rj I think 12v is more like a 2/71 GM diesel like they used on the old Allis Chalmer dozers. (Takes off running before Don gets back to his keyboard.)

 

 

This was my reaction to that:

.....Took a min to sink in!

 

I need my truck to run everyday too...but if I break it, I do have something else to drive...like my 93 S10, or the old 95, or Mom's new Suburban (she doesn't "need" go anywhere)

And to all you naysayers...a 12v is a 24v with emphazema, but somehow can still finish in first place...usually through massive drug use Good thing they don't use them in cycling

Chris

 

Dan - That's "Redneck Lag".... I've seen it a lot but you're the first to admit to it!

(Sorry Chris & Dan, the Devil got into my shorts this afternoon.... Funny you should mention cycling, Chris.)

Saw a woman on a bicycle today that would have embarassed Shania Twain! Her almost complete outfit would have turned anybodys head. Unfortunately, mine turned without remembering to hit the brakes, with a stop sign coming up!
I came as close to getting broadsided as you can without having a dent to show for it. An 18' Ryder box truck stopped 2" (probably 2 ft, but close) from my door handle. I did not know Okies knew that many swear words!
Still not quite myself! Sure was a nice looking bike, though... at least an H3, on my cam-o-meter!

How's that for getting back to the thread topic!

Unsigned

 

Back to Spring rates:

I'd like to expand on what Don said about spring rates, only because the sheer popularity of the "60# myth" demands it.

Most guys who build engines will tell you that a typical domestic V8 has spring rates over 60#, even in mild stock form. Never mind a performance build that turns higher RPM-- or one with higher RPM AND more pressure due to boost.

All you need to know about springs is this: F=MA

In this case, the Spring has to apply a Force (F) sufficient to maintain tappet/cam contact as the valve (which has mass M) rapidly accelerates (A) from completely closed completely open and back again.

You can use this to guesstimate what kind of spring pressures you will need to maintain valve control. But first you need some information-- peak cam rate of lift (especially on the closing ramp), rocker ratio, valve weight, engine RPM, etc.

Take that number and add a fudge factor, then you'll have to calculate the effects of boost and backpressure multiplying pressure times valve surface area to arrive at a force (in pounds).

The end result of this is a spring that's a little overkill, because cams have rounded lobes.

Having springs that are a little too stiff costs a couple HP and maybe a little faster cam wear (not an issue with Don's harder cams). But having springs too soft can cost you a lot more HP and potential engine failure.

I'll run the numbers in a follow-up post later....

jlh

 

Why is it I have to read HOHN's posts about 3 times before I catch it all??

I'll re-read again when the follow up comes
Chris

BTW, RJ...you should have stopped and asked if she was a "Custom Grind??"

 

OK fellas, here's some numbers we can put to calculate spring rates.

First, let's calculate effects of boost and backpressure (both of which tend to make it harder for intake and exhaust, respectively, to close the valve. For example purposes, I'm just going to make stuff up.

Let's say you have 50psi of boost max. The applies a force to the valve that tends to keep it open. If the valve is, say, 2 square inches in area, then it is ONE HUNDRED POUNDS of force trying to keep the valve open.

Same thing for exhaust. If you have 50psi of backpressure on that same 2 sq in valve, then you'll have 100# pushing the valve open.

The springs have to overcome these forces.

How to you calculate maximum acceleration of the valve?

First, you need to know the maximum rate of lift of the cam, in inches per degree of rotation (polar coordinates, if you're familiar with that).

Say that the max cam lift RATE is .300" in a space of 60º cam rotation. Assuming it's linear in that section (which is usually the case at peak). So we can calculate the rate as (.300/60º), or .005" per degree cam rotation.

The cam turns at half the speed of the engine, so, then we know the rate relative to ENGINE speed is half this, or .0025" per degree of ENGINE rotation

Enter engine RPM. Let's say we're figuring valve pressure for 3000 rpm.

One engine revolution is 360º. At 3000 rpm, that means 360*3000, or 1,080,000 degrees per minute of angular velocity. Call it 18000º per second. At that speed, it will only take 1/300th of a second to move the valve that .300" distance. IN other words 90ft/sec valve velocity.

So now we have to slow this valve down and accelerate it to 90ft/sec in the OPPOSITE DIRECTION, all within a space of about 60º engine rotation.

So the spring has to control an acceleration sufficient to produce 180ft/sec velocity change in only 1/300th of a second. That's an acceleration rate of 54,000 ft/second/second--- or in other words, about 1677Gs!!!!!!

Let's say the valve in question weighs 2 ounces (again, all fictional numbers). Using F=MA, we can see that this 2 oz valve when accelerated by 1677 Gs will act as if it weighed ~210 lbs!

Now, to this 210# figure we add the previously calculated effect of boost pressure or backpressure, which is another 100# of force.

That means that 310# is the MINIMUM spring pressure it would take to control the valve under the circumstances in this hypothetical example.

Still think the Stock springs are up to turning 3K rpm at high boost???


Justin

 

Hello Don,

You mentioned a cheap "fix" for valve springs on a 12 valve. I tried to send a PM, but your inbox is full. Could you also let me in on it? Something like adding shims or using different retainers? I have an in-line e-brake that at some point I want to add, and eventually want to fab up a set of twins. Nothing too terribly hot, just ~45psi so I can tow big loads up big hills without worrying so much about melting pistons. I'm also really tired of making a bunch of smoke.

Thanks,
Vince

 

Good read...on somthing I never really thought about....but makes total sence!

 

If you're gonna stay under 50psi, and 3500RPM the 60lb springs will be fine...unless it takes 65psi drive pressure to make 45psi boost!

The exhaust brake also won't need anything more than the 60lb Cummins springs!

Chris

 

JH - Very informative and well written!
Appears that better springs become an added expense with any twins, even with good, low, 1:1 backpressure like I'm getting!

Don, Guess I'll be in touch soon.

RJ

 

Thanks Chris.

 

Now I wouldn't go blindly following my advice...but when you put the twins on, check your drive pressure! As long as it's 1:1, I'd say you're fine!

Chris

 

What if your drive pressure is 70PSI:70PSI?? I dont think drive pressure has anything to do with it......its more Back pressure and Boost pressure trying to open the valves up or keep them open.....