I work with some engine people - they have said that if tumble is decreased, HP would improve, but emissions would degrade. I have read that around 3000 rpm, the TGV's just start to open (I don't know if it depends on throttle condition - 1/2 WOT, or WOT). Anybody know if there is a plan out there to add a modification to get those TGV's to stick open all the time? This might help low rpm kick. http://www.i-club.com/ubb-files/smilies/smile.gif

Actually, bypassing the TGV's should HURT low RPM performance. Intake charge "tumble" usually helps to increase torque at lower revs, since it stimulates better combustion when the cylinders aren't moving so fast. This is a nifty compromise between exceptionally long intake runners (which benefit low rpm torque) and exceptionally short intake runners (which benefit high rpm breathing). Its kind of the best of both since you can modify the intake behavior as the revs climb.

In our car, the Tumble valves are only half-butterflies anyways. When fully open, they should provide almost zero restriction.

Thanks for the reasoning.

The way I was told was that if swirl is increased in the piston (which I thought gets started by the tumble before the valve), the combustion process does not just go up and down (or side to side in H4s), which increases combustion rate, but doesn't help to push the piston as much as just a lump of mixture in the piston.

Just a thought, and perhaps something to mess around with after a few thousand miles or so.

[This message has been edited by wrxcited97 (edited March 12, 2001).]

The new Miata has a similar feature that makes the air more turbulent at low rpms. SCC disabled it to see if keeping the passageway obstruction-free would help... it didn't. They lost almost 10 hp across the board.

Actually, I think the TGVs are supposedly only used when the engine is cold (open loop).

When warm (closed loop), they are wide open at all throttle openings.

<A HREF="http://www.new-impreza.com/na/tgv.htm">More info</A>

We probably need to hit the shop manual on this one to confirm fully.

Also, this has been discussed to death previously, please search first.

Pretty well all of the emissions work (TGV, precat) is aimed at cold operation --- should be no huge gains or losses when warm.

Glenn

[This message has been edited by Glenn Wallace (edited March 12, 2001).]