Intake and exhaust runner/header tube dynamics

Internal combustion engines generally experience a pulsed flow of air in and out of them.

If the flow was steady and not pulsed, the bigger the port/runner/tube, the higher the flow. Somple as that.

However, in the case, especiallly, of the four-stroke ICE, the intake valve opens, and the air in the intake runner travels along the intake tract to the cylinder. At some point, the valve slams shut, cuasing a whole lineup of millionos of air molecules to run into it, piling into it like coils on a spring compressing, with the most-compressed coils being right at the valve. The air rushing in behind that mass of air all pilels up on it and continues to compress it and the coils then starts bounding back away from the valve.

As this air reverses along the intake tract, it can flow backwards through the carburetor, thus drawing more fuel through it, and creating an overly-rich mixture at lowr RPM.

This is one place where fuel injection (FI) shines ovver carubration. You are not fooling the A/F ratio-measuring sensore, nor the MAS, mass airflow sensors.

HOWever, if you want to get away with a bit more compression at lower RPM, you can ride that richer mixture and enjoy the fact you are not getting detonation. However, ths is not generally done.

So, further, valve events and their duration and timing will affect when the point comes in the RPM where reversion is not reacing back to the carburetor, and, in fact, the inkake valve has just experience high pressure backed up behind it when it opens.

The "negative pressure wave" proponents are IDIOTS. I don't care how big their name or business.

If the "negative pressure wave' wasa auch a valid thing, how would individual-runner injector stacks work, with the entire universe being where the magic negative wave went?

No. It is the piling up of the air directly on the intake valve that provides the often-greater-than-100% cylinder filling of the engine.. When that valve opens, a farily-high-presssure pocket of air now gets to "spring" into the cylinder, instead of back up the intake tract, only.


The higher the RPM, the less time the air mass has to reverse, and the larger you can make the CSA (cross sectional area) of the tract. Fatter ports make for a greater mass of air that is immdiately available at higher RPM to spring into the cylinder. It's not magic.

What is happening is that starting at zero RPM, and heading up the passage, the place in the tract that experiences very minimal if any reverssion move closer to the valve, which is why longer ports/tract are useful for low RPM, and shorter ones for upper RPM.

So, in short, higher RPM, shorter and fatter intake passages.

Now, on top of that, the air density changes those dynamics, also. High-vacuum idle air pressure has a longer resonance period than does higher-pressure wide open throttle WOT) air does.

So, the less-dense the air, the shorter and wider the intake passages can be.

The "sweet spot" where you make the most power will be when the air is at the highest RPM you can effectively fill that cylinder.

On the exhaust side, the longer and narrower the header primaries/exhaust tract per cylinder is, the lower the RPM will be for that most effective cylinder evacuation.

I know they look cool and all, but zoomies on the engine, even Top Fuel, are not as effective as a hader collector.

The header collector allows higher pressure pulses from one cylinder to arrive at the same time as lower pressure pulses from another cylinder.


More on the exhaust: the air is acting like a spring, again, in that when the exhaust valve shuts, one end of the "Slinky" or coil spring is not anchored at the valve, and the air stratches due to momentum away from the valve. At low RPM, that exhaust halts, turns around, and starts to head back toward the valve. With longer exhaust valve opening events that overlap the intake valve opening events, the exhaust can get all the way back into the intake port and to up the intake tract.


The key factors one tunes for in valve events are designed to maximize cylinder fill and minimize reversion, which is one reason engines do not idle at 50 RPM.


Naturally-aspirated race car engines often idle at a very high speed, because there is so much duratian and overlap that lower speeds just do not work, with the intake charge being too "poisoned" by ecxhaust reversion.

The intake tract would be more efficient if it, too had a header collector, then the cylinder split off in pairs so one was always being fed while the one right next to it was closed, so net flow to that pair in their shared tract prior to splitting apart would be positive down to the minimum RPM. This is not done largely due to space limitations.

The intake tract could go to one opening, splitting to two that each have four cylinders, all equidistant from each other in the firing order. Then those two would each split into two tracts handling two cylinders completely opposite from each other in the firing order. Reversion would be minimized in a wider RPM band, as you could make the passages larger than normal due to their length and boosting effect of pairing opposite cylinders with each other.

Few know this, however, so even race cars with far fewer limitations do not take advantage of this.

Ultimately, at the highest RPM, an intake tract that is nonexistent up to the intake valve would be conceivable, due to the sheer speed the air would need to rush past the valve into the cylinder.

The Le Mans-winning Mazda 787 with its 26b four-rotor engine is an excellent example of partially-harnessed intake tract length variations. The intake stacks shortened at higher RPM, and got longer at lower RPM, continuously variable.




Ideally, the intake and exhaust tracts would be variable on a per-PSI and per-RPM bais. However, the expense would most often not be justified for the gains encountered.

With any tract handling pulsed flow, the goal is to maximize the matching of one cylinder's higher-pressure pulse with another cylinder's lower-pressure pulse. If space does not permit, then you hve the all-runners-into-one-plenum arrangement most often seen.

All the NASCAR headers for sale on Ebay are 4-2-1 in configuration. The most efficient intakes would be about two feet tall or so and stand out vertically from the engine, in a 1-2-4-8 configuration.


Since the intake tract is usually lower in pressure than the exhaust, it is usually shorter and wider.

 

How about those old 426 max wedge engines with those staggered carbs and extremely long intakes?

Think the engineers back then were on to something.

 

both of the max wedges "413" and the "426" were deadly on the track but not good on the street and as you know the race hemi's had a similar intake also. which we all know what it did to the competition and still does.

 

Many of my favorite American muscle car models are Mopars and I've owned a number of small and big block Mopar V8's. In looking back from 1962 at NHRA Mfg Cup Winners to present I find it shocking that Dodge earned the honor just once (2016), Plymouth just once (1970). Probably no surprise to many that Chevrolet dominates with 28 (GM with 52 in total). Toyota won it's 1st last year. Ford has 6. Is it primarily GM's economic support of the sport of drag racing that's responsible for the outcomes?

 

Oh, yes, they were torque monsters, with the carbs for one side of the engine being on the opposite side of the engine with those super-long runners going to them.

 

You mean these?



From a recent post here:

Show Off Your Engine Bay

 

that's the "long ram" it was not worth a darn, but looked good though.

 

I think it worked as they hoped. As I recall, one of the main issues with that long ram air intake was that it was prone to cracking.

 

the main reason that i personally saw was called heat, the carbs set on top of the exhaust manifolds, in traffic at a stop light they would sometimes die because of the heat and overly rich. they did not run worth a crap, I would set the floats lower to help this a little and that's about it a little.

 

Not ALL ideas are GOOD ideas…