Interesting autospeed article on ram air intake

[MechEngVT]

To make generalizations one of the things common among commercially-available cold air intakes is that they replace the stock air filter box and short hose to the throttle body with a long tube placing a reusable filter near the front of the vehicle and somehow duct the stagnant air from the nose to this filter.

The "wave theory" brought up by whatthe could more than explain how such a setup would improve fuel economy. By using a longer intake duct you are actually tuning the intake to improve volumetric efficiency at a lower engine speed. Engines do not pull air at steady flow but rather in pulses as each cylinder fills only once every two revolutions and no two cylinders fill in unison. CAIs often produce a sound because they eliminate a resonant chamber (the air box) and replace it with an organ pipe (the tube) thereby allowing that unsteady flow pulse to resonate and produce sound. Since sound is an alternation of positive and negative sound pressures you can acoustically tune your engine to shift the torque curve to a lower engine speed. When it is in tune the positive air pressure pulse will continue past the throttle and hit the open intake valve improving port velocity (increasing swirl and chamber turbulence, improving fuel burn and reducing knock tendency). The acoustic pressure pulse induced by the unsteady intake flow will likely exceed any "ram" pressure seen at normal highway speeds.

If the torque curve were improved VERY low in the engine speed range, below 2000 rpm, this could very conceivably improve fuel economy by allowing more high-load operation without lugging or allowing earlier upshifts. Shifting torque to a lower speed could shift a BSFC iso-efficiency island lower as well. I envision improving ultra-low speed torque (and efficiency) improving hypermiling by allowing a shorter-duration higher-load pulse followed by an EOC, effectively decreasing the percentage of the time the engine is on and under load.

As much as hypermiling depends on DIY and testing to compare I'm confused by the overt hostility to a mod just because it is perceived as a "performance" mod for racer-types. I tend to think most people's perceptions are wrong and would rather see data. If someone could do a good a-b-a on any mod and prove a FE improvement I'm all for it but debating the merits in the absence of data using only preconceived notions or dismissing an improvement because it's negligible (it's *still* an improvement) seems like a waste.

[whatthe]

Originally Posted by MechEngVT

To make generalizations one of the things common among commercially-available cold air intakes is that they replace the stock air filter box and short hose to the throttle body with a long tube placing a reusable filter near the front of the vehicle and somehow duct the stagnant air from the nose to this filter.

The "wave theory" brought up by whatthe could more than explain how such a setup would improve fuel economy. By using a longer intake duct you are actually tuning the intake to improve volumetric efficiency at a lower engine speed. Engines do not pull air at steady flow but rather in pulses as each cylinder fills only once every two revolutions and no two cylinders fill in unison. CAIs often produce a sound because they eliminate a resonant chamber (the air box) and replace it with an organ pipe (the tube) thereby allowing that unsteady flow pulse to resonate and produce sound. Since sound is an alternation of positive and negative sound pressures you can acoustically tune your engine to shift the torque curve to a lower engine speed. When it is in tune the positive air pressure pulse will continue past the throttle and hit the open intake valve improving port velocity (increasing swirl and chamber turbulence, improving fuel burn and reducing knock tendency). The acoustic pressure pulse induced by the unsteady intake flow will likely exceed any "ram" pressure seen at normal highway speeds.

If the torque curve were improved VERY low in the engine speed range, below 2000 rpm, this could very conceivably improve fuel economy by allowing more high-load operation without lugging or allowing earlier upshifts. Shifting torque to a lower speed could shift a BSFC iso-efficiency island lower as well. I envision improving ultra-low speed torque (and efficiency) improving hypermiling by allowing a shorter-duration higher-load pulse followed by an EOC, effectively decreasing the percentage of the time the engine is on and under load.

Great post. I couldn't have summed it up that well. Cylinder filling and length of intake duct were my thoughts exactly. A longer pipe should lower the rpm that this theoretical improved filling occurs at.

The question of do you want a 2, 3, 4, or 5 foot pipe on the end for your car should find it's own answers on a forum like this. I'll try different lengths just to see what works.

I tend to think most people's perceptions are wrong and would rather see data.

I couldn't agree more with you on this as well. It seems all too often that you come across a post on a car forum where the poster says 'that sucks' without a list of reasons or data to show why.

[jonr]

IMO, CAI will hurt you. A tuned intake system could help you (some?), but only at a a very limited rpm/load range. More applicable to stationary engines driving load controlled generators at fixed rpm.

Low rpm is not an all good thing either - piston speeds are lower so the conversion of heat into movement is less efficient.

[garys_1k]

Acoustic effects significant to FE lie between the throttle valve and the intake valves. Nothing upstream of the throttle will change the acoustic tuning at these part loads.

Keep you air filter reasonably clean and you'll do fine, unless you want high RPM power. FE calls for limited amounts of air so efforts spent tweaking out a small pressure drop are most definitely spent elsewhere.

[Vanner]

It goes without saying a-b-a controlled experiments, especially with different lengths of pipe to test the applicability of Mech's wave theory, would be far and away more conclusive and useful than a forum theoretical discussion. Keep us posted, whatthe. Until then we don't have any.

