Ok, not necessarily a thirdgen issue, but a general issue, so its still tech related lol.

Ok, what I am trying to understand is the interaction in the motor of higher intake charge velocities. I have heard over and over that the higher the velocity of the intake charge the more low end torque you can expect, this being demonstrated by the differences in the mini-ram and the Super-Ram if I am remembering old posts correctly. So my question is why? It seems like the faster the charge is going the better it would be for high end in that it would be able to fill the cylinders faster. Now I am sure thats something thats always asked in beginning level mech eng. courses, and I admit I have had none....I'm a CS major w/ cars as a hobby....and electronics as a hobby...and theoretical physics as a hobby lol.

Ok, after writing this whole thing out, I think I understand how the intakes create their intake charge velocities. The mini-ram sits right on top of the manifold, so there is very little restriction offered by the intake, making the only real restirction in the intake manifold, and the heads, so the only accelleration is there. The superram has long intake runners, which obviously cannot supply their cylinder all at once so the intake charge is accelerated from the time the air enters the runners, to the time it gets into the cylinder. So even a carb, the entire volume of air to fill a cylinder cannot move thorugh the openings in the carb all at once. Which I imagine is why trips are nice, but probably need some tuning to keep the fuel to air mixture ok since even a 750 CFM carb is enough to supply a 350 motor up to more than 8000 RPM goin on that these are 4 cycle motors, 2 cylinders whould be on the intake cycle per RPM, or 1/4 the motor, 350/4 is 87.5 cu in. that times 8000 is 700,000 cu in. divided by 12 inches in a ft cubed, or divided by 1728 =405 CFM I have never worked on a carb intake, but I have seen the 4 openings, I assume for a 4 barrel carb, (meaning it has 4 openings for air to pass through I am assuming?) that means 1 hole for 2 cylinders. That hole isnt the size of the cylinder, so by saying its a 750CFM carb, I would guess that it can supply and atomize fuel at that rate, but any higher intake velocity caused by a higher demand for air will result in the fuel to air ratio being thrown off. Either way though, that carb makes a restriction so incrasing the distance from the restriction creates a longer vacume makes for more accelleration makes for higher intake velocity. Am I right? It all seems to make sense at least in generating the speed lol.

Why you might ask would I be asking about this, well, I got into an arguement w/ a few ***** freinds, who really dont know much about cars, and I bagan quoting some of the stuff I had seen here.....but then actually started thinking about it, and realized I don't really know why higher intake charge velocities increase low end torque. One of them was sick of his civic which he followed the standard ***** rhetoric, and got a cold air intake for, and "exhaust system" for. But he hated the fact he has no power off the line, and I thought well I am sure we could give him some low end, by taking some from the high end by increasing the intake charge velocity....but, biw I am thinking its not so easy, before I put any thought into it I just thought throttle body spacer. Now I am not sure that would really do anything. I got to thinking that because another ***** I was talking to (I keep hoping I will get through to these guys!!!) said well I have cold air induction which is pretty long, shouldnt that increase my low end? I was at a loss....now I think I understand why it doesnt effect his low end, in that it is supposed to be able to flow more volume that his 1.6 motor will ever request. For the other guy though, I imagine we're looking at a custom intake to get low end torque off an import lol, even though his gearing generally keeps him in the 3000 rpm range lol.

So in summary, I guess I could have just posted Why does a higher intake charge velocity increase low end torque? lol. But writing it out helped....somewhat lol

 

Let's start with the basics of how an engine works....

First off, a 4-stroke motor with 8 cylinders fires 4 per revolution. The 4 strokes are intake, compression, power, and exhaust. Intake occurs while the piston is going down, and lasts ½ of a revolution; compression occurs while the piston is going up, and lasts another ½ revolution; etc.

So: a 350 CID motor at 7000 RPM fills half of its displacement each rev, or 175 CI; 175 CI * 7000 = 1225000 CI per rev; Divide that by 12^3, the conversion factor from cu in to cu ft, the answer is 708 CFM. In a variously perfect world, that's how much air would be flowing.

But in reality, it's very rare that an engine fills and empties exactly its volume at any given speed, for a number of reasons; not the least of which are all the restrictions in the intake and exhaust tract, and the timing of valve events (cam profile) which are designed to favor the most efficient operation over some given band of RPMs. The name for the percentage of fill that the cylinders actually experience is "volumetric efficiency". Most stock engines manage about 85% cylinder fill at their peak HP RPM; a well-tuned high-performance engine sometimes gets close to 100%; and by careful matching of parts to RPM range, it is possible to obtain over 100% fill, but only over a very narrow RPM range, and at the expense of efficiency at all other RPMs.

