When an engine running at any given RPM, it only draws as much air as it can thru any carb. If that is the case, what makes an 850 CFM carb a poor match for a small CI engine? What function of the 850 CFM carb does not work well at say 300 CFM? Is the volumetric efficiency of the engine limited by too big of a carb? Is that where the mis-match becomes apparent?

It's like drinking water through a straw. If the the straw is too small, you stay thirsty. If it's too large (say, like a hose-sized straw), you get a lot of turbulence and mixing in the straw, but you may not actually be able to pull the fluid up by the time you run out of lung capacity. If the straw is just right (like a Big Gulp straw), it hits the spot.

Basically, an orifice that is too large will tend to swirl the air, prohibiting it from traveling in the direction you want it to go: into the cylinder.

http://en.wikipedia.org/wiki/Bernoulli's_principle

works for pressure on aircraft also...

http://en.wikipedia.org/wiki/Bernoulli's_principle

works for pressure on aircraft also...


Dang....that's some teknacal stuff! :D

I'll take a shot at this ! Think of an engine as a air pump , drawing air through the carb , as the air travels through the carb , it draws (syphons) fuel out of the bowls through the boosters , thereby mixing the fuel with air , and on to the cylinders . A properly sized carb will have the optimum sized openings (barrels) keep the airspeed at a speed that will allow a good signal to the boosters , and let them meter the fuel properly . A larger cfm carb has bigger openings (barrels) , and on the same engine would give a weaker signal to the boosters , making it harder to meter the correct amount of fuel to mix with the given amount of air that passes through . Same as the straw example (kinda) This is a REALLY simplfied explaination , alot of other things are happening at the same time .................or ..................... How does a Holley carb work ?........I don't know , it just does !

It's like drinking water through a straw. If the the straw is too small, you stay thirsty. If it's too large (say, like a hose-sized straw), you get a lot of turbulence and mixing in the straw, but you may not actually be able to pull the fluid up by the time you run out of lung capacity. If the straw is just right (like a Big Gulp straw), it hits the spot.

Basically, an orifice that is too large will tend to swirl the air, prohibiting it from traveling in the direction you want it to go: into the cylinder.

Great example.

I like Terry's answer best. It's all about velocity (without restriction)

The answer I was looking for is Terry's. It is all about a good vacuum signal to have the carburetor operate effciently. Now if my long duration 30-30 cam can only generate 9-10 in.HG vacuum with the Holley 780 could I get higher vacuum with a Holley 650? Thanks.

Thier are many but I like this calcuator. The hardest variable to define is the Volumetric Efficency. This one lets you provide one as well as giving you a theoretical @ 100% VE

http://www.4secondsflat.com/Carb_CFM_Calculator.html

The answer I was looking for is Terry's. It is all about a good vacuum signal to have the carburetor operate effciently. Now if my long duration 30-30 cam can only generate 9-10 in.HG vacuum with the Holley 780 could I get higher vacuum with a Holley 650? Thanks.

Idle intake vacuum has nothing to do with how much vacuum you'll develop in the venturis. The vacuum in the venturis is developed by the air flowing through them, and a smaller venturi will generate a better vacuum at lower flow.

I'll take a shot at this ! Think of an engine as a air pump , drawing air through the carb , as the air travels through the carb , it draws (syphons) fuel out of the bowls through the boosters , thereby mixing the fuel with air , and on to the cylinders . A properly sized carb will have the optimum sized openings (barrels) keep the airspeed at a speed that will allow a good signal to the boosters , and let them meter the fuel properly . A larger cfm carb has bigger openings (barrels) , and on the same engine would give a weaker signal to the boosters , making it harder to meter the correct amount of fuel to mix with the given amount of air that passes through . Same as the straw example (kinda) This is a REALLY simplfied explaination , alot of other things are happening at the same time .................or ..................... How does a Holley carb work ?........I don't know , it just does !

A Very Good (and simple) explanation! *G*

I also like the quotation at the bottom of your post..
isn't it... "Too bad stupidity isn't painfull ! ".. :)

The answer I was looking for is Terry's. It is all about a good vacuum signal to have the carburetor operate effciently. Now if my long duration 30-30 cam can only generate 9-10 in.HG vacuum with the Holley 780 could I get higher vacuum with a Holley 650? Thanks.

Makes you wonder why the General didn't use a smaller carb on a street Z? Unless the rules for Trans Am racing homologation stated that the car at the track had to run the same carb that came with the car on the street.

alan

Makes you wonder why the General didn't use a smaller carb on a street Z? Unless the rules for Trans Am racing homologation stated that the car at the track had to run the same carb that came with the car on the street.

alan

It's a bit different for vacuum secondary carbs. You can call them "on demand" secondary carbs if you wish. The secondary opens when the flow through the primary is enough to create venturi vacuum that over comes the secondary diaphragm spring. Typically it would take a small block revving to very high rpm to move enough air to come close to opening the 780's secondaries all the way. It's really hard to over carb using a vacuum secondary carb. For most applications it may be wise to run a smaller carb and use more of the secondary for performance. In the case of the 302 the 780 was a good choice do to it's high revving ability.

I like Terry's answer best. It's all about velocity (without restriction)

It's about mixture control, and velocity is one of the factors. The rule of thumb I've always heard for Holley carbs is that they are have their best mixture control when the airflow through them is between 1/3 and 2/3 of their rated flow capacity.

Depends.

GM thought that a 750 Q-Jet was ok on small engines. In fact it was installed on some six-bangers believe it or not.

But if a Holley 750 DP were put on a 305 with 3.08 gears, different story.