mkubacak

I am in the process of building an aluminum air box to house a BHAF or similar filter. I want to draw air from the cowl 'a la Scotty'. The Scotty uses a 4.5" hole in the cowl. Would a 4" hole flow enough air to feed a modified motor? I would think so, since the inlet for the turbo is 4". How do you calculate airflow through a given size hole?

Tommy93w350

You need to know how much air volume is needed first. Once the volume is determined, the cfm can be calculated by the velosity of the air moving through the 4" hole.

A 4" hole could produce 1 to 100 cfm at diffent velosity rates, plus, there's a lot more to it.

SHARPMACHINE

Are you going to draw from the fender also?? If so than I would think yes.


just my .002

pmtg

call it good at 4", im assuming you have a 4" holesaw and dont want to spend the $$$ on a 4.5".

mkubacak

pgilles nailed it.

I don't think I want to use the fender hole. I will if I have to, but i was looking at just using one inlet hole if possible.

So, how much air does a HX-35 Hybrid need? What if I upgrade to twins later? Do you calculate based on the turbo, or the amount of air pumped through the engine at max RPM?

HOHN

Figure about 115 CFM for every square inch of flow area. Thus, you'd have (4*PI*115)= 1445 CFM


That should be enough for a decent amount of power.

Lary Ellis (Top)

You might want to reconsider using aluminum for the box, it will transfer heat from the engine compartment and negate the cooling effect you are trying to achieve. Thats why so many of the air systems use plastic in them

BigBlue

HOHN, define decent amount of power? My definition is 600rwhp.

600 Megawatts

1445 CFM is enough for over 1,000 HP




KP

JDGnut

How big is the stock hole in the side??? 2x6??? (that would be 12sqin..) the 4in hole would be 12.5... Just thinking that the 4in hole would be about the same as the stock filter housing... I tried a filter that had a plastic housing, with a 4in hole... with injectors and a Comp... you could hear the plastic housing colapes when you had high boost.... (after that I cut the side out of the housing...

SHARPMACHINE

I agree with JDGnut. Why do people see improvement over the stock box by putting holes in the bottom of it? I think it is because more area = less restriction through a hole.

Question for Hohn or another braniac lol = How do you come up with 115cfm per sq inch? Is this at a certain psi? Would turbulance come in to play at any point? just wondering.

Lil Dog

You need to calculate the velocity of the air through the hole to determine if you are getting close to a sonic speed with air flow. Obviously the lower the speed the less pressure drop you create. Pressure drop on the inlet side to the turbo is sensistive and a few inches of water column can cause a few PSI drop in available boost pressure.

To calculate the flow you would need to get the boost level, engine RPM and calculate the Standard CFM. Then figure out the square area of the piping and determine the flow velocity.

With zero boost you need 625 CFM at 3000 Rpm and 720 CFM at 3500 Rpm. I haven't done the calc for the boost added to that.. I need to crack the books again...

625 CFM through a 4" hole is about 119 ft/second. I believe sonic is around 1200 ft/sec. So lots of room for flow there.

vssman

Uhhhh, I wasn't paying attention during that class. I figured I already knew all I need to know about Gas Dymanics But from what I remember there's more involved than just the cross sectional area of the hole in the wall. Personally, I'd just guess and seeing that you already have a 4" hole saw cut two side by side. If you don't have the room then maybe mak it an oval.

Lil Dog

If you want to get down to scientific levels, then yes.. We could pull out our dynamics books and figure out the pressure drop factor for the opening oriface and the losses through the pipe.

Hey, I am no prof.. I just pulled out some matchbook calcs... If its not right, then lets fix it... I am willing to learn..

600 Megawatts

Yes there is more to it than that. Flow accross an orifice is a function of the pressure difference accross it. As the pressure difference changes, the flow changes as a function of the square root of the pressure change... Thus doubling the pressure difference does not double the flow, it increases it 1.414 times (sqrt. of 2) .

So as for the amount of flow through a 4" hole, it depends the pressure drop accross it.

I was not commenting on 1,450 CFM being the number through a 4" hole, I was just saying, if you want to talk about 1,450 CFM, thats enough for over 1,000 HP.

Actually, our engines at 3,000 RPM and no boost, but assuming 100% volumetric efficiency, will consume 311 CFM. You were real close lil'dog, ya just forgot its a four stroke, so each piston only consumes air every other revolution, so you divide by 2. Obviously, as stated, its gets more complicated when you factor in boost, atmospheric conditions, excess air etc etc etc. The best way to do it is to figure in terms of MASS flow needed to combust all the fuel you have and still have 15% excess air or so, and then back into a CFM number for the inlet to the 1st compressor........



BUT REALLY, if you want to make it easy on yourself, dont worry about all these numbers... 4" is TONS of area for the turbo inlet.... JUST LOOK AT THE ACTUAL INLET TO THE COMPRESSOR IN THE TURBO.... you'll quickly stop worrying over a 4" or 4.5" hole.....




Kp