FI ram air intake science??

[ManDogFish]

Can a stock tune Z33 ecu run a safe AFR with +ve pressure in the intake plenum??

I am thinking of building a custom ram air intake that would be able to produce significant air pressure at speedsover 30mph. The following calculations could be wrong but if they are right, according to them, the vq35de would have a theoretical CFM of 6.5 cubic feet/s at 6000rpm, while a 2ft^2 area ram air has the potential to get 87 cubic feet of air at say correspondingly 30mph, with 80 cubic feet/s for the FI affect, atleast a few psi right?? if 87


if this article/page on converting air speed to pressure is correct
http://www.ehow.com/how_5814125_convert ... ssure.html
say an air box is made with a direct ram air opening of 2 square feet

at 30 mph, according to the article
(30^2) x (0.0027) x 2.0 = 4.86psi
a surface of frontal surface area of 2sq.ft should face 4.86 psi of air pressure when travelling at 30mph.

If we know how much air the engine is taking is, im guessing we could estimate the left over air that would exert its pressure in the ram air box.

so at 30mph (48km/hr); 13.3m/s --> 43.6ft/s, the 2ft^2 area pushes through (87.2ft^3) of air every second.
I do realize that the pressure difference form the pressure on the car's surface makes the air flow around the car
so not all of the 4.86 psi of air pressure will be exterted on the 2ft^2 area but due to the lack of a wind tunnel, ill shall make assumptions

for 4 stroke engines
the CFM(air flow rate) = [((displacement in inches) x 6200rpm x VolumetricEfficiency)/3456]

This guy is japan was comparing the improvements in the vq35de and the vq35hr platform. The graph shows a comparison of the VE of the de engine around 1.05, so VE = 1.05

http://books.google.ca/books?id=oHmb86m ... cy&f=false

CFM=218 x 6200 x 1.05 / 3456 = 410.64 cubic ft/ minute = 6.84 cubic ft/second


so the ram opening has the potential of taking in 87 cubic feet of air,
at 6000rpm, over 30mph with some wind, If I am assuming right, this will change the delta of the stock pressure difference between on front of the intake and the end of the intake runners.

so (80cubicFt)

Im thinking it cant be that simple or I am probably assuming something wrong cuz an extra 87cubic feet of air is alot, even if I take 0.5ft^2 of ram intake area, I may get around 21cubic feet of air, 21-7=14 cubic feet of air/s for the FI effect


Does it sound like it will work??



any one know any thermodynamic formulas to relate pressure difference to the difference in volume of air going into and out of the system??

[elwesso]

I'm an engineer and I've taken classes on gas turbine design and advanced fluid mechanics, and I can tell you that it's not that simple. I'd have to dig through some of my stuff from "back in the day"...

Air is an ideal gas (or can be treated as such), so you can use pv=RT... If temperature is constant, p1v1=p2v2...

In regard to your question of the air going into/out of the system, you can use the 1st law of thermodynamics for an open system, which is...



It looks complicated, but it's not as bad as it looks. Q is the rate of heat transfer going in (or out) of the system, W is the rate of work (power) going in/out of the system... Both Q and W will be 0. m(dot) is the mass flow rate (in lbs/s or kg/s).. the mass flow rate will be constant throughout the system, because even if the volume is being reduced, the total "amount" of air is not changing. Simply multiply the density of air by the volumetric flow rate (CFM) to get mass flow rate. Remember density of air varies highly on temperature.

h is the enthalpy, which is defined h=u+pV.. That is a value you look up in a table at a given temperature/pressure.

You can also neglect the g/gc(z) term because there's going to be no change in potential energy.

However, what you're suggesting won't really work, because you're not takikng into account that it takes extra energy to compress the gas.. Remember the engine is just an air pump, so it will naturally only flow as much as it "wants". When you try and jam more air into it, it takes additional work to maintain a higher pressure inside the intake/manifold..

I don't have a decent calculator handy, otherwise I'd try and crunch some numbers for you, just because I'm a nerd...

[ManDogFish]

Thanks,
i'd believe you even if you werent an engineer...

due to my field of mechatronics engineering.... i never covered much much mechanical eng related thermo...or I dont remember it, or I never went to the class or all of the above

yea i forgot 1st law of thermo and ideal gas law existed...lol

I thought it wouldnt work at first.... and then i remembered yamaha used ram air on their 'carburetted' r6 and actually gained a few whp at higher speeds....
and those were little tiny ram air vents....with and air box under the fuel tank....as they were carburetted, i would assume there would be no perfect tuning for low and high speeds as on low speeds it would run rich and lean on high speeds due to the ram air....

and i think one day I tried holding a bucket shaped object out of the car window at a high speed....and the force of air was somewhat a lot...
made me assume that it would create significant pressure atleast 1 psi from the entrance of the bucket to the back end

so I thought if yamaha's ram air uses that little ram air opening for a 14k rpm 600cc engine.... a 2 square feet area should be more than adequate enough for a 6000rpm 3500cc engine

I do think it would work though.... yes it does take more energy to maintain a +ve pressure but the the force by the air at higher speeds would be a lot with a 2 square feet ram opening. Specially when the engine is only consuming 6-8 cubic feet of air per second at the redline....

