Time to go Stage II!

[Alky V6]

For a blown application, where you begin to raise intake pressures above cylinder pressures at the time of the intake valve closing event, intake valve spring seat pressures become critical. You may be able to increase intake pressures to very high levels, but that doesn't mean the cylinder has equalized to that same pressure value when the intake valve closes. Only so much flow through a particular intake runner and intake valve size is possible. Or, to put it another way, as the speed of the intake charge flow through the intake runner and past the intake valve reaches a certain point, returns in the form of power increase in relation to an increase in intake manifold pressure begin to rapidly decrease.
So you begin to add big spring seat pressures to better control the valvetrain for ever decreasing returns in power, and the decreasing value in valvetrain dependability and longevity.

 

[Alky V6]

How much boost you run in relation to cylinder pressure at the time that the intake valve is closing, and how much exhaust backpressure you have in relation to cylinder pressure at the time that the exhaust valve is closing will dictate how much valve spring seat pressure will be required.

Pressure differential across the valve, times the area of the valve.

 

[Alky V6]

This explains why some people require much more seat pressures than others. Look at the boost levels and exhaust turbine sizes (exhaust backpressure).

My Stage I setup ran up to 7,800 rpm with a max of 30-31 psi boost, and a better than 1:1 exhbp to intbp ratio with as little as 170-180 lbs. spring pressure (worn springs) on the seat.

 

[Alky V6]

Might be a good idea to know your exhbp to intbp ratio when attempting to pick out valve springs.

 

[Alky V6]

Area of a valve diameter of 2.08 = 3.396 square inches.
Area of a valve diameter of 1.625 = 2.072 square inches.
A pressure differential of 20 psi on the above exhaust valve would equal 41.44 psi.
On the above intake valve, it would equal 67.92 psi.

The area would actually be less since we'd only be concerned with the ID of the valve seat to calculate the area being acted upon on the backsides of the valves, not the complete diameter of the valve.

 

[Alky V6]

Just as an exercise, let's try this.
The ID of the seat for a 1.625" diameter exhaust valve, I'm going to guess is 1.350".
I'm going to assume that 220 lbs on the seat is a good pressure for a naturally aspirated application using Ti valves for a redline of 9,000 rpm. Now, let's switch to a turbo application.
Let's shoot for a boost pressure of 45 psi with an exhbp to intbp ratio of 2:1. That will give an exhaust side backpressure of 90 psi at an intake boost pressure of 45 psi.
The area for a diameter of 1.35 is 1.43.
90 psi exhaust backpressure times 1.43 equals 128.7.
We still have cylinder pressure acting on the cylinder side of the valve. 45 x 1.43 = 64.35
128.7 - 64.35 = 64.35 pressure differential.
220 + 64.35 = 284.35 seat pressure.

 

[norbs]

That seems a way big difference from NA to turbo are you sure these calcs are correct? No one compensates for a turbo when you get your cam card?

 

[Alky V6]

That seems a way big difference from NA to turbo are you sure these calcs are correct? No one compensates for a turbo when you get your cam card?

I don't know.
I added something to the calculation in my last post.

 

[Alky V6]

Let's look at my case.
Assuming a 1.43 area on the backside of the exhaust valve.
Target intake boost level: 42 psi
Exhbp to Intbp ratio: .90:1
Exhaust backpressure at 42 psi int boost: 42 x .90 = 37.8
Exhbp acting on the back of the exhaust valve: 1.43 x 37.8 = 54 psi
Pressure acting on the cylinder side of the exhaust valve: 1.43 x 42 = 60 psi
Pressure differential across the exhaust valve: 6 psi favoring the cylinder side.
220 - 6 psi = 214 seat pressure? I actually need less seat pressure?

 

[norbs]

This is all theory how will you know your exh bp unless you measure it on the new combo?

 

[Alky V6]

This is all theory how will you know your exh bp unless you measure it on the new combo?

