Just curious to see if any of you guys ever ran into valve float that was associated with running high boost pressures. I was thinking about it one day. I mean, it is really a possibility seeing as how the extra pressure in the intake charge could actually try to keep the intake valve open. If this were to happen, then piston to valve contact would almost be guaranteed. Would simply running valve springs with a higer seat pressure be more benefical at this point especially as a safety measure? Just looking to see what you guys think about it...

 

Force on valve = Surface area (2.02" intake) times pressure (20 PSI)

2.02²∙π/4 ∙ 20 = 64 pounds pushing "down" on the intake BUT the intake valve closes about 120° Before Top Dead Center AND the piston is compressing the intake charge so there is little or no pressure difference between the top and bottom of the valve.

 

Yes, that may be true but I am talking more about the effects when both the intake and exhaust valves are in overlap. I would imagine that would be where the critical tolerances would essentially be compromised. Assuming people out there are using a regular stock head with stock valve springs that have maybe, what, 80-90 lbs. of spring pressure, the 64 lbs of holding force on ANY of the valves allows such a high risk that the spring will not properly dampen the opening and closing oscillations. This would lead to valves bouncing off of their seats and causing engine damage.

 

Run stiffer springs on the intake and lighter on the exhaust. The increased exhaust back pressure reduces the required spring force.

 

Huh? How is increased exhuast manifold pressure going to act on the valve differently than increased intake manifold pressure?

 

I don't understand his reasoning either. It would either act the same as his intake example or need a higher pressure spring on the exhaust. Either way, the only true was to know is to find the before turbo back pressure and go from there.

 

I used to wonder about this, too. However, guys who build boosted motor for a living told me basically, it just doesn't happen. At some extremely high boost levels it can have a small effect but on your average high performance boosted engine it's just not a factor.

 

Hugh MacInnes also talks about this in his book. He also felts it was a non-issue.

 

When the valve is open pressure will be the same on both sides of the valve making no difference whatsoever. When the valves are closed pressure in the chamber is greater then any boost pressure that you’ve got in the intake or in the exhaust manifold and it will keep the valves closed, for that matter, the increase in cylinder pressure due to the boost should be greater then the boost pressure so if anything boost will help, not hurt.

I suspect that the original source of this BS is some combination of the fact that you can add boost to an engine and extend it’s RPM range as well as a boosted engine with fairly mild parts can make power levels that would normally get much more serious parts to make, the end result being someone suddenly finding that their stock, 100Kmile long block goes into valve float at 6500rpm where it never did it before the boost… even though it never saw over 5000rpm before the boost…

 

MacInnes theory was basically the same as what Mark just stated above.