So, you would expect that on a moderately warmed-over 350 on the street, with perhaps 92% VE, you'd see about 652 CFM.

But that still doesn't tell you what size to make the carb or intake tract. Remember that the numbers we have calculated assume perfectly uniform flow. But an engine doesn't flow that way.... it flows in pulses. Each cylinder's fill event is a pulse of air flowing through the intake. In between pulses the flow is much less. The system has to be sized to feed the demands that actually exist during the peak of the pulse of flow, i.e. when the engine is actually consuming air, not some average value. This means that the flow rating of the carb (or throttle body) needs to be somewhat larger than the value calculated above, usually by around 20% for best results.

But even that is a compromise, because there are other factors that affect it. Remember that an intake has runners, each of which feeds a cylinder; and a plenum, which is behind the throttles, and sort of stores air in between intake events, and thereby smooths out the pulses somewhat. So.... the larger the plenum, the less of a pulsation effect there is at the carb, and the lower the correction factor given above will be.

Keep in mind that the correct carb size is not what will feed some monkey-spank about how high the driver will "spin" the motor; it works the best when calculated at the RPM of peak HP for the particular engine combination.

Velocity is a different matter altogether. Remember that air has mass, and therefore inertia. That means it requires the expenditure of energy to get it moving, and once moving, it requires energy to stop it. That's the principle behind "ram" intake effects: the runner is made long and relatively small, which causes the air to be accelerated to a relatively high velocity during the intake event, which means that even as the piston approaches the bottom of the intake stroke and therefore quits changing the size of the cylinder ("sucking") as much, the air contnues to rush in, thereby filling the cyl a little bit more.

But there's a penalty for that. The longer and narrower the intake is made in order to promote this effect, the more restrictive it is to flow. So it's a direct trade-off: velocity at low RPMS when the flow reuirements aren't very great, to promote low-end cylinder fill; or a large runner with low velocity and little or no "ram" effect at low RPMs, and no efficiency boost at low RPMs.

Torque is simply the result of cylinder fill. Period. Given a particular stroke length, torque is directly proportional to the number of gasoline molecules burned. Peak torque occurs at the RPM corresponding to the most complete cylinder fill.

Horsepower is the time rate of torque. In other words, HP is directly proportional to the number of gasoline molecules burned per unit time. It takes air to burn fuel, so it's also a measure of air molecules per time. But, air molecules per time is just another way of saying CFM; so, in its most basic form, HP = flow.

For the CAI in front of the throttle body to have a noticeable effect on cylinder fill, it would have to be so small, as to create a flow restriction at an RPM equal to the peak horsepower RPM of the motor, divided by half the number of cylinders (for a 4-stroke engine). So if this hypothetical r¡cer had a motor that made peak HP at 5000 RPM, and he actually installed an intake duct that created a ram-air effect, it would begin decreasing HP at around 2500 RPM.

It is not worth arguing with people like that. Let him shoot himself in the foot while he polishes his wood that way, he will simply slow himself down, like most of those people do with most of what they do to their cars.... rather like 2 of them I got stuck behind in traffic while they were "racing", which slowed them down so much (holding their cars in low gear way too long) that traffic leaving the light piled up behind them.

My fingers are tired.

 

RB just saved me a lot of typing...

 

yeah I relized earlier today that there are 4 cycles, not 4 revolutions, its 2 revo9lutions for 4 cycles LOL. Ok, the ram effect is what I was looking at. As for the *****, the one who was looking for low end, he's not your typical *****. He'd really like low end torque, and some speed, and a back seat, and cant afford a 96 impalla SS lol. The other ***** is a ***** through and through, and keeps trying to disprove the old saying "There ain't no replacement for displacement" However, he seems open enough to new ideas, that if I can make things make sense to him, he just might see the light The first guy said he wouldn't car about losing his high end, if we could make his motor peak lower, that would be great reason being (go figure on this honda blunder) the motor is a high end motor, but the trans is geared to keep the motor in the 3000-5500 RPM range. So when he floors it, suddenly it has power for about 1 second or so, then it shifts, and theres nothing again. It drives him nuts. I say shouldn't have gotten a slow car then lol. Being a nice guy though, I figured I'd investigate and see if there was anything we could do to help the thing....looks like nothing short of forced air will really help....I'm not talking turbo though, the lag would just increase the effect...but a low boost super shouldnt cause any major problems up high, and should be enough to create the "ram effect" at the lower RPMs The question is, does anyone make such a beast lol, making hondas make low end torque is not a popular thing to do....

