Fuel pressure for a naturally aspirated BBC

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OLD MAN JONES
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Fuel pressure for a naturally aspirated BBC

Post by OLD MAN JONES » Fri Feb 08, 2013 9:54 pm

I have a BBC 509 Merlin, that produced 620hp @ 5500 rpms and has an 850 Holley Marine carb on it.

? is, what is the correct fuel pressure at or near WOT?
I am having a hard time to get it to produce more than 4500 rpms, with a 2.37\1 gear and a 85" 3 blade R. The prop pitch has not changed and it would turn 5100 with the previous motor, which dynoe'd at 70 less hp and torque.... What's up with that!
Timing, plugs, wires, and all else seem just right, but I am currently only getting 4.5 Psi @ 4k + RPMS. Is this my problem?

Thanks to all
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Rich Andrews
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Re: Fuel pressure for a naturally aspirated BBC

Post by Rich Andrews » Fri Feb 08, 2013 10:41 pm

You've got a BBC Merlin...did u hit the lottery?

Something seems wrong,real wrong cuz that's a bad motor...real bad. Got any specs to look at?

Heads? Cam? Carb? Pics?
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Re: Fuel pressure for a naturally aspirated BBC

Post by pipedog » Fri Feb 08, 2013 11:01 pm

Hey Old Man I meet you at the worrior ride in 2a, I gave you the throwable floatation cushion. I have been a drag racer for years (I know airboating is a diff animal, but HP is HP) and you should be fine with that fuel pressure. I had a fuel problem once, the car ran awsome (lean is mean, but will burn up a motor) so I didn't know it and burnt some pistons(learned a good lesson). Your probably at the minimum pressure, but my experience you should still make full power. If everything is good how about you timing? Check and see if it is set the same as when they dynoe'd it. One time I had a motor lose power cause of a worn cam lobe, hope that is not your problem! Other than that I don't know :dontknow: I hope you find the problem. It was nice to meet you and hope to get down your way one of these days :thumbleft: Ps all dyno's are said to be different, but thats a big difference from 5100 to 4500.
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Deano
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Re: Fuel pressure for a naturally aspirated BBC

Post by Deano » Fri Feb 08, 2013 11:36 pm

OLD MAN JONES wrote:. . .has an 850 Holley Marine carb on it . . . I am currently only getting 4.5 Psi @ 4k + RPMS . . .
I believe that is your problem. That carb won't have any pressure problems until after you get above or beyond 7 psi. I would guess that an engine that size might be OK at 5.5 or 6, but am reasonably sure 4.5 psi ain't gonna get it. The bigger Holleys are designed to run at about 5 -7 psi. A healthy big block can start having problems with only 4.5 psi of fuel presure, and that can happen with less motor and less RPMs than what you are expecting.

With that having been said, volume and pressure are inversely proportional so if the fuel lines, fittings, etc are smaller than 3/8", that is likely the problem. Pressure won't offset volume at higher RPMs. Hell, for all I know, over 600 hp may need 1/2" line to avoid the problem you're having. I don't know about that, I expect others here will though.

Not to say that it couldn't be something else, but given what I've seen and what you are describing, I would expect the RPMs to go up when the the fuel delivery (pressure and/or volume) does. What kind of fuel pump are you running?

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Re: Fuel pressure for a naturally aspirated BBC

Post by rbjscott » Sat Feb 09, 2013 12:34 am

6.5 Lbs. is what carb. builders have told me for my 502. Call Holley tech. line or send E-mail question.
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Re: Fuel pressure for a naturally aspirated BBC

Post by air-roberts » Sat Feb 09, 2013 5:03 am

yes me too i'd like to see 7 lbs i even run up to 10 lbs oin my loig when running nos
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Re: Fuel pressure for a naturally aspirated BBC

Post by never enuff » Sat Feb 09, 2013 11:34 am

needs more fuel pressure,at least 7 psi. or more

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Re: Fuel pressure for a naturally aspirated BBC

Post by pipedog » Sat Feb 09, 2013 5:33 pm

I agree with Deano you need the volume. that size motor you should have a big pump that requires a regulator so you can adjust your pressure. Line size should be a -10 from tank to pump -8 from pump to regulator and 2 -6 from regulator to duel feed carb. that will give you plenty of fuel and be able to adjust the pressure. I'm still not convinced that fuel pressure is his problem. With his nice boat and bad a$$ motor I'm sure he didn't skimp on a fuel system :dontknow: 4.5 is at the minium for sure like I said before, but to lose 600 rpm's :dontknow: Obviously try increasing fuel pressure and make sure you have volume. Good luck and let us know what you find out :thumbleft:
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Re: Fuel pressure for a naturally aspirated BBC

