I've been following cylinder head discussions this past couple of weeks and am having problems with the concept of too much intake. Can the reason why that is a problem be explained simply or is this something that requires a very technical explanation?

The intuitive answer seems to be that for a normally aspirated engine, more intake volume is good because there is more air available to be sucked into the cylinder and surplus wouldn't matter. That is apparently wrong.

Of course, I don't understand why long tubes are good for torque but can't support high RPMs and so I doubt that I'm going to understand this. That is the challenge. Can you explain it so that I can understand?

 

small diameter intake runners mean the velocity will be very high (compared to a larger diameter, and the same flowrate). Therefore, the air column will have a lot of momentum, and will help "stuff" more air into the cylinder. This is why small volumes work well for low-end torque. The problem with this design is that, in the upper RPM ranges, the small diameter cannot support a sufficient flowrate (can't get enough air into the engine), so top-end horsepower suffers.

By the same token, large-diameter runners will have a lower air velocity at low RPMs. When the valve opens, the column of air is not moving very fast, and therefore has to accelerate to fill the cylinder. This means not as much air gets in, and you lose low-rpm torque.

I'm sure you can fill in the blank about large-diameter runner high-RPM horsepower.


I have books that can explain it much better than me; maybe go to Barnes & Noble and sit down with one for 10 minutes if you're still confused?

 

If you want a good tech read on the subject

The bible;

The Scientific Design Of Exhaust And Intake Systems by Philip SMITH

"a thorough explanation of the physics that govern the behavior of gases as they pass through an engine, and the theories and practical research methods used in designing more efficient induction manifolds and exhaust "

 

Keep in mind,if youre thinking about L98 engines,the stock TPI intake was designed for a 305 motor and was decided good enough to use on the 350.They started off with a intake that was small anyways.So theres alot of discussion about going with better flowing/larger intake set ups.

That said, there are improvements that can be made over the stock design,by going with larger plenum/port sizes and/or shorter runners,depending on engine combo selected.Its all about balance.

And yes theres a line you gotta draw before it becomes too large.You cant go too small or too large.Its not simply just slapping something on way bigger and expecting results.

The same goes for an exhaust...cant go too large or too small either,it will screw the flow up.

 

All good advice above. I've heard it compared to blowing through a drinking straw, and blowing through a paper towel roll. The straw will have good velocity, but not much flow; the big tube; big flow but not much velocity. The key is finding the right compromise for you, and your motor. Lengths involve harmonics and resonance. There are formulas for this stuff.

 

A specific cubic engine has a finite amount of air it can pull. As stated above, the larger the conduit, the slower the delivery. If you have 10 CUFT, you can move it a little at a time fast, or a lot at a time slowly,but it has to fill in the same amount of time. In an engine, the faster the air moves, the better.

The issue becomes based on the CI of the engine, at what point can it simply not move any more air. Once you get the maximum amount of air the engine can move based on it's size, then you have to figure out what is the SMALLEST delivery system you can use with that given volume.

Then, of course, introduce the cam and a whole other set of factors come into play.

IMO, Since the last place the air travels is through the intake valve, the HEAD runner size is more important than the intake runner size. Think of a commercial AC system. Every see that the vents closest to the fan are larger and they get smaller as you get further away, that is to keep up "port velocity".

So I would say that given that position the intake can flow more than the heads and it wouldn't be the problem. however, the heads need to flow enough that the motor is taking in the maximum air the surrounding components can support, meaning the cam and cyls.

All that being said, I don't think you can really have an intake with too much flow, as the end result into the cyl is based on the heads, the intake is a reservoir for the heads and the heads will take what they need while they need it.

OF COURSE you get into the dynamics of intake pressures and such, which, can effect the delivery into the cyl to a degree. However, that is over my head.