Hi chaps,

I'm a new comer to turbo engines, just wondering what the point of the VVC/Vtec type arrangement on the STI is?

Looking at the graphs on the PPP web site, both the b4 and after plots show low (PPP) to non existant (std) power below 3500, then obviously a big jump in the mid range, then power tails of at about 6K onwards.

Now what is the valve timing doing here, the power band isn't exactly huge, what happens at high revs - the red line is 7.5K - can't the turbo flow enough air for the engine at those revs?

IIRC the Jap STI has a redline of approx 8K, does the power tail off at 6K on that too?

Just a bit confused as to why they bothered when the engine has no real top end (compared to the midrange anyway) and its pretty much over at 6500rpm, no bottom end either.

Would the bottom end be even worse without the variable valve timing?


Cheers

Rich B

 

The hard part with the VVC is that it is on the intake only, as opposed to VTEC which is intake and exhaust. The main advantage is that you get better mixing with a low profile, long durration, cam at lower RPM's, which in theory should improve the torque. If you can get the information, I would bet that at higher RPM the cam profile is the same as the stock WRX.

 

Rich the power band is actually more usable than those graphs show
Just imagine if you had all the power right up to 500rpm from the red line, would be good eh? but the second you went past it it would be like hitting a brick wall as you would have no real trail off, your power would need to drop in 500rpm what its doing in 3k rpm.
It may look like it doesnt have much power, but that PPP'd STi is still pushing 250lbs of torque at 7000rpm, not too bad eh
You also have to remember that once you have inertia you dont need to keep all that power there, you can drop it off (thus the reason you get a big push into the seat) because if you kept it there you would still be stuck in that seat (which is quite scary )
Prodrive have done an excellent job with the STi and as you can see, the in gear flexability has improved tremendously (loosing 3seconds in some places )
This wont be a dissappointing car, trust me

Tony

 

If my foot is on the floor I want to be stuck in the seat - that is the whole point A boost curve that tails off as soon as it peaks is just wimpy. Nothing wrong with an engine that keeps making more power higher up the rev range, quite rewarding really, but dropping the boost at high revs does make the engine last longer. You can run lots more ignition advance and get away with a top mounted inter"heater" more convincingly.

There is no need to have a big power drop just before the red line. As long as you don't bounce off the rev limiter all the time. If I wanted big taper at the top I would buy a TDi.

Low compression small capacity engines will always be naff off boost.

The VF30/4/5 whatever it is in the various STis (JDM, UK, RA) has quite a big exhaust housing, and can flow well at the top end.

Tony it is not 250lbft at 7000 RPM. That would be about 333 BHP. now there is a nice idea...


[Edited by john banks - 10/8/2002 5:44:09 PM]

 

the variable valve timing is mainly for emissions purposes, it does help a bit with the power but only under heavy load. it will eventually work it's way down in to normal imprezas as emissions are getting harder to conform to.

 

John,Bit like a Bike then?

To be honest, it is all over a bit quick on the STi 7 but I'm hoping the new PPP/a.n.other performance upgrade () by bringing the "Boost Window" down by 1000rpm will sort this out.

TBH, it's only when you're not really going for it that you want a wider powerband, when you're going for it, you don't really care about keeping it spinning

Matt

 

Fit a boost controller...the turbo can easily boost and hold 1.1 to 1.2 bar over the whole rev range, right to the redline. Then the torque curve is/feels very flat...and the acceleration only stops when you hit the speed limiter...Make sure you run high RON fuel when you do this, though...