What carb should I choose?

BEB · #1 (**permalink**) 03-05-2004, 03:11 AM

I am having difficulty choosing a carb.

You all seem to have different opinions on whether a Double Pumper or a Vacuum Secondary is better.

Car/engine details:

AK 427 Cobra replica - 2600lb(1180kg)weight, 5 speed manual gearbox, 3.31 diff. It is used as a Street/Strip car.

409ci smallblock Chevy V8 (400 + .040), Dart Pro 1 aluminium heads, 2.02/1.60 valves, 23 degree valve angle, 200cc intake ports, 64cc combustion chambers, 10:1 CR, Edelbrock Performer RPM intake, 34 degrees total timing, 14 degrees at idle, MSD 6AL ignition, MSD Pro Billet 85551 non vac.adv. distributor, KB hypereutectic pistons, 15 Hg idle vacuum, Competition Cams XE268H-10 hydraulic cam - duration @.050=224/230 and duration @.006=268/280, Holley Blue Top electric fuel pump, 1 3/4" headers + sidepipes, Holley Powershot 14" air filter, NGK BCPR6EY-11 spark plugs (.045" gap). Max. RPM 6000.

On an English engine dyno it made 413 bhp and 503 lbft torque.

I've narrowed the choice down to the following:
1) Barry Grant Speed Demon 750 Vac. Sec. or 650 Double Pumper.
2) Edelbrock 650 AVS or 750 Performer.
3) Holley 700 Double Pumper.

What do you suggest and why?

Thanks Guys...

DAVID GAGNARD · #2 (**permalink**) 03-05-2004, 04:29 AM

I would go with a Holley 750 double pumper.........according to Holley's formula (cu in X max rpms divided by 3456) your engine needs approximatley 700 cfm of flow at 6000rpms,so I would put at least a 750 cfm on it and I'm partial to double pumpers...............

David

CSX 4027 · #3 (**permalink**) 03-05-2004, 04:37 AM

My engine builder swears by these guys and I have always had the best luck with them. Go to
www.aedperformance.com
Check out all their series and especially their troubleshooting page. They are alot better to deal with than the other more household name brands and you will get one trick piece.

vettestr · #4 (**permalink**) 03-05-2004, 05:26 AM

What is your choice in terms of street or strip? For a strip carb choose the Holley 750 or a 780 double pumper with mechanical sec. This semi controlled fuel leak will give best times once tuned properly but will be at the expense of street performance and any MPG you have now. A better double use carb would be the Edlebrook AVS but more people have a hard time fine tuning this carb and if going this direction the Carter Comp series 650 that this is a knock off from is even better. This carb has some 37 possible adjustments and hard to beat if all correct. I was raised with the Carters and shown all the mods to fine tune this carb. The book about Performance mods for Carter is pretty good and lists many of the tricks Dad showed me years ago. For Drag racing only the Holley is more popular and more people can tweak it with you.
I would try 4 more degrees of initial timing to 18 and back off mechanical 1 or 2 if needed. If bumps against started when hot just stroke gas pedal once before turning key to flood a bit so it spins up before trying to light against starter

need4speed · #5 (**permalink**) 03-05-2004, 09:17 AM

If you are interested in a Demon carb then I recommend that you go to their web site (www.barrygrant.com) and either use their Demon Selection Guide, or contact them for a recommendation based on your engine specs. They are very helpful. I personally have a Mighty Demon and so far am extremely pleased with it.

Tongue Pirate · #6 (**permalink**) 03-05-2004, 01:26 PM

I tend to prefer double pumpers fro a couple reasons.

#1. One less theory to rely on. When you push down the secondaries will always be there because they are "mechanical"

#2. If your carb size is not 100% correct you don't have to change spring sizes in the vacuum diaphram to get them in sinc. I also like having that second set of squiters to get me going

The only reason I would go with vacuum secondaries would be for gas mileage & I don't see that being a big factor in this type of car.

All of the above are, of course, my opinion & some will of course see the subject quite differently

BEB · #7 (**permalink**) 03-06-2004, 03:59 AM

The general agreement is to use a Double Pumper then.

What about the optimum size based on 409ci, max. 6000 rpm, etc.

I thought a Holley 700 would be right (something like a p/n 4778)or should I go to a Speed Demon 750/Holley 750, etc. I don't want to go too big.

Tony Radford · #8 (**permalink**) 03-06-2004, 05:30 AM

Demon is the way to go. A number of the guys here in Atlanta have recently replaced their Holleys with Barry Grants. I've never heard anything but thumbs up remarks about performance improvements. For a couple of hundred dollars difference, it's a no brainer.

Jmar · #9 (**permalink**) 03-06-2004, 05:35 AM

I'm using a Holley 750 DP on my SBC (350) with a cam similar to yours, 15 inches of vacuum at idle, an auto with a 2500 RPM converter at 2500 lbs. Yours should run well with 750 CFM.

Shelbytexas · #10 (**permalink**) 03-10-2004, 07:42 PM

Take a strong look at the DaVinci - try the 750 seriously modified and blueprinted Holley to flow 830 cfm. It's a mechanical secondary with incredible throttle response and NO bog upon opening of the secondaries. I just installed on a my 427 (3400 lb shelby). It is very precise. No choke, though - takes about 60 second to warm up.