Secondly, I'm going to do something dumb and defend myself against these oblique attacks on my character, but I don't understand why this has to be personal. "Overt hostility?" I think not. I do not care whether CAI's are performance mods, but that is what they are designed for. So when someone comes by and says "btw, they increase fuel economy too, here's my hunch as evidence" a little red flag goes up in my head. Now Mech has finally presented a theoretical explanation, good, lets test it!

Finally there is no such thing as a negligible improvement, it either offsets the cost of installation, labor, and additional noise or it doesn't.

I guess, whatthe, my list of reasons for saying "that sucks" is no explanation towards how its possible (until Mech's post), no data to the contrary, and no adoption from the factory (except in sports cars). Manufacturers take great pains to tune their intakes (albeit mostly for noise reduction), and if CAI's were worth it they'd be everywhere. Again, sorry this is personal, but it seems all too often that you come across a post on a car forum where the poster says 'this is magic' without a list of reasons or data to show why.

[whatthe]

Stock fuel cut on my GT is 7500rpms. Max FE for me is going to be below 4000rpms and if I'm hypermiling I should theoretically never see above 4000 again. Therefore, my factory intake is poorly tuned for the 750-4000rpm range and it is an area worth investigation. Although, I will say that I do agree that CAI is not necessarilly the way.

Why do you say that acoustic tuning will not be effected at part load? As I see it, the air is still pulsing even past the throttle valve. If you go home and stick an 8 foot section of pipe on your intake, go for a drive, and tell me that it didn't make a difference at part throttle.... I will be incredibly surprised. I have an exhaust/intake theory book that has a few graphs showing the effects of cylinder pressure/filling against valve opening for a few different lengths of intake. This length of pipe was placed before the "carburetor/intake manifold" (older book).

From an incredibly limited experience stand-point, and a premature update for Vanner, I put a 6-7 foot length on my GT yesterday, put the wideband on, and went for a drive today. The pipe is longer than the theoretical values I was thinking of, but I used some old intercooler pipes and common sense says it's easier to make it shorter than longer later. There have been some interesting changes that I can better get into after a tank of gas.

[tjts1]

Some people will never be convinced no matter how overwhelming the evidence. Oh well.
http://www.karlsnet.com/mopar/ramair.shtml

[garys_1k]

Originally Posted by whatthe

Why do you say that acoustic tuning will not be effected at part load? As I see it, the air is still pulsing even past the throttle valve. If you go home and stick an 8 foot section of pipe on your intake, go for a drive, and tell me that it didn't make a difference at part throttle.... I will be incredibly surprised. I have an exhaust/intake theory book that has a few graphs showing the effects of cylinder pressure/filling against valve opening for a few different lengths of intake. This length of pipe was placed before the "carburetor/intake manifold" (older book).

From an incredibly limited experience stand-point, and a premature update for Vanner, I put a 6-7 foot length on my GT yesterday, put the wideband on, and went for a drive today. The pipe is longer than the theoretical values I was thinking of, but I used some old intercooler pipes and common sense says it's easier to make it shorter than longer later. There have been some interesting changes that I can better get into after a tank of gas.

Tuning upstream of the throttle valve will change the acoustic signature (sound levels) apparent to the driver and outside environment, yes. Car makers add Helmholtz resonators to the intake boxes to quiet certain frequencies that would sound objectionable to the driver.

But those effects are lost on the other side of the throttle valve, inside the manifold. Acoustics there, which would cause local pressure wave optimizations right at the intake valve (for improved volumetric efficiency) are almost entirely (>90%) caused by reflections between the throttle plate itself and the various parts inside the manifold and intake ports. The throttle valve is still more than 50% closed at those operating points where we run for best FE, so that plate is a HUGE chamber end reflector for acoustic waves inside and outside the manifold.

Now, open the throttle valve and yes, the entire intake system becomes important. A plate at 90 degrees to the flow will have a minor acoustic impact. Racers and those looking for wide open or no, (i.e. diesel) throttle operation should look at the impact of the whole system for tuning effects.

A bit off topic, but for part throttle FE I think we'd be better off looking at a tuned exhaust system. That's always "wide open" and adjusting its length for constructive interference acoustics at the exhaust valve (where constructive means improved breathing) will work at ALL throttle openings. With a better exhaust flow you ought to reduce the total engine friction, but overdoing it could cause some of the incoming intake charge to be pulled right out of the exhaust valve. Reducing cam overlap could help optimize that.

[jonr]

I agree, either tune for resonance from the throttle downstream or work on the exhaust. Possibly something could be done with rubber diaphragms instead of long pipes (think drum vs organ pipe).

75% or so throttle is best for acceleration, but we don't do much of that and so are most often at less than 20% (ie, mostly closed).

[Formula413]

Originally Posted by johnpr

I'm aware of what the factory rated them, the truth is you put an un-modded t/a next to a ws6 and race them and there is no definate which is the faster car. this has been debated time and again on ls1tech.com and each time it comes out the same - they are equal cars.

Correct. As another F body guy I can attest to this, I have seen dyno sheets. The widely varying ratings for different LS1 engined cars (Z28/Trans Am, SS/WS6, Corvette) are just marketing, not reality. All LS1 powered cars made very similar power.