Post by OLD MAN JONES » Sat Feb 09, 2013 10:28 pm

Thanks to all who replied, I knew my o.a. fuel pressure was a lil' low, and I do have 3/8 " fuel ines with a Holley blue pump and regulator. It is now set to 7 lbs and I have adjusted the throttle linkage to now move to full throttle and she has gotten up to 4950 RPMS...
A little better now and it snaps up quick, i just don't understand why a motor with 70 + more h.p. & torque dosen't produce more total rpms. with the prop at the same setting as previous engine? Any suggestions as to why ?
I don't want to get into a pissin' match, just asking for honest opinions from experienced people. I will gladly continue to post results as I go thru the motions.

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Re: Fuel pressure for a naturally aspirated BBC

Post by mcm4 » Sun Feb 10, 2013 3:58 pm

I will add my 2 cents.

Does your new engine have a single plane or dual plane intake. My 555 bbc came with a single plane, when I switched it to a dual plane, with no other changes I gained 400 rpm.

Also, if your engine is set up to produce hp on the street or dragstrip, it might/probably/could have way too big of heads and valves on it, too much air

also, your cam could be producing its torque at too high of rpm, check your lift and duration at 050.

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Re: Fuel pressure for a naturally aspirated BBC

Post by mcm4 » Sun Feb 10, 2013 4:43 pm

I will add my 2 cents.

Does your new engine have a single plane or dual plane intake. My 555 bbc came with a single plane, when I switched it to a dual plane, with no other changes I gained 400 rpm.

Also, if your engine is set up to produce hp on the street or dragstrip, it might/probably/could have way too big of heads and valves on it, too much air

also, your cam could be producing its torque at too high of rpm, check your lift and duration at 050.

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Re: Fuel pressure for a naturally aspirated BBC

Post by pipedog » Sun Feb 10, 2013 8:26 pm

OLD MAN JONES wrote:Thanks to all who replied, I knew my o.a. fuel pressure was a lil' low, and I do have 3/8 " fuel ines with a Holley blue pump and regulator. It is now set to 7 lbs and I have adjusted the throttle linkage to now move to full throttle and she has gotten up to 4950 RPMS...
A little better now and it snaps up quick, i just don't understand why a motor with 70 + more h.p. & torque dosen't produce more total rpms. with the prop at the same setting as previous engine? Any suggestions as to why ?
I don't want to get into a pissin' match, just asking for honest opinions from experienced people. I will gladly continue to post results as I go thru the motions.

:salute: :old_glory: :salute:
Old Man Jones you should look up fuel pumps and see what size pumps substain how much HP! The holley blue is on the small size for the horse power your motor makes! Just trying to inform you.
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Re: Fuel pressure for a naturally aspirated BBC

Post by wilemotorsports » Sun Feb 10, 2013 9:24 pm

YOU SHOULD HAVE ABOUT 32/36 DEGREES OF TOTAL TIMMING ON THAT MOTOR IF YOU HAVE A VACIUM ADAVANCE DISTRIBTOR YOU SHOULD DISCONECT THE VACIUM ADAVANCE AND HAVE WHAT THEY CALL TOTAL TIMMING OF 32 TO 36 DEGREES WHEN CHECKING THE TIMMING . ALSO IF YOU HAVE A FLAT TAPPET CAM IN THE MOTOR CHECK TO SEE IF YOU HAVE SCRUBED A LOB ON THE CAM SHAFT . IF YOU HAVE A HYDROLIC ROLLER CAM YOU WONT HALF TO WORRY ABOUT THIS PROBLEM . BBC ARE BAD ABOUT WIPEING OUT THE CAM LOBES ON BRAKE IN PERIODS BECAUSE THE EPA HAS TAKEN THE ZINK OUT OF TODAY OIL GOOD LUCK MARSHEL

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Re: Fuel pressure for a naturally aspirated BBC

