James Bailey

The pancake manifold used on the 930s with the TINY intake ports is a perfect example of how different a manifold should be when boosted..... yet it all seems so simple

ptuomov

In my thinking, the only differences between a boosted intake manifold and a normally aspirated intake manifold are the air density and air temperature.

The higher density amplifies a lot of things, including inertia effects and wave energy. Also drag I think. The higher temperature speeds up the waves and effectively shortens the runners. The sound of speed effect is proportional to the square root of the absolute air temperature, while density and pressure cancel out.

For both the boosted manifold and the normally aspirated manifold, the engine is not really "sucking." It's the pressure in the manifold pushing air into the cylinder. In the case of normally aspirated engine, it's slightly under that 14.7 psi ambient pressure. In the case of boosted engine, it's that 14.7 + the boost.

If someone is bothered by the torque dip between the flappy closed and flappy open modes, there are options. If this is just about tuning being difficult, one option is to disable the flappy and fix it in either position.

If this is however more about the torque curve shape, one can move the left titty away from the right titty by adding plenum spacers. Or one can partially fill some of the plenum inside with aluminum block filler epoxy mix, and squeeze the ******* together:



That quick and easy Helmholtz approximate formula says that one would need to add about 275 cc of block filler on both sides (15% reduction in plenum volume) to move the left titty from 3150 rpm to 3400 rpm. Call that the "push-up-bra" for the S4 intake.

ptuomov

Why do you think these intake manifolds gave the best performance for the boosted boxer six?



I personally think that this kind of manifold makes more power on a turbocharged boxer six:

James Bailey

[QUOTE=ptuomov;10500219]In my thinking, the only differences between a boosted intake manifold and a normally aspirated intake manifold are the air density and air temperature.

You might wish to look at 911 and 930 intake manifolds to see how very different they are....

ptuomov

Two questions:

Which model year? From the 1970's to until about model 996, I think the "911" turbo manifolds slowly converged towards the earlier normally aspirated manifold shapes, minus the Varioram system which I believe was N/A only. After 996, they start "overdriving" the intercoolers with a anti-tuned intake manifold that intentionally reduced cylinder filling, but that's a different story. Also, I wouldn't underestimate the importance of air cooling needs on the intake manifold design in the air-cooled model years.

Second, does the small port and runner areas observation apply to all boosted cars or just turbo cars? I think you really only see that with production turbo cars, and the reason for that is to get the turbo to spool up at a low rpm. For the same reason, the hot side of the turbos is usually undersized relative to what would make the car the fastest. For someone trying to make more power from a production car it's usually better to switch to more N/A type intake manifolds and larger turbo hot sides.

victor25

lately I have started to just completely remove the flappy plate on installs. It make for a much smoother power and torque curve

GregBBRD

This sounds like the "glass half full" or the "glass half empty" paradox.

However, since the lower pressure in the cylinder occurs at virtually the same time the intake valve opens....and builds as the piston goes down...it's suction, to me.

If you ever get to play with a flow bench, you will quickly see that there is a huge difference between blowing through an intake manifold and sucking through one......

ptuomov

It's all semantics. I am armed with high-school physics and English as the second language.

Whether you call it sucking or blowing, in an engine with an intake manifold plenum it's the same whether it's a boosted or N/A engine. The static pressure is higher in the manifold than in the cylinder, and that pressure pushes the air to the cylinder.

Getting deeper into semantics, the apparent difference between what is usually called blowing and what is called sucking is that blowing can be directional whereas sucking can never be. That is, the higher pressure pushing the air in somewhere can be either non-directional static pressure (air is still) or directional kinetic pressure (air is moving in some direction). One will get different results in a flow bench or real life if the higher pressure upstream is static or kinetic. Sucking vs. blowing is just semantics, whereas kinetic vs. static pressure has substance.

After the digression: As long as the air is slowed down with a diffuser into a plenum, like it is done in most well designed intake manifolds, it's mostly just that static pressure doing the pushing. Boosted or not boosted engine, it's the same thing.

John Speake

..... in addition to those thoughts we have to bear in mind that these are essentailly pulsed systems.

Cheburator

MK, for the life of me, I am tired of explaining that it is not possible. I bought an entire assembly from a crashed E39 M5 for my project. The 928 bonnet (hood for you 'mericans) line does not allow for the ITBs on cylinders 1 and 5, and for their trumpets without major mods to the bonnet and the silhouette of the car, which kind of defeats the purpose. There is no space to mount the actual air box on top. It is that simple. Trust me, when it comes to being ingenious with our 928 race engine, we are hard to beat and we still had to be inventive to make the ITBs fit...

We even made more horsepower on a real dyno than you, from a 5.4ltrs block...

ptuomov

What specifically do you have in mind?

John Speake

That pulsed is rather different to linear steady flow, and could explain why equal length diameter runners behave so differently...

PortlandTom

Talking about sucking air is roughly analagous to talking about pushing something with a rope. Molecules of a gas experience minimal attractive forces, but significant repulsive forces. (which increase as temperature increases) So air moves from a high pressure region to a low pressure region, not because the molecules in the low pressure region attract outside molecules, but because molecules in the high pressure area get away from each other by moving into the low pressure region.

Hope this is not too far off-topic. It's an interesting topic that often causes confuson.

BC

This is so utterly important so as to be stickied!

Everything from intake design (and port and valve), or exhaust design, cooling duct design, brake cooling, rear wing design, etc.

All of it is us trying to make air move a certain way, when it will always go the way it wishes.

ptuomov

BC -- What's the inside area of the stock manifold plenum cover? Or to ask it in another way, how much volume does a 10mm plenum spacer add?