Not enough fuel flow for 572? - Team Camaro Tech
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post #1 of 15 (permalink) Old Aug 28th, 16, 06:39 PM Thread Starter
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Geoff
 
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Not enough fuel flow for 572?

Hi guys,
I installed my 572 from Smeding performance over the past few weeks in my '68 Camaro and I'm troubleshooting some power issues. If I get on it in say 3rd or 4th I'll feel it start cutting out slightly (like a misfire) between maybe 2500-3500 rpm. I've checked plugs and wires and they all look good. It's got a mechanical "high flow" fuel pump on it with 8AN lines throughout and a 1/2" pickup in the tank. I've got clean filters (checked them last night) and up top is a fuel log with a Holley billet return style regulator on the firewall side of the log. I've got the spring cranked down and the best I can get is just over 6 psi. My pressure sender is right at the front of the log so I can monitor pressure on the dash. I can see it drop at times to 4 psi when I'm on it pretty hard. This is all below 4000 rpm since I'm waiting to work out little things like this before I try taking it up higher. Shouldn't an adequate pump maintain pressure below 4000 rpm? I'm wondering if it's not big enough for this motor. I don't have details on it but can contact smeding to see what it is. They dyno'd it with an electric pump at 7 psi and it made great power.
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post #2 of 15 (permalink) Old Aug 28th, 16, 07:44 PM
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Don
 
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Re: Not enough fuel flow for 572?

Carb size, 850/950 ? Need more WOT fuel pressure.

cid x rpm / 3456 just for starters, You do the math.

Don
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post #3 of 15 (permalink) Old Aug 28th, 16, 08:10 PM Thread Starter
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Re: Not enough fuel flow for 572?

It's a quick fuel 850. The equations tell me minimum fuel needed I realize that I'm just surprised by the pressure occasionally dropping a couple pounds. Makes me think either the pump isn't flowing what it should or the regulator is allowing too much flow on the return side.
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post #4 of 15 (permalink) Old Aug 28th, 16, 08:34 PM
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Re: Not enough fuel flow for 572?

Quote:
Originally Posted by ggsz28 View Post
It's a quick fuel 850. The equations tell me minimum fuel needed I realize that I'm just surprised by the pressure occasionally dropping a couple pounds. Makes me think either the pump isn't flowing what it should or the regulator is allowing too much flow on the return side.
I believe pressure is just part of the equation, fuel volume is needed here. I presume the electric pump you dyno'd was putting out maybe 110 gph or more. You must have had the correct psi / volume to support the dyno so since you are back to reality in your car, you may see whats missing.

Let's see what the other guys say that have this much motor for the street.

Don
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Zdld17:69 Z/RS,306, NOR141111, 9N554XXX, 12A, X3G, 59/59,723, AFR 195,CCC282/290HR, TKO 600, BU1122B1E Owner since Dec 1968

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post #5 of 15 (permalink) Old Aug 29th, 16, 06:28 AM
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Re: Not enough fuel flow for 572?

Do you really need a fuel return line? Try it blocked off. If that is too much fuel, try a fuel regulator valve and adjust

Till it runs right. It seems you have 1/2" fuel line. That should be plenty of volume. Years ago I had hilborn fuel

Injection on my 57 gasser. It would quit running at 6,000 rpm. Finally figured out Too much fuel was returning

Back to tank. Just my own experience. Hope it helps

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post #6 of 15 (permalink) Old Aug 29th, 16, 06:59 AM
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Re: Not enough fuel flow for 572?

Quote:
It's got a mechanical "high flow" fuel pump ... fuel log with a Holley billet return style regulator
Unless we are talking about a cam gear driven positive displacement fuel pump used with alcohol and nitromethane mechanical fuel injection your bolt to the block pump has a single spring above the diaphragm that pressurizes the fuel at six psi.Including a pressure regulator with this system is just adding a restriction to the fuel line.

Quote:
with 8AN lines throughout and a 1/2" pickup in the tank
Just like the weakest link in a chain the smallest ID line that you use is the restriction in your system. If you used a half inch line for the supply up to your fuel pump the carb only sees what can be forced through the 8AN (3/8th inch ID) fuel line.

Quote:
I'm just surprised by the pressure occasionally dropping a couple pounds.
Fuel pumps trade pressure at dead head (idle) for total volume at zero head pressure (the fuel line level with the pump pick up open to the atmosphere) as your fuel flows through the carb. Keep in mind the pressure inside the fuel bowls is zero as it is vented to the atmosphere. There is only fuel pressure to the inlet to induce fuel to flow.

Quote:
Do you really need a fuel return line?
Yes if you have an electric fuel pump, no if you are using a block mounted mechanical fuel pump. An electric fuel pump is not pressure regulated the way a mechanical pump is. It requires an external pressure regulator which is a spring powered needle and forced into the seat that is adjustable by preloading the spring. You drop pressure by restricting the size of the orifice that the gas has to flow through. What is not consumed (allowed through the regulator) needs to be returned to the tank to allow fresh fuel to be drawn and pressurized to the full pump pressure.

