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| Fuel psi spikes | Login/Join |
DRR Sportsman |
I’ve swapped back to two alcohol carbs this year and I’m having an issue in low gear with fuel psi spiking. -10 line to the belt pump -10 line to a magnafuel 10” long 4 port log with a BLP bypass regulator mounted at the end of the log, 4 -8 lines to the carb. My Racepak shows the fuel psi jumping up and down some but then jumps up 1 1/2 pounds half way through low gear and the RPM will drop at that spike when trying to run over 4 psi. On the Dyno whe had 8.5 psi in upper rpms and showed that in the car also but wouldn’t run. The only way I can get rid of the rpm dip is lower the psi to 4psi going down track. Started out with my Ron’s 0 pump and the blp bypass, changed the bypass to an APD with same results. So I order a new waterman .500 pump and still having the same issue. When the psi jumps up the 02s fall so that tells me what I’m seeing in psi spiking is correct. I took the new carbs to another builder and he did find a few small things but still doing the same with psi. Different pump, different bypass, started out with -8 line pump to log and changed to a -10 pump to log and nothing has phased it. High gear the psi is up and down but the AF isn’t going way rich like it is in low gear. I’ve ordered a Holley 4 port double adjustable bypass to try and remove the magnafuel log. Anyone had or seen this issue ? Getting pricey changing all this and nothing helps. I’m about to the point of putting me Terminator’s back on | ||
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| DRR S/Pro |
best regulator i have run, on a 1000hp gas carb with a 13 gpm enderle pump, same reg was on a 2300 hp blow thru setup, I pulled my apd reg off for this one.... never a issue https://aeromotiveinc.com/prod...le-bypass-regulator/ J.R. Baxter ""Fathom the hypocrisy of a Government that requires every citizen to prove they are insured ..but not everyone must prove they are a citizen." 2024 Miller Rolla Competition Engines ProCharger Hoosier Tires Abruzzi | |||
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| DRR Pro |
I had a fuel pressure problem when I switched from MFI to EFI this summer using the same 0 pump with a Holley EFI fuel regulator. I switched to a 0-1/2 pump and overdrove it 10%. Better but still not correct. I contacted Aeromotive Tech department and they recommended the correct regulator that fixed it. | |||
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| DRR S/Pro |
Dual carbs present there own fuel issues vs a single carb... I usually find that higher float levels and higher fuel pressure vs a single carb setup help. When I ran a mechanical pump I fed it with #12 and then #10 on the pressure side. Another thing to look at would be your fuel return line, does it just have a bulkhead fitting on the top of the fuel tank? If it does then you can be aerating the fuel in the tank, more of an issue with smaller tanks and if the return line is close to the feed line. Have you checked the needle and seats, 8.5 psi sound high to feed a motor with 2 carbs. Also how old are the floats, I assume they are the black foam type? If they are old they can become soaked with alky which makes them heavy and requires a lower than normal float setting relative to the needle & seat, this eventually causes fuel starvation, extremely lower fuel levels in the bowls, due to limited float drop,,, when this happens the needle can't drop enough to clear the seat to allow full fuel flow because the float is resting on the bottom of the fuel bowl. This is caused from an alky soaked float. I assume the obvious things have been check such as the tapper floats facing the rear of the car, tank vent line large enough and free flowing etc? | |||
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| DRR Trophy |
I didn't see where you said what size return line you were running from the regulator. It needs to be at least the same size as the feed line from the pump to the regulator. Some will say to run one size bigger than the feed line. | |||
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DRR Pro |
top 38 is dead on honesty is the best policy,insanity is a better deffense 1.036, 6.16@ 224 | |||
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| DRR Sportsman |
Tank is a 2 gallon mounted up front and the bulkhead is in the top. Same as the injection was mounted. Carbs are brand new and i did change to the APD bottom feed bowls and those do have a wedge float. Return line is #8 witch is smaller since I changed the pump to log to 10 but it did the same with the #8 line feeding it. I bought the pump from Rupert and he recommended 10 feed and 8 to the log with 8 return. Rupert is also the one i took the carbs to for him to see if he could find something wrong in the carbs and only found a few issues that wouldn’t cause whats going on. I have a smaller pump gear so i may try speeding the pump up if the new regulator doesn’t fix it. BLP and Rupert said the .500 pump was plenty big enough to feed 800 hp small block. In my head im thinking the line and big log just and the #8 lines to the carbs may be to much area for the pump to keep full so speeding up the pump or the smaller area of the bypass and removing the log may help if thats the issue. As far as psi as is the only way it will go down the track is 4 psi in high gear and the floats set so fuel is at the top of the sight glass. Any higher psi the spike causes what i assume is a flooding condition and the rpm drops, when the psi goes back down the rpm levels out rest of the pass. AF looks like chit in low gear and the car cant be consistent as is | |||
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| DRR Sportsman |
