port size cam duration,rpms

[grumpyvette]

PORT SIZE FLOW AND THE RELATION TO CAM DURATION

 

 

FIRST, This will not be anything more that a brief glimpse into a subject that takes years to understand fully and I’m sure there are a few people on the site that can give more exact info! This is meant to apply to the 350-383 sbc engines most of us are useing

My purpose is merely to give an idea as to the relationship between the factors and yes IM ignoring several minor factors to make things easier to understand like dynamic compression and valve timing overlap

But lets look a a few concepts

(1) There are 720 degrees in a 4 cycle engines repetitive cycle of which between about 200degrees to about 250 degrees actually allow air to pass into the cylinder, (the valves open far enough to flow meaningful air flow) and the piston has a maximum ability to draw air into that cylinder based mostly on the engines displacement and the inertia of column of air in both the intake port and the suction (or negative pressure the PROPERLY designed headers provide) this produced a max air flow thru the ports, the greater the volume of fuel/air mix effectively burn per power stroke the greater the engines potential torque production, the faster you spin an engine the greater the NUMBER OF POWER STROKES PER MINUTE, and up to the point where the cylinder filling effectiveness starts falling off due to not enough time available to fill that cylinder the torque increases, above that rpm or peak torque it’s a race between more power stokes and lower power per stroke

(2) look at this diagram

(3)

As air enters an engine it normally travels thru both an intake system and the cylinder heads intake port to eventually pass into the cylinder thru the valve. The valves in a normal small block corvette engine are between 1.94 and 2.08 in diameter, that’s between 2.9sq inches and 3.4 sq inches of area, but because the valves require a seat that at a minimum are about 85%-90% of that flow area we find that the intake port even with out any valve has a max flow of not more than about 90% of the flow thru a port of valve size. Or in this case 2.46 sq inches-2.9 sq inches of port area, Since you gain little if any flow having a port that’s substantially larger than the valves AT NORMAL ATMOSPHERIC pressures and since you can’t substantially increase the valve sizes for several mechanical reasons you must improve efficiency, this is done in two major ways, you can match the intake port length and cross sectional area to the engines most efficient rpm range on the intake side, to build a positive pressure behind the intake valve as it opens and match the exhaust length and diameter on the exhaust side to provide a negative pressure to help draw in more volume this will require the cam timing match that same rpm range of course. By experimentation its been found that air flow port speeds in the 200-320 cubic feet per minute range are about the best for a chevy V-8 now lets say you have a 383. 383/8=47.875 cubic inches per cylinder, the rpm range most used is 1500rpm-6000rpm so that’s where are cam and port size must match, you can do the math , (47.875 x ½ engine rpms = cubic inches, divided by your cams effective flow duration, (use 210-235) as a default for a stock cam) x 720 degrees/1728 (the number of cubic inches in a cubic foot) to get the theoretical max port flow required (I will save you the trouble its 250cfm-275cfm at max rpms and about 2.4-2.9 sq inches of port cross section, depending on where you want the torque peak, or use this handy calculator,

 

Intake Runner Area = Cylinder Volume X Peak Torque RPM 88200

Or this helpful site http://www.newcovenant.com/speedcrafter/calculators/intake.htm

Either way you’ll find that you’ll want a port size in the 2.4sq –2.9 sq inch area

Now use this calculator to figure ideal port length, REMEMBER youll need to add the 6†in the cylinder head to the intake runner length to get the total length and you can,t exceed the engines REDLINE RPM which with hydrolic lifters seldom is higher than 6400rpm

http://www.bgsoflex.com/intakeln.html

 

 

Ever wonder why your engines torque curve gets higher with the engines rpm level until about 4000rpm-5500rpm(DEPENDING ON YOUR COMBO) but fades above that rpm level?

well it depends on several factors, first as long as the cylinders can fill completely you get a good fuel/air burn so you get a good cylinder pressure curve against the piston each time the cylinder fires, THE ENGINES TORQUE CURVE INCREASES WITH THE NUMBER OF EFFECTIVE POWER STROKES PER SECOND, at very low speeds there’s not enough air velocity to mix the fuel correctly or produce a effective ram tuning effect but as the rpms increase the cylinders fill very efficiently until the rpms reach a point where the cylinders just don’t have the time necessary to flow

enough air through the valves to fill the cylinders , remember a 5000rpm the intake valve out of 720 degs. in each cycle opens for about 250degs of effective flow even with a hot roller cam, now that’s only about 35% of the time and there’s 41.6 intake strokes per second , that’s only 1/60th of a second for air to flow into the cylinder

