holly 390cfm carb vs edelbrock 500 cfm

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holly 390cfm carb vs edelbrock 500 cfm

Postby herpapabear18 » Mon Feb 23, 2004 6:14 pm

I have a 1981 ford 300 with about 20,000 miles on it. It is all new with stock size parts. The only mods are a offy c manifold. It has no emissions of cats. I am running a holly 390 4brl carb. I also have a almost brand new 500 cfm 4brl in my shop. What carb is best to run. Also what mods should i do to the carb (jets) to make it work best. I know verry little about carbs so almost everything is new to me with these
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Postby Guest » Mon Feb 23, 2004 8:13 pm

Can't help ya on the jets, but for your motor it seems that the 390 would be more suited to than the 500.
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Postby SuperMag » Mon Feb 23, 2004 9:01 pm

Unless your motor is making 250 HP or more, the 500 CFM carb will do nothing for you, except possibly decrease your mileage. On a stock motor with a properly tuned 390, I would doubt that your secondaries are fully open at WOT as it is.
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Postby broncr » Mon Feb 23, 2004 9:40 pm

I've found that #46 or 47 jets are best for me - at an altitude of 6,000'. The Holley folks say you should run 51's at sea level & decrease the size by 1 - for every "approximately" 2,000" above sea level.

By that figuring, I should be running 48's, but have found them to be too large/rich.

Are you still running the stock exhaust manifold? Changing to EFI's or headers would be your next "best bet".

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Postby caveman » Thu Feb 26, 2004 1:51 am

I have a carter 500 on my 225 slant six. 500 cfm is excessive, but the carter/edelbrock-copied design only actuates secondaries as vacuum demands. My opinion, the carter design is superior to holley for smaller engines--look at the sizing of primaries vs secondaries. spreadbore carters have small primaries, which you run on 99% of driving. Holley is squarebore, all 4 same size. Also, it is so easy to tune the carter--no need to remove top for metering rod changing, strip kits available to set accelerator discharge, vacuum level step-up rods move, jets, etc.
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Postby herpapabear18 » Thu Feb 26, 2004 12:35 pm

anyone have any suggestions to websites on info how to step by step rebuild the holly?
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Postby SuperMag » Thu Feb 26, 2004 2:37 pm

There are a few here about halfway down... http://www.carbs.net/books.asp
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Postby TazzieTim » Fri Feb 27, 2004 4:24 am

On Web site Addo posted for Barry Grant road demon carbs its says. the "525 cfm four barrel is designed for 260ci engines and up and runs with cams of 210 at 50 or less duration".
What surprises me about this is that if this is the manafacturers recommendations for a 260ci at this duration it would mean that it would still be suited to well less that 200 HP (eg, estimate 2V 250 with 210 dur or less between 140-175hp depending on duration). Of course the carb would still be ideal for much more HP than this.
As for the actual advantage over a 390 4bbl who knows?
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Postby TazzieTim » Fri Feb 27, 2004 4:32 am

If it were me with a 300 ci with reasonable HP I would definely try it. The 300 may like the bigger carb bores at low rpm. Fine if others disagree?
Cheers Tim
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Postby MustangSix » Fri Feb 27, 2004 9:01 am

IMO a 390 is a teeny carb better suited for 2.3 liter four bangers. :| Nobody on this board would recommend that carb for a 5 liter V8; why is it the proper size for a 5 liter I6?

I'd try the 500. A vacuum secondary carb won't overcarb the engine and the primaries are not so overwhemingly large that you'll see any degradation in throttle response. If you see a big drop in performance or economy, you could easily switch back.
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Postby broncr » Fri Feb 27, 2004 9:37 am

[quote="MustangSix"]... why is it the proper size for a 5 liter I6?

from the Holley engineer's chart on carb sizing:

300 ci. displacement (requires) 380 cfm @ 5500 rpm .

"add about 10% (cfm) for high performance."

350 cfm @ 5000 rpm + 35cfm for high performance = 385 cfm.

For MOST 300's, a 390cfm is adequate. It's been fine for mine - I've frequently mentioned that I'm not interested in a 5K+ redline - which is where the 390 begins to be insufficient for a hi-po 300.

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Postby MustangSix » Fri Feb 27, 2004 10:54 am

I think the Holley chart is very pessimistic and is intended to keep most novices from overcarbing their cars. More is not always better!

A reasonable range on a 300 would be between 390 to 600 cfm for vacuum secondary carbs. Keep in mind that the secondaries will only open in response to engine airflow requirements, so even at the upper end of that range, you are not really overcarbing. A 500-550 cfm vac secondary Carter or Edelbrock is just about perfect on a relatively stock engine this size, IMO.
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Postby SuperMag » Fri Feb 27, 2004 12:35 pm

MustangSix wrote: IMO a 390 is a teeny carb better suited for 2.3 liter four bangers. :| Nobody on this board would recommend that carb for a 5 liter V8; why is it the proper size for a 5 liter I6?

