Opel hi-po cylinder head comparison

[TGSI Racing]

Yea… what Rally Bob said about harmonics. You can destroy your valve train if the harmonics of the spring match the RPM of the engine. At the risk of being a bit ling and tedious, I'll explain and… except for a little math… try to make it simple.

The "harmonic" of concern is the "frequency" at which the valve train will oscillate. If the engine runs at that frequency, you are in trouble. In reality, it's not too hard to figure out with a little physics. Taking all the calculus out, here's a couple of simple equations:

Frequency = {sqrt(k/m) / (2*Pi)}

The units cycles per second or CPS or Hertz or Hz… I'll use Hz
k is the spring constant (see formula below)
m is the mass of the moving portions of the valve train in kilograms
(1/2 of the spring mass and all of the rocker arm mass is a close approximation and does away with some nasty calculus)

The spring constant is determined as follows:
Starting with the free length of the spring, compress the spring and measure the pressure (convert to kilograms). Simultaneously, measure how far the spring was compressed (convert to meters).
NOTE: 1lb = .4536 kilograms and 3/4" =. 01905 M; 1/2" = .0127 M is usually pretty good… that is 3/4" from free length. Try to use 3/4" if the spring doesn't coil bind.

Now use this formula to find k:

k = {(pressure * 9810) / x }

The units here are Newton/Meters (N/m)
9810 is 1G in meters per second squared
x is the distance (in meters) the spring was compressed for the measurement

Note that you need to be careful with units. Everything I have shown is in kilograms and meters. Convert the way you want but be very careful.

Now that you have all of that figured out, just pick a combination that puts the frequency of the spring and mass well above the max frequency of the engine. Oh yea… the frequency of the valve train:

Valve Frequency = RPM /120 Hz

This all becomes really easy with the "magic" of the computer. If you have just a little bit of programming skills, the program to calculate the spring frequency would not be too hard to write yourself.

As you can see, selecting the correct valve spring is just not a matter of some guy at the local machine shop pulling a number out of his butt to make it stiffer. Too stiff … you pound the engine to death and eat up HP. Too soft… Mr. Valve meets Mr. Piston. Wrong Frequency… valve train destroys itself. Change the mass of one part… start over again.

[madhatterpdc]

I love how you guys can hammer out these calculations and make it seem as though it is coming right off the top of your head (it probably is!!)

I just wanted to add one comment to TGSI's very comprehensive explanation of how to calculate the harmonic frequencies (it is rare that I can make recommendations at this level), But I think that a lot of users will have an easier time of plugging the equations into an Excel spreadsheet and then let the cells calculate each other and return a value. A lot more simple to the PC semi-literate than programming, just about as complicated as using a graphing calculator.

[madhatterpdc]

Let's try this, it's dirty but I think it gets the job done. TGSI may want to check my equations and conversion factors.

Please enter your values in the "Empirical Units" column and it will convert to Metric for you.

All of the numbers that I have used for example (valve train mass and spring tension and distance to compression) are completely random and have no real world basis for any engine that I am aware of.

Let me know what you think!

(Attachment edited 7-25-03 15:00 EST, cosmetic and organization changes only)

[madhatterpdc]

has anyone had a chance to check my work here. I want to make sure that I drew out the equations and conversions correctly.

[GTJIM]

We old 'boomers' do our calculations on the back of envelopes ... with the help of a slide rule (if greater computational power be needed!).

[wrench459]

Originally Posted by RallyBob Don't forget spring harmonics.... ...I ended up using a set of identical springs, but reduced the spring tension by 5 lbs on the seat, and the harmonics 'went away'. In truth, they didn't go away, ALL springs have harmonics, the point at which they occurred just changed. So, if you have harmonics occurring at 20,000 rpms, but the engine redlines at 8000, you'll have no problems. If it happens at 7800, you're in trouble. Change the weight of the valvetrain parts, or the shape of the cam lobes, and the harmonics will change again. Weird science.... Just another fly in the ointment to consider..... Bob

Whats your thought's on conical springs Bob or anyone else?
It's my understanding they tend to help cancel out some of the harmonics early in valve lift.

I can't prove that they work other than the motor is still in one piece. Beginner's luck?:

[RallyBob]

Originally Posted by wrench459 Whats your thought's on conical springs Bob or anyone else? It's my understanding they tend to help cancel out some of the harmonics early in valve lift. I can't prove that they work other than the motor is still in one piece. Beginner's luck?:

I haven't run them on an Opel. I'm sure there are a lot more choices now compared to when I first looked at them. Back then I had one choice, I *think* it was for the Corvette LT-4. Heights and rates didn't work for me and my cam choices as I recall.

Nowadays, there are probably a lot more options, but I haven't tried them, or even investigated them. Guess I'm pretty happy with my current setup (Crower Ti retainers and double 1.25" OD springs), since they've been very reliable. Been using them since '95 or so. Doesn't mean there's not a better solution however!