Carb size help

[soopertruper]

I'm building a new motor for my truck. Its a 363ci using a 3.5625 stroker crank bored 30 and 5.7 rods. My comp will be 10.2:1. Not sure what cam yet, but it will be pretty big and a roller. The engine builder wants me to use a 750 dp, the formulla calls for a 630cfm carb, and Holley says a 650dp. Who do I listen to? And what decides vacuum vs. mechanical 4 barrels? Thanks Damon

[jusbo]

you decide vac* or mechanical secondaries.....I personally would go for vac*, because the pressure(manifold??? <- Probably needs correcting) would open the secondaries up when they are really needed.



You could go with a 700 cfm, then change jets if the need arrises.

[coppergmc]

Just be realistic when you think of the rpms that you will turn the engine and the engines use. If you are using a high stall converter (3000 plus) go with the double pumper. The vacuum secondaries work well with stockish converters.



I like the responsiveness of the 650dp for a true street strip 327-383 and have used them to good effect on larger engines. The 750dp works well too. If you are running a dual plane intake without a spacer, bare in mind that they like 750s a little better. It is kinda hard to go wrong as long as you have the carb tuned correctly for what your engine needs.

[soopertruper]

I'm using a 2800 stall, and a performer rpm manifold and don't really see myself shifting past 6000 max.

[coppergmc]

I'd probably use the 750dp. If you find a 650dp that is readily available don't hesitate to use it.

[MARKYMARK]

i'm running a 830 dp on my 350 with 11.5-1 comp and way to big roller cam. unreal throttle response. 750 vac carb wouldn't idle because of the very low vacume

[Scotty_S-15]

I'd probably go with a vacuum secondary 750. I've swapped back and forth between my vacuum secondary 750 and my DP 750 on my truck, and the ET doesn't change. But the vacuum secondary is easier on the street, and better gas mileage. I see an increase of from 6 mpg with the DP to 7 mpg with the vac. sec.http://www.s10v8.com/images/smilies/icon_razz.gif ( I never really checked)

......... Another plus of the vac. sec. carb is that you can't give it too much gas. That is, driving down the highway, crack open a DP 750, your engine might jump to 2800 RPM, and while it'll go, it'd probably go FASTER at those lower speeds with the vac. sec. carb, 'cause it will open just the amount that's needed at that load. But the DP will sound meaner under those circumstances ...... and you might SWEAR it's faster. But it's pobably NOT.

........ And for the same reason, using a vac. secondary is a little more forgiving if you're oversizing the carb.

[rudedogg]

i have a 650 dp that has only dyno runs on it (4)to be exact it actually belongs to my boss he wants 300.00 its really NEW!

[adh383]

did u mention heads you were using?

[soopertruper]

Not exactly sure on the heads. The only thing I know is they are factory GM aluminum heads with a center bolt valve cover. The machinest is on vacation and I won't be able to get to them till friday, and never thought to take the casting #'s before I dropped them off.

[soopertruper]

Those aluminum heads are just temporary, cause the budget only allows for a roller cam or heads right now, so I figure its easier to swap heads later than a cam. Undecided on edelbrock e-tecs 170cc or dart pro 1's 180cc runner

[adh383]

U might want to use a vacuum carb--750 holley

[soopertruper]

Vacuum 750 it is!!! Thanks for your help guys.

[OTHCUSTOMS]

im being told my 750 dp isnt enough for my new 406. im told i should go 850dp. seems like 1 thing always leads to another.

[Scotty_S-15]

On 2007-07-02 22:46, OTHCUSTOMS wrote:

im being told my 750 dp isnt enough for my new 406. im told i should go 850dp. seems like 1 thing always leads to another.



I wouldn't go jumping into that just yet. If you know and liked the way your 750 Holley worked, I'd go with that tried and proven carb. You can always upgrade once you've got any other bugs worked out.

........... To put carb sizes in perspecitive: Last week NASCAR was racing a road course at Sonoma, and I heard the announcers BSing about the carbs. They were saying that some guys were using 750 cfm Holleys to gain a slight edge on mileage, while other guys were using the 830 cfm Holley, because of a mere couple HP advantage. That's right, "a mere couple HP". And that's on a 750 to 800 HP engine. http://www.s10v8.com/images/smilies/icon_wink.gif

[OTHCUSTOMS]

i was thinking that myself, few people are saying though do to a 4 way adjustment on the 850 compared to a 2 way adjustment on my 750. i will be able to adjust it to my motor better.

[soopertruper]

See, thats why I'm asking this question! 750's on a 700-800hp motor. OTH- there is a holley dp that has 4 way adjustable idle screws, and quick fuel and proform also sell a conversion kit to make it a 4 corner idle system.

[OTHCUSTOMS]

yeah sooper.... where do they sell that conversion kit???

