cutting down a wastegate spring

gsmith660 · 02-20-2009, 04:14 AM

Anybody got any guidence on how much I should cut down a .8 bar spring to get
.5 bar, is the spring rate linear so for example 1 ring on a 1 bar spring gets me .8 therefore 1 and half should get me .5 bar, am I in the ballpark.

beepbeep · 02-20-2009, 04:18 AM

Cutting springs makes them harder. It also shortens the elastic region.

Craig 930 RS · 02-20-2009, 12:29 PM

Yep, and get the correct spring.

911nut · 911nut's Garage

I think that if you are able to cut one down and grind in get the proper taper on the end that you cut, it could work out.
Do a search on wastegate springs. I posted the length of a .7 bar spring in one of those threads. With a little interpolation you could arrive at the length of an .5 bar spring.
A shorter spring will exert less force when captured in the wastegate than a longer one. If you have a mechanical boost gage you can check your work. If you don't have one, time to install one before you experiment.
Good luck and be sure to post your results for the naysayers

gsmith660 · 02-20-2009, 03:23 PM

does anybody know at what pressure on the bottom of the diaphragm opens the wastegate should it be what ever boost pressure is?

fredmeister · fredmeister's Garage

Quote:

Originally Posted by gsmith660

does anybody know at what pressure on the bottom of the diaphragm opens the wastegate should it be what ever boost pressure is?

Yes it sees the boost pressure. As long as the upper chamber remains vented to the atmosphere.
You don;t want to cut the spring because like others said this increases spring rate. It will likely not reduce the preload much like you are attempting to control because the spring is compressed a huge amount in the housing, but because of the higher rate will actually make it worse.
Why are you talking 0.5 bar anyway that is aweful low for a 930 engine? Unless you are planning on installing a boost controller to get back to 0.7-1.0bar with the 0.5bar spring.
Don't mess around with this stuff unless you have a good understanding of how the wastegate works and how springs are designed. Take a step back and look at the design or do a little research on google to see how things work before making changes.

gsmith660 · 02-20-2009, 03:45 PM

Quote:

Originally Posted by fredmeister

Yes it sees the boost pressure. As long as the upper chamber remains vented to the atmosphere.
You don;t want to cut the spring because like others said this increases spring rate. It will likely not reduce the preload much like you are attempting to control because the spring is compressed a huge amount in the housing, but because of the higher rate will actually make it worse.
Why are you talking 0.5 bar anyway that is aweful low for a 930 engine? Unless you are planning on installing a boost controller to get back to 0.7-1.0bar with the 0.5bar spring.
Don't mess around with this stuff unless you have a good understanding of how the wastegate works and how springs are designed. Take a step back and look at the design or do a little research on google to see how things work before making changes.

Its not a 930 engine its a science experiment with a 2.7 motor and just about the same number that say dont cut the spring also say cut the spring. It makes sense to me that if you reduce the overall length then the preload would also decrease if you can point me to a site that will refute that I am willing to look.

gsmith660 · 02-20-2009, 04:33 PM

Gonna get a tial, my stock wastegate now for sale I just got it to acuate with 10 psi air.

9dreizig · 02-20-2009, 04:33 PM

please send pictures when you blow the engine up!! LOL
+1 on don't cut it!

Rob 930 · 02-20-2009, 04:51 PM

gsmith660,

It's a curiosity to some that cutting a helical coil spring (such as the one in your wastegate) will indeed stiffen it. Any design textbook on springs (such as Machine Design by Deutschman or Mechanical Engineering Design by Shigley and Mischke) will confirm this. There are formulas that explain it, but they're too much work to enter here. To convince yourself, take any old spring from a hardware store and measure how much force it takes to deflect it a known amount. Then cut it in half and measure again. You'll find it's stiffer - that is, it takes more force to move the spring the same amount when the spring is shortened. Think of it this way: if you have a long torsion bar and a short torsion bar (of the same diameter), which one is stiffer? It will be the shorter one. A coil spring is essentially a coiled up torsion bar, so a shorter coil is stiffer. Convince yourself...

gsmith660 · 02-20-2009, 05:01 PM

I get the impression that I have worn out my welcome here your responses are getting terse I am only trying to learn and I thought that this was the right place to ask questions.

