NA Power: Intake Manifold Volume Solution?
 

I've read in several places on this forum that one of the obstacles to NA power is the intake manifold, specifically that the plenum volume is insufficient.

One example from @celek :

From this thread: OFT Tuning to 8,500 RPM

I've also seen a couple of attempts to resolve this issue by completely redesigning the intake manifold, with the flaws @celek pointed out above.

What if a simpler solution/test of this hypothesis was attempted?

If the plenum= volume from the throttle body to the intake runners, then why not simply add volume by adding spacers behind the throttle body? It's not a complicated shape and the "plenum" on the stock manifold is not large enough (IMHO) that the diameter change would matter (right?).

Additionally, since spacers would be simple to fabricate (I'm thinking a rectangular block of plastic, cut into different lengths to test and 5 holes drilled through, one big one and four for bolts. A fancier approach could be a length of pipe with flanges on each end for the manifold and TB to attach to.

"Why haven't you tried this yourself?" you might ask.

Well, I'm nowhere near a tuner that could test and tune, I don't have an intake tube that's adaptable to the the resulting changes in physical position of the TB, and, frankly, I'm not on that level of "tuning" yet.

To sum up:
1.) Fabricate several different lengths of spacers, basically throttle body-diameter tubes, to go between the TB and the intake manifold.
2.) Sell first-born for money for dyno time.
3.) Bolt each one on, tune :iono: , then try the next size, and so on till one works.
4.) If none of that works, I have other ideas, all involving hacking the plenum and epoxy.

Thoughts?

@celek, @Tim Radley, @Element Tuning

Intake plenum volume is not that hard to tune, so I can't imagine why Subaru would get it wrong.

There are already TB spacers and IM spacers out there, but I am not sure if any of them give solid gains.

Doing more reading on this subject, working my way back through Jeff Hartman's "How to Tune and Modify Engine Management Systems" to grapeaperacing.com and on to their white paper on induction systems, page 13, suggests that we're looking at a plenum volume of roughly 40-50% of total cylinder displacement (for 7000-7500 rpm). So 1 liter.

At 65 mm diameter, that's an 9-11 inch tube. Hmm...

I don't agree with the original hypothesis. If it were the heads out flowing the manifold, it would get choked further as RPM's increase.

The IM spacers increase runner length and shift the power lower in the RPM band.

There's also good evidence that a good header and tune totally eliminates the torque dip without going near the TB and IM.

mmm.. Yes. I've seen this exact argument elsewhere and I think you were involved. My point is that this is an easily and relatively cheaply testable hypothesis. Also, we're not talking about intake runner length, we are manipulating plenum volume, albeit by adjusting the effective length of the plenum.

It's a volume vs flow issue. I can verify that the IM is indeed the limiting factor.

Most manufacturers/cars have intentionally undersized plenums to boost low RPM efficiency for MPG.

Second suggestion, slightly more involved/expensive would be to hack the legs off a stock pre-2017 manifold (used ~$75) and epoxy in a box or cylinder. Problem with that is adjusting the volume to experiment/ building more iterations.


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Oh! I forgot to include the volume of the existing plenum. That makes this much more reasonable. (Anybody know what the stock plenum volume is?)


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Wonder if that is the same case for the red intake manifolds. Is anyone tuned on the 17s yet?

I have a 17 with the red intake and no, I'm not going to hack at it (yet) :lol:



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Very interesting topic, I sadly dont have anything to add to it, but Im all for learning so, in for the updates :)

dosent jim crawford make spacers?

eta: yeap

https://crawfordperformance.com/coll...t-brz-frs-gt86

Yes, but those are for the intake runners. This is for the plenum.