[OTHCUSTOMS]

yeah i found it sooper but for the amount that they are seling that i might as well go with the 850 my buddy has. he has a 850 proform w/ the 4 way adjustment a $600 carb that he will sell to me for $300. and its prctically new.

[grimpuppy]

On 2007-07-02 23:06, Scotty_S-15 wrote:

They were saying that some guys were using 750 cfm Holleys to gain a slight edge on mileage, while other guys were using the 830 cfm Holley, because of a mere couple HP advantage. That's right, "a mere couple HP". And that's on a 750 to 800 HP engine. http://www.s10v8.com/images/smilies/icon_wink.gif





I would be willing to bet my left nut that a 750 Nascar carb flows way more than 750 cfm. Same with the 830 carb. Plus these motors are optimized and tuned to the gills. They set them up to work with the carb they decide to use, so they only lose a couple horsepower with the smaller carb. I would bet they also run at about 110% VE for most of the race.

[rayL]

i would also bet grimms left nut that these carbs dont flow 750. http://www.s10v8.com/images/smilies/icon_razz.gif

[Scotty_S-15]

On 2007-07-05 09:24, rayL wrote:

i would also bet grimms left nut that these carbs dont flow 750. http://www.s10v8.com/images/smilies/icon_razz.gif



Me too. http://www.s10v8.com/images/smilies/icon_biggrin.gif Grimpuppy said that the NASCAR motors are "optimized and tuned to the gills" and that might be an understatement. Those guys probably spend more for a carb than I've got in my whole truck.

.......... I guess there's a reason big BBCs in the 800HP range run 1050s. http://www.s10v8.com/images/smilies/icon_wink.gif

[grimpuppy]

I am running out of left nuts pretty fast http://www.s10v8.com/images/smilies/icon_eek.gif

[grimpuppy]

Holley says 650cfm carb for your application. But look at Holley's system max II kit, they recommend a 750 carb for it. If you run it through their formula, it shouldn't be a 750 either. They like to keep street carb recommendations small. It cuts down on the huge carb, huge cam won't run worth a crap tech calls.

[akajoker]

I have a question since this post is getting fairly lengthy, am I the only person running a Demon? they are probably the MOST tunable carbs out there, Just a question http://www.s10v8.com/images/smilies/icon_biggrin.gif

[rayL]

On 2007-07-05 20:24, akajoker wrote:

I have a question since this post is getting fairly lengthy, am I the only person running a Demon? they are probably the MOST tunable carbs out there, Just a question http://www.s10v8.com/images/smilies/icon_biggrin.gif





i run a demon. Folks try to tune em like a holley and thats where they run into problems

[soopertruper]

What makes a demon better/different than a regular holley? I'm open to suggestions!

[rayL]

Contoured air entries, Removeable venturi sleeves on Race & King Demon, Smooth concentric venturi bores, Billet metering blocks - no porosity to upset carburetor calibration, Billet baseplate - no broken ears and a solid base for throttle shafts, Large capacity float bowls, Replaceable boosters for added tuning capability, glass sight windows for setting float levels

this is just a few things. No carb is any better than the nut behind the screwdriver.

[akajoker]

On 2007-07-05 23:40, rayL wrote:



Contoured air entries, Removeable venturi sleeves on Race & King Demon, Smooth concentric venturi bores, Billet metering blocks - no porosity to upset carburetor calibration, Billet baseplate - no broken ears and a solid base for throttle shafts, Large capacity float bowls, Replaceable boosters for added tuning capability, glass sight windows for setting float levels

this is just a few things. No carb is any better than the nut behind the screwdriver.









Ray, you are right most people use to running holley's do try to tune them like a holley, but that's not the case, when a demon is tuned right, and they are not that hard, mine came with a tuning sheet for guidelines(but I live 5 miles from Barry Grant Headquarters so I had one of their guys tune it) they are great, I'm running a mild .30 over 350, cast iron heads, 2.02/1.60 valves, flat tops, .520 solid lift cam, and using a 650 DP Demon, I pulled a holley off, brand new, but had somewhat of a "dead" spot in the rpms, I tried a Edelbrock 750, it ran smooth as anything, but didn't have the power the holley or Demon did. I love my Demon!

[grimpuppy]

I have nothing against Demon carbs... But in Holleys defense, if you took a Holley carb to Holley headquarters and had them tune it, it probably would have run pretty good for you without the dead spot.

[akajoker]

On 2007-07-06 09:19, grimpuppy wrote:

I have nothing against Demon carbs... But in Holleys defense, if you took a Holley carb to Holley headquarters and had them tune it, it probably would have run pretty good for you without the dead spot.





LOL grim you're 100% right about that, being i'm close to BG was just a benefit. the Engine guy I do alot of powdercoating for is starting to use Quick Fuel carbs over the demon's, says they are ready to run out of the box, quick fuel claims it too..never seen one running in person so I can't say