CA's Again

[SteveS]

Thanks to Maxrace and his posts, I am comfortable in calculating cross sectional areas based upon several different formulas. My question, though, is there a formula to prescribe ideal or suggested CA if only RPM, CID and number of cylinders is known?

[bill jones]

-One thing that you also need to think about is how fast the valve opens and closes, like if you have a hydraulic or flatappet cam vs a mechanical roller and whether or not you have high ratio rockers that makes some significant differences.

[SteveS]

Bill, this is a DOHC design with buckets.

[Old School]

Steve, there is a website called WALLACE RACING for Pontiac enthusiasts that have a great number of very useful calcalators. The one you might be interested in is the calcuation of CSA with no choke from RPM. This formula calculates the rpm range for a particular CSA. I have also used Jim McFarland's formula for calculating the torque peak in and engine. The formula is: the smallest CSA of the intake track x 88200 divided by the cubic inches of one cylinder. Example 557 big block chevy.
3.9 x88200=343980. Divide that by 69.625 =4940. This seems a little low and there is a considerable spread in rpm difference in the two different calulations somewhere in the range of 2500 rpm, maybe more. Peak horsepower should occur with 1300-1600 rpm of peak torque in most applications. I have no dyno so I am dealing with the math theory world instead of real life experience. Maybe both calculations are a little skewed? Hopefully someone more experienced than I can expand on this and clarify.

[SteveS]

Thanks Old School. The Wallace Racing calculator that requires only stroke, bore and RPM appears to calculating a CSA by using a velocity of .55 Mach. On the other hand, one of the other calcultors advises going above .50 Mach.

[Old School]

There is another site, RB RACING, for motorcyles that has a calculator for INLET RUNNER AND PEAK TORQUE and several others of interest. This uses the same formula as I found that McFarland spoke of. The discrepancy between the rpm spread of the two formulas has me confused. Hopefully someone can explain??

[John Wallace]

SteveS is right.
I use the .55 Mach number with the CA to figure the equation.
The Choke point of the port is the limiting factor on making more HP at a higher rpm.

The low RPM number is probably where the engine is being choked at with the corresponding CSA. If it was on a dyno, it would probably show a leveling off or gradual decrease in HP after that rpm.

[maxracesoftware]

My question, though, is there a formula to prescribe ideal or suggested CA if only RPM, CID and number of cylinders is known?

a pretty good Formula to use is :

CA = ( Bore * Bore * Stroke * RPM * .00353 ) / 614 FPS

where RPM equals Point of Peak HP
and CA is the smallest or minimum cross-sectional area in the Int System

but be careful, that is only the theoretical minimum area,
and due to different port shapes at the minimum area the real Choke velocity can differ, to be more accurate, you must use a Pitot Velocity Probe.

[maxracesoftware]

I have also used Jim McFarland's formula for calculating the torque peak in and engine. The formula is: the smallest CSA of the intake track x 88200 divided by the cubic inches of one cylinder. Example 557 big block chevy.
3.9 x88200=343980. Divide that by 69.625 =4940. This seems a little low and there is a considerable spread in rpm difference in the two different calulations somewhere in the range of 2500 rpm, maybe more. Peak horsepower should occur with 1300-1600 rpm of peak torque in most applications

yes the 4940 is too low

a more accurate Formula is

RPM = ( 614 * CA ) / ( Bore * Bore * Stroke * .00353 )

where RPM = point of Peak HP
614 feet per second (.55 Mach)
CA= 3.900 minimum cross-sectional area

in the above example the ballpark RPM point of Peak HP = 7600 to 7700
and that should put RPM point of Peak TQ around 6000 to 6200 RPM

[SteveS]

a pretty good Formula to use is :

CA = ( Bore * Bore * Stroke * RPM * .00353 ) / 614 FPS

where RPM equals Point of Peak HP
and CA is the smallest or minimum cross-sectional area in the Int System

but be careful, that is only the theoretical minimum area

Thanks. Is not that also based upon a Mach .55 derivation where 614 fps=.55*1116fps?

[maxracesoftware]

Thanks. Is not that also based upon a Mach .55 derivation where 614 fps=.55*1116fps?

