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Topic Review (Newest First)
04-29-2004 12:03 PM Actually, the manifold discussion is retreading old territory but the carb discussion is 10X as much! Do a search, everything that can be said about carb selection has been said. And it is an emotional subject!!
04-29-2004 11:47 AM
killerformula Great thread! Could probably go in the advanced forum. One thing that hasn't been mentioned yet is carburetor size. I think of the link between the combustion chamber and the air outside as a sort of pipeline in which all sizes of the pipe have to compliment each other. For instance, it doesn't make sense to build a 400 small block with 215cc runners and a 600cfm carb. Its just not going to work very well- the sizes of the "pipeline" don't match well. I think carburetor size, as willy's touched on, is really very important for smaller cube engines. Probably the biggest mistake made in hotrodding is over-carbing a 350 Chevy. Very few people have the build-up it takes to make use of 750 CFM in a 350 chevy. The large venturi's are going to slow the air down and make the motor sluggish and sloppy in the low R's (probably the most important RPM range on the street, given that races are won and lost at the starting line).

The way your engine breathes is analogous to blowing through a straw. If you blow through a mcdonald's straw, you can get some pretty good pressure going on the other side if you put your hand in front of it. However the total volume of airflow from such a straw is pretty low- Kind-of like the small, restrictive, low RPM motor that's making good low end power, has great vacuum and small ports. If you tried to blow through a garden hose, however, you'd probably get some pretty weak pressure at the other end- kind of like a smaller cube motor (like a 350) trying to suck air through something like a 750 cfm carb at idle. The total airflow, however is of course much higher (something that a high RPM motor would take advantage of).

04-29-2004 08:45 AM
brainsboy Willy has some good points. I just want to add that finding a good intake and head runner size is not easy. You need to do the math to find a good mixture of Velocity/volume. You can find mathematical formulas to calculate the correct runner size. Usually as volume go's up velocity gos down, and vise versa. This is where experience comes in and may be why one motor makes 410hp but its almost identical twin is struggling to make 310hp. Its all in the details. Velocity vs. volume is something that is very debatable and many people have different opinions.

04-29-2004 08:44 AM
Cstraub That disscussion is in the Internals Forum of the board. sorry.

04-29-2004 07:53 AM You are pretty close. As I always say, engine design is a series of compromises; what works for low speed economy and torque usually hinders high speed performance and vice versa.

Large manifold volume doesn't really reduce average manifold vacuum but it does reduce mixture velocity which is just as bad at low speed. Low speed in intake plenums & runners allows gasoline to condense and settle to the bottom of the conduit messing up distribution and air fuel ratio. Also, low velocity has dynamic cylinder charging implications - cylinders aren't 'supercharged' with slow speed gas as they are with high speed gas. However, as engine speed increases, the relatively small volumes (flow area of the conduit to be more precise) in a stock type manifold begin to restrict fluid flow and hurt performance. Friction losses overpower the benefits of high velocity and cylinder filling is again less than optimal. Single plane manifolds tend to reduce low to moderate speed velocities and hurt bottom end performance relative to split manifolds but they really benefit top end performance. For big cubic inch engines that have plenty of bottom end power even with too big intake equipment benefit on the top end much more than they are hurt on the bottom end. Small engines are much more sensitive to giving up bottom end performance so more care must be taken in component selection.
04-29-2004 07:48 AM
Cstraub 59vette,
go to we are in deep discussion their trying to teach people not to OVER cam, but we started with CFM needed for CID for a certain RPM.

04-29-2004 07:22 AM
Theory: Intake volume vs cylinder volume / engine vacuum

having had and still having problems with my engine combo, I got interested in this subject.

My theory is the following:

A large intake volume Iv compared to a small cylinder volume Cv leads to low idle vacuum and related problems.
If carb is on the large side problem worsens... requiring you to run a smaller carb than would be normally expected....

Consider this:
A torker single plane intake has and RPM range of ~2500 to 6000 for a 350cui engine. For simplicity we set the relation Iv/Cv =1.

Putting that intake on a 283cui engine with a lot smaller cylinder volume we now have that Iv/Cv = 1.24.

Now the intake volume is suddenly 24% larger compared to the cylinder volume on this small motor

This means that when ever we need to fill the cylinder we can now draw from a 24% larger volume, meaning that the "pressure drop" in that volume will be smaller (less vacuum) thereby drawing less air through the carb lowering the velocity which make the proper carb function harder to obtain.
This also makes it necessary to have a smaller carb compared what is normally expected in order to create velocity through it to atomise the fuel properly.... Carb acts a a restrictor to get vacuum in the manifold.

This resembles the theory about small cylinder head intake runner volume for smaller motors.
Compares to install a set of 220CC heads on a 283 cui engine, which is much too large....

The larger volume will also move the useable RPM band up, in order to compensate for the larger volume we must add more rpm to ontain the optimum air speed as the cylinder volume is not variable.
In turn this 2500-6000rpm manifold might now be a
3000-7000 rpm manifold....

This really means that installing a single plane manifold on a 283 for street is nuts....
In order for it to work reasonably it must be a dual plane on which the runners will be better sized for the cylinders, and a 350cui idle-5500 rpm manifold =idle-6800 for a 283cui, would probably be optimum for such a small motor

Does this make sense or does anyone think different...?


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