Need to downsize from my 750vac sec Holley. Want your choice of carb and why! - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-11-2005, 12:45 AM
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Need to downsize from my 750vac sec Holley. Want your choice of carb and why!

This is a follow up on the 'Holley idle inconsistent' thread. I'm going to replace my 750 Holley as its venturis are way too large to get any decent atomization during the warming up period. Engine won't run great until the intake manifold is nice and warm so the fuel can be kept in suspension with the air. Until then, it tends to foul the plugs during warm up and chugs. Once warm idles nice.
I live in Australia so I don't have your luxury of paying the cheaper prices than you do. But one of my choices is the Holley Street Avenger 570 which is actually well priced here at only $353US. The other choice is the Barry Grant Road Demon 625 priced here at $450US, $120 more than what u guys pay.
Engine specs are
327SBC
Flat top pistons set at .020" in the bore and with 4V reliefs to give a compression ratio of roughly 9.35:1.
Unported 291 Fuelie heads that are lightly milled, with back cut valves and springs rated to .490" lift.
Performer EPS manifold
Speed Pro 214/224@.050" degree, .444/.465 lift cam with 112 degree lobe seperation
Rams horn manifolds
Porsche sourced Hall effect HEI ignition
TH350 trans currently with stock converter
3.55 ratio
Body weight should be around 2600 pounds.

In current form road performance is very good with top end grunt being as you'd expect with such a large carb'. Low end pick up reasonable. I'm just sick of the unnecessary fuel wastage as it always smells rich around idle and fuel consumption is scary.

My heart goes towards the Road Demon but my head says Avenger. I might even consider an Edelbrock carburetor, but your reason must be bloody good if you're going to recommend me to use one.
Are all three carb types roughly of the same air cleaner mounting heights? I've got limited room to grow under the bonnet lol.

To BGTech. What makes the RD625 suitable for engines with less than 220 degrees cam duration? Is it the power valve used? If I get one of these carbs and go fatter with my cam duration, what will be the effect? What would I do to make the carb workable for such a cam?

There's scatterings of Holley 600's over here as they're considered paper weights and I might even score one at a good price. Are these acceptable? But seeing a rebuilt one's not much cheaper then I'll continue to look at the new carbs mentioned B4.

I'm just learning here and, like you guys, I don't like throwing money down the drain. Hence me asking you these questions even though I'm a M/Mechanic (and yes, I can spell too). All my years at working on cars I've never had to change spark plugs due to heat ranges being unsuitable or having to go to a larger or smaller carb like my issues. I always thought my carb was a 600 or a 650 and was'nt until I realized it was a 750 that the alarm bells rang. Looking forward to your replies. Rob

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Old 04-11-2005, 01:14 AM
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Holley guy

I would get a Holley 600 DP. There is the option of a remanufactured one from directly from Holley. They are alot cheaper and are redone by the same guys who build them.
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Old 04-11-2005, 04:45 AM
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I have the 670 Street Avenger and love it. Personally I would stay away from a double pumper.

Vince
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Old 04-11-2005, 06:50 AM
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Club 327,


With the automatic transmission, and stock converter I’d stay away from a mechanical secondary carburetor. A 625 Road Demon™ should work extremely well on your combination. The idle, and transition circuitry is engineered for the smaller cam durations with these carburetors. Check with the guys at Victorian Performance, they should be able to help you out.
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Old 04-11-2005, 07:32 AM
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Dumb question; why can't you mail order from Summit or Jegs and get US prices? Shipping CAN'T be $100!! and I agree something in the 600 - 650cfm range should be perfect.
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Old 04-11-2005, 09:13 AM
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Holley's Street Avenger carbs work fine but they tend to be jetted on the small side, out of the box - read the plugs. Also Check the vacuum your motor is making and replace the power valve as needed for optimum enrichment. Since you are running an automatic with factory stall, I'd recommend a 50cc accelerator pump to help in off-idle transitioning. For street, I'd definitely go with vacuum secondaries.
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Old 04-11-2005, 09:39 AM
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Id say listen to tech...I personally probably would have chosen the 570 Holley assuming your staynging under 5500 RPMs with it.
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Old 04-11-2005, 10:31 AM
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downsizing

