Ok Carb setup that has the pressure set at 7psi vs 9psi.
Bowls are vented to the atmosphere and assuming seats are not blown off with the 9psi. Also assume that fuel volume is sufficient in the application to maintain fuel bowl level at WOT.
What effect does that the pressure have on the AFR/performance of the car?
In my mind none. In a fuel-injected setup when the pulse width of the injector at X; with more pressure, you will get definitely more fuel delivery in that timeframe. But with a carb I don't see it. So why all the fuss?
exactly why all the fuss?
Both my cars run gas, have 6.5 fuel pressure going down the track and ~ 12.7 AFR at the stripe.
Are you talking about gas or alcohol. Gas being much more sensitive to tuning changes then it can possibly need an emulsion adjustment with the higher pressure because the fuel would be higher in the bowls. If you tune to that pressure then it isn't necessarily an issue.
I don't see why anyone would need to run 9lbs pressure on gas though.
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Not really sure what your asking here. First it sound like your trying to compare fuel pressure in a FI setup vs a carb, if so it's apples and oranges! One has nothing to do with the other. Ya, the so called float level may be higher with 9 psi vs 7 but have you ever seen what the fuel is doing in the bowls on a run? The difference in fuel levels in the carb bowls at 7 vs 9 on a run is like trying to set your float levels on your car sitting on a boat in the ocean during a hurricane! In a so call static condition which doesn't exist on a running motor with a carb, even on the dyno, the higher float level would supply more fuel to the motor for a given signal, but it's no different than filling a 5 gallon bucket and then adding 5 drops more, the change is insignificant!
Based on these 2 assumptions, there should be no difference in performance, nor any reason to change anything else, IMHO.
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The application is alcohol. But don't see if making a big difference on gas either. Unless float level is changed and in the assumptions, it is not.
The comparison to EFI was not to compare the two but to say in an EFI situation it DOES matter on pressure.
I was basically just confirming that the actual feed pressure is NOT that significant. The assumption of adequate flow would address keeping the float level at the same level in both instances. While I understand the hurricane thoughts the level being the same and correct address that...
At the end of the day, I am continually hearing guys talk as if a small pressure change on a carb is making a difference. I never agreed with this thinking but thought perhaps I was missing something.
Line #5 from Mark.W.. The man with the plan that just plain works on foaming /G force issues with fuel. lower pressure cuts down on foaming.
On the dyno with my alky carb, we were able to make the engine noticeably richer/leaner with pressure changes.
I have to imagine that the same changes would be twice as impactful with gas.
I would say there is no effect on A/F as the fuel pressure is only keeping the bowls full. My problem with the question is your 2 assumptions.
Hotrod or perhaps the National Dragster had a fuel pressure carb test where they found the bowls were not staying full and higher pressure did improve the ET & MPH of their super stock test car.
Higher "pressure" on a regulated mechanical pump just means that you lessened the restriction before the inlets. A bypass style regulator is really the way to go to maintain steady fuel supply at the needle and seat.
Pressure and volume are 2 different things.
If the inlet needle is working there's no rise in the level, but the fuel can be more agitated and effecting the emulsion circuits.
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the bowl level does affect richeness.i have seen where raising the pressure would keep the bowl full.once you are able to keep the bowl full there should be no difference at 7 or 9 unless the float allow the fuel to be at a higher level in the bowl..as we all know there is many variables in this equation.
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Volume through an orifice is directly proportional to pressure. Therefore, the bowl will fill faster with the same float drop with more pressure. This video shows the chaos inside the bowl during a run. https://youtu.be/s-q1eJu1Wz8 The front bowl becomes aerated as the fuel drops below the n&s. The rear jets would surely suck air if not for the jet extensions.
I would argue spiking the pressure to 11psi during full throttle would be a good thing. I have done this in the past with good results. Then again it was the setup I started with and have no clue if I fixed anything. It seemed to work just fine. The video shows there is a lot more going on then just keeping the bowl full.
That's my experience of 10 years running a chassis dyno - raise the fuel pressure on a carb, the float level goes higher,fueling gets richer.
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