 

A supercharger (either positive displacement or turbo) throws the whole induction theory into a spin. While sonic pulses still occur in the intake tract, providing extra charging at some range of RPMs, the effective difference at other RPM ranges is minimized by the higher intake pressure. Basically, with boost in the intake tract, the limiting factors are not so much the intake tuning and runner sizes (within practical limits, of course) but the size of the valves, valve lift, and valve timing. You'll want perhaps a little less overlap with a blower cam profile, since scavenging by the exhaust to aid cylinder charging is not a significant factor.

Just my opinion, but most higher-revving engines are designed just for that by the bore/stroke ratio, and attempting to produce more lower RPM torque can get expensive and provide limited satisfactory results. It's like trying to make a Aurora V-6 produce 300 ft/lb at 2,000 RPM. Without HMX or nitromethane in the cylinders, it isn't going to happen.

Face the facts. An engine that needs to rev to 6,800 RPM to make 130HP isn't going to have a lot of torque at ANY point in its RPM curve. Do the mechanical engineering math (divide by 5,250) and determine the torque at any given point. You just don't net a lot, so even a 50% increase (which may not even be possible) isn't going to provide dramatic results.

If your buddy is really ambitious, he may be able to devise a means of fusing two Honda engines together, approaching more of the displacement range of the smallest SBC engine.

 

See above for the answer.... Unless he can get more gasoline molecules in there, he won't increase the peak torque. And he can't do that unless he also gets more air molecules in there, which he can't do without some sort of boost. N/A all he has to work with is the same as the rest of us have to work with, which is 14.7 psi more or less (1 atmosphere).

Sounds like a typical r¡cer who doesn't understand how engines work. Most of them fall into that category. Again, I find it not worth my time arguing with such a person, whether they're a r¡cer or a "domesticer"; let him waste his time and money and not go faster, it's no skin off your nose.

"Never argue with an idiot.... he will drag you down to his own level, and then beat you with his greater experience"

 

It's so beautifully universal, too.

flow = speed
pressure = power

It works for engines. It works for hydraulics. It works for electronics.

 

All of the information about velocities is correct, but it is not the real reason for this phenomenon. Resonance is the real reason why runner length and diameter, and plenum volume determine where an intake manifold will make peak power. If you remember from Vibrations and Acoustics in college(if you took those classes), the ruuners and plenum can be modeled as a helmholtz resonator. As the intake vallve closes, a pressure wave travels up the runner and into the plenum. The air in the plenum acts as a spring and sends the wave back down the runner to the intake valve. If it is designed correctly, the returning pressure wave will be greeted by an open intake valve on the following intake stroke. The pressure wave then enters the cylinder, forcing more air in and thrus making more power. On the other hand, if designed incorrectly, the pressure wave will return to a closed intake valve, hit it, and bounce off creating a low pressure area just behind the valve as it is opening. This would then be even worse because there would be no air right behind the valve and significantly reduce the amount of cylinder filling.

This was my senior project in college for the Formula SAE racecar team.

I have all the formulas and info I used if anyone is interested.

 

Resonance is the issue in certain specific applications, such as TPI. It is a very minor factor in any of this for almost any other induction system. That's not really what his question was about, nor does it apply very universally.

You're exactly right about how it works though. No question of that. Your description of the phenomena involved is exactly how TPI is designed, word for word; and it's exactly what makes it suck as a "performance" system. The runner dimensions determine the operating RPM range, which is not then subject to any further change without taking it off and putting something completely else in its place; and the dimensions used result in a total sacrifice of any chance to make any meaningful power above about 1.5 times the tuned RPM, in favor of maximizing the "tuning" effect. In the case of TPI, with its 22" runners resulting in an "acoustic length" from cylinder to cylinder of about 4 feet , this results in its "tuned" RPM being about 3600 RPM and its "brick wall" or "cliff" RPM being about 4500, due to the speed of sound being in the neighborhood of one foot per millisecond (subject within the runners to the varying effects on the speed of sound from the temperature of air in the runners, the pressure drop across the air filter, etc.).

I've posted the caculations on this site too, even though I'm not an engineer by trade. I'm a mathematician and physicist, but since I enjoy certain minor life details like paychecks, I'm not employed in that field at all; I'm in broadcasting, where lots of people with no marketable skills seem to find a home.