Am i wrong assuming that it would create a +ve intake manifold pressure? keep in mind the car can go faster

yes compression would take energy but,(2 squared feet area) out of the potential 87 cubic feet air flow, 7 is taken by the stock NA engine, A significant part of the left over 80 cubic feet of air flow would have to contribute to air compression in the ram air box right? its not possible for a much much much smaller air intake opening to generate the same intake manifold pressure...

I guess calculations can tell if its significant or not....although i stopped studying like 8 months ago...i cant remember anything

The calculations i think would be much more complex than that...even if temperature if ignored... I mean, the main issue I have is finding out how much air would actually go into the system, either by mass or volume... it would be great it you could redirect me to something useful

[elwesso]

Also another thing to consider is that your air filter is going to effectively reduce the velocity of the air..

I think I could bust out my old fluid mechanics stuff and come up with some pretty accurate estimations... $20/hr.. LOL!!!!!!!!!!

An area you could look at is inlet design on gas turbines.. It's a lot of the same.

[BusyBadger]

It looks complicated, but it's not as bad as it looks.

[jerryd1987]

something everyone forgot to mention is that the stock engine management has no way to tell there is additional pressure going to the intake, to do that your going to need something to retune the ecu(admittedly im not sure how the fi kits do it since everything ive messed with had the maf pre turbo not post like this platform) or run a map compatible system like the haltech(this is what i run for my turbo and honestly its the preferred way ive tuned anything) the haltech already has a onboard map too

[elwesso]

Remember that the manifold pressure is really the secondary affect of what's really happening. When the piston goes down during the intake stroke, most people think that the engine is "pulling" or "sucking" air into the cylinder, but that's not the case. What it's really doing is creating a pressure differential, where the cylinder is at a lower pressure than the ambient (manifold) pressure, and the atmosphere is actually "forcing" air into the cylinder. Most might argue that its kind of 6-half dozen- of the other, but understanding that the difference in pressure is what ultimately affects how much air the engine can "pump" is key to this whole subject.

Old school cars (carbs and early FI) use MAP sensors (manifold absolute pressure) and air temperature sensors to calculate the mass flow rate of air into the engine. These were orginally used because pressure and temperature was easier in those days. Ultimately, this is the ONLY thing the engine caress about is the mass flow rate of air going into the engine, so it can be combined with the proper amount of fuel. Today, Mass air flow sensors measure the mass flow rate of air directly, "negating" the need for MAP and IAT sensors. Note that most Nissans 90-95 model years did NOT have intake air temp sensors, because they simply weren't needed. Today they're used for other reasons, but we won't get into that.

On your point though, regardless of if the MAF is before the turbo or after the turbo, or there is no turbo, we assume that the only path of entry for air is "through" the MAF, therefore the air will be metered. There are pros and cons to having the MAF on the pressure side of the turbo or not, but we won't get into that. When you effectively raise the ambient pressure, all that means is that more air can get "forced" in by the ambient, and the MAF will meter this properly. Even if you added a turbo, the MAF can still properly read the amount of air going into the engine, but other things must be compensated for boost, which again we won't get into. HOWEVER, at relatively minor differences in pressure (say less than 5-6 PSI) the stock engine management can handle that properly in many cases (don't quote me on that because it's a vague statement).

After all, ambient pressure is lower by about 1PSI at 2000ft above sea level. Also, air DENSITY is inversely proportional to air temperature. Air is about 15% more dense at 30°F than it is at 100°F, which means that you're also injesting more air at lower temperatures, which is calculated by the MAF. I think everyone can tell their cars feel a little stronger when its cooler out than when it's hot out. Being at sea level when it's cool out maximizes your power.

http://www.engineeringtoolbox.com/air-a ... d_462.html
http://www.engineeringtoolbox.com/air-t ... d_771.html

[jerryd1987]

1) no the stock ecu cannot handle differences of 5 or 6 psi feel free to try it, wont take long for a blown engine betting withing 100 miles.

2) the intake air temp sensor is used for fuel and spark correction nothing more nothing less its built into the maf sensor, theres a completely extra map for air temp corrections(as well coolant) i know because ive looked at it in stock ecu's

3)again wrong on the maf, if the maf simply was able to read the airlow regardless of where its placed then we wouldnt need to tune the cars. checked into it and denser air removes more heat from the maf so thats how the computer calculates for boost but doing this is ineffective which is why EVERYONE converts to map sensors for more then stock block power. maf sensors apparently have as much as 7% error whn determining compressed airflow rates making pre turbo far better.

