super charged valve spring pressure

[Leny Mason]

I need some expertise on valve spring Pressure naturally aspirated versus super charged, my Cammer has stock cams and I need to know what spring pressure would work the best it is super charged and Bret said Jay may know or some one else, thanks Leny Mason   

[CaptCobrajet]

A cammer won't require as much valve spring as a pushrod engine, NA or supercharged.  I'm sure Jay would have info, and also Mr. John Vermeersch.  John would have knowledge of the original blown cammer set-ups.

[Joe-JDC]

On the SBFs with Vortec or Pro Chargers, we found that increasing the seat pressure ~ 20# seemed to work well to 14 psi increase.  I can't speak to the SOHC specifically.  Also, the exhaust duration was increased 4* on camshafts to compensate for more exhaust blow down time, but found that that was not really necessary because the blowers worked well with stock camshafts and more spring pressure on seats.  Also, found that blowers did not really need a special grind because they worked well with typical high performance camshafts.  Was more fun to install a blower on a dead stock shortblock and add high flowing heads.  Idled smoothly, and sounded stock until you got on it.  Joe-JDC

[scott foxwell]

Never found increasing spring pressure on a blower deal necessary. Pressure differential between cylinder and int. port doesn't change with boost.

[jayb]

My take on this is when the valve is closed, pressure on the back side of the intake valve opposes the valve spring pressure.  So, when the intake valve closes it can bounce off the seat due to the supercharger pressure detracting from the valve spring pressure that is trying to keep the valve closed.  This effect will show up on the dyno graph just like a valve float condition.  On the dyno with my supercharged FE running 17 psi of boost, I lost about 400 usable RPM going from naturally aspirated to supercharged, and I think it was due to that effect, although there's no way to prove that.  But when I increased the intake valve spring pressure by 60 pounds, I got the RPM back.

The way I calculated how much additional pressure I needed was to figure out the area of the back side of the intake valve and multiply that by the peak boost pressure.  I have 2.25" intake valves in my supercharged engine, so neglecting the area of the valve stem the area of the back of the valve is 1.125*1.125*3.1416, or 3.97 square inches.  So, using about 4 square inches and 17 pounds of boost I figured I needed to add 68 pounds of seat pressure to the valve.  I added 60, and that solved the problem.

I'm currently revamping that supercharged engine and will be running it on the dyno again sometime in the next few months.  I'll see if I can repeat that experiment when I do so, and check to see that the results are the same.


 

[scott foxwell]

My take on this is when the valve is closed, pressure on the back side of the intake valve opposes the valve spring pressure.  So, when the intake valve closes it can bounce off the seat due to the supercharger pressure detracting from the valve spring pressure that is trying to keep the valve closed.  This effect will show up on the dyno graph just like a valve float condition.  On the dyno with my supercharged FE running 17 psi of boost, I lost about 400 usable RPM going from naturally aspirated to supercharged, and I think it was due to that effect, although there's no way to prove that.  But when I increased the intake valve spring pressure by 60 pounds, I got the RPM back.

The way I calculated how much additional pressure I needed was to figure out the area of the back side of the intake valve and multiply that by the peak boost pressure.  I have 2.25" intake valves in my supercharged engine, so neglecting the area of the valve stem the area of the back of the valve is 1.125*1.125*3.1416, or 3.97 square inches.  So, using about 4 square inches and 17 pounds of boost I figured I needed to add 68 pounds of seat pressure to the valve.  I added 60, and that solved the problem.

I'm currently revamping that supercharged engine and will be running it on the dyno again sometime in the next few months.  I'll see if I can repeat that experiment when I do so, and check to see that the results are the same.


 

So how do you respond to my above comment? It's not a guess, it's physics...

[Joe-JDC]

Worked with a mustang shop exclusively for about 15 years during the late '80s and into 2004, and we built some of the quickest and fastest mustangs on the planet during that time.  Many magazine articles written on them.   Every one with blowers had to have more spring pressure with more boost.  Those engines didn't take physics in development training.  Not going to argue the point, but experience has proven the need for more spring pressure to keep the valve train from killing itself under boost.  Joe-JDC

[Falcon67]

Also watch your carb return springs, if applicable.  Watched a guy crunch a brand new car because he could not shut it down in the burnout.  Blower draw held the blades open and he did freak and didn't go for the kill switch.