9dreizig · 02-20-2009, 05:12 PM

You've not worn out your welcome here, and nobody was getting terse ( go to OT if you want terse LOL), just alot of engineering types here with limited communications skills ( you guys know who you are

) .
Around here if you ask for an opinion you'll get it , nobody is shy here either..
I think Rob's explaination summed it up pretty good..

TimT · 02-20-2009, 05:22 PM

Quote:

You've not worn out your welcome here,

+1

There was a little misinformation regarding springs posted above

Analogy to a spring... say you have a lever 4 feet long you are using to lift something....

If you cut that lever in half... so you now have a 2 foot lever... and it will take more force to lift that something..

A coil spring is just a big wound up lever.. cut a coil spring in half you have a shorter stiffer spring.... cut a coil of that same spring you still have a shorter stiffer spring of an unknown rate

Keep your .8 bar spring... and get a boost controller...

9dreizig · 02-20-2009, 05:27 PM

Actually Tim, if I understand boost controllers, you can never dial in less boost than your spring,, so he'll still need to solve the less than .8 bar issue as all a boost controller does is effectively squeeze down the atmosphere end of the waste gate..

gsmith660 · 02-20-2009, 05:27 PM

Yeah I stay away from OT I have given up the idea of the stock wastegate and opted for a tial too bad the piece looks rough because I have had it on the shelf since 82 the 79 it came off of had very few miles when the owner torched it when he couldn't make payments the turbo is the same but got hot enough that i had a new core put in it back then. I dont intend to blow up the motor, alot of folks dont have faith in the 2.7 but mine has all the updates and has run flawlessly. the 2.7 I have on the shelf got overheated and it still made the trip from new orleans to missouri all I had to do was stop to fill up the oil and add a little gas. This is a test bed to learn and then i intend to put a 3.2 together and transfer it all over to it and bump up the hp.

gsmith660 · 02-20-2009, 05:30 PM

Thats true that is why I am getting a 5 or 7 psi spring and adding a boost controler to dial in the desired boost using an accurate boost gauge.

TimT · 02-20-2009, 05:34 PM

Quote:

so he'll still need to solve the less than .8 bar issue as all a boost controller does is effectively squeeze down the atmosphere end of the waste gate..

Yes you can't get lower boost than the spring with a controller. Cutting a spring in half or cutting a coil off will only increase the minimum boost..

He the o/p should contact TIAL and see if they have low boost springs available.. they probably do

911nut · 911nut's Garage

Hooke's law states that a spring applies a force that is linearly proportional to the distance of it's equilibrium length. In other words, take two different lengths of springs made from the same steel and wire diameter. Compress the springs to an equal distance the longer spring applies a greater force than a shorter one.
That's why longer and longer springs with the same wire diameter are used in the stock wastegate to raise the pressure at which the wastegate opens. The longer spring is crammed into the chamber above the diaphragm. Since the longer spring has been displaced a greater amount than a shorter one, the force applied by the spring to the diaphragm is greater.

gsmith660 · 02-20-2009, 07:33 PM

Quote:

Originally Posted by 911nut

Hooke's law states that a spring applies a force that is linearly proportional to the distance of it's equilibrium length. In other words, take two different lengths of springs made from the same steel and wire diameter. Compress the springs to an equal distance the longer spring applies a greater force than a shorter one.
That's why longer and longer springs with the same wire diameter are used in the stock wastegate to raise the pressure at which the wastegate opens. The longer spring is crammed into the chamber above the diaphragm. Since the longer spring has been displaced a greater amount than a shorter one, the force applied by the spring to the diaphragm is greater.

That was kinda what I was thinking but didn't know where I had seen it, at one time in my life I had studied all that but it is ancient history (Navy nuclear power school) that is kinda obvious when you look at the 1 bar spring and the .8 bar spring together they are same dia. wire same number of coils but the 1 bar spring is a little bit taller.

gsmith660 · 02-20-2009, 07:34 PM

but I am still going to get the tial.