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Demon vs Holley parts interchange

April 8 2011 at 8:23 PM

Doug  (Login 6T9Stang)
Member
from IP address 64.184.48.13

Hey all!

I should say right off the bat I don't have much experience tuning carbs. But I like to learn & can be pretty methodical when I have to.

My main question - does anybody have a link or know off the top of their head what parts will exchange between Holley & Demon carbs?

Basically I'm wondering if I'm going to be able to find all the parts I need to tune the Demon (Speed Demon 750 with vac. secondaries) correctly since Barry Grant is now defunct. I'd hate to spend $$$ and then have to turn around & buy a Holley anyway (although I have been eyeballing the Ultra Double Pumpers).

My other concern is whether the 750 Speed Demon is just too big for a 357" motor. It idles rich & occasionally will diesel a little at shut-down when it's hot. I didn't realize until after I bought it that BG rates their carbs differently because of the wet/dry flow differences. The only adjustments I've attempted so far are the 4-corner idle screws because we barely got the car on the road before time to put it up for the winter, plus I knew I'd be swapping the Weiand X-celerator out for an Edelbrock Performer RPM this spring.

In case it helps:
Aussie 2V heads
Comp 282S Magnum cam
MSD ignition
manual transmission

Thanks for any help - Doug

 
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AuthorReply


(Login 62highboy393c)
Member
76.115.161.223

Re: Demon vs Holley parts interchange

April 8 2011, 9:15 PM 

I dont know about what will interchange I only run Holleys, I think the 750 Demon carb should be fine, the cam is making the idle rich, you will just have to get used to it, and a high idle will make it run on after you shut off the engine, do you run premium gas? If you dont try it, it will help.

 
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tinman
(Login tinman351)
Moderators
71.57.55.107

it's a lot of carb

April 8 2011, 9:39 PM 

BG recommends a 650

http://www.barrygrant.com/demon/default.aspx?page=11

Holley & BG are rated at different specs, a BG 750 flows a lot more than a Holley 750 at the same depression, Mighty Demon 750 is said to flow 952 cfm

you may have a hard time getting it tuned, if at all?

as you're saying idle will be rich and mid-full power will be lean due to poor signal. you'll have to lean the idle circuit at the idle restrictions and richen the jets until it AFR's right and even then you'll still have transition problems

pretty sure a good old 650 double pumper will rock

there'll be phantoms, there'll be fires on the road... and the white man dancing

 
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6T9Stang
(Login 6T9Stang)
Member
64.184.52.39

Whole Lotta Carb

April 10 2011, 2:42 PM 

tinman posted:
"Holley & BG are rated at different specs, a BG 750 flows a lot more than a Holley 750 at the same depression, Mighty Demon 750 is said to flow 952 cfm"

Wow - I knew there was difference in the ratings, but 200 cfm is quite substantial! Before I found out BG went under, I was looking at their 650's.

 
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6T9Stang
(Login 6T9Stang)
Member
64.184.52.39

re: gas & idle

April 10 2011, 1:08 PM 

Dan - I only run 93 octane in this one. Idles nice @ 800 rpm after it's warm, and will even go as low as 600. Just smells of unburned fuel & might make your eyes sting a little if you're in the garage & there's no breeze :o

 
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Mike
(Login 418_hipocoupe)
Member
75.16.41.201

gas & idle

August 30 2011, 5:30 PM 

Do to the cam overlap, what you are smelling is unburned hydrocarbons, actully caused by a lean idle. Try richening up all 4 idle screws a tad, probably eliminate the engine run on.

 
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(Login Falcon67)
Member
75.88.8.113

BG carbs

April 9 2011, 10:35 AM 

There is very little that interchanges. Time to shop for a new carb IMHO, or start buying up old Speed Demons so you have a parts pile to draw from.

1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.

 
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dion
(Login xdclevo)
Member
202.63.49.193

Re: BG carbs

August 30 2011, 4:36 PM 

>There is very little that interchanges. Time to shop for a new carb IMHO, or start buying up old Speed Demons so you have a parts pile to draw from::
..

I think you'll find a fair bit is interchangable, with atleast aftermarket parts if not Holley. With the 750 Demon i have, the only original parts are body and baseplate, rest is bolted on stuff when some fiddling was done to it, boosters everything changed. This is my 750 Demon:

[IMG][linked image][/IMG]



 
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6T9Stang
(Login 6T9Stang)
Member
64.184.52.39

Bottom Line

April 10 2011, 2:46 PM 

I've done quite a bit of reading here on the forum over the last few days, searching through a lot of the 'carb recommendation' threads, and my head hurts! happy.gif

I've seen some people say Clevelands like more carb than your typical small block, and others say most people over-carb. The 'carb chooser' on the Holley site picked me out a 650 VS, yet I've seen it posted by a respected contributor here that a 700 DP was too small for an engine very similar to mine (same displacement, cam, & aussie heads).

