ok, so we all know that optimal intake diameter is 25% larger than TB diameter.

What method would you use for intake length? i recently saw someone use what appears to be a modified helmholtz equation (which i thought was more for acoustic design - it doesnt incorporate pressure). would a bernoulli equation be more appropriate?
does anyone have any insight?
does anyone care?
thx

 

No insight to offer, although I think its important, people usually just buy something on the market instead of designing on they're own. When they do they're just at Home Depot pieceing **** together.

 

Well, i think the Helmholtz would be appropriate since you are determining the proper length by resonant frequencies, much the way a speaker cabinet port is designed. I love this kinda stuff, any other geeky engineers lurking around


[Modified by nfn15037, 2:44 AM 7/26/2002]

 

Mista Bone had an AWESOME tech paper about this very thing. Maybe he'll post it up or provide a link.

 

I have very little to add to this except that I'm defintaely interested in what everyone comes up with.

 

mmmm, tech papers...

where is mista bone... come on man, post it!

 

This is a reprint from July 1999 Hot Rod, story by Steve Magnante from

Ramming the Rat

What is Ram Tuning?

Ram tuning surfaced in the racing community during the early 1950's. The idea was both simple and quite complicated, depending on how deep you dig. On the surface, ram tuning simply takes advantage of the inertia contained within a moving column of air/fuel mixture (only air on FI) as it comes to a stop against the closed intake valve. Traveling at close to 100 mph through the intake runner, you can imagine there is plenty of energy waiting to be released. By adjusting the length ot the intake runners, this energy can be used to improve cylinder filling when the valve opens again. The more pent-up energy waiting to crash the gate, the more air to pack the chamber. But there's much more to it than this.

It all begins at the piston. After expelling spent combustion gases through the open exhaust valve during the exhaust stroke, the piston reaches TDC. At approximately the time the piston begins its journey back down the cylinder during the intake stroke, the intake valve opens and suction draws in a fresh air/fuel charge through the intake tract. By the time the piston reaches BDC, the intake valve is closing, and it is here that we can began our examination of the occurences along the length of the intake tract. For these few milliseconds, we aren't concerned about what is happening inside the cylinder or the combustion chamber. Our attention is focused on the column of air that exists between the the back of the closed intake valve and the opening of the ram tube or, on a carbureted engine, the entrance to the plenum directly below the carburetor.

The instant the intake valve closes, it initiates a chain of events within that column of air. A set of four of these events will always occur in a particular order. These four events constitute a harmonic cycle. Each event involves a change in the pressure and velocity of the air/fuel mixture in the tube. These changes always begin at one end of the tube (either the closed valve or the open end) and progress to the other end. This progression, or trasversal, occurs at the speed of sound; a harmonic cycle consists of four traversals.

The first traversal is the result of the fuel mixture adjacent to the valve coming to a sudden stop. As it does, it builds pressure which is equal to the product of the air density, the air velocity, and the sonic velocity. As the incoming molecules of air hit this pocket of air that is already stopped, they also stop, and the volume of stalled air grows in size, creating a division between the two zones; this "front" is like a weather front on the evening news. The front separates regions that are at different pressuers. This front moves away from the valve toward the open end ot the tube at sonic velocity. It is the front between the two regions that traverses the tube at the speed of sound, not a portion of the air/fuel mixture itself. Each wave actually moves through the air itself without being of it.

When the front reaches the open end of the tube, the molecules of air nearest the open end begin to flow away from the opening of the tube. This volume of zero pressure air extends toward the closed valve at the speed of sound, constituting the second traversal, where the air in the tube is at zero pressure and negative flow. Then, the air at the closed valve experiences a negative pressure equal in magnitude to the positve pressure initially developed when the valve first closed, thus initiating the third traversal which also travels away from the engine. Once the front reaches the end of the tube, the air inside is entirely stagnant, and at the negative pressure, air once again begins to flow onto the open end of the tube. This represents the beginnig of the fourth traversal, where all the air in the tube is at zero pressure and has positive velocity traveling toward the engine.

With this fourth traversal, one harmonic cycle is completed, although it's difficult to imagine all of this taking place in a short timespan between valve closure and valve opening. The beginning of the first cycle of the next harmonic occurs when this flow region reaches the closed valve, and, again the pressure rise is equal to the product of the density of the charge, the velocity of the charge, and the speed pf sound, which----at 60 degrees F---- is equal to 13,420 inches per second! Shope divulged that tests conducted at Chrysler in the mid-'50s concluded that the most effective time to open the valve was after the third harmonic----the intake valve is opened just as pressure rise begins to occur at the head after the 12th traversal. These experiments also showed that when tuning for the fourth harmonic, the intake runner had to be too short and couldn't contain enough air/fuel mixture to completely charge the cylinder. And efforts to tune for the second harmonic resulted in extremely long runners and the acceleration of an excessive air/fuel mass, so the third harmonic was the most advantageous.

Chrysler testing resulted in a formula to calculate where the ram effect will come into play. To wit: N x L = 84,000, where N represents the desired engine rpm to tune for and L is the length in inches from the opening of the ram tube to the valve head. Shope explains: "Let's say you're running at Bonneville with an engine that develops peak horsepower at 8400 rpm and want to tune for maximum ram effect at that level. Then, L should equal 10 inches, as in 8400x10 inches=84,000." To achieve ram tuning at 5500 rpm simply divide the constant, 84,000 by 5500 rpm. The result is 15.27 inches, the ideal distance for the intake tract as measured from the opening of the ram tube to the valve head.

The effects of ram tuning reveal themselves as blips in the horsepower and torque curves, which can either be tailored (by manipulating runner length) to coincide with, and enhance, the power peak or to bolster some other area of the power curve. In other words, just because a given engine may make maximum power at 7000 rpm doesn't mean you have to utilize the benefit of ram tuning at that speed. In fact most cases, you wouldn't. A drag-race engine, for instance, would have its intake system tuned for a speed a bit above the midpoint of the engine-speed range. Is ram tuning responsible for where peak torque and horsepower are made? Well, it can shift the peaks a few rpm, but the more dominant contributors are found elsewhere (camshaft, compression ratio, and so on). But there is no doubt ram tuning can be a useful tool for enhancing the power curves.

***********NOTE**********
How this is exactly applied to our Honda 4 cylinders with the common plenum feeding 8 valves is what I'm trying to research. My 1.5L shows a big peak in the power bands at 4800 rpm with the AEM intake installed. There is also a peak at 2400 rpm, which is the result of the second harmonic.


Charles Tague, a.k.a. Mista Bone
10/8/1999

 

ahhh, one of the TOO articles, of course!

thanks man

 

no my friend, a article I copied from Hot Rod and posted on ENDYN BBS.

Info is only 40+ years old.

 

Mista, have you heard any info about the ENDYN BBS re-opening?

 

Endyn eh? so what ever did happen to that company? I recall reading some crap Atomic Performance (or someone that works from there) was writing about how shady Endyn really was and screwed one of hte atomic techs over, but never heard about any resolution.

 

Larry shut down the BBS, it was "taking up to much of his VALUABLE time."

Th e webmaster still has all the files though, I was hoping for a archive.