But, that isn't what his question is about... it was about the inertia effect of air moving through a runner.

 

I agree with that, except it's all relative to the desired RPM range you want from the engine. As RPM changes, so do the valve timing events, as well as the resonance of which you're speaking.

I agree with RB on the basic principals because he's right. But to go through the trouble of finding what length runner to use for a 1.6L Honda motor to increase low RPM torque would be like shooting pigeons blindfolded. You have more of a chance getting **** on than anything.

 

I knew there was a reason I liked you.. Yet another nailhead hit with elegant precision.

<- Ex-engineer at two television stations.

__________________
1989 Rally Sport
1986 Z28 ASC McLaren convertable SE #41

"Mars needs women."

 

Ex- major-market TV & radio station chief engineer... nowadays I work at Harris designing radio studios for all the chief engineers that have too many stations to take care of (consolidation, without naming names ) and don't have time to do it themselves!

 

TransamGTA350, I am interested in seeing those formulas. I am on the Oklahoma State Formula SAE team...
Hope you dont mind giving your information over to the competition!
Thanks!

 

Charge velocity is related to the intake cross-sectional area. This general principle is know as the bernuelli effect. This is what happens when you put your thumb over the end of` a hose to make it spray farther. Charge velocity is critical in a WET manifold becaue the air must be going at the proper speed to keep the fuel suspended and properly vaporized. Too slow and the fuel begins to "fall out." Too fast and the intertia of the fuel (since it is much heavier than air) doesn't allow it to make corners and it "falls out" that way too.
Charge velocity is crucial in a dry manifold, because the speed of the air as it swirls around the intake valve also serves to atomize the fuel. The amount of fuel you can burn is related to not only the amount of air in the cylinder, but also the amount of air around each fuel particle. With a large drop of fuel, you can only burn the fuel that is on the outside of the drop. As the drop burns it must find more air to complete the combustion. If it can not find enough air by the end of the power stroke, then it is exhausted as a hydrocarbon.
Other things to concern yourself with is the amount of energy it takes to combust fuel. Energy and heat are relatively the same thing. If the fuel has enough heat in it that only a small portion more will initiate a reaction with oxygen it will be more apt to complete the reaction. That is where hot fuel injection research comes into play. The only bad thing about that is if the fuel becomes too hot it will boil, which makes it much harder to inject/venturi. In an injected application, you can heat the fuel much higher, because the high pressure also increases the boiling point.

Engineering is so cool

 

But it does apply universally. The TPI setup was designed to perform around 4500rpm where as a miniram, or superram is designed for around 6000rpm. The both use the same principle, just tuned for different outcomes. There is no certain plenum/runner combination that will yield "tuning", and others that will not yield "tuning". They will all have a natural resonance at a certian frequency, although it may be so far from the operating range of the engine that it seems that it isn't there. The car that I designed the intake for made peak power at 12,500rpm, so it works all over the rpm range. But don't get me wrong, I am not saying that the other factors such as charge velocity are not important, because they are. Those factors have to be met first, or the system does not work. But charge velocity only has to do with runner diameter, and resonance tuning for whatever application you are dealing with determines the runner lingth and plenum volume.

JMatlock88,
I would be happy to give you my equations and formulas that I used, I just can't send you my design for obvious reasons. My sn is IrocMGO on AIM.

 

The TPI system is designed to operate at 3600 RPM.

The tuning effect of acoustic waves bouncing around inside the intake tract is only one of a number of things that control how the intake system performs. While such waves exist to some extent or other in any intake tract, their effect is so small as to be swamped by other things going on at the same time.

In the case of a typical factory carb intake, the runners are so short and the plenum so small, that the tuning effect *if any* would occur at 6500-7000 RPM. Work it out: speed of sound, length of runner. That's all you have to know. In a MiniRam, with 6" runners from plenum to valve, it's even higher. But the runners are so large compared to their diameter, and the runner volume so small as a result compared to the plenum, that the retuen wave is mostly damped the instant it exits the runner into the plenum, and so contributes virtually zero energy to the next cyl filling event.

The vast majority of intake designs are not deliberately tuned at all; any "tuning" they happen to exhibit is purely accidental. And even when you calculate the modes for reinforcement and then try to find them on a dyno, they're not there. Too small of an effect to affect the engine's output.

The real world is sometimes alot different from the classroom or the drawing board.

 

Ah, yes, engineering is so cool. Finding an engineering job these days, on the other hand, is not.