I do have real world numbers from the Stage I project. The exhbp to intbp ratio was in the range of .90:1 at 28 psi boost.
I do expect it to be different with the Stage II combination, especially at 42 psi boost. If it ends up being 1:1, that would still be nice. That would calculate to the same seat pressure as a naturally aspirated application. If 220 seat pressure is good for a n/a application, then going with a seat pressure of around 287 should more than cover it.

 

[norbs]

So basically your saying if the bp is 1:1 nothing changes for spring psi na vs turbo?

 

[Alky V6]

So basically your saying if the bp is 1:1 nothing changes for spring psi na vs turbo?

I don't see why it would, except for maybe minor pressure fluctuations due to pressure harmonics.

 

[turbobitt]

So basically your saying if the bp is 1:1 nothing changes for spring psi na vs turbo?

This is true and the added benifit of turbo vs. supercharging.

Allan G.

 

[turbobitt]

I don't see why it would, except for maybe minor pressure fluctuations due to pressure harmonics.

Thats a problem when analyizing a single cylinder system vs. a complete system.

Allan G.

 

[Alky V6]

Thats a problem when analyizing a single cylinder system vs. a complete system.

Allan G.

A very good point. Let's explore this.
An exhaust valve opens every 120 degrees of crankshaft rotation with an evenfire crankshaft. As one exhaust event is proceding, when during that event does another exhaust valve open?

 

[Alky V6]

From the point that first exhaust event begins and goes through its cycle, 2 other exhaust events will have begun before the first event finishes. In my case, a fourth exhaust event will begin 30 crankshaft degrees after the first event has ended (seated). The question is, what sort of pressures are being contributed by the other exhaust events as the first exhaust event is ending (valve is seating)?

 

[Alky V6]

As the first exhaust event is ending (valve seating), the third exhaust event is just peaking blowdown pressure measured in that cylinder's exhaust port. According to the sim, that is about 88 psi on that exhaust port. How much of that pressure is being seen at the port of the first exhaust event?

 

[Alky V6]

Let's use the 88 psi peak pressure presented by one cylinder blowing down as another cylinder's exhaust valve is closing.
Assuming a 1.43 area on the backside of the exhaust valve.
Target intake boost level: 42 psi
Exhaust backpressure at 42 psi int boost: 88
Exhbp acting on the back of the exhaust valve: 1.43 x 88 = 125.84 psi
Pressure acting on the cylinder side of the exhaust valve: 1.43 x 42 = 60 psi
Pressure differential across the exhaust valve: 65.84
220 + 65.84 = 285.84 seat pressure. Very close to Crower's suggestion for my particular application.

This is assuming an exhbp to intbp ratio of 1:1. If we were talking about a 2:1 ratio, I'm guessing that number would be added to the peak blowdown pressure value, adding another 60 or so psi to the required spring seat pressure. 285.84 + 60 = 345.84. Real close to what Chris was suggesting is needed.

 

[turbobitt]

Let's use the 88 psi peak pressure presented by one cylinder blowing down as another cylinder's exhaust valve is closing.
Assuming a 1.43 area on the backside of the exhaust valve.
Target intake boost level: 42 psi
Exhaust backpressure at 42 psi int boost: 88
Exhbp acting on the back of the exhaust valve: 1.43 x 88 = 125.84 psi
Pressure acting on the cylinder side of the exhaust valve: 1.43 x 42 = 60 psi
Pressure differential across the exhaust valve: 65.84
220 + 65.84 = 285.84 seat pressure. Very close to Crower's suggestion for my particular application.

This is assuming an exhbp to intbp ratio of 1:1. If we were talking about a 2:1 ratio, I'm guessing that number would be added to the peak blowdown pressure value, adding another 60 or so psi to the required spring seat pressure. 285.84 + 60 = 345.84. Real close to what Chris was suggesting is needed.

Now look at all cylinders at it's lowest cylinder pressure, and see if it matches to the peek exhaust pressure event that may not be happening on the same cylinder, given peek pressure differential. Keep in mind that the valve just need to be near closing to be effected.

Allan G.