Spring pressure

we have found on our combo ( fuel head) that for every .010 you lose on the gasket....you gain real close to 1/3 of a point in compression. so, in your case if you indeed have a true 12 to 1 and put .020 thicker gaskets on that should give you real close to 11.4 to 1. you might want to check it with that formula that Eli posted. i went through all this when we cc'd our motor over the winter. hope this helps
Darren

 

Thats great info ! , Thanks everyone

 

how many valves are open???

That is absolutely correct, and you don't have to be a rocket scientist to figure that out. The boost pressure is the pressure in the manifold and would be higher if less valves were open and lower if more were open, but considering that the number of valves open at any one time is kind of a constant, the boost pressure is also.

That said, I agree with the theory that to a point, more pressure is better and definately has resulted in less maint. There is way less chatter and wear on the valves and seats, and the springs last much longer when you start with a higher seat pressure. We use springs that install about 625 and change them when they get down to about 425.

Clint

 

what brand spring would be best for around 450 seat. i know everyone probly has there own opinion..so lets hear it and why....

 

I have springs that PAC made for me, Part # 1320 and 1353KJS. One is a dual, the other is a triple, both install at 480 lbs, but have Hi Freq. over 35000 and have spring rates at 1000lb. open pres = 1300 plus, Working well on many TA/FC and Pro Mods. I also stock a full line of PAC springs, Need info? Call me 216 390 4561 Ken Juszczyk / Motorsports unlimited, PAC Spring sales and dist.

 

effect of boost on spring pressure

With a 2.375 valve you have probably got about 2.25 inches in diameter exposed to manifold pressure(deduction for the seat contact area) This calculates to 7.071 square inches (2.25Xpi). Manifold pressure is in pounds per square inch so at 50 psi of boost you would have 353.55 pounds pushing the valve open against spring pressure(7.071X50). This would result in a theoretical zero seat pressure at max boost with 353.55 static at the seat.
You would still need to have enough spring pressure beyond that to control the valvetrain in a running situation.
Jeff Johnsen

 

Isn't he formula for area piR2. The 2.25 is diameter. Wounldn't it be 1.125 squared x pi. or 3.97 sq/in? Also I have seen this discussed for a while but nobody ever seems the factor in the valve stem. Wouldn't this reduce the actual area of the valve some what? Or would a standard 11/32 stem valve not make a big difference?


Corey

 

Boost on back of intake

Whether Jeffs calc is correct or not I think there is more to this than meets the eye.
If all the talk about boost on the back side of the valve was 100% real then some of the engines out there would never be able to shut the intake valve. Surely there is very little weight on the back of the valve when it is still open as the pressure is allowed to escape through the seat into the cylinder. As soon as the valve actually shuts and sees full manifold pressure on the back of it, it is similtaineously being forced shut by the rising cylinder pressure on the chamber side. There is probably almost manifold pressure in the cylinder before any compression starts and by the time the intake shuts the piston is well on its way back up the bore and moving fast. I dont know what the running compression pressure is but it must be considerable by the time the intake shuts. Of course then the plug fires at 20-40* BTDC and at that stage you cant open the intake even if you wanted to due to the massive cylinder pressure. Im not sure that the calculation of seat area X boost is a very accurate one without allowing a factor for cylinder pressure working on the other side. I reckon if you took the common calc and divided it by at least 5 would more accurate.
Lets hear what others have to say here- especially some of the more experienced racers and engine builders.
Jim

 

Corey is right, its (radius squared x Pi)

 

basic hydraulics
piston sq. in x psi = force
take the stem sq. in from the the valve sq. in x the blower pressure = amount of spring you need for static

if you have 50 psi on the boost gauge you have 50 psi pressure
plain and simple
not a little less when running
lets do the math
11/32 stem = .0927 sq in
2.375 = 7.452
minus stem = 7.359 sq in
x 50 psi boost = 367.96 +plus the valve weight, spring pressure required to hold at static closed
add in for pushrod and lifter at 10K better have some good springs in there

 

My take on it is that one thing seems to be certain, with high boost blown engines more seems to be better (up to a certain point of course) when it comes to seat pressure on the intakes. Insufficient seating pressure plays hell on the valve seat as the valve rattles in the seat and finally is slammed shut by rising pressure from the combustion chamber. All you have to do is look at a spintron video of the way a vavle stem moves around on the spring side to imagine the way the valve head reacts as it seats with insufficient pressure.

Of course with turbo engines its all together different because even though they may have the same manifold boost numbers the pressure differential will be less ( if not I want to know who the engine builder is ).

Pac Man what are your feelings on a triple vs a double? With all other factors (seated pressure, rate, frequency) Does the triple manage surge better but give up slight HP due to increased mass and friction? or is my thinking flawed here.

 

Whoops



Corey

 

Valve


Comax Racing

 

Ok; the area of the valve is pi r squared, not as I said..........it was late.
The exposed area is about 4 square inches ( I'm sure you include the stem area as it will move under pressure). At 50 pounds of boost you would be losing about 200 pounds of seat pressure.
The discussions about valve open and equal pressure in the cylinder are irrelevant for two reasons, when off the seat, the valve has progressively more pressure and secondly, that is not the rotational position of concern. The point of concern is as the piston is approaching TDC before the intake starts to open. The exhaust is still open but nearly closed, the piston has slowed its upward travel and there is basically no cylinder pressure to offset the 200 pounds pushing the valve open.
The tight spot we measure for clearance is just before TDC for the exhaust valve and just after TDC for the intake. With full boost against the valve, the lifter hits the ramp on the cam and sends the valve, supercharged, toward the piston. I do believe that this is the problem time in the event cycle and boost does have this effect.
Jeff Johnsen

 

Now all you need is a static race engine with boost to apply the theory.

The actual application proves out that the straight forward calculating boost pressure on back side of valve takes away all that spring pressure does not pan out. There's dynamics on both sides of the valve, not a static condition.

Also almost always when running more boost, therefore more power, it leads to running more RPM's which is why the increased spring pressure pays off.

Blessings..........Ron Clevenger.

 

Thanks Ron, I don't need a static race engine to try the "theory" on; I tune a 5.30 second blown alcohol dragster in the current, real world and know that boost has a significant effect on the intake valve's required spring pressure.

 

Hope you dont tune that thing late at night

 

Ok; I didnt throw the sarcasim out there, I just responded to it. I probably should have ignored it. Maybe it's late again.
Jeff

 

Sorry if I came off sarcastic, didn't intend it to be taken that way. Thought it was a discussion on ideas.
Good luck on tuning...............blessings...........Ron.