECU into mild boost increases. But IIRC the biggest power gains are from the up-pipe and underdrive pulley.

[ArthurS]

I think I am going to just save up and get stage zero and a torque converter. After that chunk of change, ill spend some time on rims and tires. I dont think I will go past a stage zero, but who knows.

[AtomicLabMonkey]

Turning up the boost will almost always work as the brute force way to make more power with a forced induction car, but I've read plenty of examples of supercharged and even turbocharged mustangs realizing some pretty big power gains from smoothing out the intake flow path as well as the exhaust. I don't know how well it would correlate to a 2.0L engine, but changing out the intake manifold on a blown 5.0L can be worth 40-50hp on it's own because the decreased pressure drop through the manifold flows so much more air.

[sperry]

I think the flat 4 layout pretty much kills the potential of gains from intake manifold porting. Same deal with the ehaders on a flat 4.. not much to work with...