Idle Check Pressure

Discussion in 'PSI Superchargers Tech Questions' started by Fuel Cars, Jan 27, 2019.

  1. Fuel Cars

    Fuel Cars AA/AM

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    In general, does raising the idle check pressure richen the idle system?

    Also, I understand that the amount of leak down will need to be adjusted but is there generic rule for how much based on idle pressure increase, i.e. 8 psi to 10 psi, 80% to ?

    Thanks in advance for answers, rookie still learning.
     
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  2. Mike Canter

    Mike Canter Top Dragster
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    You pretty well hit it right on. The norm is 8.5 to 9.5 with 82% leak down. The percent leakdown will put you in the correct area but then you should note throttle response off of idle. If the snap of the throttle results in a slight hesitation then you are lean, if it blubbers then it is too rich. You want a smooth aggressive snap of the throttle response. If your eyes are watering during warmup then you maybe a little rich. Once it is right then measure the leakdown and make a note where it is so you can always check it and adjust it. Now when I say 82% I am saying idle return capped off and BV outlet hose disconnected. Also depends on the leakdown tester you are using.
     
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  3. Mike Canter

    Mike Canter Top Dragster
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    Oops, just read your question again. The answer is no. If you raise the idle pressure then you will have to reduce the percentage of BV leakdown to be at the correct AFR. So if the idle return is at 15 lbs then the leakdown may have to be 75%
     
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  4. Fuel Cars

    Fuel Cars AA/AM

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    So raising the idle check pressure leans the motor with all things being the same?

    I would have thought the higher the idle pressure, the more fuel in the system since less is being bled off, that's why I'm a rookie, thanks Mike.
     
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  5. Mike Canter

    Mike Canter Top Dragster
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    Nope, don’t believe I said that raising will lean it out. If you increase the idle check spring pressure you will raise the fuel pressure at idle and it will richen the mixture if you do not readjust the BV leakdown to compensate for it.
     
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  6. Fuel Cars

    Fuel Cars AA/AM

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    Mike; ok, got it and thanks again, and, no general rule of thumb for raising the idle check pressure by, let's say 2 psi, reduce leak down by maybe 1%, or, more like percentage of pressure increase to leak down percentage decrease?
     
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  7. Mike Canter

    Mike Canter Top Dragster
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    No general rule other than previous knowledge.
     
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  8. Mark Leigh

    Mark Leigh Member

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    Just following along -- Paul is this a Blown Alky deal with a pedal clutch ? Or ?
     
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  9. Fuel Cars

    Fuel Cars AA/AM

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    Yes, blown SBC on alky but with a powerglide.

    Ever since I ran some bad alky, a series of small stuff have been going bad, the latest is my idle pressure is way down, less than 5 psi and the car idles but it doesn't respond like it used to, I got a new idle pressure assembly and spring kit and have ordered a 0-30 pressure gage to accurately check the springs.

    My question on here was to confirm which way I needed to go, sometimes I get upside down and need a nudge to get right side up, and to me, this is best part about this site, real racers helping others, lots of knowledge here, thanks Will.
     
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  10. Mike Canter

    Mike Canter Top Dragster
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    Did you also buy some pressure adjustment shims. When I say 8 1/2 to 9 1/2 that is crack pressure. I would check all the check valves and see where they are at.
     
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  11. Mike Canter

    Mike Canter Top Dragster
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    Forgot. Check the poppets for sidewall scoring and freedom of movement. They should not leak when below the crack pressure.
     
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  12. superpro453

    superpro453 Member

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    Always a good idea to check that you're not running a Gorr dual circuit BV which needs the Idle Check set much higher...
     
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  13. Mike Canter

    Mike Canter Top Dragster
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    So on the Gorr Dual Circuit BV what is the setup. What do you set the idle return at
     
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  14. superpro453

    superpro453 Member

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    We don't run one anymore (sold ours to Kim Christiansen). From memory, it's around 58lbs. setting it to 10lbs, resulted in a burnt piston which caused the RC gaskets to push out, oli getting under the tyres and a somewhat expensive crash....
     
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  15. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    Raising idle check pressure is another way to richen the fuel system at idle without messing with the barrel valve.

    I have played with this over the years with clutch cars. In Top Alcohol, you can't stage on a two step with a clutch, so spool position is critical to getting the right stage flow at idle. So if you want to richen idle flow without changing your stage flow, you can bump the idle check up some.

    Now another thing to remember if you have some normally open leanouts that have a check valve keeping them closed at idle, you need to make sure they are always higher than your idle check or it will start cracking them as you raise the idle check.
     
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  16. Fuel Cars

    Fuel Cars AA/AM

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    I bought the spring kit from Good Vibrations and it comes with the shims, also, the whole check spring assembly is new.
     
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  17. Fuel Cars

    Fuel Cars AA/AM

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    As a follow up, I tested all the new springs and found one at 8.5 psi, installed it and the idle is great and the response is back, thanks to everyone, specially Will, appreciate the straight answer.
     
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  18. promodracer

    promodracer Member

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    How hi. I run this one.
     
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  19. superpro453

    superpro453 Member

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    According to the document on the Gorr web site - "

    This barrel valve is designed to use a #8 idle check at 65 psi, failing to do so may result in not enough fuel pressure during staging. "

    Looks like I underestimated it in my earliest post.
     
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