Vent is #8 and not blocked. It’s odd that the fuel psi with injection was so smooth all the way through the run and with the carbs its moving up and down 1/2-3/4 pound | |||
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| DRR Pro |
Dumb question because I don’t know anything about carbs. Why are 2 carbs needed for an 800 hp sbc? Does only 1 carb produce less HP? Is that the advantage to using 2 carbs? | |||
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| DRR S/Pro |
They aren't with regards to air flow however it does help for fuel flow, an 800 HP alky deal is a 1500 plus gas hp deal with regards to fuel flow so 4 needle and seats are better than 2! | |||
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| DRR Sportsman |
They are good for a little hp 25/50. My main reason for the intake and duel injection/carbs is consistency. Yes a single can be deadly but ive proved to my self the sheet metal intake and duals are flat nasty if everything is right. Car was the best its ever been with the twin terminators if I didn’t miss the tune. My issue with the injection was changing tune with weather and elevation changes. Was hoping the carbs would have a tune for cold/cool and warm/hot | |||
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| DRR S/Pro |
A 2 gallon alky tank is too small unless your towed up. Dumping bypass fuel back to the tank without a dip tube can also be an issue. On your fuel bowls, if those are the ones that were designed to remain fuel of fuel, every racer that I know that got those when they came out a few years back has taken them off. Don't know the particulars but they had issues and had to run a bunch of fuel pressure. Something is wrong for sure and it's a fuel flow issue which is why it shows up in first gear the worst, highest fuel demands. Your missing something stupid in the fuel system. | |||
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| DRR Pro |
Ok, I understand this now. Thank You. | |||
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| DRR Sportsman |
Ive had the 2 gallon cell since 98 when i built the car. With a single carb or the injection never an issue. It’s something small im sure but finding it is gonna drive me crazy. The carbs came with billet bowls but i knew right away i had an issue with them. Fuel was at the bottom of the sight glass and wouldn’t raise when raising the floats. Called the original builder and he said they wouldn’t raise until i got the psi up in the upper rpms. I said ok ill try it but i knew better. Floats set at .450 with 3 psi will raise if you set them at .550 with 3 psi. After the first week i ran it i ordered the APD bowls (no foam) and the fuel would raise as it should but still didn’t fix whats going on. | |||
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| DRR Sportsman |
I called Waterman/Aeromotive and they say the spike is from to much volume and the bypass isn’t big enough. They want a 10 bypass. The regulator I’ve ordered is set up for a 10. He also said I could change the crank pulley from 18 to 16 tooth and slow the pump down to help the issue | |||
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DRR Pro |
Not to hijack the thread my question is if your mechanical fuel pump is good up to 1200 hp but motor makes over 1350 what symptoms will you see? BBC 655 SR20. 4 different Convertors, different cam, different intake, and it runs the same. Makes no difference what we change. We are getting the same ET and MPH. Being told the mechanical pump is the problem, wants us to go to a electric fuel pump. About .1 off ET it should run. Does that make since of pump being to small???? Thanks in advance Brian Mollison 2013 American Dragster 2020 Sumerduck Dragway VA Top ET Champion 2017 Sumerduck Top ET & SSS Track Champion 2017 MAC Top ET Track Champion 2009 Sumerduck Dragway Va Top ET & IHRA SSS Track Champion 2008 IHRA Division 1 Top ET Bracket Final Runner-up 2007 Colonial Beach Dragway Top ET Track Champion | |||
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| DRR S/Pro |
I have to say that although not impossible, but its very likely the motor makes much less than 1350 HP. Without more info I would say the items you changed are not the restriction! If your having fuel delivery issues you would likely see them at their worst in the top of first gear, this is where the motor sees its highest fuel delivery demand due to high rpm/second acceleration rates vs in high gear. | |||
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| DRR Trophy |
I agree with Top 38. Have you changed what's on top of the intake? | |||
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| DRR Pro |
It’s a APD 1600 CFM 2.5” blade carb. Have tried a 1680 carb and ran the same ET and MPH. Only thing we haven’t changed is the mechanical fuel pump and it has a fuel log Brian Mollison 2013 American Dragster 2020 Sumerduck Dragway VA Top ET Champion 2017 Sumerduck Top ET & SSS Track Champion 2017 MAC Top ET Track Champion 2009 Sumerduck Dragway Va Top ET & IHRA SSS Track Champion 2008 IHRA Division 1 Top ET Bracket Final Runner-up 2007 Colonial Beach Dragway Top ET Track Champion | |||
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| DRR S/Pro |
If it were as easy as bolting on larger and larger sized carbs we would all be running with top fuel! Well maybe not quite that fast,, lol. The bottom line is the carb has to pull fuel to make good power, CFM alone is not enough. On a similar motor a smaller 2.350" throttle plate carb made 15 more HP than a 2.400" carb! Provide specifics of the changes of the intakes and there state of porting, cam spec's etc. What fuel are you running, weight and gearing of the car. Dyno HP and Torque at what RPM, etc. | |||
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