General Category > Quadrajet Carb Talk and Tips
Testing power piston springs
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:
--- Quote from: 77cruiser on January 31, 2025, 05:27:08 PM ---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.
--- End quote ---
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.......
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