I would recommend going to a carb no larger than a 600, and with vacuum secondaries. The reason for going with vacuum secondaries, is that unless you have the need for the larger carburation, your secondaries will remain closed until your engine needs it. The Vacuum secondaries are tuneable to when you want them to start opening, which with your engine, I would put in a spring that would allow your secondaries to start opening at around 2500 RPM, at WOT, and be completely open at around 5000-5500 RPM. You will find better driveability as well as better gas mileage with a smaller carb. A 570 CFM, in my opinion would be just the right size to go with, as that will give you all you need out past 6000 RPM, and then some. Overall I think you will find superior performance in almost every instance, over the 750, with the exception of if you are wanting to turn well over 7000 RPM.
As for the 600 CFM carbs, Ive never found any problem with any of them, and the 600 seems to be, for the most part, the universal carb, since it will work well on small displacement engines, and supply the larger motors as well.
In my observation, the biggest problem is not with the carburetors, but with the people messing with them. ]
Be careful of what you are buying when you go with a used carb. As a rule, Ive found that the main reason people are willing to let go of a carb at a deal price, is because they have messed them up. You could well be buying someone elses junk.

Last edited by Max Keith; 04-11-2005 at 10:39 AM.
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Old 04-11-2005, 03:43 PM
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Quote:
Originally Posted by willys36@aol.com
Dumb question; why can't you mail order from Summit or Jegs and get US prices? Shipping CAN'T be $100!! and I agree something in the 600 - 650cfm range should be perfect.
Customs taxes or duty fees, whatever they're called, plus shipping would make it pretty near the same wouldn't it? I'd rather deal with a local company just for the convenience.

I agree with the other fellas on the 570 Holley or 625 Demon. Either would work fine.

Larry
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Old 04-11-2005, 03:58 PM
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DEFINITELY go with vacuum secondaries no matter which carb you choose. They give you the best of both worlds, good bottom end responsiveness and no loss at top end either. Double pumpers were invented for drag racers who didn't need all the secondary opening jewelery 'cause they are WOT all the time anyway. Can be made to work but not intended for street use.
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Old 04-11-2005, 05:18 PM
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carb

Just as an opinion, the smaller the carb the better low speed response will be. Its called port velocity. you don't use the car for hi speed, so get some awesome throttle response with a small carb, plus the benefit of much better gas mileage.
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Old 04-12-2005, 12:35 AM
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Thanks to all for your replies as they're all of merit. Looks like I'm on the right track here and noticed no recommendations so far for the Edelbrock carbs. To Willys36. You asked a not so dumb question. I've bought parts from Northern Auto parts twice now. First time was great, paid no duty and minimum charge for shipping. 2nd time not so great. Freight bill came to only $40 when I entered c/card details......cool(intake manifold, cam, lifters, timing chain set, etc). Got my Visa statement only to find they slugged me an extra $110 out of my account for shipping without any warning or whatever. Then I got my delivery......great. But only to receive a tax bill from UPS for a total of $150, which covered customs duty, import duty and freakin GST! Man was I fuming. Parts were $400 and ended up paying $280 for freight and taxes.......that sucks big time - not fair. Funny how only once out of the 4 overseas purchases was I taxed. And in reality if I had a warranty issue I'd be cactus. So it's local here I go.
I'd never consider a double pumper. How do they offer greater performance? I mean really, an extra squirter and the secondaries are open on demand! Once throttle's fully open a vac should hold its own.
How does a stock converter dictate the type of carb you need? I may go up in converter stall one day, but no bigger than 2500, and no less than 2000rpm as I intend to put a Crane 278H inside her.
One more thing. The Bary Grant 625 Carb offers more cfm over the 600's and 570 Avenger. Is this achieved by larger venturis? Or is it achieved by the contoured and streamlined air horn entries? My guess is the latter which would mean great vacuum signal for efficient fuel atomization but less restriction for more air flow at full noise!
I'm gonna be in contact with VPW, seeing their prices are fair and they're the only distributor of BG carbs. I'll keep you all up to date and appreciate your inputs.
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Old 04-12-2005, 06:32 AM
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[QUOTE=club327]
How does a stock converter dictate the type of carb you need? I may go up in converter stall one day, but no bigger than 2500, and no less than 2000rpm as I intend to put a Crane 278H inside her.
One more thing. The Bary Grant 625 Carb offers more cfm over the 600's and 570 Avenger. Is this achieved by larger venturis? Or is it achieved by the contoured and streamlined air horn entries? My guess is the latter which would mean great vacuum signal for efficient fuel atomization but less restriction for more air flow at full noise!
QUOTE]

