Carb sizing for boosted applications.

[InlineUK]

Hi,
I've been searching for ages trying to find a formula to give carb cfm for boosted applications. I found loads of sites with cfm calculators but couldn't find any reffering to boost.

But now I have finally found one so i thought i'd post it here.

CFM = ((CID x Max Rpm)/3456) x ((Boost Psi / 14.7) + 1)

(This is assuming 100% VE so the result would need to be adjusted accordingly.)

Can anyone confirm whether this is correct/accurate?

Cheers
Nathan

 

[Does10s]

Well putting in Kelly's numbers I get 966.9 CFM.

So that's over twice what the 450cfm carb is "rated" for.

But if you take our carb, 450 and multiple that times the boost pressure in "Bars" (2.15 in our case) then you get 967.5 CFM.
That's pretty close to the formula.
That might be another way of going about it.
But there's no way I'd put a 1000cfm carb on a 250.


Later,
Will

 

[Bort62]

Well, think about it.... CFM is a measurement of volume. If you increase the pressure, you will move more air by mass w/ the same volume. It is higher density.

What that means is, if you running 2 bar boost, the CFM into your turbo will be ~ twice that flowing through your carb.

So 967 into the turbo inlet matches the 450 through the carb pretty well.

Of course, that is 450 at a certain pressure drop - which is not really applicable under boost.

 

[InlineUK]

From what you guys are saying i get the impression that when selecting a carb for boost you use the same equation you would when selecting a carb for NA?

I had been thinking of using a Demon 575 blow through 4bbl carb for my motor. Do you think this would be too big for a street motor?

I was recommended this carb by the tech/sales at Barry Grant, but i'm not sure whether they recommended it because it would be suitable or because it was the smallest blow through they do?!

 

[Bort62]

From what you guys are saying i get the impression that when selecting a carb for boost you use the same equation you would when selecting a carb for NA?

I had been thinking of using a Demon 575 blow through 4bbl carb for my motor. Do you think this would be too big for a street motor?

I was recommended this carb by the tech/sales at Barry Grant, but i'm not sure whether they recommended it because it would be suitable or because it was the smallest blow through they do?!

For what motor?

Better to have a little small of a carb on a blow-through motor than too large.

 

[StrangeRanger]

I think that equation works for a draw through detup but not for a blow through.

 

[InlineUK]

For what motor?

Better to have a little small of a carb on a blow-through motor than too large.

Sorry, forgot to say!
Its for a 250. I'm using one of Mikes Alu head/intake and Twin T3s running 7-10psi.

 

[wallaka]

For what motor?

Better to have a little small of a carb on a blow-through motor than too large.

I'd say that is a good rule of thumb for anything, boosted or not.

 

[InlineUK]

For what motor?

Better to have a little small of a carb on a blow-through motor than too large.

I'd say that is a good rule of thumb for anything, boosted or not.

So what size range would work for the 250?
350-500cfm? 400-600cfm?

 

[StrangeRanger]

Blow through or draw through?

 

[InlineUK]

Blow through or draw through?

Blow.

 

[mustube]

I'd like to see a picture of the twin T3 set up. That's cool!

 

[InlineUK]

I will post loads of pictures once its all together.

 

[XPC66]

Hi,
I've been searching for ages trying to find a formula to give carb cfm for boosted applications. I found loads of sites with cfm calculators but couldn't find any reffering to boost.

But now I have finally found one so i thought i'd post it here.

CFM = ((CID x Max Rpm)/3456) x ((Boost Psi / 14.7) + 1)

(This is assuming 100% VE so the result would need to be adjusted accordingly.)

Can anyone confirm whether this is correct/accurate?

Cheers

Nathan

That formula is pretty crude and doesn't take into account adiabatic efficiency nor engine effciency.

If you are going blow through, you need to consider that carburettors are basically velocity meters. The venturi acts to increase velocity pressure and reduce static pressure of the air stream. The difference between the static pressure in the venturi and the static pressure fuel bowl causes fuel to be pushed through the jets.

If you have a boost application the velocity will remain fairly close to non boost. However the pressure /density ratio changes due to adiabatic inefficiency. This means the static pressure drop will be greater in the venturi. It just so happens that it correlates to roughly the same size venturi requirement as for normally aspirated.

 

[InlineUK]

I've been emailing a company called CSU who come highly recommended from another forum.
They modify 750 holley carbs for blow through and custom calibrate to engine/vehicle specs. They think one of those carbs would work really well on my engine without loosing any bottom end power, which i understand is one result of too large a carb.

What do you guys think? Anyone used a CSU carb?

 

[kirkallen143]

CSU is a reputable company in the turbo world and good luck with contacting them, I hear they are hard to get ahold of because of a booming business.

I calculated your 250 @ 75%VE and 5500 max rpms w/ 10psi boost applied and get CFM=501.38.

Not saying this is the right or wrong cfm for your motor, but every motor is different. I don't think you will be able to purchase a blow-thru carb and put it on your motor and go (speaking plug and play type scenerio).

With the formula above and my motor (200ci) it says that I needed 364 cfm at 10psi and @ 75%VE (5000 max rpm). I first had the 500cfm 2-barrel holley (which is really 353 cfm at 1.5"Hg) on the single turbo 200 inline six and it never ran right. When I switched to the 350 cfm holley 2-barrel (which is only roughly 250 cfm @ 1.5"Hg) tuned it right on spot, and it has never run better.

What gives?

To be honest, I couldn't tell you from my knowledge alone. Pressure differential? Couldn't even start a conversasion on the subject. But, I have come to find it takes research and developement, and from that alone is what I have found to be anyone's solution. Be sure to keep notes and record your changes/progress as you go along.

So try the 525 cfm Demon, it will take A LOT of messing with before you get it right, trust me and what other have gone through. Or go with the holley 450 cfm double pump 4V, which will also need to be tweeked by you or a reputable shop like CSU. It all depends on what you want to do, and how you want to do it. I can't be the one to tell you where to start, but you have to start somewhere. We will let you make that call.

Only from my mistakes that I have made, do I now judge my experience.

Good Luck.

Kirk

 

[XPC66]

There are so many variables to consider and really a baseline is required to assess engine efficiency and cam performance.

Just working on a generic 250 cu in engine the density ratio @ 10psi in plenum is around 1.73 (0.265 kg/sec) with IC and 1.4 (0.178kg/sec) without. The power range would vary between 240 and 190kW, with and without, even higher if the head and cam had decent performance.

The best way I have found in sizing a carby is to work on aggragated throttle plate size , in this instance something around 80mm would be my initial pick.

 

[InlineUK]

Thanks guys, some good info there.

I have managed to contact CSU via email and they are confident their carb would work. But as you say, there are many variables to consider, so there aren't any guarantees.
I guess i'll have to make a decision and give it a try.