Correct air intake pipe size for 4x4 snorkel ?

[TMcRally]

On a Porsche Cayenne twin turbo, it makes around 500 flywheel horse power.4.8l V8 X 7,000rpm red line

I want to run 2 independent snorkels, each snorkel will be just under 2m from air box to open end.

I'd like to keep the ID to the smallest commonly available size but I don't want a restriction.

Is there a simple formula I can use ?

Thanks

 

[Lou Scannon]

There are various ways to estimate the volume air flow. Or, if you can log data, you can get it directly from your ECM.
Once you have the volume air flow, use the Moody diagram with a starting pipe diameter to calculate the pressure drop. Make sure to include the equivalent length of any elbows.
Iterate the diameter until you find a compromise between size and and restriction that you like.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[LittleInch]

Your issue is that you don't know what the acceptable pressure loss is.

I would look at the existing inlet size and then double the diameter of your snorkel.

So if it's 3" inlet use a 6" pipe.

This sounds like one hell of a 4X4. Any pictures?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GregLocock]

You'd probably want to keep the full throttle manifold air pressure to within 1 kPa of ambient. So work out your likely airflow at 7000 rpm, plus a bit for the turbos, and then use a duct resistance equation.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[SwinnyGG]

Greg's mention of keeping pressure drop below <1kPa below ambient is a good 'rule of thumb'.

Fortunately for you, this is pretty easy. 4.8L @ 7000 RPM is around 900 CFM.

Two separate ducts means they don't have to be huge. By my quick math 2 4" ducts will do the job without much drama. I didn't account for any bends, which you'd want to if you really care that much, but there's enough margin that I doubt it matters.

You may want to ask yourself, however, if you really NEED snorkels. Or if you really even want them. If this is just for cosmetic reasons (I mean... I see the hilarity in building a balls-out off road vehicle on the Cayenne platform from here) then have at it.

But if it's not... do you REALLY think you're going to ford 1m + depths in a Cayenne? Adding snorkels will protect the intake, assuming they are sealed well enough - which is a non-trivial goal to accomplish - but there are other things under the hood that will likely not survive being immersed in water. Where is the ECU? How is the turbo oil supplied? Where is the PCV? Where are the trans/diff vents?

 

[LittleInch]

I get about 600 cfm, but haven't allowed for the turbo effect which could double or triple the airflow.

But agree that a snorkel seems a bit much unless you're doing it for dust issue?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

20 or 25 horsepower per square inch of intake area ought to be in the ballpark. Doesn't matter turbocharged or not.

 

[TMcRally]

Very much appreciated, thanks to you all.

It looks like 2 x 4" intakes is a safe bet, I was hoping for smaller but it is what it is.

..................

Littleinch, I haven't got pics with me but I will take some "as is" and post up, it is a work in progress (very slow).
Reduced dust is just a bonus, the snorkels are for crossing water.

SwinnyGG, The intention is to do some reasonably aggressive off-roading. It's not common but I do occasionally have water come up over the bonnet when driving taller vehicles than this one and the intake on these is behind the grill above the radiators. You only have to get it wrong once to have a bad day. It has front, rear and center locking diffs as well as low range, air suspension to raise it. I'm modifying it to fit 34" tyres (aiming for 35" but might not quite make it) I should end up with 330mm+ absolute clearance, i.e. no diffs, spring hangers etc hanging down. Computer is up high under the dash inside the cabin, air breathers are not bad standard but are raised. PCV/AOS is a sealed system and on top of the valve cover. Turbo oil system? not sure if I need worry about it, (do I, you've got me worried now) it is engine oil feed through a sealed system (let me know if I'm wrong). I do worry about thermal shock directly on the turbos as they are low, my thoughts are not to work them hard around water crossings and cross my fingers.... eeek!!! triple caution.

Again, thanks all
Dave

 

[SwinnyGG]

Re: turbo oiling - some systems use a non-pressurized drain back to the pan, which can wind up under vacuum and isn't designed to seal against immersion in water. I know nothing about the Cayenne, so maybe its a non-issue, but on some engines it can be.

