As an example, I want to use L92 heads, as the reference datum for this discussion.

I do not know how to correctly calculate coefficient of discharge taking into consideration the valve-to-bore spacing. No calculator or formula I have found asks for bore size, so coefficient of discharge always increases with valve size. I do not feel this is an accurate representation of what is actually happening.

I have found valve spacing measurements of:
LS1/2/6: 1.91"
L92/LS3: 2.01"
LS7: 1.97"
But I do not know the relationship between these valve spacing measurements and the center of the cylinder bore.

Using the 2.01" spacing of the L92 valves for this example, are each of the valves 1.005" away from bore center?

How do I calculate how much of the curtain area is shrouded by different size bores and with different sized valves?

Once again, using L92 heads as an example, how would I calculate the coefficient of discharge for 2.165" valves on a 4.00" bore compared to 2.165" valves on a 4.065" bore?

Now, with the same L92 valve spacing of 2.01", what would the coefficient of discharge be with 2.08" valves on a 4.00" bore?

I understand that 2.08" valves would probably require new valve seats to work in L92 heads. I also understand that the engineers at GM decided that the L92 heads and 2.165" valves work just fine on 4.00" bores.

Does a larger valve always guarantee more flow? Does a smaller valve that is less shrouded by the cylinder even stand a chance to compete with a larger valve despite how shrouded the larger valve may be?

For the L92's intake cross sectional area, I have found 3.19in^2. Which is ~2.02" in diameter. Which is 93% of the stock 2.165" intake valve. Everything I have found says you actually want the CSA to be around 90-91%, but I haven't found anything written by GM engineers responsible for choosing L92 intake valve sizes.

Also, from my limited understanding of this, the major effect of a large CSA compared to the intake valve is the reduction of the back cut or transition distance from port to valve. And that negatively effects low rpm and part throttle airflow.

Would the installation of new valve seats with smaller internal diameters prove to be beneficial in restoring low rpm and part throttle airflow? Or would it just prove to be a restriction at all rpms?

I suppose my main question is this:
Would new valve seats with an internal diameter of 1.872" (90% of 2.08) and 2.08" valves in any way be beneficial when using L92 heads on a 4.00" bore?

Any help in understanding these concepts would be greatly appreciated. Thanks in advance for your time and patience.

 

Discharge Coefficient is a calculation that requires a measured variable; CFM. You have to flow the heads on a Flowbench with a specific bore size before you can calculate the C/D. With all the different valve job profiles, port configurations, and chamber shapes, you can't just plug in the bore size to the calculation like a correction factor.

C/D does not always increase with valve size. In fact, it normally decreases if the port is left unchanged. There are different ways to calculate C/D as well depending on what CSA you're using. Some use the valve area, some use the curtain area, some use the curtain area up to a certain point until the Venturi is the smallest CEA.

No, the valve spacing is the spacing between the centerline of the intake and exhaust valves. It doesn't have anything to do with the location of the valves to the bore. That's often called "pierce points" and is often an X/Y coordinate.

The LS7, for example, was a tighter valve spacing than the LS3 heads, but the exhaust valve will the cylinder wall in a 4" bore. That's because the intake and exhaust valves are closer togetehr, but they're moved way over to the exhaust valve side of the chamber to deshroud the big intake valve.

You might need some kind of CAD software to figure that out, as well as defining what "shrouded" means. Is .100" off the bore wall shrouded? Is .200"? That itself isn't really agreed upon among head porters.

A larger valve usually only guarantees greater flow at lower valve lifts because of the increase in curtain area. The valve job also plays a role as a 45 and 50 degree seat will change things too.

That might be at the pushrod pinch or maybe the flange, but the smallest CSA at the Venturi is more like 2.7in^2. My 823 castings had a Venturi of 1.86" in diameter before the valve job was cut. It's now 1.88" after the valve job.

Greg Good advocated smaller valves, but he was more worried about shrouding and wet flow characteristics. I think he liked putting in 2.100" valves and could get away with that on the stock seats. With a stock Venturi of 1.86", I would think you could still put in 2.080" valves and be under 90% of the valve diameter. However, with the rest of the port still being so large and lazy, I think you will still have many is the same issues when feeding a smaller 6.0/6.2L shortblock.

 

KCS, thank you for your response. I was going to refer to the center of the bore as the (0,0) origin when speaking of the valve spacing in relation to the bore, but I wasn't sure if people would understand what I meant. Knowing about the pierce points, even the terminology used for it, will help me further my research.

I can probably model the curtain area of different relations between bore, valve size, and curtain area @ different flows and lifts once I find the pierce point coordinates for each of the valve spacing variations. It shouldn't be difficult to change the size of the bore or the valves within the model once the pierce points are set. Then, modeling the diameter of the curtain area for .200", .400", .600" (for example based on stock L92 head flow data) shouldn't be hard at all. Finding out at what lift the curtain area hits the cylinder wall, and how much of the curtain area is blocked by the wall at peak valve lift for different sized valves in different sized bores seems like it would be very useful data to have.

A lot of the CAD programs I have used are capable of calculating the actual percentage of the overall curtain area that would be blocked by the cylinder wall, as compared to the theoretical full curtain area available. Plotting these percentages in a spreadsheet would be beneficial as you could continue to run different combinations through the model and compile a large set of data points to plot a graph and semi-accurately estimate the "optimal" valve size for each bore size and each valve spacing variation. There are plenty of flow charts out there from different head designs and different valve sizes, all tested on various bore sizes, to run through the model.

I know that the relationship between the port volume and valve size also play a large role in the overall flow. As does the port shape. I, personally, like smaller volume intake runners and cathedral port heads, but I have been thinking a lot about using an LY6 as the base of my build. Just figured that I would probably have the heads rebuilt anyways, so why not optimize them as much as possible for the 4.00" bore they will be used on? And I like to do my research before making decisions, so here we are.

Thanks again for the informative reply.