Would this have a positive effect on power? I saw in the July 2008 GM High Tech Performance on pgs. 32-33 it has an Impala that had an addition to the intake plenum installed to create more volume. Their 'T' design doesn't seem very efficient but in theory, would say adding more volume on top of the plenum make more power?

 

Depending on the mods to an engine, it only uses X amount of air with an induction system. As long as you have enough air, providing more air isn't going to make any difference.JMHO

 

Well, people say the reason the LT1 doesn't make much power below 3k rpm is that the intake runners are too short, so I guess what it sounds like you're taking about is like running a single plane intake and that would increase your power.

 

No its not a single plane, it is an LT1 intake that looks like the top panel was cut off and an extra plenum was welded/bolted on to increase the volume.

 

I know the new Edelbrock intakes have longer runners, but not much longer. In theory that should help the powerband through all the RPM. I haven't seen any back to back dyno tests that show a difference, though. Maybe once I can get some money together I'll do one.

 

I thought Edelbrock claimed something like 1/4" longer on the runners. While longer runners move the peak torque to lower RPM, such a small difference would not likely be noticable.

The plenum provides a "reservoir" of slow moving air, for the runners to pull from. The shorter runners increase the impact of the valve opening and closing on the plenum. Sudden pressure pulses from the valve opening and closing can cause pressure waves in the plenum, if it isn't large enough. The larger the plenum, the less variation in MAP as the valves open and close. The less impact adjacent cylinders have on each other. But the plenum also decouples the throttle body from the runners, and too large a plenum volume can induce lagging response to throttle movement. Bigger isn't necessarily better. But directionally, an engine that moves more air is going to need a larger plenum volume.

I wonder if Edelbrock may have sacrificed a bit of plenum volume to achieve the "air gap" feature?

 

Yes, they did increase the length 1/4", but don't say anything about the volume. I thought Lloyd ported one...

 

I know, I was comparing the advantages of a single plane to what you want to do, but Injuneer broke it down better than I ever could.

 

Lt1 runners are so short that they are effectively untuned, basically their tuned frequency/rpm is way outside of the range that the engine can run in, so you don’t get any tuning effect from them. That may sound like a major disadvantage, but what you have to realize is that for every tuned range there is effectively an equal area that actually hurts power by about the same percent. LT1 intakes basically ignore both and result in a flatter power band.

WRT to plenum volume… the issue with the pressure pulses that injuneer mentioned is that they are negative pulses, where you are decreasing the air (air pressure) available to the next cylinder. Essentially, an individual runner (IR) intake setup tends to make the most power out of any setup, and the larger you make the plenum, the closer it gets to being an IR setup. As he already said, the issue with a larger plenum is that as it gets larger and keeps the cylinders from affecting each other, it does the same to the throttle body, and you loose responsiveness to throttle position changes.

OTOH, the general rule of thumb is that you can usually go 1-1.5x the engine displacement before you start getting a truly measurable decrease in responsiveness, and you’ll find that the LT1 manifold is nowhere near there even on top of the smallest engine