I've seen some impressive flow #s posted for lt1 heads, but the higher flowing numbers are usually achieved with the larger valves. However, some of the porting gurus I've noticed are able to achieve similar numbers with stock sized valves. My questions are, is it necessary to ungrade to larger valves when a smaller one will suffice? At what point will it be necessary to increase valve size...ie...stroker motors, forced induction? How much cfm can be gained from a lt1 head that flows 270/197 @ .600 with stock valves, by upgrading to a 2.00/1.56 valve combo?

 

I'm also wondering, what is the maximum peak flow that has been achieved with an lt1 head? I've seen #s posted in the 280s @ .550.

 

How much cfm can be gained from a lt1 head that flows 270/197 @ .600 with stock valves, by upgrading to a 2.00/1.56 valve combo?
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you can get a good idea of potential flow by calculating what the Flow CFM per square inch of valve area is ??

example=> suppose the LT-1 valve OD is 1.900"
and flows 270 cfm

Valve_Area = 1.900 * 1.900 * .7854
Valve_Area = 2.835 sq in

Flow CFM/Sq.inch = 95.228

new valve = 2.000" od = 3.1416 area
3.1416 * 95.228 = 299.17 potential cfm if heads can be efficiently ported to keep up with new valve area

 

That's a good formula for that, works only when the head has enough material to enjoy the new larger valve.

Some of the guys getting big numbers with stock valves are probably treating the stock valve like a larger aftermarket valve when thye do the valve job, trading long life for flow most likely.

Now racing flow benches is not the best thing to do on top of that. I've seen 285cfm and 295cfm LT1 heads flow 275cfm on a more down to earth bench.

Bret

 

Is it possible this approach is overly simplified - and I honestly don't know the answer, just trying to "learn"?

Prorating flow based on cross-sectional area would seem very "approximate" at best, and limited only to the point in the valve event where port face area exceeds valve curtain area. Before that point (0.500" lift) flow is "roughly" proportional to the perimeter of the valve seat, as flow is initiated in the anulus at the seat, and then becomes a "curtain" as lift increases. In these flow conditions, since nothing is being "squared", the gains for a larger diameter valve are not as dramatic - linear with diameter, rather than to the second power. Throw in the issues of shrouding and it would seem like its almost pointless to predict flow as a function of valve diameter, other than to say bigger may be better, until shrouding becomes an issue.

Or am I overcomplicating things with a bunch of theory that really doesn't represent the real world?

Stock LT1 intake valve is 1.94", which reduces the results of the simplified analysis to 287cfm.

 

Throw in the issues of shrouding and it would seem like its almost pointless to predict flow as a function of valve diameter, other than to say bigger may be better, until shrouding becomes an issue.

Or am I overcomplicating things with a bunch of theory that really doesn't represent the real world?
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Yes, you are "over complicating" a very simple Flow concept !

Flow CFM per square inch of valve area values
will give you a good idea of the "potential" possible Flow CFM

SuperFlow publishes CFM/Valve Area Sq.Inch Charts in their manuals

Represent the real world ??
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Yes, it represents the real world , ..by giving a good estimate of the "potential possible Flow CFM " .

Real World example=>
530 cfm @ 28" ProStock Car with 2.530 int OD = 105.425 cfm/area

suppose you want to build a ProStock Truck engine
with a 2.230 OD intake valve ...what would be the "flow design target" ??? 105.425 cfm/area with 2.230 = 411.76 cfm needed

in real world reality...this in fact gives a very good indication of potential possible Flow CFM that could be achieved and a flow design target .

530 cfm and 412 cfm are pretty honest flow numbers
if you asked around, you might get numbers between
530 to 550 cfm for ProStock Cars, and 412 to 430 cfm for old ProStock Trucks class , depending upon how honest the FlowBench is ?

my previous Post was just a good quick example of calculating what the Flow potential might be between various valve ODs

that method was a bunch quicker than doing all the math required to post all the flow CFM potential values at the different curtain areas as the valve moved thru the Lift range in .050 or .100 " increments, between 2 different valve OD's !

after all, the Curtain Area = Valve Area @ .25 L/D Ratio
for the 2.530 OD = .633" lift
for the 2.230 od = .558" lift
for the 2.000 od = .500" lift
for the 1.940 od = .485" lift

if you take a look at this picture in this Link
you can see i put more value in determing various Curtain Area
values (like Curtain Area Discharge Coefficient) , than just Valve Area values,
but each gives you certain information to judge how good a job you are doing , and what possible potential there is left for you to discover

http://www.maxracesoftware.com/Flow_Max_Intake_1.jpg

 

It all depends on who does the head work...I like the 2.02/1.60 combo. And the 30 deg backcut really does make a big diff on the flowbench. When I put in a bigger valve, I always cut a little out of the chamber wall to un-shroud it.

Flow vs. Velocity

What is the operating rpm level of the motor?