Testing power piston springs

[Cadman-iac]

  Since I have an extensive collection of QuadraJet parts, I figured it would be nice to know exactly what the springs where rated so I could make an informed decision about which one to use.

 On another forum I found a lot of information on Quadrajets, and they explained how to test your springs so you could figure out which one to use.
 They recommended using an old core body and applying vacuum to the base of the piston bore.
 So I took a core and pressed a 90° plastic vacuum connector  into the port for the piston, used some long 5/16" bolts to keep it elevated so the fitting was not being pushed to either side, and I connected a vacuum gage with a "tee" fitting,  a vacuum source with another "tee", and a valve that can open and close with a turn of the knob to create a controlled leak.
  I used my A/C evacuation pump for the vacuum source, and after putting the spring I wanted to test in the bore I installed the power piston and turned on the vacuum.  Using the gage to tell at what vacuum the piston begins to move downward by slowly closing the valve to change the amount of vacuum applied to the piston, I was able to accurately determine at what vacuum the piston begins to move and at what vacuum it reaches maximum travel. Then I wrote that down and bagged it for later use.
 I tested over 70 springs like that and I now have a collection that has 19 different settings from 2-6"HG to 9-16"Hg. The first number being the minimum vacuum required to move the piston, the second number being the maximum vacuum required to fully seat the piston.
 Once I determine what vacuum the engine achieves at idle and what it makes at cruise speed, I can pick the best spring to use in the carburetor.

 Here's a few pictures of what I used. The valve is a simple rotating type I think was for an aquarium or something.
 Hope this helps if you are wanting to test your springs so you know what you have on hand to tune with.

 I also used this method to test the vacuum advance unit on the distributor.  It helps to know how much vacuum it takes to get maximum advance and when it quits.

 Rick

[novadude]

Nice idea!  if you have tested the commonly available springs from cliff and others, can you share your results? 

[quadrajam]

Yeah, what novadude said.^

[Cadman-iac]

Nice idea!  if you have tested the commonly available springs from cliff and others, can you share your results?

 Novadude,

 I have only found out about Cliff's website from browsing through answers to questions I posted on Google, so I wasn't aware that you could buy parts from him. I found his book in a bookstore years ago and have been using it, but I confess I hadn't thought to look for parts sources from the book. I've always done things the old school way, I'd go to the parts store. It's only been recently that I've started looking online for answers and parts.
  I used to be a partsman at a GM dealership for years so I had gotten parts through them. I bought 3 10 packs of springs over 30 years ago and still have probably half of those left, plus what I've pulled from my carburetor collection. I've been retired for 20 years, so I guess you could say I'm outdated now.
 Unfortunately I don't have any way to tell if any of the springs I have are equivalent to what Cliff offers. If you have an old core that you can use to test with, you can use your engine as a vacuum source with a long hose going to your test carb if you don't have another vacuum source you can use.  You will need a vacuum gage of course. The valve I used is for an aquarium or I think it might be for a drip watering system. My son and I were doing a hydrogen experiment for one of his school projects many years ago and we had bought a bunch of tees and valves for the project and I can't remember where we got it,  maybe Lowe's hardware? Since then I've been using that stuff for all kinds of things. One valve was used to regulate the vacuum to an EGR valve when I was having some issues with bucking on another truck. I've been collecting parts and vehicles for 50 years, so I have a bit on hand, and I have the space to keep it. You might say I have my own wrecking yard.

[Shiny]

Rick, that's a labor of love that could genuinely help a lot of people if you were willing to share.

I understand it would be difficult to assure apples to apples, but if you're willing, pull out your calipers and add the spring dimensions to your table.  For example, if you add the length, coil OD, the wire diameter, and the number of turns, it would be relatively easy to match across different sources.  There are online calculators that could even estimate spring rates.

I've attached a table I used when I was trying to make a reasonable guess on selecting a PP spring.  I don't know where it originated, but someone like you did a lot of similar work and tossed it into the ozone.  Thanks to the originator and perhaps it may help you.

I used the same approach as you on both my distributor advance and on the needle/seat holding power against fuel pressure (trying to diagnose a flooding/nozzle-drip issue).

So I like your effort to actually quantify these things!  Beats the heck out of folklore!  And with the chaos in the aftermarket parts world, my confidence in the distributor advance specs was sketchy at best.

Mike

[GSP7]

I tested Cliffs 4 springs , light blue, orange, long green, plain color, with the stacked quarters($.25) weight method on top of the power piston with each sprg. set in a stripped main body  .

Vacuum test like above would be better though 

[68rs/ss]

Interesting. How did you determine which spring to use in a particular engine? I am thinking you would need to choose a spring that would hold power piston down at cruise. Giving some throttle or increased load on engine would drop engine vacuum. Did you pick a spring that started to lift power piston as vacuum dropped or choose one that lifted after a couple of inch's drop of vacuum? I Had tried this very thing with a spare core , but did not have a motorized vacuum pump for constant vacuum other than a hand vacuum pump, which didn't work. Good stuff. Phil

[Cadman-iac]

 Hi Mike,

 I could do that, it will take a while to get all the information and post it, but that makes sense. Better to share the information with facts and dispell the rumors.
 The only issue is how accurate my vacuum gage is.  Another thing that may affect the results between mine and yours and others is the altitude, I'm at approximately 4600 feet, so at sea level would that change the reading a bit? Just curious.
 I'll start on this tonight, good idea, and thanks for the table on the springs, that could prove useful.

