Smaller Cold Air Intake vs. Larger Intake with hot air

[boosted300]

Apologies if already discussed, I searched cold air intake and air filters and wasn’t successful in locating what I was looking for. 

I did review the Show your RB thread to look at the engine bay layouts of others with RB motors and for the most part, it appears most people have their air filters within the engine bay in some configuration or the other. 

Currently I do as well

 

 

The obvious reason, as I’m sure is the case for most, is convenience/simplicity combined with an aversion to start cutting things in the engine bay:  There’s really no room to route a 4” intake pipe anywhere unless you want to start cutting holes in the radiator support.

The obvious issue is the detrimental effect of the hot air from the fans pumping right into the intake. 

 

For lack of an air temperature gauge I tried my best to measure the difference.  I used an infrared laser temp gun to take some temp readings.  With the engine at operating temp, I waited for the to turn fans on, and watched as the temperature of the surface of the air filter rose to approximately 215 degrees. 

I then put a dummy filter in the area in front of the radiator support, just below the intercooler piping on the driver side, right in front of the lower 2.5” hole of the support.  Everything else being equal, the temp of the surface of this air filter was approx. 100 degrees. 

 

 

Assuming the rule of thumb that every 10 degrees of temperature (colder) equates to a 1% increase in HP is accurate, the assumption could be make that there is approx. 40-50 HP to be gained from a cold air intake. 

 

However, I personally have a strong aversion to cutting a larger hole in the radiator support if it can at all be avoided. 

 

What’s the alternative?  I’ve looked at all angles of the engine bay and don’t see any realistic, efficient, and aesthetically appealing (yes this is important to me!) method of plumbing a 4” cold air intake. 

 

That being the case, the thought occurred to me of using a reducer to plumb from the 4” turbo inlet down to the 2.5” existing hole in the radiator support, knowing that this obviously would create a significant restriction to air flow.  Taking volume, air speed, turbo efficiency etc. out of the equation (because I’m no M.E. and couldn’t figure it out anyway!), a simple area calculation shows a 61% reduction in area available for air flow.  Would this equate to a 61% reduction in power?  I highly doubt it.  But would it result in a greater HP reduction than HP increase provided by the colder air?

 

I also considered fabricating a sort of collector with a 4” inlet to two 2.5” pipes running to each of the holes on either side of the radiator support.  Even this would result in a 22% reduction in area available for airflow, and also would be a bit unsightly. 

 

So, finally, here is my question:

 

Which would result in a greater HP net result? Or perhaps I say, which is the lesser of two evils?

• A 4” unrestricted inlet with 200+ degree air temps.
• A 2.5” restricted inlet with ~100 degree air temps.

 

 

I know there’s some really technical guys on here so please share your opinions with if you don’t mind.

 

Thanks in advance.

Edited January 11, 2015 by boosted300

[johnc]

I assume your RB is turbo charged. If so, the intake air temp at the air filter is basically meaningless if you are running an intercooler. Also, under hood temps with the car sitting still are also meaningless. And a1% horsepower increase for every 10 degree drop on IAT is BS.

 

So, I'm not sure where to go from here to provide an answer.

[LanceVance]

Have you considered ducting cool air into the inner cone of a dual cone filter?

 

Nice cool air force fed into the inner cone while at speed, and zero restriction on what the outer cone can pull in sounds like it would give you a reasonable compromise between the two options you mentioned.

[boosted300]

I assume your RB is turbo charged. If so, the intake air temp at the air filter is basically meaningless if you are running an intercooler. Also, under hood temps with the car sitting still are also meaningless. And a1% horsepower increase for every 10 degree drop on IAT is BS.

 

So, I'm not sure where to go from here to provide an answer. 

 

I respectfully disagree John.

 

To a certain extent, the use of an intercooler makes the temps at the filter irrelevant.  However, in my opinion, if you just look at the picture of my engine bay, it seems apparent that the heat extracted by the intercooler, gets plumbed right back into the intake.  This is exacerbated by the fact that there is not one, but two heat exchangers that are having heat extracted from them.  Once I put the AC system in, it will be heat from three heat exchangers being extracted and pumped right into the intake.  It's like a vicious circle that seriously decreases the efficiency of the intercooler.  I tried to illustrate what I mean in the following pic.  It's not a great illustration as I lack a good photo editor, but I think you get the gist of what I mean.  The yellow line shows the flow of heat from the intercooler/radiator, into the turbo, back into the intercooler, where it gets extracted and pumped back into the intake, etc. etc.  This can't possibly not affect the efficiency of the intercooler.

 

 

Also, while I agree that, with fans off, underhood temps with car sitting still might be meaningless compared to the underhood temps while the car is moving...with the unvented hood closed, even with the car moving, once the fans kick on, hot air is being extracted from the heat exchanger and being pumped directly into the intake before it has any chance to escape the engine bay downward. 