Noooooooooo! Not again! :lol:

The 300 has a significantly lower redline than the 302. And the heads don't flow nearly as well. So while the engines may be of the same displacement, the "out of the box" appetite for air is significantly less. That having been said, it makes no sense to put anything bigger than a 390 on the 300 unless you've done the requisite head and valvetrain work necessary for the higher airflows.

The Carter/Edelbrock AFB: Yes, it is a simple matter to change metering rods vs. the Holley Jet system. But the AFB is a MECHANICALLY actuated secondary; the secondary throttle blades are linked to the accelerator linkage, and start to kick in at 65% throttle whether you want them to or not. Airflow through the secondaries is controlled by a pair of "air valves" that are regulated by a counterweight. This is a very crude method, and generally doesn't work worth a damn. (Think of the counterweight system on an automatic choke. The old Series "D" AFBs omitted this problematic system altogether.) And a counterweight system is going to be totally useless on an offroad vehicle subjected to all kinds of off-attitude "input."

Another thing to consider is the mixture differences between the primary and secondary sides of a carburetor; the secondaries are jetted richer to avoid detonation problems at full power output. If you overcarb an engine, you are delaying the opening of the secondaries due to the increased air volume necessary to open them. This delays the point at which you introduce the richer mixture, creating the potential for all kinds of detonation problems; overcarbed engines are real buggers to tune and make run right.

The Holley carburetor addresses the need for secondary fuel enrichment through a complex system of vacuum actuated secondary jetting and power valves. While this increases the level of difficulty in tuning the carb, it does provide for a much more precise and efficient way of tailoring the fuel needs of an engine under a variety of different conditions.
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Postby rancherlee » Fri Feb 27, 2004 3:02 pm

At 4000 rpm My Holley 390cfm was pulling 3hg vacumm in the intake and 5hg @ 5000rpm, that IS NOT good and means that It was too small (I had a Mechanical secondary carb so I KNOW all 4 barrels were open). Its ran decent with the holley after LOTS of tuning. I bolted on a Edlebrock 500cfm that I had sitting around and out of the box it ran better than I could ever get the holley to. I went from 13mpg with the holley to 15-16mpg with the edlebrock and It held 0-1hg vacumm at 4000rpm and 1-2hg of vacumm at 5000 rpm WOT and pulled better from 4000-5000rpm.
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Postby SuperMag » Fri Feb 27, 2004 3:08 pm

I see from your sig that you've done the headwork. What all did you have done?
(It should be noted that the original poster has an essentially stock engine.)
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Postby TazzieTim » Fri Feb 27, 2004 3:39 pm

Have a look at this if anyones interested. its a carb sizing test done on dyno
http://www.chevyhiperformance.com/techa ... index.html
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Postby SuperMag » Fri Feb 27, 2004 5:14 pm

I saw that... good article. It's interesting to note that the while the 390 stifled power output on the built 383 (no surprise there), it was still able to supply enough mix to generate the rated torque for that engine.

Hey Jack, there's an idea for an FSP project... flog a dyno-mule 300 with a bunch of different carby and EFI set-ups... :D
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Re: holly 390cfm carb vs edelbrock 500 cfm

Postby broncr » Sat Feb 28, 2004 2:04 am

[quote="herpapabear18"]I have a 1981 ford 300 with about 20,000 miles on it. It is all new with stock size parts. The only mods are a offy c manifold...."

As you might have noticed, carburation is an interesting subject. Personally, I'd stick with the 390 for now :lol: . The quoted Holley chart was for a stock (70-75%) volumetric efficiency. Once you exceed that, it can get a bit complicated.

"No offense" Fordman, but my 390 is working quite well for me, and I'm drivin' in the real world. Maybe it's the altitude... :lol:

broncr

PS - I'll volunteer the lil mule for a couple dyno runs - if someone will loan me some bigger carbs & pitch in some $$$. The Carb Shop here (Denver) does dyno tunes for $130 (& ask me how I know :wink: )

2/28: Oh yeah, - I already have a 600, just need the "in between's.
Last edited by broncr on Sat Feb 28, 2004 10:49 am, edited 1 time in total.
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Postby rmt » Sat Feb 28, 2004 9:28 am

'fordman' - The 302's exhaust ports are crap and almost half the size of the 300's exhaust port.

I'm not sure about "half the size" but smaller seems logical enough since the 302's cylinders are considerably less displacement than the 300-6.
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