 Gsp7,
  I've read about that method, but I couldn't relate the washer weight vacuum readings, they didn't really correlate to me.
 I can't recall where I read about the method I used, it may have been the same place I saw that washer idea. I had been looking at the digitalcorvettes website I found after asking a question on Google.

 Between them and Cliff's website I think you can find anything you need on a Q-Jet, but Cliff sells parts, they don't!
 
 I'll get back with what I find when I'm finished. Thanks for the suggestion.

 Rick

[Cadman-iac]

Hey Phil,

 I have a vacuum gage installed in my Suburban and watched it as I changed throttle position while driving.  Taking the average cruising vacuum rate, and the lowest reading when going WOT, I chose a spring with a  maximum vacuum about 2 inches below the cruising rate, and just below the minimum vacuum at WOT, so the piston will raise completely at WOT but remain down while cruising.
 Up until I found these sites, I had to rely on what was in the carb when I started and hope it was correct.
  I wish I had known all this stuff years ago, it would have made my life a lot easier.

 Rick

[Cadman-iac]

  Mike,

 One thing that I noticed while testing the springs was that some would read the same but look very different.
 I'll have to go through them to give you the  specs on them, but I didn't expect that.
 One had closely wound coils and short, while the other had wide coils and longer if memory serves. I can be more precise after I get the information compiled.

 Rick

[Cadman-iac]

Mike,
 Another question, what format should I use to lay all this out? I'm only writing it down in a notebook and then typing it out on my phone, so what would work best if you were going to use the information?

  Thanks, Rick

[77cruiser]

I did something similar, didn't have an AC vac. pump, I had an old air compressor plumbed the intake side to the tank, I think it would go to about 22 inches of vac.

[Cadman-iac]

I did something similar, didn't have an AC vac. pump, I had an old air compressor plumbed the intake side to the tank, I think it would go to about 22 inches of vac.

 I would think that would be sufficient for testing your springs as your engine doesn't make that much vacuum even in deceleration except maybe at sea level. Here at 4600 feet I'm lucky to get 20HG on decel.
   Rick

[Cliff Ruggles]

Power piston springs are there to lift the PP with quick or heavy movements of the throttle based on engine load or when quick acceleration is needed.

This allows us to set the carb up for lean operation at very light engine load and quickly richens things up when engine load is increased.  As a side note do NOT forget about the VA canister here as it also adds additional timing to allow us to effectively burn that lean mixture, but only when engine load is very light and engine vacuum is really high.

I don't want to get into writing a book here but most if not all tuners don't incorporate the VA as part of this tuning or look at it's cut-in and full apply points when tuning with carburetor PV's or PP springs.

Anyhow, it's easy to over-think PP spring purpose and use.  The springs have a cut-in point and opening rate. This is based on the number of coils, length, and wire diameter.

I offer a kit with four springs, color coded.  They were designed and tested to offer the tuner options for various set-ups.   

I've found with very few exceptions that using the strongest spring that stays down at idle vacuum has proven to be the best choice.  This simply happens because ALL power piston springs will have the PP down at high vacuum and light engine load. 

When you quickly go to heavy and full throttle ALL PP springs will push the PP to full height as they should.  At the same time the accl pump adds fuel to avoid a lean condition.

The carburetor, by design will also richen up the mixture due to the pressure differential created above and below the venturi areas and increased air flow thru the carburetor. 

Matter of fact you can completely eliminate the PP and primary metering rods and select the ideal main jet size and the carb will function flawlessly in all areas.  You just woln't get the precise metering control via using the PP and metering rods especially if your carb has an APT system.

I know that's hard for many reading this to get their head wrapped around, but think of how many MILLIONS of carburetors that are out there in use on various applications that run straight off a main jet and or a pilot jet system and don't use metering rods or any kind.........some food for thought......

[Cliff Ruggles]

I'd also add here is that you can use your carburetor to see what the PP and spring are doing for you with no rigging of any kind.

Cut a small piece from a plastic drinking straw and insert it into the front vent (not possible on some Pontiac carbs but will be on all others).  Make sure it's long enough so you can see some of it when the engine is off and PP at full height.

The straw will sit on the PP and move up and down as it does.

Attach a vacuum gauge to a manifold vacuum source.

You can then watch the drinking straw and vary engine vacuum to see what it is doing. 

Engine vacuum can be raised by increasing engine RPM and also engine timing.  Lowered by slowing the engine RPM's and/or retarding timing. 

It also helps to mark the drinking straw with a magic marker to note it's full up and down positions.

Using this method you can see what the PP is really doing in a dynamic scenario on a real engine.......