ho to calculate venturi size/shape in intake manifold

[cosworthmk1]

Hi all, I need some advise on venturi.

I intend on having a go at casting my own manifold for an engine im building. I Want too use Fuel Injection and ITBs and was wondering if i needed a venturi in the intake runner at all, and if so how would I calculate the shape and size of said venturi.

Any help/equations would be greatly appreciated.
Cheers Shaun.

 

[GregLocock]

What is this venturi supposed to do? Where in your intake will it be?

Cheers

Greg Locock


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[tbuelna]

cosworthmk1,

You probably don't want one in your injected engine's intake manifold. It would only produce an unwanted flow restriction.

Venturis are used in carburetors to create a controlled zone of below ambient air pressure so that fuel will flow through the metering circuit in the desired direction.

Hope that helps.
Terry

 

[cosworthmk1]

Greg

If i had needed to have a venturi in the intake it would need to be after the throttle body, but then again this is why im asking people on here as i didnt know myself.

Thanks for your input Shaun.

 

[cosworthmk1]

Terry.

Thanks for your input Terry this is very helpful as im more familiar with carbs and nots so familiar with design of componants for fuel injection.

Once again Thank you for your help Shaun.

 

[patprimmer]

No.

A carburettor needs a venturi to draw and meter fuel into the air stream.

FI needs no such restriction.

Regards
Pat
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[patprimmer]

Are you a student.


Regards
Pat
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[cosworthmk1]

Pat

Hi Pat, I am a marine systems apprentice in NZ but also did a pre-apprenticeship course in automotive unfortunately tht course didnt goin into such depth as tht would have come with the apprenticeship afterward but i went with marine systems instead.

Thanks for your input.

Cheers Shaun.

 

[TrackRat]

It's quite common to use a 5-7 degree taper on the intake runner up to the throttle plate on an individual runner/TB to increase air speed without restriction of air flow. You don't normally have a true venturi on a FI intake system unless you're attempting to correct for a system that was designed too large for the application.

 

[patprimmer]

A venturi does not fix low airspeed as the air speeds up as it passes through the restriction, but slows down again imeadiately it exits the venturi restriction.

A taper is a valid design as it gradually accelerates the air as it moves toward the valve seat area.

Regards
Pat
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[TrackRat]

A venturi shape placed close to the valve increases the port velocity for better cylinder charging when the port area is too large to start with. It's a bandaid.

 

[evelrod]

If we are talking about a "two valve" engine, then, yes...A venturi/choke like cut just below the intake valve seat has shown to increase port velocity and, in some engines this has produced a bit broader power range....In some! I have done at least one that this practice simply killed the top end. Don't know why cause it was a customer engine and I did not have time to tinker with it...just reground the port a bit to moderate the adverse affect (I did not cut the port, I got it that way).

I use the undercut intake port on my vintage racing 1594cc Lotus twincam two valve crossflow hemi combustion chamber head with good results (197.2hp @ 8100 rpm---135.2 lbs/ft tq @ 6800). However, I tried it on my 1380cc Austin five port and it was a huge waste of time. Unlike the Lotus, the Mini heads are a dime a dozen and I could afford a little experimentation.

Additionally, I'm pretty sure undercuting would not be practical on a four+ valve head. I have no practical experience with one, though.

Rod

 

[patprimmer]

Rod

OK I agree that a venturi effect where the valve and valve seat area is part of the venturi so that the airspeed through the most restricted part is increased.

Obviously if the most restricted part goes supersonic, it is counter productive.

Do you think it was the drop in cross section, or the better lead into the valve curtain area that gained the power and possibly it only works if there is no shrouding to obstruct the discharge end of the venturi where the valve and seat are the neck down point of the venturi in effect.

Regards
Pat
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[TrackRat]

The venturi I was referring to is farther back in the port not at the valve seat/throat area. It was used as a crutch for an oversized runner system. In the seat/throat area you do whatever it takes to get the desired results.

 

[cosworthmk1]

Hey Guys.

Thanks for all your feed back and help this is giving me some serious ideas on how to go about casting my intake.

 

[evelrod]

Pat

I think it's probably a combination of things that cause an increase in some and decrease in other similar head designs. There is (oh, how I hate this) the "luck" factor, for want of a better term. In my case, I firmly believe it is the tapered port design, hemi cumbustion chamber that makes the In valve port choke work. Over the years, we tried several different heads and this mod is what worked. Incidentally, I got the idea back in the early 70's when I bought a Brian Hart cylinder head that was so configured. Further talks with David Vizard got me started in the future mods.

So, why does it cause so much power loss in the Mini's head? Dunno, unless it is the fact that the port design is something really horrible in design.....OH! Wait! That's the problem!!!!!!!!!

Cosworth

From my limited experience in actual port design....It always seemed that the smallish ports produced the best, broadest power curve. In road racing small sedans, Tq is often a better determinant of lap times than ultimate HP...Given the limited gearbox choices and obvious aero deficiencies. Just my experience, anyway.

Rod