Do I need stiffer springs for boost?

This is one of those old wives tales that I am always amazed to see still floating around. It is based on a sliver of truth but is generally grossly misunderstood and misrepresented. It seems most often you hear people saying this will cause intake valve float which really does not make sense in any typical context. Let’s start with some basic engine dynamics.

At TDC on the exhaust stroke both intake and exhaust valves are open for a brief duration for what is called cylinder scavenging. The exhaust mass leaving creates a low pressure zone behind it and that low pressure zone will help accelerate the intake charge to start filling the cylinder. Perfectly optimal cam timing would result in the exhaust valve closing right as all the exhaust has left the cylinder and fresh air is just reaching the exhaust valve. Naturally that is only at one RPM and at other RPMs there may be some slight variance with either a little fresh air escaping through the exhaust or more often a very small amount of exhaust still trapped in the chamber. At this point the piston is already traveling down at a very high rate of speed causing a low pressure area in the cylinder. The exhaust valve is closed and now pressure whether it’s just atmospheric pressure or boost pressure is filling the cylinder. Because the piston is traveling so fast the low pressure zone remains at the top of the cylinder past bottom dead center so the piston is traveling up the cylinder but air is still flowing in through the intake valve. Optimal timing for the intake valve to close is when cylinder pressure and port pressure have equalized and air has stopped flowing into the cylinder. At this point as the valve is closing pressure on either side of the valve face is roughly the same. As the valve closes the piston is pushing a high pressure wave at a very high rate of speed up toward the combustion chamber so now that the valve is closed the pressure in the combustion chamber is pushing the valves closed.

In reality there are two situations where intake pressure could overcome spring pressure. The first would be if your cam timing were far from ideal. Even then it’s a very long shot because it would have to be while the exhaust valve was open and at that point the intake valve is very much open and flowing air past it so the pressure the air is putting on the back of the valve face is going to be pretty negligible.
The other is the most likely scenario and would take a pretty extreme setup. For example say you had a 1 square inch valve and 40 lbs of seat pressure. If you were running 45 PSI boost then it is actually possible that the boost could push the intake valve open from a seated position at a point where the cylinder pressure was below atmospheric pressure like at the end of the exhaust stroke.

Where I believe these rumors come from and where they may have some truth would be in the world of supercharged V8s running high boost such as in drag racing. If you have a big 2 valve V8 with say a 2.5” intake valve that means you have almost 5 square inches of valve area. Now if you are running 45 PSI boost you would need 225 pounds of seat pressure just to hold the valve closed against the boost pressure at any point the cylinder pressure reached atmospheric while the intake valve was closed. That is the only time this pressure difference should be an issue though because again when the intake valve is closing there will be almost no pressure differential on the two faces of the valve. So as long as you have enough spring pressure to keep it closed on the exhaust stroke you are fine.

Now turbo motors actually have the opposite problem and it seems very few people are aware of this. Most people talk like you will have atmospheric pressure on the exhaust side of a turbo but that is only true after the turbo. The turbine is driven by pressure and that pressure at best will roughly equal the boost pressure so with a turbo optimally sized on the big side for overall flow and power you hope to get as close to 1:1 ratio as possible. That means if you are making 10 PSI boost you have 10 PSI pre turbine backpressure (PTBP). In this case your whole system operates pretty much as an NA motor would. Cam size, overlap, etc would all be the same as NA theory. The only downside is lag and to a greater degree boost threshold. A big turbo means it won’t spool as early or as fast. So you may choose to run a smaller turbo for quicker spool and response. This means you are pushing the turbine side past it’s sweet spot and in this case your PTBP will start to climb higher than 1:1 especially as you revs increase. So if you are running a very small turbo that could mean you have as much as 2:1 based off gauge pressure so 10 PSIG boost means 20 PSIG PTBP.

 

In this case there could potentially become a point where you would need to be concerned about that pressure affecting your exhaust valve but it would still take a pretty extreme situation and more than anything would really tell you that your turbine sizing was way too small. If there is any chance of that pressure pushing or holding the exhaust valve open then it means that during overlap it is pushing exhaust back into the chamber and possibly even into the intake runners. Naturally this would kill performance as you would be replacing fresh oxygen in the chamber with already burnt exhaust.

 

courtesy of webmatter.de