If you were running a looser converter like a 3000 RPM or higher there would not be as much load when you first hit the throttle, so the secondary butterflies of a vacuum secondary would not open up as quickly. This is where the mechanical secondary would give better performance. The opposite is also true though. With a tight converter a mechanical secondary can open to quickly limiting performance. The venturi area on our 625 is slightly larger than the 570 Holley carburetor. Which exact cam are you going to be using? That number looks more like a Comp number.
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Old 04-12-2005, 07:51 AM
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Quote:
Originally Posted by club327
To Willys36. You asked a not so dumb question. I've bought parts from Northern Auto parts twice now. First time was great, paid no duty and minimum charge for shipping. 2nd time not so great. Freight bill came to only $40 when I entered c/card details......cool(intake manifold, cam, lifters, timing chain set, etc). Got my Visa statement only to find they slugged me an extra $110 out of my account for shipping without any warning or whatever. Then I got my delivery......great. But only to receive a tax bill from UPS for a total of $150, which covered customs duty, import duty and freakin GST! Man was I fuming. Parts were $400 and ended up paying $280 for freight and taxes.......that sucks big time - not fair. Funny how only once out of the 4 overseas purchases was I taxed. And in reality if I had a warranty issue I'd be cactus. So it's local here I go.

I'd never consider a double pumper. How do they offer greater performance? I mean really, an extra squirter and the secondaries are open on demand! Once throttle's fully open a vac should hold its own.
How does a stock converter dictate the type of carb you need? I may go up in converter stall one day, but no bigger than 2500, and no less than 2000rpm as I intend to put a Crane 278H inside her.
OUCH! I had a similar experience sending a gift to someone in Canada. I made this wooden model train model for a dear friend and sent it UPS.

I put in a $1500 value for insurance purposes on the UPS form. About 3 weeks later the package got sent back to me as undeliverable with a COD bill attached for $280! Seems Canada looked at the insured value and placed a COD import tax. Luckily my friend was not at home (he would have accepted it and paid the stupid charge) and his wife rejected the package. I gave it to him during one of his frequent visits to Kalifornia and he hand carried it free across the border.

No matter what torque converter you run, you will be happier w/ vacuum secondaries on the street. With a Holley or one of the fancy $$$$Holley clones, simply play with the secondary spring to adjust opening rate. A properly tuned vacuum secondary carb will ET heads-up with a double pumper on the strip yet give you streetability for Friday night Sonic Drive-In runs.
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Old 04-12-2005, 03:49 PM
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To BGTech, I may consider the following cam if I have a moment of madness. Or might go for the lesser 272H.

Chevy SB Adv Dur Int 278 Exh 278 Dur @ .050 Int 222 Exh 222 Deg Lobe Sep 110 Valve Lift Int .467 Exh .467i

Good to fair idle, daily performance usage, good mid-range torque and HP, mild bracket racing, auto trans w/2500+ converter, 2700-3200 cruise RPM, 9.5 to 10.75 compression ratio advised. Basic RPM 2400-5400.

Yeah Willys36, the taxes are into everything. Just be glad you're not an Australian farmer. They're now paying water rates/fees on the water in their dams, as it's figured the dams are taking the water that would otherwise flow into the water catchment areas. You just can't win. And to think my old man used to make his own moonshine in suburbia and got away with it!
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