 

[gruntguru]

In general, turbocharged engines will tolerate a larger intake depression since the resulting power loss can usually be offset by a small increase in compressor pressure ratio.

je suis charlie

 

[Lou Scannon]

But it's not a free lunch. There will be a calculable loss of thermal efficiency due to greater turbine work, as well as a calculable increase in compressor out temperature, which will most likely translate to an increase in intercooler out temperature. This could impact knock-limited power.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[TMcRally]

Can I ask about the shape of the snorkel air entry point.

I have seen all kinds, some that are intended to ram air in through an oversize mouth pointing forwards and then some the complete opposite, with an angled cut that points backwards and some trying not to catch rain water.

Are there pro's and con's that I'm not seeing.

 

[TMcRally]

[highlight #EF2929]NOTE; I POSTED AN IMAGE OF THE CAR, WHEN I CLICKED ON THE IMAGE THE LINK TOOK ME TO MY COMPUTERS HARD DRIVE SO I DELETED THE LINK[/highlight]


Littleinch,

A rough picture of the bull bar being fabricated. Nothing is easy. To fit the bigger wheels the intercoolers need to come forwards, I tried to do this without re-routing the plumbing for them. Then they start to become more exposed so the mesh in front of them is cut from 10mm mild steel plate and is part of the mount footprint of the whole bar.

I will have 2 round pipes coming off the bar, one at the top and one from the bottom of the side plates on each side. The bottom one will angle up and join the top one at the corner of the car then run around under the head lights. The square opening in the middle is for a winch mounted on a trailer style hitch. Having it removable allowed me to use it on the back if needed, also reduced weight up front and improved the approach angle.

 

[Lou Scannon]

To ice the cake, you could apply a modest bell end at the inlet. Orientation of the inlet is not critical. Ram air starts to be appreciable at highway speeds and up.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[SwinnyGG]

Are there pro's and con's that I'm not seeing.

It won't matter much as far as performance. If it were me, I'd want to avoid ingesting rain water or road debris, which means pointing the intakes somewhere other than straight up or straight forward.

I did once see an aftermarket snorkel 'kit' which had a shielded opening at the top to protect against water intrusion, but had a drain hole drilled in the bottom of the lowest elbow so that anything that made it inside could exit. Made me chuckle. Probably don't do that.

 

[LittleInch]

The image upload has changed recently, but you should browse to the file and then open and click upload.

If you try to copy and paste instead of drag and drop then you could get something like you describe. It's worth trying again.

Ram air effect is a speed dependant thing, but you have to be moving quite fast to get any appreciable effect. Can't see that being important for this type of vehicle.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JayMaechtlen]

The airflow is proportional to actual horsepower generated - I wouldn't think you'd be doing max power while fording, bur ??? maybe sometimes. When the exhaust system is cooled by immersion in water you will probably get less boost pressure. (cooler exhaust to turbo) I can't see that being a big deal.
- I imagine it has an intercooler - anything susceptible there?
- power steering fill/vent?
- trans fill/dipstick?
- brake master cylinder (yeah, those caps are normally sealed well nowadays)
Sounds like it will be an amazing toy!


Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[SwinnyGG]

I wouldn't think you'd be doing max power while fording

The snorkel is permanently mounted - you don't just stop and put in on prior to a crossing. You need to account for power at full load.

 

[romychaj]

a pressure drop through the snorkles wont matter a bit, because the turbo system will keep boost pressure to whatever it has been programmed regardless. (thats what turbos were invented for, to keep airplanes operating at high altitudes.) of course if you exagerate and choke the inlet to 50 kpa there will be a noticable efficiency drop (higher turbo exit temp.) as the operating point moves off the map, that however to a large extend will again be compensated by intercooling. all you really wind up with is a higher rotor speed.

you can use 0.6 liters/sec./hp for a ballpark calculation, btw.

edit: so what im saying is look at the area of the stock intake, divide by two and be done.

 

[Lou Scannon]

Gratuitous pressure drops are as unwelcome in a boosted engine as they are in normally aspirated engines.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz