F.A.S.T./O2 over correction problem

[Craig Smith]

Rob,

If the engine is misfiring at all, it will definitely cause the O2 sensor to be inaccurate. Sounds like you're on to something now. Like I said before, closing the plug gap and seeing a noticeable performance increase generally indicates the need for a stronger ignition. On the valve spring thought, keep in mind that for every pound of boost you run, you should multiply that pound of boost by the surface area on the back side of the intake valve and subtract that number from the seat pressure on your valve spring. If you make some quick calculations based on this, you can see how easy it is to wind up with very little seat pressure.

Hope you'll get this all figured out!

Craig

 

[njrob6]

Let me see if I got this right:

Im running 22 psi of boost, multiply that by 1.90" (intake valve size)
That gives me 41.80 then subtract that # from the seat pressure of the springs i am using, which is 110lbs seat pressure. That give me 68.20

Is 68.20 the amount of seat pressure the springs have under 22psi of boost? Does this mean i should go with a valve that has 150lbs of seat pressure?

Rob

 

[carbuff]

Rob,

I think you're off slightly... The 22psi is the pounds per square inch, so you have to multiply by the area of the valve, not the diameter (unless 1.9" is the area, but that's not what you said).

Since I'm not a GN person, I could be way off...

Anyway, if I'm right, you have:

Area = pi * radius ^ 2 = 3.14159 * (.95")^2 = 2.835

Pressure on the valve = 2.835 in^2 * 22psi = 62.4 pounds

Take the 110# of seat pressure - 62.4# and you have 47.6# of seat pressure! Even worse than your calculation...

Ouch!

Bryan

 

[Craig Smith]

Bryan hit the nail on the head. I'll bet it's now a little easier for you to imagine where your miss is coming from though. Call our buddies at Comp Cams, tell them what you have and what you are doing, and they can give you a valve spring that's right for the job.

 

[njrob6]

So what I need is a spring that has at least 160lbs of seat pressure to keep it closed during 22 psi of boost? If I get one that has 160lbs of seat pressure and i decide to run 26psi of boost, will it start to blow open then?
What is the formula you used to come up with the surface area of the valve (.95)?

Thanx for all the info,

Rob

 

[carbuff]

The surface area is equal to PI time the square of the radius. If the valve is 1.9" in diameter, the radius is 0.95". Square that, and multiply it by PI. That's the equation I was trying to show above...

Hope that helps!

Bryan

 

[Lance]

The formula for finding the surface area of a circle is: Pi R^2

The actual result using the valve diameter is not accurate unless you factor in the valve stem and the radius of the tulip. But it does demonstrate the need for factoring in the valve size on a forced indution engine when selecting a valve spring. I know this firsthand because I found out the hard way. 200 lbs on the seat wasn't enough valve spring at 7000rpm and 17psi boost with a 2.125" intake valve. It's overkill but now I have 300 lbs on the seat and it took care of the miss I had.

Lance

 

[njrob6]

Ok, I put on a new set of Crane dual valve springs rated at 165 lbs seat pressure @ 1.75", no more miss. I ran the car today, I went for a license test so I had to take 6 runs, there was only enough time for me to take 4 runs, so I didnt take my full blast runs yet. But it ran 108mph in the 1/8 mile so it should run good on a full pass. So far so good.

Rob