pipe diameter for snorkel

I've read a number of write ups on home built snorkels, and wondering if anyone has actually done the math on what size of pipe you need to ensure your engine gets the volume of air needed to run properly?

Any engineers out there care to give us the lowdown on what size would be 'best' or most efficient, just to supplement the excellent write ups availble....?

Hehe, you trying to get someone here to do your homework for you?

Hehe. but good question though. I'd like to know that answer myself!

File 05 said:

Hehe, you trying to get someone here to do your homework for you?

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well....I...um...errr....uh....yeah, I guess I am. BUT, it's an honest question because I am willing to admit that I don't have the technical knowledge to even start figuring out the size of pipe necessary to feed a 4l engine.
I am guessing that you would have to calculate in the highest volume of air that could fit in a cylinder, the compression ratio, max rpm, the volumetric efficiency (whatever the hell that is - it just sounds good) of the pipe and elbows, etc, etc..
"Dammit Jim, I'm a physician - not a aero-dynamics technician....!!"

Measure the inside diameter of the intake pipe to the throttle body. Round up to the next easiest available size. You can verify by doing a detailed search of writeups to see if someone gave dimensions. The throttle body is 62-65 mm, depending on your upgrade, so that's a start.

To be really sure you have no restrictions, use large radius bends at the 90's. That's where all the engineering takes place in tight systems to keep airflow up. Basic exhaust header principles apply: smooth bends, consistent size, and efficient length. It's just flowing the other way. If you insert the air filter somewhere in the line, bumping up the size to it won't hurt.

I'm not entirely sure that just going with the current throttle body measurement covers all the bases.

Consider this - and I'm just flapping at the gums here - assume you have a really large shop with a 2" pipe running all the way around the shop. this 2" pipe is attached to a shop vac at one end, and closed off at the other end. Along the pipe are several ports which are usually closed off unless you are using the sawdust producing tool at that port.
I have been told, and I believe it to be reputable, that you would have greater suction at the tool station closest to the shopvac, than you would at the station furthest from the shopvac. My simple mind can't figure out why this is but apparently it is. I think the solution in the shop example is to have a smaller diameter tube at the furthest from the shop vac.
So relating this to the engine (which creates vacuum like a shopvac), maybe it would be more efficient to have a throttle body sized tube nearer the throttle body, and decreasing in diamter as you get closer to the intake above the windshield......??
Having said all that, I do see that the snorkel manufacturers do use a consistent diameter tube for the whole thing...

I don't know, that's why I'm throwing it out there...

an intake needs volume and speed, not suction pressure like a shop-vac

According to the guys that make the Airflow snorkels for the XJ, you need 3" to over come the friction of the bends and on the walls over the longer distance.

run some dryer ducting through your firewall and use a stock type flat a/f
dust and water are now less of an issue unless the water is over your knees or you cant breathe yourself i have seen this done and it works well your "snorkel" is much shorter and more efficient this way ask me about it

Just some rules of thumb, every bend cost 15% velocity. Going from a smaller to a larger diameter costs velocity. Going from a larger to a smaller diameter reduces flow. An oval, square or rectangle, with the same area of the desired circle has nearly the same flow.
The fewer bends the better, a constant diameter is preferred and a near constant area is nearly as good.

Three inches. I know someone who runs 2" and it 'chokes' the engine. So 3 inches.

Randy Curran said:

Three inches. I know someone who runs 2" and it 'chokes' the engine. So 3 inches.

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Now you know why.