Doing it Gangsta

Tom S · Sep 27, 2011

So if I was reloading a typical sharply pointed rifle cartridge and put the bullet in backwards so the point was in the case, would there be this huge reduction in velocity bacause the gases did not have a surface perpendicular to the bore to press upon?

I am not saying BB is wrong on his theory but I cannot totally wrap my head around it either.

TARM · Sep 28, 2011

Technically to accurately project the area of a valve where manifold and or drive pressure can exert force on you do have to calculate the area its being applied to. While you can use the flat surface area of the face that is not accurate as the force is being applied to area on the backside of the valve. But point in fact you do also need the area of the valve face as well as force is being applied there as well. Its not just as simple as boost or drive pressure.

For the force from boost and drive pressure you need to take the area of the valve minus what is covered by the seat (so inner and outer seat measurement will get you that) then you need to also subtract the area of the stem. That will get you the surface inches squared that you can now multiple boost PSI to get the pressure countering the spring,

This force will only matter when the valve in question is fully seated.

There are of course countering forces from pressure inside the cylinder and if you want to correctly account for those then you would be using the valve face surface area. Its a dynamic event and situation so there many factors going on.

But honestly IMO its a lot to do and still will not get you 100% there.

The principle job of the valve spring is to ensure the valve follows the cam profile and closes solidly. Where bounce or flutter happen is when this does not happen. RPM and cam profile valve train weight are the primary specs that dictate spring rate. Boost does have an effect but its to a lesser degree IMO.

With that said I personally would error on the side of more than vs too little spring pressure.. That does not mean overly excessive spring pressure. Point being is as long as we are under the threshold of damage to parts its best to error on the side of more if this is a perf setup.

I think there is much more going on that effects the needed spring pressure than the increases of boost directly. RPM tend to climb at a faster rate and we tend to go higher in rpms band. This is a dynamic event and has to be looked at as such. After doing a good amount of reading on the subject even the engineers can not fully agree on this stuff when it comes to boosted engines. What it comes down to is an educated guess to start out then making adjustments based on experience. The whole idea is to prevent flutter/float and on the other side not to go so high as to have needless wear or strain/failure of other valve-train parts.

My only real issue in this thread was the stated 72 lb and 92 lbs for the OEM and 910s when set to OEM spec height they are 110 and 130 respectively. So I wanted to know how that was computed so I could then understand where the 175 lb on these new springs rate in relationship to those and what the install height is etc... I am not saying either way whether in terms of proper spring pressure. My feelings are if you plan to push your engine and its a higher perf build they are likely a good choice. When you go with larger injectors with more air capacity you are increasing your rpm limit and from that alone there is more force. Add to that a new cam profile with faster rate which adds to that need even more. Then finally account for the effects of the higher boost as needed.

Break in the springs running them easy and letting cool to take the initial set before romping on them. Then hope there is no float ( popping) when you take them up to redline. If there is add shims to get needed tension or order new ones.

The guys that are install new/reground cams you are installing new lifters as part of the install, yes? Welding the cam gear on to prevent walking as well?

Big Bore · Sep 28, 2011

Ok so I've been playing with this valve seat area and PSI and how to calculate it.

First of all, I was completely wrong on the PSI subject. I was applying the Force per unit Area formula ( F/A = P ) incorrectly as 09Stroker tried to tell me several times. Per being the key as he stated, and the reason I made the mistake is irrelevant and dumb really, I made it and that's that, but as I was looking through calculations and saw it, rest assured I got a bit red in the face. Yea, I feel really cool right now. My apologies to those who were trying to wake me up while I dozing at the calculator.

I was correct in how to calculate the area of the valve being affected by boost. There is a formula that calculates the force applied to a radius or angles using vectors and it definitely diminishes as the angle increases past a certain point until it reaches 90* where it becomes 0. I have arranged to meet with my future son in law who is an experimental physicist and works for an aerospace company so he can check my work and help me make actual calculations using an intake valve from a 7.3 head and possibly make a graph based on varying PSI in the intake tract. Maybe Doug or someone else can find this useful when selecting springs.

Tom, the bullet will leave the barrel at the same velocity unless gas cutting occurs. There is no other place for the gasses to go, and the resistance in this case is the friction in the bore, which will remain the the same regardless of which way the bullet is pointing. I've loaded bullets backwards for various reasons, and its also a "trick" I've heard of used by big game hunters in Africa. Remember you are talking about an explosion that happens in nano seconds resulting in peak pressures of 50,000 to 60,000 psi in most high powered rifles.

