RoadRocket L98

Everyone who knows engines knows that a properly matched and tuned engine will always run better than a money dump that has been thrown together carelessly and without regard to whether the parts go work together or not. Balanced is better so to speak. Now this may make no sense at all, it is just something I have been thinking about. You get maximum torque at low RPMs with a Tuned Port setup because of the unusually long and narrow path the air has to travel to the combustion chamber. Im not sure why, but Ive heard the TPI system has a ramming effect on the incoming air charge. Anyway, there are two sides to the combustion cycle (ie. intake&exhaust) and to meet the needs of the process, there are two devices. These are the intake runners and exhaust runners in the heads and the exhaust manifolds (or headers) & intake/external TPI runners. These both serve the same purpose, except on different sides of the combustion cycle. You could almost call headers 'exhaust runners'. I admit that sounds bizarre, but for our purposes, it makes it easier to understand. Now, since matched components work better together, the engine would theoretically be more balanced if the intake runners were the same length as the header primaries. To put it more simply, since headers for TPI engines have unequal length primaries unlike the tuned headers made for Mustangs, the intake system could be made to have longer intake runners to match the longer primaries of the header on certain cylinders. What need does the air charge have to get in fast if it cant exit just as fast?

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'83 Firebird 2.8
'89 Firebird Formula 350
2nd owner, red/grey, t-tops, dual cats, and a 3.27:1 rear
K&N, TB bypass, No MAF screens, CAI, full Accel ignition

jmd

iTrader: (4)

It certainly has velocity. As to whether it's truly "tuned" is another question.


Well, not quite. One has air drawn by vacuum. One has air forced.

Nope. There are characteristics far beyond what you consider.

Including temperature, & thusly density of the intake / exaust charges. Also including exhaust scavenging effects (I'm not saying log manifolds have that ) vs. the vacuum drawn of the air charge in the intake.

Although it would be nice if everything about the velocity and other characteristics of the intake charge worked to have an inertia to very mildly "force" the air & fuel into the cylinders, it just doesn't happy w/ a stock setup.

Lengthening the intake runners would add throttle response time, and not gain anything except for those who want torque, and not any rpm use.

bjankuski

What determines the length of an intake runner or exhaust runner is the speed of sound in each of the runners. If the runner is designed correctly, the speed of sound can be used to actually ram the air into the cylinder head at the exact moment that the valve on the cylinder head is open. This same principle can also be used to draw the exhaust out of the cylinder head. By building the correct length header you can use the sound wave to draw the exhaust out of the cylinder head.

With that in mind the intake and exhaust systems will not be the same length because the speed of sound in the intake and exhaust systems is not the same. The hotter the air, the faster the sound waves travel. This means the exhaust system will always have longer runners then the intake system because it is operating at much higher temperatures then the intake system.

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1988 Formula 5L/5sp 3.45 gears, SLP cold air kit, MAS gutted, TPIS air foil, TPIS AFPR, MAC headers, Mac under drive pulleys, MAC cat back, Relocated MAT.
Best ET street tires 13.85
Best 60 FT street tires 1.930
Best MPH street tires 99.29

88IROCs

BJ,

The speed of sound is a constant - 625 mph or 1000 km/h - and does not vary by tube size(though any pressure in the tube can vary the speed at which mach is reached).

The optimal exhaust event occurs at the rpm when the scavenging pulse arrives at the intake valve, as that valve is opening. As such it is governed by the timing of the valve opening event. The exiting exhaust plug moves past the exhaust valve at the speed of sound, but loses most of it's velocity in the exhaust port and header primary tube.

Optimal cylinder filling in a TPI system is achieved by taking advantage of reflected wave effects. These occur because the column of air in the intake tract(between the valve and the plenum) is constantly in motion. The two most advantageous reflected waves are the second one(most velocity) and the the third one(most density). The Chevrolet TPI system is designed to use the third reflected wave, to achieve high cylinder filling over a broad rpm range, which results in a flatter torque curve(the second reflected wave can achieve higher cylinder filling, but over a narrower rpm band, resulting in a peaky torque curve). Again valve timing, in concert with the length of the intake tract, is critical to taking advantage of the desired reflected wave.

JMD,

RoadRocket is correct in stating that the TPI intake does create a ramming effect when moving air into the cylinder. That is because: due to reflected wave effects, the air is already in motion before the intake valve opens. However what's missing from his theory is two things:

1. there are four cycles - intake, compression, combustion and exhaust. during parts of any one of these cycles one or both valves can be in motion. the intake valve actually begins to open during the exhaust cycle, stays open during intake cycle, and closes during the compression cycle. the exhaust valve opens during the combustion cycle, stays open during the exhaust cycle, and closes during the intake cycle.
2. because both valves are simultaneously open during a few degrees of crankshaft rotation(valve overlap), they are related inasmuch as the exhaust event influences the intake event and vice versa. and once again we come to valve timing to determine how much one event influences the other.

[This message has been edited by 88IROCs (edited March 02, 2001).]