Finally no one has mentioned it as of yet, but a BBC is not terrible efficient. It normally has a BFSR (Brake Specific Fuel Ratio) of 0.737 to 0.824 compared to a 0.500 for a SBC. The BFSR is the ratio of gas to horsepower and is the flow rate measured in pounds of fuel per hour compared to the total horsepower produced. Since gas is fairly consistent in weight (though it is a mixture and as such it varies) of around 6.2 pounds per gallon so you can calculate how many gallons per minute the motor needs based upon it's consumption rate from dyno testing (the BSFR is one of those numbers that get printed on the dyno sheet for your use). A Top Fuel motor consumes 71 gallons per minute: your mileage may vary.

Big Dave
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post #7 of 15 (permalink) Old Aug 29th, 16, 07:17 AM
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Re: Not enough fuel flow for 572?

What Dave says and Boyle's Law - Pressure is restriction to flow.
Mechanical pumps provide volume of fuel upon every stroke in filling a bowl, like pouring milk into a bowl of cereal.
Remove the regulator and pump should fill a quart bottle in approximately 1 minute at idle rpm.
The added benefit of a return line is keeping fuel in motion preventing the fuel from vaporizing - bad event as gasoline is now a gas.
Pumps and carbs don't do a gas (fog/mist).

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post #8 of 15 (permalink) Old Aug 29th, 16, 07:44 AM Thread Starter
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Thanks guys. The regulator is downstream from the carb such that it's regulating pressure on the upstream (carb side) and has a return line to bleed pressure. In other words fuel doesn't flow through the regulator before getting to the carb. I understand that the bowls are at atmospheric pressure so would it be accurate to say that as long as I see some positive pressure (at least a couple pounds of pressure) at the rail to continue filling the bowls then I shouldn't be starving it for fuel? 8AN should be big enough for this setup. I listed the pickup size in case there were questions about it being a restriction. Honestly I'm used to fuel injected motors and this is my first carb motor I've actually worked on.
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post #9 of 15 (permalink) Old Aug 29th, 16, 08:21 AM
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Re: Not enough fuel flow for 572?

The fuel pressure gauges (liquid filled) are notoriously inaccurate and when they get hot from engine bay heat can/will read zero even with 6-7 psi.

I don't think lack of fuel is the problem.

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post #10 of 15 (permalink) Old Aug 29th, 16, 09:29 AM
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Re: Not enough fuel flow for 572?

One troubleshooting trick you can do -
Take Camaro out as normal to a less traveled road,
Do you're power run,
When it cuts out, leave foot on pedal in same spot - turn of ignition and coast to side of road,
open hood, remove a bottom bowl screw to see if fuel drains into cup.
No fuel - no power - decrease pressure setting
Has fuel - no power - check ign coil temperature, it may be overheating - swap in a cold coil brought along.

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post #11 of 15 (permalink) Old Aug 29th, 16, 10:20 AM
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Re: Not enough fuel flow for 572?

I would have to agree it's not a fuel problem .At that low of an rpm it might be the other way around if you have an automatic and weak gears. How does the car act in first or second wound up a bit and you floor it?
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post #12 of 15 (permalink) Old Aug 29th, 16, 02:03 PM
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Re: Not enough fuel flow for 572?

When I was running out of fuel nearing 7500rpm in drive at about 120ish (at the strip) it was like the new car speed limiters, just lost power, but pumping the gas peddle would make it surge....this is how I found the issue.

Aeromotive SS pump off my buddies 9.teens 565BBC Nova....no issues now. (yes, electric pump)

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post #13 of 15 (permalink) Old Aug 29th, 16, 02:40 PM Thread Starter
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Re: Not enough fuel flow for 572?

Thanks for the tips guys. This is an electric fuel pressure sender (not mechanical). My next step will be to block off the return line and see if it makes a difference or to just pull the regulator off completely. If not then I'll move back to ignition which is what I thought it might be at first. I can try a different coil for pretty cheap. This new motor certainly generates some heat under the hood, at least a lot more than the 350 that was in it before and the coil is sitting on top of the intake. This is a new MSD blaster 2 coil but I have heard of them having issues out of the box (on some different forums). The only other thing I can think of which I don't believe to be the culprit is the TSP ready-to-run distributor. I did a small amount of welding on the body and exhaust after installing the motor without disconnecting the battery but I wouldn't think it would mess up the ignition module in the distributor.
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post #14 of 15 (permalink) Old Aug 29th, 16, 06:10 PM Thread Starter
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New development...checked my voltage across the primary side of the coil and it's only 3.8V! I'll run a new switched wire off the fuse panel later this week. The coil is only 0.7 Ohm which helps but if it's < 4V then the high voltage side will be 1/3rd of what it should be as well.
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post #15 of 15 (permalink) Old Aug 31st, 16, 09:49 PM Thread Starter
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Well the coil voltage turned out to be fine. Measuring it with the engine running on the primary leads doesn't work because it's a fast switching ground so my multimeter was just reading an average of 4V. I verified it's getting 12V by testing the negative lead to a ground. I also checked the resistance on the coil pri and sec windings and it's fine. Tonight I removed the fuel pressure regulator and blocked the return line with no luck...still feels like it hesitates below 4000 rpm and is pretty sluggish under 2000. I'm reading over 7 psi at idle now and maybe 6 while on the gas pretty hard. At this point I don't know what to do unless the ignition module in the distributor is bad.
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