As long as the bullet seals properly it will accelerate at the same rate regardless. When it leaves the bore the external ballistics will take over and of course it's flight path will be completely different, namely it will shed velocity quicker and trajectory will not be anywhere near as flat.

Now, if that same bullet were undersized for the bore and gasses could escape around it, you would definitely see a difference in velocity. It's frontal area in resistance to the gasses would be affected differently just as it would when bucking wind during it's flight bath. IOW the flat base would resist the gas pressures more than the tapered profile of the bullet nose as the gasses are escaping around the bullet. Cup the base of the bullet and it would increase the resistance and the velocity.

Big Bore · Sep 28, 2011

TARM said:
This force will only matter when the valve in question is fully seated.

There are of course countering forces from pressure inside the cylinder and if you want to correctly account for those then you would be using the valve face surface area. Its a dynamic event and situation so there many factors going on.

I think the most important point during the cycle is when the chamber pressure is at it's least. That would be the number I would be interested in.

Tom S · Sep 28, 2011

Looking forward to the data on the valve vs boost pressure. Nice to have some cool high tech discussion.

juniort444e · Sep 28, 2011

Too many big words for my peanut. I quit.

Just a quick thought tho, someone mentioned something about bigger isnt always better referring to the bigger locks i assume, but upon further investigation on dougs page, i noticed that those locks are double the size BUT the retainers are 30g lighter. So one would think that there isnt any gain in weight only gain in strength and a possibility to shave weight too. Just my 2cents.

TARM · Sep 29, 2011

What it comes down to I think is the effects of VE but specially the VE effects in the intake and exhaust when taking into account sonic limitation of flow. As you go to a VE that is below 100% you will have a pressure difference across the valve which is further applied with boost. Cylinder to manifold. This is a figure that needs to be accounted for.. It all gets so technical and in depth only a engineer could enjoy it and not end up hurting your head.. LOL

I will state IMO its only one small part of a number and Cam profile, RPMs(peak and rate), Valve train mass are what is critical first. Then adding tension for the effects of boost.

As a general rule if I had to put a number on what I have learned from reading way to much on this for diesels it would be 1 to 1 for what to add in for each psi of boost in spring tension. So take a 110 OEM spring subtract out for the 17-20psi its good for. Take it to 80-93lb/inch for spring for a NA app. Take it to 50psi boost take it to 130- 143lb/inch 75 psi boost from a set of compounds 155-168 But also do not forget any changes in peak rpms if you are going past 4K would be my guess.

Anyways thats what I came up with of what to add after you had already factored in what was needed for it from RPM, cam profile, valve train spec in a NA setup. This is the most basic of rules of thumb kind of factoring

genie144 · Sep 30, 2011

So on a daily driver - without knowing what your plans are for air and fuel... How were these parts spec'd for the build? And forgive my ignorance... In low boost, low RPM situations (aka daily driving), wouldn't the spring pressures be a bit much?

Sam

jwlandry · Sep 30, 2011

For all that are questioning the strength of the lifters on this application or if they have been chosen correctly for a Daily Driver, Then answer me this....

A 6.0 and a 7.3 Lifter is the same size.

A 6.0 runs 2 springs at 125# of seat pressure a piece, so 2(125#)=250# ------- These are STOCK springs.

So if a 6.0 can handle that much seat pressure and turn 4500rpms, then why cant a 7.3 with the same size lifter handle 175# of seat pressure and turn 4000rpms?

these figures are rough numbers but are really really close to specs.

jwlandry · Sep 30, 2011

TARM said:
My only real issue in this thread was the stated 72 lb and 92 lbs for the OEM and 910s when set to OEM spec height they are 110 and 130 respectively. So I wanted to know how that was computed so I could then understand where the 175 lb on these new springs rate in relationship to those and what the install height is etc... I am not saying either way whether in terms of proper spring pressure. My feelings are if you plan to push your engine and its a higher perf build they are likely a good choice. When you go with larger injectors with more air capacity you are increasing your rpm limit and from that alone there is more force. Add to that a new cam profile with faster rate which adds to that need even more. Then finally account for the effects of the higher boost as needed.

Here you go. This is where the measurements of the springs come into play. The blue thing you see in the head is a valve spring height micrometer. It accurately measures the distance between the valve spring seat and retainer.

And as for the seat pressures. They come from a chart that Doug has. Like he told me, He could buy a digital spring tester but since there is already data on the springs that he has chosen as well as the stock 7.3 springs there is no need to do it again.