 

While the difference may not be 200 degrees to 100 degrees...it may still be 180 degrees to 100 degrees...which is still a significant difference. 

 

Even if the 10 degrees = 1% rule is inaccurate, I live in South Florida where temps are typically mid to upper 80's... When the temps drop to the 50's turbo cars feel like they are on nitrous.  That's only a 30 degree difference. 

 

I can't help but believe that an 80 degree reduction in intake temps would make a decent difference in HP.

 

Could you lend your opinion as to the effect the reduction in intake diameter might have?

Edited January 11, 2015 by boosted300

[Phantom]

I actually did an experimental study on this when I was a senior mechanical engineering student.  I was trying to determine the benefit of "ram" induction.  Here's what I found out. "Ram" induction basically doesn't exist at normal legal speeds. Once I exceeded 80 MPH the inlet pressure started to rise pretty rapidly which indicated a reasonable ability to achieve increased air density. The side effect of my study, however, had to do with air temperature.  I found, like you, that the under hood vs. above hood temperatures differed by about 100 degrees, while the car was moving.  This was on about a 75 degree day in northern Idaho. That is actually worth about 5%, not 10%,  worth of HP increase.  Remember when you're looking at air density vs temperature that you have to start form absolute 0. 200 is not twice 100 as it's actually 660 vs. 560. The big three in those days were claiming about a 15 HP increase on their vehicles equipped with shaker hood intakes. I wasn't able to do a dyne testing in those days so I wasn't able to confirm my numbers.

Here are the best options I can suggest to you:

1) Cut a 4" diameter hole in your radiator support and reinforce it with a 4"O.D. section of thin wall tubing so you can basically have
   4" all the way through, or

2) Taper down to 2.5" through the radiator support but be sure to be at 4" on either side of it. The very short period os 2.5" diameter will cause a slight pressure loss but will be minimal compared to the advantage of 4" on either side. Then be sure that you're pulling the colder air from in front of the radiator.

FWIW - it doesn't matter whether a car is boosted or NA, colder air is better. Even on an intercooled engine the starting temperature has a direct effect on the exit temperature out of the intercooler. Cooling capability is based on the differential temperatures and the total exposure time of the two fluids to each other. Those specific values, however, are a very complex relationship and can't be covered in a short reply.

[johnc]

I agree that getting the air filter in front of the core support is a good idea. You can also run it into the front of the wheel well and protect it with sheet metal. You will have to clean it more often in that position.

 

Air temperature is just one part of the air density calculation. Humidity and barometric pressure are other components. I've instrumented the engine bay of my own racing 240z and saw engine bay temps within 20 degrees of ambient at speeds over 25 mph. It was a NA engine and I made mods to get the air out of the compartment.

 

A turbo engine will be hotter. Moving air filter out of the engine bay will give more room to get air out.

[boosted300]

I actually did an experimental study on this when I was a senior mechanical engineering student.  I was trying to determine the benefit of "ram" induction.  Here's what I found out. "Ram" induction basically doesn't exist at normal legal speeds. Once I exceeded 80 MPH the inlet pressure started to rise pretty rapidly which indicated a reasonable ability to achieve increased air density. The side effect of my study, however, had to do with air temperature.  I found, like you, that the under hood vs. above hood temperatures differed by about 100 degrees, while the car was moving.  This was on about a 75 degree day in northern Idaho. That is actually worth about 5%, not 10%,  worth of HP increase.  Remember when you're looking at air density vs temperature that you have to start form absolute 0. 200 is not twice 100 as it's actually 660 vs. 560. The big three in those days were claiming about a 15 HP increase on their vehicles equipped with shaker hood intakes. I wasn't able to do a dyne testing in those days so I wasn't able to confirm my numbers.

Here are the best options I can suggest to you:

1) Cut a 4" diameter hole in your radiator support and reinforce it with a 4"O.D. section of thin wall tubing so you can basically have

   4" all the way through, or

2) Taper down to 2.5" through the radiator support but be sure to be at 4" on either side of it. The very short period os 2.5" diameter will cause a slight pressure loss but will be minimal compared to the advantage of 4" on either side. Then be sure that you're pulling the colder air from in front of the radiator.

FWIW - it doesn't matter whether a car is boosted or NA, colder air is better. Even on an intercooled engine the starting temperature has a direct effect on the exit temperature out of the intercooler. Cooling capability is based on the differential temperatures and the total exposure time of the two fluids to each other. Those specific values, however, are a very complex relationship and can't be covered in a short reply.

 

Good info, thank you. 

