I had to take a break from this and come back to it cos overthinking has started to fry my brain a bit
sorry if its changed since you read it.
Black Grouse wrote:
On this sort of arrangement, the plenum is almost always under negative pressure - because the cylinders are sucking and the throttle body is throttling even at Wide Open Throttle (WOT) - maximum air flow occurs at WOT and the plenum is at slight negative pressure and is there just to even out the suction pulses from each cylinder.
Is it possible to buy/make a larger diameter throttle body? It would make the idle and low throttle less driveable but potentially improve WOT flow, which is ok for a race engine.
So the diamter of the TB isn't really a limiting factor on this engine.
For reference the 1KR-F20 race engine makes 120hp (claimed) with a restricted 20mm intake on its plenum - The standard TB on the 1KR-FE is 40mm.
The best thing to do would be to optimise the topology of the TB instead which is in part what I'm trying to do here. Its shape can be improved on to further increase airflow by means of improving air velocity (or more accurately: by reducing drag) - That was kinda where the initial experimentation with porting the TB and shaping the bellmouth in the induction pipe etc started, as
documented in the cylinder head rebuild thread; and what brought me to this point here. Of course that improved velocity ties into the whole 'distributing air through the plenum' thing.
If you did put a bigger TB on I think all it would do is make the throttle feel on/off in how the engine responds. It would also have a negative effect on this pressure distribution through the plenum due to the reduced velocity of air entering the plenum, so cyl.1 would have a large pressure differential whereas cyl.3 would have a comparatively smaller differential as that area of the plenum is subject to lower pressure most of the time.. as you can see here:
This is a rough approximation of airflow based on induction velocity with cyl.2 as a pressure outlet but it illustrates the point.
PetrolDave wrote:
What's the real benefit/effect of these tuned length inlet manifolds that are appearing even on the 2019/2020 bugs?
I heard the idea was to get a pulse resonance going that created a slight positive pressure by the inlet valve at certain rpm ranges to improve the cylinder filling and hence improve mid rage torque?
The pulse timing of the intake is relative to the length of the intake runners on the inlet manifold that run from the plenum to the intake ports and is a different topic altogether.
It's exactly the same premise as getting the right primaries diameter/length on the exhaust manifold, so that when the positive pulse wave created from air slamming against the closing inlet valve is reflected back up the runner on one cycle, as that positive pulse escapes the end of the runner into the plenum it pulls air behind with it - the resulting negative pressure wave going back down the runner, and the positive in-rush that chases it, coincides with valve opening event of the next cycle to force more air into the chamber at that instant the valve opens when the piston is at the top of the exhaust stroke/beginning of intake stroke but not actually moving (to suck air in).
You can see therefore how the length of the runner with the fixed speed of the pulse wave -speed of sound - determines at what rpm this happens. After this, the pull from the piston moving down again takes over once it's further into the intake stroke. It is a different topic to what I'm covering here though they are interlinked..
Plenum pressure I'm interested in with the induction system design is more to do with whats happening at valve closing, the momentum the air has, to force more air in at the bottom/end of the intake stroke when the piston is barely moving just before the valve closes - and this momentum in turn would create a larger positive pressure pulse wave when it hits the closed valve, influencing this pulse wave timing at valve opening and thus everything plays nicely together.
The timing of the event is optimised - the effect drawn out over a larger rpm band for a given intake runner length - with VVT, so its not so much that one thing replaces the other but that they're all tuned together for the best VE. The reason this is prominent at low/mid revs is because the piston is moving slowly; so that period at the top (or bottom in my case) of the piston stroke when the piston isn't moving up or down at all is exaggerated and that's when these tricks come into play.
I think one of the coolest examples is the LaFerrari that has infinitely variable intake length that 'breathes', like a trombone slider, with engine RPM.
The technology has actually been around for years and was banned from F1 in 2006 for being so effective.
The actual pulse wave timing effect is present on every engine because its just a physical property of fluid dynamics; it contributes to how any engine behaves the way it does but engine designers obviously integrate that into their design - VVT is an economical way of exploiting it further - and sometimes if they put a lot of resources into designing around the pulse-wave timing aspect (like dual length runners), the marketing then reflects that expenditure
Tuned intake length and pulse wave timing is a pretty basic (albeit complex) fundamental of engine design that's been around for decades though.
There's a good article/test covering intake length that I linked in the OP of this thread:
"I think, in hindsight, the smaller internal diameter had a similar effect to increasing intake runner length -
If you're interested here's a great write up on the effect intake length has on an engine's power curve: https://www.emeraldm3d.com/articles/cat ... th-intake/"
Obviously I didn't have as much undertsanding of the process then because I alikened the pre-TB intake diameter to
valve opening runner length as opposed to
valve closing plenum pressure distribution via induction velocity; but the results are similar nonetheless.
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