750 or 850 Demon

dlowe01 · #1 (**permalink**) 02-09-2004, 04:55 PM

Was wondering what would be better for a 408 storker. 750 or 850 Demon?

blykins · #2 (**permalink**) 02-09-2004, 05:04 PM

Well, Demon advised me to use a 750 with my 425W. It's too small.

ENTDOC · #3 (**permalink**) 02-09-2004, 06:36 PM

I need a 750 if your interested in selling. chuck

blykins · #4 (**permalink**) 02-09-2004, 06:40 PM

Well....that sounds like a good offer....but I think I'm gonna wait until I get the car finished. I'd like to see how much of a hp gain I'll get....so I'll have to wait until I'm able to get the car on a rear wheel dyno.

If you're still wanting a Demon, I have a good contact to get them cheap. Mine is a Mighty Demon....

Tongue Pirate · #5 (**permalink**) 02-09-2004, 07:13 PM

It's really hard to say without knowing how the powerplant is built & what you are using the car for. Generally speaking the 750 is probably the carb for more applications than the 850. I ran a 454 under a 750 Speed Demon with a decent sized cam & it was more than enough.

powerpagers · #6 (**permalink**) 02-09-2004, 07:59 PM

Hey;
A 750 would not be too small. There are adjustments and jets to exchange. Is either one still for sale?

Perf. Eng. Eric · #7 (**permalink**) 02-09-2004, 08:39 PM

What duration is the cam? That will determine if you will want to use a speed or a mighty Demon. We will use a mighty if the duration goes above 244@ .050. This type of carb seems much more driver friendly. I have found the 825 mighty works best on the dyno on the 408/418 stroker enginers if they have a good head ( like the AFR 205). Cam type and lift will also be a factor in the selection of the right carb. Hope this helps.

dlowe01 · #8 (**permalink**) 02-10-2004, 07:18 AM

The cam is a Comp cams 284 H with 240 intake and 246 exhaust duration and .541 intake and .544 exhaust lift. The heads are world products with 2.02 intake and 1.6 exhaust and a mild port job. Am leaning towards the Edelbrock air gap at this time. Compresion is 10.1:1. Will be mainly for street and few road course events a year.

viking · #9 (**permalink**) 02-10-2004, 07:58 AM

750 should be plenty, works great on my 427 FE

Zoom This · #10 (**permalink**) 02-10-2004, 03:14 PM

My 418 has an 850 cfm Holley and has proven to be enough cfm. The power dropped off on the dyno at high rpm with a 750. Motor is .576/.574 dual pattern cam with Edelbrock Victor Jr heads and intake. Main jets are 80 on all four corners.

Chaplin · #11 (**permalink**) 02-10-2004, 03:41 PM

Assuming your max rpm is 6500 and 100% volumetric efficiency (VE) (which you will not achieve with a naturally aspirated motor), you would need a carburetor that flowed 767 cfm.

How To Calculate CFM:
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency

(Your example: 408 CID x 6500 RPM = 2,652,000 / 3456 = 767.4 CFM)

Below is from Holley:
Most Street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.

Therefore, assuming you achieved the same VE as a fully modified race engine, (95% VE), the max CFM necessary would be about 728 (95% of 767). Therefore, the 750 would be fine. You could use the 850 also, but you may suffer some loss of power and responsiveness on the bottom end because of decreased velocity of the air/fuel mixture.

FWIW, I just went through this drill with the techs at Barry Grant and they suggested a Might Demon 750 for my 428 with the comp cams 292H. I just bolted on the card tonight, but didn't get to test it out yet.

dlowe01 · #12 (**permalink**) 02-10-2004, 04:06 PM

Thanks for all the info. I think the 750 mighty demon should do the trick.