408 help????

Super Victor since it has large cubes you vcan get away with it, 750cfm carb should be fine, AFR205 or 225 would be fine, with a flat top I am assuming around 12:1 compression?? I would go with a larger cam however...

How high do you want to spin it?

dont mean to break up a post..but i need ricks help

hey rick....i recently built a got a 347 short block for my car...and i topped it off with afr185 heads and used the trickflow street intake that i had b4...what i was wondering is..shoudl i switch my intake to track heat?? im not sure how to feel bout my dyno #'s..331rwhp..and 367rwtq untuned and 12 degrees timing...now i can get u the cam specs dont have them with me...but i know its a comp cams special for nitrous cause thats what i had built this motor for is to run like a 75 or 100 shot without splitting the block.....the cam card said something like a .584 lift....do u think i can switch that intake just the upper and not lower? the car is street driven but also sees good track time....im just looking to get a good 410rwhp....and 425 rwtq...with a 75 shot..

yea i have a feeling that my intake is choking me a lil....im onna run the trick flow track heat...right now i have 15/8 bbk longtubes and they barely squeeze in im not sure if the 13/4 wil fit...the long tubes i have now are like a hair away from my steering column....i am running a 75mmtb and a 76mm mass air meter....

aleborjas said:

ok ill get a smaller carb 750 Race demon is ok or an other brand better than that? i whant to have like 10.1 or 10.5 CR how can i do because i whant to use pump gas, and what do you mean with how high i want to spin it?

Where or wich company makes 1 3/4 LT's for 94' with 351?
like arround what ET's can these engine do with 250 shot on a 94' stang stock weight?

Click to expand...

You'll need a large dish to acheive that compression ratio. How many rpm do you want to turn?


Accufab and Kooks are the first I think of for headers for a SN series car.

Rick 91GT said:

You'll need a large dish to acheive that compression ratio. How many rpm do you want to turn?


Accufab and Kooks are the first I think of for headers for a SN series car.

Click to expand...

Rick,you think a 750 is enough for that motor?

Personally I would look at the BG Mighty Demon 850 with annular boosters.

We were runing a 950 annular RS Race Demon with something simalar,a little samller cam (hyd. roller) and turning 6800rpms.That was on top of a Super Victor with a 1 3/4" Wilson burst panel spacer.

btw,we also got by with a compression of around 11.5:1 on pump 93

Rick, you think with that much nitrous and ci that a 750 is ok? I'm over carbureted with a 1000cfm

FWIW, I'll lay out my combo and #'s for you.

418 cubic inch, box stock TFS "R" heads and super victor intake, 1000cfm demon carb, custom hydro roller, 252/262@ .05, 110ls, .576/.576 lift, 1 7/8 kooks. Thats the basics, on pump gas the motor made 581/539 and in my 3300lb sh!+box with the 5spd the car went 11.55@126.66 on a shakedown pass shifting at 6-6100rpms. from 5600-6700 motor holds right around 560hp. Peak power came at 6500, torque at 5000. It's a little soft on the hp #'s down low (under 4k) but with the tourque it makes down there you dont notice it in the car. From 4,000 to 5,000 rpms it picks up 200 hp. Ok, enough rambling.
mike.

I forgot about the N2O, but yes we have had pretty good succes with a 750 Holley on a 408ci motor, 11.3:1 compression, COMP N2O soild roller, although you'l find most with 850cfm carbs and they are fine.

Mike I am always surprised at how your car runs with that 1000cfm carb, I know of 8 sec combos with less carb.

If you want a great N2O style carb look at a Pro Systems Here is an article Patrick James wrote about carb size... FWIW I have never been dissapointed with a Pro Systems carb.

