Author Topic: Air box / velocity stack space issue  (Read 1448 times)

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Offline QikNip

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Air box / velocity stack space issue
« on: November 04, 2018, 04:39:23 PM »

After lots of measuring and scheming I've decided to duct fresh engine air from the very generously sized (and hopefully high pressure) windshield cowl, rather than using a scoop or NACA vent in the hood. To that end I've been measuring and cutting poster board all day mocking up two identical air boxes for each of the DCO 50 Webers ... and scoping out how to plumb air from the cowl to the air boxes given the tight space constraints under the hood. BTW, I'm unable to use one box due to space, fuel line and linkage constraints. The engine is spinning at 8,100 RPM's and the cars is running 150-152 in the three mile.

To further complicate things, the back throat stack on the rear carburetor is crowded by the master cylinder and brake booster, limiting the depth for both air boxes to 3.25". The boxes dimensions are 6 high" X 5.25 wide" X 3.25 deep". Overall the boxes seem adequate at 112 cubic inches each (it's a 122 cubic inch engine), but I'm concerned about the 1.6" distance between tops of the 2" diameter stacks and the air box wall opposite them. Of course the firing order is such that only one carburetor throat in each box will draw at a time. The space limitation also includes the plumbing from the cowl to those boxes. That will require feeding air into them from the back side of each through a .75" X 5" rectangular slot (3.75 square inches), with that air being delivered by a shared 30" long 2" X 6.5" rectangular shaped duct.

With all that as background, does anybody see any (or several) flashing red lights to this plan?
Rick
P.S. Sorry I can't add pictures to make this a bit less confusing!
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Offline Crackerman

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Re: Air box / velocity stack space issue
« Reply #1 on: November 04, 2018, 07:26:09 PM »
When manufacturers want to run an airbox/velocity attack setup with low hood constraints, they bend the stacks to fit and enclose it all.

The v8 BMW m5 comes to mind. The I take is a work of art with the lid removed.

Offline Dynoroom

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Re: Air box / velocity stack space issue
« Reply #2 on: November 04, 2018, 08:16:43 PM »
BMW M5 with the lid off...
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Offline tauruck

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Re: Air box / velocity stack space issue
« Reply #3 on: November 04, 2018, 09:03:17 PM »
No hijack intended. My mate Guido is a very clever Dane and unfortunately he loves Rovers.
Similar system on his motor but each stack had a plastic butterfly. Some were seized, some broken
and only two worked. We ripped them all out. He says the car's never run better. We did however have
to break the box open along the plastic weld and use my Epoxy adhesive to reassemble the junk.
BM clearly has a better system but it still looks like a bag of snakes. :-D

Online stay`tee

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Re: Air box / velocity stack space issue
« Reply #4 on: November 05, 2018, 05:03:08 AM »
The velocity stacks on Goldenrod come to mind, thay were shoehorned into a constrained ramair system,,
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Offline Lemming Motors

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Re: Air box / velocity stack space issue
« Reply #5 on: November 05, 2018, 07:15:49 AM »
For a 122 cu in (1,999 litre) engine, assuming its naturally aspirated, at 8,100rpm that's 285 cu/ft per minute air consumption (if my calc. is correct). Its very similar to my own specs. but we established its a different engine.

I am planning to use throttle bodies and have spoken to two (UK) suppliers - with knowledge of my intended power outputs, revs etc. and they both confirmed that the clearance from the top of the rams horn to the inside top of the air box should be approx. 1 inch (or more). This discussion was based on 'off the shelf' air boxes enveloping all 4 ITB's and me trying to reducing the depth by cutting one down. I cant verify their opinion. Don't know if that helps.Since you are using two air boxes per your space restrictions I wonder if they should be linked together with a balance tube in case the input airflow (pressure, if any) favours one over the other?

I cant comment on the 3.75 sq inch input duct for each box - its a bit smaller than the aftermarket air boxes offer but they are probably giving me enough for a much bigger capacity engine. They are 4" diameter so 12 sq inches. Your 7.5 sq in (combined); flow characteristics of a thin wide duct aside, that area is equivalent to a 3.1" round duct inlet.

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Offline QikNip

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Re: Air box / velocity stack space issue
« Reply #6 on: November 05, 2018, 07:57:32 AM »
For a 122 cu in (1,999 litre) engine, assuming its naturally aspirated, at 8,100rpm that's 285 cu/ft per minute air consumption (if my calc. is correct). Its very similar to my own specs. but we established its a different engine.

Yes it is naturally aspirated and your calculations are correct :)

I am planning to use throttle bodies and have spoken to two (UK) suppliers - with knowledge of my intended power outputs, revs etc. and they both confirmed that the clearance from the top of the rams horn to the inside top of the air box should be approx. 1 inch (or more).

Can I assume that your velocity stacks are  positioned vertically and that by clearance you are referring to the distance between the open end and the "roof" of the air box?

Since you are using two air boxes per your space restrictions I wonder if they should be linked together with a balance tube in case the input airflow (pressure, if any) favours one over the other?

The balance tube is an interesting idea. I do wonder however if it might introduce a "harmonic" wave issue...


« Last Edit: November 05, 2018, 07:59:43 AM by QikNip »
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Offline Lemming Motors

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Re: Air box / velocity stack space issue
« Reply #7 on: November 05, 2018, 08:49:41 AM »
The ITBs air intake - yes - velocity stacks - and yes, my reference was from the top of the stack (its open mouth if you will) to the roof of the air box.

Harmonics I know nothing about. I am aware of a discussion about the open distance (roof to stack) with regard to harmonic reflections but that is also too advanced for my needs (if I can build the engine with some spare horses - can you ever have too many?).
A Bonneville Lakester please barman.
Certainly sir; a lick of salt, a sip of gas and a twist of Lemming. More Lemming sir?
Just a squeeze.

A Squeeze of Lemming it is sir.

Offline QikNip

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Re: Air box / velocity stack space issue
« Reply #8 on: November 05, 2018, 10:40:37 AM »
The ITBs air intake - yes - velocity stacks - and yes, my reference was from the top of the stack (its open mouth if you will) to the roof of the air box.

Thanks … and emoge below

(if I can build the engine with some spare horses - can you ever have too many?).  :-D
138.0 G/CPRO 4/18 Arkansas Mile Challenge Record
145.632 G/CPRO 8/17 Bonneville Record
149.825 G/CPRO 8/18 Bonneville Record