Cylinder Head Airflow (Another Light Bulb Came On)

[Kevin Shasteen]

This is kind of long-but very informative. Dont get confused; I read it for a couple hours till "That Light Bulb" went off-then the dynamics of the process were clear.

 

I was reading How to Modify & Build SBC Cyl.Heads by David Vizard again for the umpteenth time. I was attempting to bridge the gap between the Port Velocity, Max.CFM & Chamber Volume together in an attempt to understand their interrelationships.

 

I know it depends on Velocity, CFM & the needs of the engine; but what got my attention were a couple formulae he gave for determing Max.RPM & Max.Output (HP) based on the cylinder head.

 

The first actually has two formulae w/in the process & addresses the "Power Limiting Port Area". Vizard claims just like the length of an intake manifold's runners will determine peak rpm...well, the Intake Port's minimum corss section area will determine a cyl.heads maximum efficiency! Vizard claims this "LPV" is the determining factor w/in the cy.head that aid's to the torque curve peak-and after its reached-the curve falls/plumits downward.

 

Further, this "Minimum Port Cross Sectional Area is found right at the entrance of the cyl.head's intake port (adjacent to where the pushrod comes thru) as Chevy has to limit it's port width due to the pushrod's location-this limitiation in port size also limits Airflow. As Vizard explains; air weighs .078 lb/cu.ft. Its a known that if a port velocity exceeds .55 to .60 of the speed of sound then the port itself becomes "limited" in efficiency; (port velocity is good but too much is not good).

 

Vizard claims that w/the thermal qualities of an engine sound will travel at 1200 fps; so 690fps is about as fast an Airflow Velocity one would want for all out performance. 690 fps represents 57% of 1200.

 

If you're planning on building a perf.engine & are not sure of what cyl.head requirements you need you can play around w/the following #'s. The minimum port area measurement must be made in sq.in. To determine peak power at a target rpm the minimum port cross sectional area (LPV) can be determined by using the following forumula.

 

LPV = .00353 x RPM x S x B^2/CA

 

LPV = Limiting Port Volume

RPM = Max. RPM for a Perfromance Engine

S = Stroke of the crank

B = Bore of the engine's cylinder

CA = Minimum Port Cross Sectional Area in Sq.In.

 

If you already have a set of heads & would like to know what their Max.HP is capable of-then this formula is one you can play around with as an aid to cam selection.

 

A conversion factor from a choice of three conversion factors must be chosen-and they are:

 

1) 177,780 = for a flat tappet cam

2) 184,136 = for an Endurance Race Roller Cam

3) 195,558 = for an all out Pro Stock Drag Cam

 

The formula is as follows:

 

Max.RPM = CA x 177,780/S x B^2

 

Take a SBC 350; Vizard uses the 186 Heads w/an Minimum Port Sectional Area "CA" of 2.2:

 

2.2 x 177,780 / 3.48 x 4.03^2

 

391116 / 3.48 x 16.2409

 

391116 / 56.5183

 

RPM = 6920

 

As far as Peak HP he gives the following:

 

*Note1: Formula assumes a 10.0:1 Comp. Ratio

and an adequate intake/exhuast system

*Note2: You will also need to determine what your choice of cam w/rocker ratio factored in gives as max.valve lift; the cyl.head Mfg should have airflow measurements at .050 increments-you may have to call for the specs or have your cyl.heads tested on an airflow bench.

 

Conversion Factors:

 

1) 1.95 = Flat Tappet Hyd.Cam

2) 2.0 = Flat Tappet Solid Cam

3) 2.05 = Hyd.Roller Cam

4) 2.1 = Solid Roller Cam

 

Conversion Factors for Engine Capacities (he only gave conversions for SBC-that's what the book is about-so nothing else was included)

 

1) 1.1 = 400ci

2) 1.0 = 355ci

3) .95 = 327

4) .9 = 302

 

This formula is:

 

CFM at Max.Valve Lift x Cam Type x Displacement Conversion Factor = Max.HP

 

His example used a 400 w/041 heads midly ported, and a Hyd.Comp Cam w/.511...so he assumed @ .500 after pushrod flex. .500 is a figure head porters use anyway as a flow measuring point; the 041 heads flowed 220 CFM at .500 lift.

 

Work the formula as follows:

 

220 x 1.95 x 1.1 = 459.8 hp

 

Cool, huh! If one is using Low Comp.Cy.Heads then you can deduct 20-50hp.

 

One addit'l formula; His test are done at 25" of water pressure; not all test are done at 25". Some are done at 28", 12", 5". Suppose you need to know the results at 25 but the tests were done at 28": use the following:

 

Flow @ NPD = SQRT(NPD/OPD)

 

NPD = New Pressure Drop

OPD = Old Pressure Drop

 

NPD = SQRT(25/28)

 

NPD = SQRT(.89296)

 

NPD = .945

 

You can take the .945 & multiply it by the test result #'s at different .050 increments for your knew results.

 

I know this was a long post; but the idea is one I think answers a lot of questions we have regarding cyl.heads; thought I'ld pass it along.

 

Kevin,

(Yea,Still an Inliner)

 

[ May 05, 2001: Message edited by: Kevin Shasteen ]

[Kevin Shasteen]

Vizard's book is so full of information about the SBC's airflow-there's only one way you can grasp it.....read it a dozen times/then read it another dozen times; and so on, and so on.

 

Each time I read it (scan thru it) I learn something knew.

 

About perf.books in general; when I find a knew one at the book store-I scan thru it looking for those neat little side remarks/it seems each author has their own perspective & experience/knowledge to add; everything else in between (usually) is pretty much the same.

 

But Vizard's book is full of one fact after another; he's been modifying cyl.heads since the 50's: very good stuff!

 

Kevin,

(Yea,Still an Inliner)

[pparaska]

Cool stuff, Kevin. I need to buy that book!

[Guest Fast Frog]

Kevin, you're awesome!!

 

You're a walking encyclopedia of automotive books and articals to read!! However, you're still behind DavyZ in number of posts to this forum!!