Hints & Tips

How to correctly size a carb.

Carburetors are rated by CFM (cubic feet per minute) capacity. The traditional calculation for sizing a carb to your engine goes like this:

CFM= Engine CID x Peak RPM x Volumetric Efficiency / 3456

So for a built-up/modified SBC, or strret engine, we're probably looking at numbers something like these:

CID = 350

Peak RPM = 6000

VE = 75 to 85% (depends on your heads, cam and a billion other things) so let's split the diff and go with 0.80. Note -- If it's a really efficient, great build, it could be 90%, a full blown - pro built race engine could be 100 or 110%!

For a result of: 350x6000x0.80 / 3456 = 486.11 CFM. Heck, even if we were 90% efficient we'd only be at 546.88 CFM!

This would tell us that going with a 600 or 650 CFM 4BBL would be WAY more than enough, but suprize, we'd be wrong! There's one other thing that the "standard rule of thumb" equation doesn't take into account, and that's the manifold pressure drop. 4V carburetors are rated at 1.5 inches (Hg) of pressure drop (manifold vacuum) and 2V carburetors at 3 inches.

But, 1.5 In Hg is a bit starving for a HiPo engine. At Max RPM, you probably want this to be more like 1.00 or 0.80 In Hg. Again, let's split the difference and say 0.9. Now, in order to get the proper CFM use the following math:

Real/Final CFM= Calculated CFM / Square Root of (Desired In Hg / Rated In Hg) --- Or from our example:

CFM = 486.11 / sqrt(0.90/1.50) = 627.24

OK, that's a little more like it!

OR, because of this you can just apply a fudge factor as follows:

Use 110% to 130% higher on single-plane manifolds. Example: If the engine needs 590 CFM, select a carburetor rated in the range of 650 to 770 CFM for a single-plane manifold. A 750 would be right. An 850 probably would cause driveability problems at lower RPM. A 1050 probably would cause actual loss of HP below 4500 RPM. For dual-plane manifolds use 120% to 150% higher.

Why I use a Vac Gauge instead of another in my Pod.

What I can tell at a glance!!

A high, steady vacuum reading is the sign of a healthy engine. If you're at altitude, these readings may be lower (supposedly by 1 inch of mercury for each 1000ft above sea level, roughly). A low vacuum reading means you have poor piston rings or oil rings. You may also note lower vacuum readings when running high overlap camshafts or changed camshaft timing.

If the engine passes that test then snap the throttle open and closed again. Doing this quickly should reveal a vacuum drop to perhaps 0-3 or so and then it should snap back to 25 before settling back to 17-22 again. This means the valves and rings are OK. Snapping the throttle and getting a drop to 0 and then a climb to only say 20-22 will possibly indicate worn rings. Basically, if it only snaps to about where it was or just a hair above, you may be dealing with worn piston rings. It should snap temporarily higher (3-4� inches or so) and then settle back. A sticky valve can be spotted by seeing a vacuum reading that occasionally drops from normal by about 4 inches of mercury.

A burnt valve will behave the same as a sticky valve, but will drop much more, say 6 inches of mercury (and then return back to normal, in a cycle).

Worn valve guides can be detected by a vacuum reading that vibrates back and forth between 14 and 19 or so very quickly.

Weak valve springs if you suspect weak valve springs, rev the engine to say 4000 rpm and see if the needle swings back and forth rapidly. Then raise the revs to 5000 or even 6000 rpm, if your valve springs are weak, you'll see the swinging get wider as revs increase.

A vacuum leak would be indicated by a low reading of say 5 inches of mercury or so. Though, a very low reading can be caused by extremely advanced ignition timing or incorrect camshaft timing as well.

Headgasket issues can be found by seeing a floating needle between 5 and 19 inches of mercury. This could also be caused by a intake manifold gasket leak or a faulty injector. You can check the oil for coolant if you see this behavior however (and vice versa) to confirm your diagnosis. Oil can be tested for coolant since it'll be impossible to see in it.

Really strange behavior such as a vacuum indication that shows a slow movement between various readings could mean a leak in the intake manifold, bad idle air/fuel ratio, or perhaps a clogged PCV system.

EMAIL: mkpony@mkpony.com