Yes, approximately

one more thing...
CA = ( Bore * Bore * Stroke * RPM * .00353 ) / 614 FPS

is just what the one effect, that is, the effect of the minimum cross-sectional area would have on an Engine

even that Formula calculated you needed a certain min CA to attain a certain Peak HP RPM, its obvious that if you installed a stock OEM cam, you would never attain that RPM


so the CA Formula is one aspect

you could look at all the various other Engine parts and their effects

the CA might show you 7000 RPM limiting effect
the Carb CFM might show you 6000 RPM limiting effect
the Cam specs might show 5000 rpm limiting effect
the Valve diameters, the curtain areas, etc might all show you differeing RPM limits

so its the "Total Engine Combination" and choosing all the correct dimensions that will determine if an Engine makes good HP/TQ numbers
all have to be working together

[PFM]

Hello all,

I still scratch my head with all of this, the results don't quite line up. The bore is 3.75" the stroke is 2.77" the minimum port cross section is 2.125 sq in, the average port cross section is 2.40 sq in. If you listen to Jim McFarrland and believe PK TQ happens with an average port speed of 240 to 260 FPS (and this seems to fit my dyno sheets), this would be around 7000 RPM for this engine, a bit high but possible. If PK HP happens at say .55 MACH or 614 FPS based on "minimum" cross section then in my little six PK HP should come in at 9500 RPM. Now fun as this would be a spread of 2500 RPM from PK TQ to PK HP seems a bit too large to me what is wrong here? The formulas are clear the math not too tough, my spread sheet works OK but the results just seem off.

Any help would be great.

PFM

[bc]

Does the stroke play any role in what the minimum cross section for a given rpm? For example to engine that are the same cid like: 4" stroke and 4.25" bore compared to 4.525" stroke and a 4" (i know its a little extreme). Wouldn't the longer stroke engine want a bigger minimum cross due to the faster moving piston?

[John Wallace]

Try This Calculator

[maxracesoftware]

same equation as
CA = ( Bore * Bore * Stroke * RPM * .00353 ) / FPS

1.5676 area = (3.75 * 3.75 * 2.77 * 7000 * .00353) / 614

RPM = ( FPS * CA ) / ( Bore * Bore * Stroke * .00353 )
where;
RPM = point of desired Peak HP
FPS = Feet per Second
CA = Cross-Sectional Area in Square Inches (smallest measured)

9489 rpm = (614 * 2.125) / (3.75 * 3.75 * 2.77 * .00353)

all that equation tells you is you have enough "Min Cross-Sect. Area"
to not be a direct Choke problem till 9500 RPM
so what's limiting your Engine right now is not the CSA
but more a Time Area- Average Curtain Area problem,
for example, you might have a 4.000 CSA , but have .400" max Cam Lift
and 200 deg duration, ..theres no way your Engine will ever reach the RPM where the min CSA of 4.000 inches is the limiting factor.

to tell what RPM the Time Area - Average Curtain Area is limiting you need different set of equations.
the
RPM = ( FPS * CA ) / ( Bore * Bore * Stroke * .00353 )
is mostly used to determine "if" the CSA is the problem when you do
have enough valve size OD , Cam Lift/Duration, etc.

the above is only an "Area Rule" type equation that does not include the other Engine Part's effects.

you would have to Post at least this bare amount of Data to get a better idea of what combination of Engine Part's Specs are really limiting your Engine's RPM Range ??
Cylinder Bore Size..... =
Crankshaft Stroke...... =
Connecting Rod length.. =
Number of Cylinders.... =
Engine RPM at Peak HP.. =
Compression Ratio...... =

Number of Intake Valves =
Intake Valve Diameter =
Intake Valve Stem-Dia. =
Max. Intake Valve Lift =
Intake Duration @ .050 =
Degreed Int. CenterLine =
Camshaft Lobe Centers.. =

Number of Exhaust Valves=
Exhaust Valve Diameter =
Exhaust Valve Stem-Dia. =
Max. Exhaust Valve Lift =
Exhaust Duration @ .050 =

Also need what Heads Flowed CFM on Intake and Exhaust
and an idea of Intake + Carb CFM Loss percentage wise