 

If I do give in and decide to cut, I think that 3" to possibly 3.5" is the largest diameter I can get away with anyway as I believe that might be all the spacing between the radiator and the wheel well will allow.  So it would still require a transition from 4" to 3" at the very least.

 

As a result, if I go the tapering route, the best I may be able to get away with due to space constraints would be 4" to 3" down to 2.5" on the engine side, then back to 3" to 4" in front of the radiator support.  It will be a tight space in front for such a dramatic flare which will likely result in needing to go with a fairly compact filter. 

 

So in a nutshell, in that specific example, do you feel that the gains from the colder air will offset the loss from the reduction in tubing size?

Edited January 12, 2015 by boosted300

[boosted300]

I agree that getting the air filter in front of the core support is a good idea. You can also run it into the front of the wheel well and protect it with sheet metal. You will have to clean it more often in that position.

 

Air temperature is just one part of the air density calculation. Humidity and barometric pressure are other components. I've instrumented the engine bay of my own racing 240z and saw engine bay temps within 20 degrees of ambient at speeds over 25 mph. It was a NA engine and I made mods to get the air out of the compartment.

 

A turbo engine will be hotter. Moving air filter out of the engine bay will give more room to get air out.

 

Thanks John. 

 

Since I'm so reluctant to cut, I would not go the wheel well route. 

 

Yes I would agree, due to the turbo in my car and a lack of any modifications to evacuate the hot air, there's probably a greater than 20 degree temperature differential. 

 

Regarding your race Z, what modifications did you make to get air out of the compartment?  Due to the fact that I have the stock, non-vented hood, once the fans kick on there's 2600 cfm of hot air being pumped into the engine bay, pressurizing it to a certain extent and it's unknown what effect the open bottom has on evacuating that hot air, ie does it suck it out, does it recirculate, etc.  In either case, once they are on it doubtful that the filter would be able to access any of the outside air that rushes by underneath the car as the hot air from the fans will most likely be displacing any external air imo. 

[Leon]

So in a nutshell, in that specific example, do you feel that the gains from the colder air will offset the loss from the reduction in tubing size?

 

I think that you'll eventually realize that no one can really answer that question unless they have an engine simulation software at their fingertips and the willingness to help. Even then, it's still just a guess/approximation albeit a better one than people just flat out guessing.

 

This question can be put to rest in one of two ways:

 

1. You are satisifed with some sort of theoretical explanation for why it's better one way and not the other. Maybe even a simple "yes" or "no" from anyone that seems qualified will do the trick.

 

2. You actually build the two different intakes and test them back-to-back to see which works better.

[johnc]

I cut two 3" holes behind each strut tower and made a belly pan that ran from the core support to the crossmember.  That forced the hot air back and down under the car (the belly pan created a low pressure area under the engine compartment) and out into the wheel wells which are traditionally low pressure areas.

[(goldfish)]

I seem to remember JeffP doing some testing to the size of his intake.  Depending on the HP levels you are looking for 3" maybe enough.

[johnc]

Generally true.  A lot of air can flow through a 3" tube.  More then enough for 500 hp.

[aongch]

So far I have not overheated even in heavy traffic, and will monitor when tracking soon. I have the vented hood, would this be one of the best upgrades? Seems to extract quite a bit of the engine compartment heat, 

 

This might be a temporary solution, not having the filter elements on the other side of the support for best positioning:

 

[helghast7]

I think your simplest solution would be to put a smaller filter and put some ducting in. Really the only way you can see how well your intake is doing is by installing 2 IAT's (1 before the intercooler and one in/near the manifold) and driving the car in different scenarios. Not only will that show you the temperature of the air coming in but how well your intercooler is really doing its job, which frankly is the only thing that really matters.

 

I can tell you from my past rotary experience that ducting is probably the biggest help with this stuff, next to IC location which we already have a decent spot compared to most cars.

 

My question for you is, real driving conditions, cruising speeds and WOT what are your intake temperatures now, before any changes?

[boosted300]

Unfortunately I don't have IAT sensors so I really don't have any way of taking readings while driving.  That would certainly give me a definitive answer to my question though!

 

I agree that ducting is very helpful, however in this case, with the filter in such proximity to some fairly high flowing fans, I would think any ambient air entering through the ducting would easily be overcome/overpowered by the hot air from the fans. 

 

With everyone's greatly appreciated input, I have a two different set-ups I'm thinking of testing out. 

 

The consensus seems to be that a 3" inlet would be sufficient for what I'm trying to achieve.  500hp in a sub 3000lb car is fine for me.  I'm not trying to be the fastest guy in town...I never could be in this town anyway!

 

However, before I go that route (which would actually most likely be the simplest route) I have one other setup I may want to experiment with first because, as I mentioned before, if at all possible I'd like to avoid any cutting of the radiator support.