[scott foxwell]

Worked with a mustang shop exclusively for about 15 years during the late '80s and into 2004, and we built some of the quickest and fastest mustangs on the planet during that time.  Many magazine articles written on them.   Every one with blowers had to have more spring pressure with more boost.  Those engines didn't take physics in development training.  Not going to argue the point, but experience has proven the need for more spring pressure to keep the valve train from killing itself under boost.  Joe-JDC

lol...never mind.
But WHY did they need more pressure?
It wasn't boost.
More boost in the induction, more boost in the cylinder when the valve closes. The pressure differential never changes with more boost.
Think about it.

[plovett]

How do you know the differential pressure is the same?  I'm not saying you're wrong, and I get that there's more pressure in the cylinder to counteract the higher pressure in the intake.  But is it exactly the same as naturally aspirated?

paulie

[scott foxwell]

How do you know the differential pressure is the same?  I'm not saying you're wrong, and I get that there's more pressure in the cylinder to counteract the higher pressure in the intake.  But is it exactly the same as naturally aspirated?

paulie

If your intake port is keeping your intake valve open because of pressure, are you not closing the valve too soon? Hopefully when you close tine intake valve, you've put as much air into the cylinder as you can, whether it's @ atmospheric or 45# of boost. Pressure delta should be zero or very close.
I think a lot of people "assume" the pressure behind the valve thing because it's somewhat intuitive but they fall short of thinking about the other side of the valve. If adding spring pressure to a boosted deal "makes more power", it's somewhere else.

[jayb]

Scott, I see your point, I'm just relaying my actual dyno results.  Maybe at high RPM the boost pressure doesn't completely fill the cylinder, so that a pressure differential does exist?  Things are happening pretty fast at 7000 RPM.  I'm not sure on the actual cause, but I know that more spring pressure was needed on the intake valve to get my engine back to the NA redline. 

[mbrunson427]

If your intake port is keeping your intake valve open because of pressure, are you not closing the valve too soon? Hopefully when you close tine intake valve, you've put as much air into the cylinder as you can, whether it's @ atmospheric or 45# of boost. Pressure delta should be zero or very close.
I think a lot of people "assume" the pressure behind the valve thing because it's somewhat intuitive but they fall short of thinking about the other side of the valve. If adding spring pressure to a boosted deal "makes more power", it's somewhere else.

That makes a lot of sense. Here's another one for you that kind of makes you scratch your head at first.....

I was modeling some rods for an aluminum rod company in college. We did some studying on a supercharged motor rod, and an NA rod. We were trying to compare the two so they were for the same displacement, only parameter change was boost vs no boost. Before getting a hold of the rods, I was assuming that the supercharged motor rod would be much more substantial, wrong! The supercharged rod was lighter and had less beef to it. I was puzzled and I asked him about it. He explained that the added boost in the engine helps keep the rods in a compressed state, rather than rapidly swinging between compression and tension. So the supercharged motor made more power, but actually needed less rod.

[scott foxwell]

If your intake port is keeping your intake valve open because of pressure, are you not closing the valve too soon? Hopefully when you close tine intake valve, you've put as much air into the cylinder as you can, whether it's @ atmospheric or 45# of boost. Pressure delta should be zero or very close.
I think a lot of people "assume" the pressure behind the valve thing because it's somewhat intuitive but they fall short of thinking about the other side of the valve. If adding spring pressure to a boosted deal "makes more power", it's somewhere else.

That makes a lot of sense. Here's another one for you that kind of makes you scratch your head at first.....

I was modeling some rods for an aluminum rod company in college. We did some studying on a supercharged motor rod, and an NA rod. We were trying to compare the two so they were for the same displacement, only parameter change was boost vs no boost. Before getting a hold of the rods, I was assuming that the supercharged motor rod would be much more substantial, wrong! The supercharged rod was lighter and had less beef to it. I was puzzled and I asked him about it. He explained that the added boost in the engine helps keep the rods in a compressed state, rather than rapidly swinging between compression and tension. So the supercharged motor made more power, but actually needed less rod.

This is very true, especially on turbo engines where there is constant pressure even @ overlap.

[cammerfe]

Over the years, I've discussed the use of aluminum rods with the manufacturers on several occasions. Another characteristic of the aluminum is that it's less rigid than steel. I've been told that the 'cushioning effect' of the aluminum is of benefit. Early aluminum rods were known to stretch whenever a car was 'motored' down---pulling the big end egg-shaped, to the detriment of the bearing shells. Putting more 'meat' in the rod cap and providing a 'pin' for the shell all helped. I've never seen an aluminum rod with less 'bulk' than its steel counterpart.

KS