Undivide intake ports in 2-valve heads with paired intakes

[PackardV8]

On a normally aspirated engine, there is an ideal intake tract length and area for a given RPM.

However, for pressurized-induction, does this apply, since doesn't the manifold pressure prevent resonant tuning? It would seem so, since some supercharger intakes for OHV8 two-valve heads with paired intake ports are simply a plenum with four short undivided legs to the ports. At the head surface, there begin divided ports. Here's the question - what would be the effect on a supercharged engine with paired intake ports if the divider wall was removed as far as possible back into the head?

thnx, jack vines

 

[TDIMeister]

Resonant tuning most certainly does apply for forced-induction engines, although most OEMs leave it out because it's a significant extra cost that can be offset simply by fractionally higher boost pressure.

There are ways of pairing ports and groups of cylinders in such ways as to achieve resonance tuning with given length runners at low- and high RPM, respectively. For example, a 6 cylinder engine can have a manifold switch between a large plenum volume with all 6 cylinders fed together and 2 smaller plenums of groups of 3 cylinders each. The 6-runner grouping demonstrates better low RPM volumetric efficiency, while the 3-runner pairs are better at high RPM due to less destructive pressure wave cross-talk. This, along with variable runner lengths, is a fairly common feature in high-output normally-aspirated series production engines and is mostly responsible for production engines reaching BMEP values previously in the realm of racing engines (~14 bar for NA engines). Tuning a V8 is more tricky, especially a typical cross-plane crank one because you don't have alternate bank firing. (the firing order goes something like L-R-L-R-L-L-R-L.

However, doing this correctly is NOT left to guesswork but rather some knowledge of acoustic theory and same basic equations, culminating in 1-D gas exchange simulation or even 3-D CFD.

 

[PackardV8]

Thanks for the clarification of my first question. So resonant tuning is possible.

What would the downside effects of eliminating the division between the intake ports down as far as possible and using a completely open plenum? On production turbocharged engines, such as Saab/GM, the intake cam event duration is so short, there should be very little overlap.

thnx, jack vines

 

[Lou Scannon]

Enlarging the plenum & eliminating the runners will tend to increase top end volumetric efficiency and also will tend to reduce peaks and valleys in the torque curve, since constructive and destructive resonances, as a function of rpm, will be reduced or eliminated.

 

[BrianPetersen]

Not having some length of separate runners (i.e. the effect of eliminating the division between intake ports) will magnify the effect of cross-talk between adjacent cylinders, particularly if the intake strokes overlap, and with normal cam timing, all 4-stroke engines with more than three cylinders will have at least some overlapping of the intake strokes.

Whether *that* is a good thing or a bad thing, depends on the specific circumstances, and for any given engine it will probably be different at various engine speeds.

Turbocharged production engines generally (but not always) have mild cam timing with not much overlap, which minimizes the effect of both cross-talk and ram tuning, and that's why it might not matter too much. The idea is that the engine itself has as much low-end torque as possible before the turbo spools up, and at higher revs, it's cheaper to just turn up the boost a little bit.

 

[dgallup]

L-R-L-R-L-L-R-L? I've never seen a V8 with 5 lefts but I guess anything is possible these days.

 

[TDIMeister]

My bad, for a GM LS engine with a firing order of 1-8-7-2-6-5-4-3 it's L-R-R-L-R-R-L-L...

 

[IceStationZebra]

I wholeheartedly agree with TDI. My simplistic explanation to non-engineering types is that the boosted air is just more dense.

BP - "Not having some length of separate runners will magnify the effect of cross-talk between adjacent cylinders and all 4-stroke engines with more than three cylinders will have at least some overlapping of the intake strokes." This would be my main concern. If you leave the runners divided you can physically move the plenum exits apart to help minimize this.

And you have to consider that the new runner will be >2x size of the original, so it will be sluggish until you really get the air moving.

ISZ