I think the bottom line is I'm asking a question that has no clear answer. Since I don't own a speed shop & a dyno, it looks like I'm just going to have to wait until I get the intake & dizzy swapped, and then let the engine tell me what it wants. Which means I'll probably have a couple of hundred invested in an Idle-Eze baseplate, power valves, and jets... and still end up needing a different carb. Or just spend the $$$ on a new carb & wonder if I really needed it :P

 
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(Login tony-can-do)
Member
142.59.65.25

Re: Bottom Line

April 10 2011, 4:09 PM 

Bottom line is it is not too much carb for what you got,just tune it for your combo and run it.would a 650 work a little better on the street only car, sure but run what ya got it'll be just fine.

 
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(Login 62highboy393c)
Member
76.115.161.223

Re: Bottom Line

April 10 2011, 4:12 PM 

I agree, a 950 HP works very well on mine, a lot better than the 850 that was on it.

 
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(Login Falcon67)
Member
65.182.95.67

Line the bottom

April 12 2011, 10:51 AM 

The deal on carbs can be simplified somewhat - LOL.

As long as the carb will meter fuel correctly for the engine, it is not "too big".

For a street or street/strip car, err on the small side. A 650 will easy support over 500 HP. People like to put big carbs on the motor, yet the guys that build alcohol carbs for the huge HP engines around here like to use 700DP Holleys as the base carb.

A VS carb can be larger than a recommended DP because the secondaries only open when there is enough air flow. You can put a 750VS on a 260, it just may not open the back barrels until 9000 RPM. I have personally tested a tuned 750VS against a tuned 650DP on a specific engine combo and found not difference in ET or MPH at the track. And I have found a difference of .02 and 2 MPH between a 600VS and a 600DP on another engine combo. It's all dependent on the build, but in general the VS carbs are more forgiving.

DPs come in 50 CFM sizes because a DP is more sensitive to the combination than a VS carb.

"carb response" is affected by many variables. Smaller primary throttle bores and better boosters drive on the street better. An 850 DP on a Torker will be lazy on a street car where a 600DP may well be rather snappy. A larger cam and still trying to run an advance curve can make a carb harder to tune. It's a lot easier to tune a 4 corner carb on a motor with a decent overlap solid lift cam when the timing is locked at 36.

Idle rules are different for a 4 corner idle carb - start at 3/4 turn out instead of the old 1 1/2 turns out for a two corner.

My personal likes:
Street 302~351 - Edelbrock carb 600
Street/Strip 302 - 600DP Holley
Street/Strip 351 - 650DP Holley
Strip 351~393 - 750DP 4 corner idle custom build, 750HP Holley or 950HP Holley

1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.

 
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James P.
(Login 70rancheroGT)
Member
152.16.52.127

Tuning

April 12 2011, 9:21 AM 


>The only adjustments I've attempted so far are the 4-corner idle screws because we barely got the >car on the road before time to put it up for the winter

It takes a while to get the carb set up on a cleveland. There are a bunch of things that could be giving you the rich idle. Not enough initial advance, power valve opening at idle, leaking accelerator pump discharge nozzles, etc.

I don't think the carb is too big, it just needs tuning. That's the fun part happy.gif

Good luck,
James P.

 
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6T9Stang
(Login 6T9Stang)
Member
64.184.48.13

Thanks, & and update

August 30 2011, 2:40 PM 

Just wanted to bump this thread to say thanks to everyone who posted with advice.

Also wanted to update & say I've got a Holley 770 Ultra Street Avenger on the car now that it seems to like. It took care of the rich idle, and no stumbles/flat spots.

Now I need to go through the Demon & see if there's anything obviously wrong. If so, I'll see if I can fix it & test it on the car.

Doug

 
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Miller Mylan
(Login 417strokers)
Member
70.77.66.15

Chech all gaskets

September 1 2011, 9:34 AM 

I was at the speed shop just last week and one of the guys was freshing up a 750 Demon He said there are ports in the base plate that don't match holley gaskets he opened the hole in the base gasket to line up! If these are not lined up the carb won't work right. I can't tell you were to look because this is second hand info but it is some thing you will have to look at!

 
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(Login 417strokers)
Member
70.77.66.15

Re: Chech all gaskets

September 1 2011, 9:40 AM 


 
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6T9Stang
(Login 6T9Stang)
Member
64.184.48.13

re: gaskets

September 1 2011, 2:55 PM 

Thanks for the heads up!

Doug

 
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