Choosing A Carb Size

One of the most asked questions we receive at PRO-SYSTEMS is, how big will my carburetor be, how much will it flow? Most customers are expecting to hear big numbers. They want to hear that their 4150 will flow 1150 cfm or that their 2 inch throttle blade Dominator will flow 1400. Unfortunately, that's not going to happen. In all the designs I've flowed, designed, calibrated and tested, it's just plain physically impossible. Builders will give out dry numbers (meaning cfm ratings without the disturbance of the fuel cone) but these numbers are not what the engine actually sees and are for reference only. Some builders still use the inflated cfm creating 28 inch rating used by an aftermarket manufacturer. Enough has been said on that subject.

But really cfm is not the main level of importance. Fuel shear, atomization properties and fuel curve are your main areas of concern.

To size a carb for the application, you're looking to achieve minimal restriction at the finish-line yet have enough signal at launch as to be sure that the booster is atomizing the fuel and supplying the proper air to fuel ratio.

Horsepower equals air flow (of course). Launch rpm/trap rpm equals a reference of the range of the air flow.

If the carb is too big or signal/curve is too poor at the launch rpms created airflow, the fuel does not properly atomize and plates out (turns back to raw fuel) on the intake. Losses of 10-12 percent of available torque at launch can easily be recognized without a lean cutout or backfire. Then as rpms increase, the plated fuel is picked up and alters the air to fuel ratio down-track as it is cleaned out of the intake. More loss of power. So you jet it down to compensate for the plated fuel being picked up and the launch gets even worse. See the dilemma.

The wider the range of rpm you're going to subject the design to, the more you need to look at the range of airflow and available options.

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

CID x RPM x V.E. / 2820 = CFM
350 x 6600 x .9 / 2820 = 737 CFM

Now you'll be a little closer.

A .9 Volumetric Efficiency (V.E.) number represents a pretty good combination and a 1.1 V.E. number represents an all out assault on the engine blocks stress handling capabilities.

Remember, if we have a heavy vehicle and a two speed we will require a slightly smaller carburetor, than a light vehicle and a stick. Also, if we have a booster/emulsion/air bleed configuration designed to operate and shear fuel at lower rpms we can increase the cfm. An increase in cfm is usually a guaranteed increase in power, but it takes a design that'll still pull and shear fuel at launch to pull that off. That's when the builder starts altering the entry and exit angles of the booster, the emulsion layout, air bleed configuration/well diameter, etc. All in an effort to fan the fuel cone to increase impact, supply the proper air to fuel ratio throughout the rpm band and emulsify the mixture prior to decrease plating for the air speed being encountered. All those mods cost money and they're not easy to do.

But return on investment is the deal when purchasing a carburetor. Oftentimes a customer is thinking of purchasing a this or a that, when the same money spent customizing his current model will yield more performance.

Remember, as we talked about earlier, the loss of torque we record at launch and the subsequent rate of acceleration you lose at the start of the race will be carried throughout the rest of the shifts. So a good leave (excellent fuel shear and proper air to fuel ratio at launch) is getting the reciprocating mass to carry this rate of acceleration to reduce E.T.'s. But if you have too much restriction at the finish-line, the mass will be slowed as a result and E.T.s will increase and none of us want that.

Rick 91GT said:

Mike I am always surprised at how your car runs with that 1000cfm carb, I know of 8 sec combos with less carb.

Click to expand...

Yeah I know Rick. When my machinist/dyno man recomended that, he had had great success with that cubic inch and carb size. Now with some of the other combos he has done he thinks we should have gone with an 850. I would like to try it, but having already spent the $700 for the carb I have I hate to spend the money right now. Maybe I will this year.
Mike.

Man I hear ya there, I was up on Charilie Boozes dyno with a 408 car and he was telling me about a 393 they just did and the 750 was the ticket and made more power then the large carb, also made the car a little more streetable. Brand to Brand and Model to Model you run into differences, it largely depends on the carb tuner as well...*

Really depends on the exact combo. I think Brandon S who I did the DARt based 364ci motor has a Pro Systems 850, but he plans to spray the house down as well...