[elwesso]

I don't really feel compelled to refute those points because that's not what this thread it about, remember this thread is not about a boosted car. Interesting that those engines have the IAT and MAF in the same sensor.. I know there's a lot of practical implications that need to be qualified with what I said, however we're talking on a "theoretical" level here in this thread, and all of my points are valid, theoretically.

I do agree with you though, for boosted setups MAP sensors are typically the way to go especially with more boost.

[jerryd1987]

theoretically though you will need some way to account for the additional airflow though because the stock computer is not able to compensate for the additional airflow. either way a real ram air setup would generate positive pressure(the old muscle cars picked up 30 hp at 70 mph with ram air setups). our cars adapt very poorly to anything with additional airflow, a cold air intake is shown to cause the car to run a bit lean, sure not enough to damage anything but the point is if the car is unable to adapt to that then definitely need a tuning solution for ram air.

as far as the aspect of ram air it works, been demonstrated on numerous different platforms, the most effective would be something similar to a do luck bumper and a hr motor,

[emhmotorsports]

Elwesso: I want to commend you on your posts, they are very well done! As an automotive engineer for engine management systems that has worked at the OEMs and in professional motorsports for the past decade, I find your engineering approach and factual backups to your claims refreshing in these online forums. Keep it up, please!

ManDogFish: I can tell you that you have your work cut out for you. As Elwesso pointed out, it is not a very trivial matter. It hasn't been until relatively recently that production vehicles (and even then only the highest end ones) have been able to use the ram-air effect effectively. It is mostly over-hyped marketing. If your're looking for big power, the most effective way is still to go with a turbo/super. With that said, I don't want to discourage you from experimenting on your car. I think you should read up on as much as you can, study past designs (F1 cars and such), and see what you come up with. You generally want to put the intake where there is the highest positive pressure on the car (probably about the location of the license plate on the 350Z). I think the orginal NISMO bumpers for the 350Z had a slot cut just in front of where the air intake is - this may be worth some experimentation. Don't expect huge gains, but try to figure out what designs/locations provide better results. Instead of focusing on the power, take a look at differences in actual airflow. You can infer power increases from the increase in airflow, which is much easier to measure. In order to truly measure the HP at the wheels, you would need the combination of a dyno and wind tunnel. The stock MAF on the Z will be sufficient for what you are looking to do, and the ECM will still likely be within its internal compensation limits (if not, it will set a code). I think your biggest issue will be finding a place where you can run for long enough period of time at high speeds.

The task can be daunting, but if you have the right mindset I think you will have fun as you're making tweaks and experimenting. The knowledge of how air behaves and flows will be very valuable here. My advice for you is that when you are seeking new knowledge, never feel bad about questioning it and waiting until you prove it to yourself before you believe it. Unfortunately, there is a lot of misinformation on the 'net, but there are also guys like Elwesso out there too.

Happy experimenting!!!

[ManDogFish]

Thanks for the technical help guys...

Turns out that my projects will be progressing a lot slower than I had expected... my crackhead mechanic turned greedy, basically asking for $30/hr for using his hoist so I have to find a another mechanic now

I was attempting to flush the clutch fluid yesterday, I lifted the car on a jack and a jack stand, and theres were huge chunks of snow and salt dripping, came back an hour after, snow still dripping... I gave up... but yea, the car drives quite amazing on 5" fresh soft snow with snow tires, I have 40% hakkapelitta R's at the front, 90% hercules avalanche at the rear with an open differential but am starting to develop a mushy slow clutch

but yea, back to the topic... I am planning to use a UAFC controller to tune the ECU...
UAFC is made by 14point7, founded by Alan....its a piggyback unit that allows you to datalog and allows you to use other inputs i think
I spoke to Alan a few times, the UAFC should work well to modify MAF/MAP signals. However the injector size has to be near the fuel flow you need. According Alan, modifying the MAF/MAP signal by a lot would probably cause the engine to change timing. I would prefer not to mess with the stock timing and knock sensor setup.
Its supposed to work like any other piggyback but it allows you to data log aswell.... for $125 it seems like a good deal...
btw he was selling them at $25 when he was working on the beta version. Some maxima, and sr20 owners seem to have had success with it.

I think it was undersold cuz people prejudge quality by price... you do not always get what you pay for.


I have suspended this project for me until I do a rear mount radiator setup cuz then I can use the whole bumper grill for ram air. This may take a long time and realistically may not happen even though I will be ordering an aluminum radiator for the rear mount soon, once I find the ideal one I could use...

[elwesso]

To relocate the radiator to the rear of the car just for a ram air seems like a lot of work for not much gain?

That UAFC sounds kinda sketchy. Basically it sounds like a way to slightly tune the stock program, which in most cases shouldn't really be needed?

[ManDogFish]

yea it is...I thought I could still build it with the radiator at the front.... but then I have currently lost hope as I do not have a garage to work in...

that was after I took the bumper the aluminum piece, took some parts off and took some measurements....its a lot of custom work unless the jeep 12" height radiator can support the vq35....which I still have to look into...