Post by bonecollector » Sun Feb 10, 2013 10:30 pm

The holley blue pump should run that motor I had a big block ford built with 850 hp and ran the blue pump with no problems.Also Glen Treadwell built my motor and told me to use amsoil z rod oil in it because of it's high zinc content. The off the shelf oil is junk now days. I only paid 12 bucks a quart for the amsoil and according to Mr. Treadwell it should last a year of average hours at 50 hours I am going to change the filter and top it off with another quart he also recomended running a racing filter like a wix racing or something like that.I agree with everyone else as far as pressure goes my big block had a Bow Laws 1150 carb and I ran 114 octane at 8 psi never had an issue good luck.
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Re: Fuel pressure for a naturally aspirated BBC

Post by flying fish » Mon Feb 11, 2013 8:21 am

OLD MAN JONES wrote:Thanks to all who replied, I knew my o.a. fuel pressure was a lil' low, and I do have 3/8 " fuel ines with a Holley blue pump and regulator. It is now set to 7 lbs and I have adjusted the throttle linkage to now move to full throttle and she has gotten up to 4950 RPMS...
A little better now and it snaps up quick, i just don't understand why a motor with 70 + more h.p. & torque dosen't produce more total rpms. with the prop at the same setting as previous engine? Any suggestions as to why ?
I don't want to get into a pissin' match, just asking for honest opinions from experienced people. I will gladly continue to post results as I go thru the motions.

:salute: :old_glory: :salute:
Sometimes engines get dyno'd with dominator carbs. Not saying yours did, but some do.

Have you looked at your plugs after a wide open run? If tuned rich, you are not achieving the correct burn and also not producing the most power possible.

I tune with an O2 analyzer. Seems easy after the bung installation in the collector.

Also, all dynos not created equal.
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Re: Fuel pressure for a naturally aspirated BBC

Post by pipedog » Mon Feb 11, 2013 9:29 pm

bonecollector wrote:The holley blue pump should run that motor I had a big block ford built with 850 hp and ran the blue pump with no problems.Also Glen Treadwell built my motor and told me to use amsoil z rod oil in it because of it's high zinc content. The off the shelf oil is junk now days. I only paid 12 bucks a quart for the amsoil and according to Mr. Treadwell it should last a year of average hours at 50 hours I am going to change the filter and top it off with another quart he also recomended running a racing filter like a wix racing or something like that.I agree with everyone else as far as pressure goes my big block had a Bow Laws 1150 carb and I ran 114 octane at 8 psi never had an issue good luck.
The holley blue pump is good to 550 hp.
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Re: Fuel pressure for a naturally aspirated BBC

Post by bonecollector » Tue Feb 12, 2013 4:18 pm

Well it ran 850hp for a whole season granted I only ran 200 feet at a time but it worked fine.
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Re: Fuel pressure for a naturally aspirated BBC

Post by Olson Boy » Tue Feb 12, 2013 10:09 pm

Holley blue may run the motor at 4 PSI but not at 7. You need to upgrade to a black or even better an aeromotive 1000 with a good regulator. 1/2'' line would be beneficial also. A motor that size needs a lot of fuel to run properly.

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Re: Fuel pressure for a naturally aspirated BBC

Post by bonecollector » Tue Feb 12, 2013 11:38 pm

I disagree I think a blue pump will work it may not be the best choice but it will work. I wouldnt comment if I didnt use one myself with no issues.But that's just my opinion.
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Re: Fuel pressure for a naturally aspirated BBC

Post by T-REX » Mon Mar 18, 2013 9:11 pm

Volume AND pressure....
1/2" line WITH decent pressure.....

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Re: Fuel pressure for a naturally aspirated BBC

Post by pipedog » Mon Mar 18, 2013 10:00 pm

T-REX wrote:Volume AND pressure....
1/2" line WITH decent pressure.....
x2... also have the regulator as close to the carb as you can get it
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Re: Fuel pressure for a naturally aspirated BBC

Post by todd glover » Mon Mar 18, 2013 10:17 pm

Call Holley and talk to your engine builder they know carbs and he or she knows your combo good luck

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Re: Fuel pressure for a naturally aspirated BBC

Post by T-REX » Thu Mar 21, 2013 6:40 am

Hi Marshall -got any updates for us?
I wanna see this beast in action! 8)

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Re: Fuel pressure for a naturally aspirated BBC

Post by Nolimit2game » Thu Mar 21, 2013 8:17 am

You need a bypass regulator and a Holley black pump. Blue might get it done with a bypass regulator but why take the chance.

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Re: Fuel pressure for a naturally aspirated BBC

Post by warrior-p » Mon Apr 01, 2013 10:19 am

6 psi on the carb - Holley blue should run it, but the next time you replace, go with a black

what cam you are running in the motor and static compression ratio
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