Its your engines ability to fill the cylinders that increases your power and the more efficiently you do that the higher the rpm level you can accomplish that at the more power your engine makes, remember the formula for hp is (torque x rpm/ 5252=hp) so moving the torque curve higher in the rpm range increases hp but at some point the time available to fill the cylinders becomes so short that efficiency begins to drop off rapidly, the peak of efficiency is reached normally in the 4500rpm-5500rpm range, and as rpms increase its a race between more power strokes per minute trying to raise the power and the increasingly less effective percentage of cylinder filling dropping the power.

Volumetric Efficiency

The volumetric efficiency of a 4-stroke engine is the relationship between the quantity of intake air and the piston displacement. In other words, volumetric efficiency is the ratio between the charge that actually enters the cylinder and the amount that could enter under ideal conditions. Piston displacement is used since it is difficult to measure the amount of charge that would enter the cylinder under ideal conditions. An engine would have 100% volumetric efficiency if, at atmospheric pressure and normal temperature, an amount of air exactly equal to piston displacement could be drawn into the cylinder. This is not possible, except by supercharging, because the passages through which the air must flow offer a resistance, the force pushing the air into the cylinder is only atmospheric, and the air absorbs heat during the process. so, volumetric efficiency is determined by measuring (with an orifice or venturi type meter) the amount of air taken in by the engine, converting the amount to volume, and comparing this volume to the piston displacement.

this increases until the torque peak then falls as the rpms increase. Here is a rough guide to match duration to port flow at different rpm level

if you’ve been following along you’ll find that you’ll need intake ports about 2.3-2.9†sq inches in cross section, and between 12†and 21 “ long (DEPENDS ON WHERE THE ENGINE IS DESIGNED TO MAKE MAX HP) and cam timing in the 215@.050 to -240@.050 lift range, as the rpms or displacement increase either the port flow or the cams duration must increase or the engines cylinder fill efficiency rpm will drop!

Now this is important, as the port flow efficiency goes up though the use of longer and larger intake ports the cam duration could remain the same or even be lower and you get more efficient cylinder filling as the rpms increase, that’s why high efficiency port designs like on the LS1 can use lower duration cams to flow similar total air flow thru the ports than the lower efficiency ports like the old fuelie heads could but at some point all ports reach max flow and an increase in the time the valves remain open at higher rpms increases the cylinder fill efficiency and that increases the engines ability to make torque at that rpm range

if you pick a smaller runner or longer runner you should pick a cam with a shorter duration to match the resulting lower torque peak that will likely result

[Guest DaneL24]

The effective flow area of the valve is the valve circumference multiplied by the valve lift...and I've found that this number is much less than the actual valve seat/port area (in a Nissan L-Series engine...haven't calculated for V8's). Since the actual valve seat is the most restrictive point of airflow in the engine, theoretically by increasing valve lift you should see some significant reduction in air velocity...in other words, have the same velocity at a higher RPM for more high RPM torque and more HP. I would think it would be best to have a fairly even effective flow area (cross sectional area) throughout the port. For example...if you hog out the ports but still have a low lift cam, you will still have low peak flow through the head so you won't gain much peak power...as well as sacrificing velocity through the port (regaining velocity only at the valve seat) and losing torque.

 

I'm curious about how the chart below was derived...I've got an idea but I'd like to hear your explanation.

 

As piston position changes, the piston speed changes per degree of crank rotation...ie the piston will be traveling slowest at TDC and BDC and the fastest at 90 degrees ATDC/BBDC. This also changes in the intermediate piston positons by rod ratio. So the piston will be displacing air at the fastest rate at 90 degress ATDC/BBDC. You also must take into account that effective flow area (valve seat circumference multiplied by valve lift) changes with relation to piston position (since the cam is synched with the pistons obviously) and effects velocity...so peak flow area occurs at peak valve lift.

 

So in theory, there are two factors that determine the velocity (purely in terms of flow area and the rate of displacement)...the piston position and the valve circumference times the lift. By combining these factors with a given displacement, rod ratio, valve seat circumference, and cam specs...you can figure out at which piston positions air velocity will be at its peak.

 

Am I correct in my assumptions?