Fiddling with a variable manifold - locking it in high rpm mode
 

My car has a variable intake manifold, that switches between a long runner'd intake with a low volume plenum, over to short runners and a reasonably large resonance chamber positioned about half way along the intake runners. This kicks in about 5500rpm and fills in a torque hole from there to the redline. Actually has quite a strong effect, takes the 2.5L engine from about 165hp @5600 to 190 @ 6500. But below 5000 rpm it hurts torque.

Now I wonder if locking this open in it's high-rpm mode would provide some low-rpm efficiency gains. Mainly by allowing the throttle to be wider open. It noticeably dulls response at low speed, need put foot down further to get the car going. Am I on to something here or would the two effects cancel each other out? Being an automatic I'm probably inviting more kick-down but I can mitigate that by putting transmission in snow mode - really resists kick-down then.

(Car is a 97 Nissan Cefiro imported to NZ from Japanese domestic market, has the variable plenum not fitted to the almost identical 4th gen US Maxima, the Japan and Middle East version had a variable intake for a kick of top end power - jokingly known as MEVI for Middle Eastern Variable Intake).

Only other mod I have on the engine at the moment is a WAI and grill blocks. It'll be a while before I can test any effects it properly, but thought I put the idea out there for now.

Reducing maximum output at lower rpms to reduce pumping losses. IMO its worth a shot and I'd love to see some good testing done to see the results.

(Assuming it works)

If instead of just locking it in high-rpm mode you make it so that it is in hight-rpm mode on low loads, and whatever mode is appropriate at hight loads, that way you don't lose any potential performance.

nice thought, but it won't work. the runners "supercharge" the engine at certain RPM's, making the engine feel peppy and giving it that punch which makes people happy.

The AVERAGE vacuum level in the engine won't change, and the pumping losses from the throttle are related to the AVERAGE vacuum.

Really interesting thought though!

I believe this to be incorrect. A pressure wave that arrives too early during the intake stroke, will increase the mean pressure without increasing the final pressure in the cylinder. And that will decrease the pumping loss without increasing engine load.

Only way to know for sure is to get a dyno chart of this engine's output, both with and without tuning valve engagement.

I did something like this when I retrofitted a tuning valve onto my intake manifold about 9 years ago:

http://www.tom-viki.com/spgm/gal/Car...0402070034.jpg

The blue line was with the tuning valve in the open (off) position, and the red line was with the tuning valve in the shut (active) position.

Interesting. On my next tank I might test it in the full time open position. I have a pretty regular commute with a few steep hills and will at least be able to tell if I'm holding a more open throttle position for the speed I usually climb at.

Long runners help build low end torque, this is good for mpg, esp. if the gearing is matched.

Having an engine with the same system, no, it won't work.

We've done street tests and dynos, and with the short runners open at low rpms, you lose a whole lot of power.

The runners don't just provide volume, the tuning of the long runners is optimized for torque. Shortcutting that will lose you said torque.

What you may want is to hook something like an aftermarket "VTEC solenoid", which allows you to select the activation rpm for the secondary runners. Set it a bit lower (maybe 4000 rpm), but not too low. Dyno data for your specific engine should tell you where the runners always open and runners always closed graphs intersect, and where it would be most useful to open them.

The point of it is to loose torque at low rpms. This allows you to open the throttle for the same power output and reduce pumping losses which should increase engine efficiency.