Estimating optimum multi-intake cfm?

[JTP173]

Say you can estimate the optimum cfm for a single intake engine (one carburettor any type), how can you estimate the optimum cfm for 3 carburettors running on the same engine? This is not for using Webers, I want rough estimate for general use assuming that the style of carburettor is not altered.

Assume it is an inline 6:

1 carburettor will be drawn through 100% of the time and the intake strokes will be overlapping.

Each one of the 3 carburettors will be drawn through only 50% of the time with no intake stroke overlap

As a result of the overlapping stroke for one carburettor the flow through each of the 3 carburettors is only 70% of that through the single setup while the carburettors are being drawn through (while on the intake stroke)

The peak-flow through the carbs on both setup is the same.

Finally the total amount to flow through each of the 3 carburettors is one third of the total amount.

Let the single carb. cfm = 400.

Ignoring changes in booster strength I would think the following would be a starting point:

(400*0.70*2)/3

Please help.

 

[patprimmer]

Some details on the manifold, engine capacity and where you want the power band would help.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules

 

[Marquis]

I'll be applying my advanced Modelling techniques (GT Power) to an old Mopar engine soon- with carbs.
The quoted "CFM" values by carb manufacturers are measured at a fixed stead state pressure drop.
For a '6 pack' set up- the outer venturis are measured at a different pressure drop the the inner. What I will do- is run the cycle simulation under steady state condition- to duplicate the flow bench the original testing was done on- at the same pressure drop (i.e run the cycle simulation with no engine- but under a steady state time basis). Then I will adjust the restriction of the carb venturi untill it matches the air flow it's rated at.
The it is a simple case of dropping the simulated component onto my correlated simulation 1 D model and optimising. Then it will be time for testing.

http://www.auto-scape.com

 

[JTP173]

"Some details on the manifold, engine capacity and where you want the power band would help"

Sorry, assume both manifolds are equally efficient. If you know the optimum single carb. cfm how can engine capacity and power-band help?

Anyway, say 250ci and rpm between 1800 and 4500. This would make the 400cfm at 1.5"Hg.

Marquis:
Using the pressure drop is a good idea (though I am not 100% on what you are doing). Setting the cfm with the actual pressure drop at wide open throttle (along with the total flow drawn by the engine in one revolution) is how I estimate the "optimum" single carb. cfm. However, I want something simple for people who have no idea and do not have access to modelling software.

 

[patprimmer]

I was after details like independent runner or large plenum or something in between and an indication of the path from carby base to inlet valve, like line of sight downhill or more tortuous.

Are you an engineer or a student or a DIYer.

Is this work related in any way or just hypothetical.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules

 

[JTP173]

I am a mechanical engineer; however, my areas of expertise are mechanical vibrations, signal analysis and damage detection.

I must admit I have not given this a lot of thought. What I am after is a rough generic guide. However, since you have asked for detail let's say:

The original system:

1 carb. leading into a "large" single plenum with 6 "long" (say around 12") sweeping runners which curve 180degs.

The new system:

Shorter, straighter, runners with 3 carbs drawing from either a large plenum or the open environment.

It is all hypothetical

 

[patprimmer]

So carb one is a tunnel ram and carbs two is three by individual manifolds each feeding a pair of cylinders.

A lot will depend on the separation of the firing order. If 360 deg apart it will almost be true IR re airflow through the carby as only one cylinder will be drawing at a time, that is unless you have a balance tube or plenum between the carby and the port.

For three by independent manifolds, about 70% sounds right

400CFM sounds big for those low rpm unless it's a big engine or the cfm rating is 3" not 1.5" depression.



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules