Carb choice

[BEB]

After much experimenting and playing around with Vacuum Secondaries and Double Pumpers between 650 and 770 cfm, I have now decided that the way forward for my 409ci Chevy is with a 750 Double Pumper.

I am down to 3 models at the moment but would appreciate advice. We already know that the idle circuit has to be richened up by way of the idle restrictor.

The question is which one?

1. AED 750 HO.
2. Speed Demon 750 DP.
3. A Holley of some description.

I don't need a choke and would prefer the choke horn removed too. I like the look and spec. of the AED 750 HO but wonder if there is something more suitable/better?

Any suggestions/feedback, etc. would be great.

Thanks,
Simon.

[Hal Copple]

i am running a Holley Pro HP 750 vacuum secondary on my 396 Windsor, all i did was re-jet it, it is perfect, without stumble, great drivability. They now made several different HP's, with billet bases and metering blocks, with all sort of (unnecessary for me) adjustable airbleeds and such.

I am very happy with my HP. I have probably 30K miles on it by now, no issues at all.

[OneFastRide]

Well, I'll put in a vote of confidence for the folks at AED. I replaced the 750 Holley that came on my 392 stroker with an AED Dominator (that's a hybrid made of 650 and 750 parts - flows 820cfm). Bolted right on, tweaked the idle adjustments, and instant performance increase.
Throttle response is incredible. Be warned gas mileage went from around 20mpg highway with the Holley to about 10mpg highway with the AED!

[need4speed]

So far I have been extremely happy with the quality and performance of my Mighty Demon 750 dbl pumper on my SBC ZZ430 engine. No choke horn on it from the factory and is very easliy tuned. It is also only about $500.

[BEB]

Do you think I would be better with a Speed Demon (and mill the choke horn off) or use a Mighty Demon?

As it is a street car I thought the Mighty Demon may be a bit 'over the top'?

[Jeff Hamilton]

I'm with Hal, I have the Holley 750 HP mechanical secondaries on a 392 W and love it. It is hands down the best carb I have ever had. It is also the one I paid the most for.
Jeff Hamilton

[need4speed]

Simon,

So far the Mighty Demon has been very well mannered as a street carb for me. If you contact Barry Grant they will be happy to give you a recommendation on which of their carbs would best suit your application. Just have your engine specs ready since they will need to know them to amke the recommendation. They are very good about not recommending too much carb since they want it to work right. Contact details are at www.barrygrant.com.

Alan

[BEB]

I've contacted Barry Grant and they have recommended the Mighty Demon 750 as they say a Speed Demon is calibrated to work best with the choke horn in place.

Does that mean the Mighty Demon is best for my application or that I should stick to a Speed Demon and not mill the choke horn off.

Any thoughts?

[need4speed]

Either would probably work fine but if you don't need the choke the Mighty Demon will flow a little better on the top end and is a little more easily tuned.

[RedBarchetta]

Mighty Demon here. Works great. I may never go back to a Holley.

[BEB]

My main concern with the Mighty Demon is whether the cam is not going to make the most of it and if the Speed Demon is better suited to the cam:

Comp Cams Xtreme Energy 268 hydraulic (XE268H-10)
Duration @ 0.006 268/280
Duration @ 0.050 224/230
Lobe separation - 110.0
Max lift with 1.5 - .477/.480

You're all pointing me to the Mighty Demon which is fine, but I do want to stress this is a street car and therefore the low down driveability is very important.
I thought the Mighty Demon was leaning more towards a race carb than a street carb?

Please advise...

[flipper35]

Try Don Gould at http://www.4secondsflat.com/ and he will set the carb up for you and if you change anything he will exchange the jets with you. He does mostly Mopar, but has done Chevy powered sprint cars and Craftsman Truck series stuff too. Does some guinea pigging for Barry Grant. Also runs a naturally aspirated 318 'Cuda that runs in the low 11's.

[flipper35]

By the way, there is a half second difference between his best runs with a 650 vs 750 (the quicker on the 318). He also had Racer Brown grind a new cam to make use of the 750.

[Tongue Pirate]

I like Holley for the looks on a cobra but demon for performance.

[RedBarchetta]

Originally posted by BEB


My main concern with the Mighty Demon is whether the cam is not going to make the most of it and if the Speed Demon is better suited to the cam:

Comp Cams Xtreme Energy 268 hydraulic (XE268H-10)
Duration @ 0.006 268/280
Duration @ 0.050 224/230
Lobe separation - 110.0
Max lift with 1.5 - .477/.480

You're all pointing me to the Mighty Demon which is fine, but I do want to stress this is a street car and therefore the low down driveability is very important.
I thought the Mighty Demon was leaning more towards a race carb than a street carb?

Please advise...

Yup, I'm running A LOT MORE CAM than you....

Comp Cam 540 lift, 240/246 @0.050, 110 degree lobe centers

For your particular application, you might be better suited with the Speed Demon (or a Holley). Based on my experiences, I would try the Speed Demon before going back to Holley though.

[BEB]

I did indeed contact Don Gould at "4secondsflat.com" & his suggestion was that I use a 650 Speed Demon Double Pumper.

Quote:
"It's not the carb size that caused the problem it's the carb TYPE, your piston speed increase is too fast for a vacuum secondary carb and no matter what spring you use you'll have problems.

You must understand how these carbs are rated...Holley's are dry flowed with only ambient air, which is great if you have a car that runs on air, especially with the price of fuel these days.

Demons are wet flowed with the fuel, so the air is heavier and more dense therefore more difficult to pull, a 650 Demon will flow about 720 on a dry flow bench and a 750 Holley will flow only about 680 on a wet flow bench.

650 Speed Demon is the right carb for your application."

So, it looks like a 750 is too big and a Mighty Demon may be too much for the cam.

Thoughts please.....

[MidOHasp]

I agree with the recommendations. Speed Demon 750 DP was recommended for my motor... it's a 408W, here are the cam specs:

CompCams "XE282HR"
Duration at .050: .232 (i) / .240 (e)
Gross Duration: .282/.290
Lift: .565 (i) / .574 (e)
Degreed at 108
C/L 112.0

Seems to be plenty of carb when it's wound tight past 6K RPM. Same cubic inches, quite a bit more cam.

[MidOHasp]

PS... 1.72 Roller Rockers

[BEB]

OK Guys,

So if I am reading your replies correctly, it looks like a 650 would be better than a 750, given my camshaft spec.

I don't want a 650 to be restrictive, but neither do I want a 750 to be too big.

Shame I can't buy a 700 Speed Demon !!!