Here is a Mathematical Formula I found on the Net for selecting a carb:
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(note: this first method seems to fall short of proper cfm's)

Take Engine Displacement (CID) and multipy it by maximum RPM and then divide it by 3456

Now you have to take into account volumetric efficiency. A high performance street engine has a volumetric efficiency of about 85% - 90% so you take your answer to the above formula and multiply it by .85 or .90 and that will give you a theoretical CFM. Get a carb with close to that theoretical CFM.

On an all out racing engine, you should use a VE of 110%.

Another Method:

Just to give you more information and another way of selecting a carb, David Vizard in his book "How To Build Horsepower" suggest selecting a 4 barrel carb with a 1.8 to 2.0 CFM per cubic inch of engine displacement. Using his advice, a 350 small block needs about 650 to 700 cfm. This would be a good range to make your selection. The smaller carb would offer better gas milage and a crisper throttle response, better driveability, and the bigger carb would be best for max hp.
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Hotwings can i ask what you are running in your car? ( engine i mean ) and can i pick your brain over a small block chevvy ( mine ).

Sure can Buddy! What do you want to know?

Here's my setup:
350 .030 bored out. 4 bolt main.
Eagle rods
9.5:1 flat top hyperurtectic pistons
block has had .035 shaved off the block deck (pistons are only .005 in the hole giving me an effective 10.25:1-10.5:1 compression ratio)
Comp Cams 280AR solid roller cam
Comp Cams lifters & pushrods
Dart II Sportsman heads (64cc chambers)
Edelbrock RPM Air-Gap intake
Holley 750 vac secondaries (I use a different carb for racing. I have a Barry Grant 775 mech secondaries. it is fully tunable including changeable venturis so I can increase or decrease the CFM's)
Crane Roller Rockers
Hooker Headers w/ full Flowmaster 50 exhaust.
MSD 6AL box w/ Blaster 2 coil.
MSD Pro Billet Distributor

The intake and heads have been port matched for better flow.

Pete

Well as you may or may not know im coming across the pond for the nationals and i want to up the hp on my sbc.

I have currently slight overbore, 10 to 1 compression, 270 cloyes cam, 4 bolt main, etc , up top i believe the heads are off a 305, very slight porting, arp 3/8 orcker studs installed, standard pushrods, twin 500 edels ( probably too much but i love em!! ) tubular headers and a home made 2 " bore system ( this is going to be re worked ) only a pair of cherry bombs as silencers, oh and i run the standard Hei ignition system, new dizzy,leads plugs.

The engine machining etc was done before i bought it, the guy had spent the best part of $3k on maching and parts but didnt build it as his wife became pregnant and he sold it to me at a fraction.

I was wondering what really is the next logical step, different heads? ( sorry but an idea of weight would be cool too! ) ((baggage)), i was toying with the idea of twin turbos etc, im mechanically skilled but this is my first real attempt at an american V8 to tune so id rather ask.

For what its worth, i can port etc my own heads and have done so on many engines, V6, 16v etc etc and have even turbocharged 4 pots.

There just seems to be so much about and for the sbc, i dont want to waste money, i was think ing of a twin tunnel ram, but that would lose me bottom end?

Maybe a session on IM would help? cheers Hotwings.

Wow, where to begin? I realize fully that you are limited by parts availability across the pond and the price of long-distance shipping. Therefore, you have to make due with what you have or can reasonably get ahold of. I don't envy your situation, but I do admire your car and your capabilities.

That being said, most folks stateside begin by ditching the 305 in favor of a 350 with 4-bolt mains. Or a Pontiac engine with some pedigree. The 305 is a smog motor and was designed for light loads in a downsized automotive/consumer environment. That you're already pushing 10:1 compression is a bit of a conundrum, because you are really stressing the internals - especially at the bottom end.

Therein lies my second point. If you want to add turbos, you'll need to seriously lower the compression. Somewhere in the neighborhood of 7.5 or 8:1. Adding boost to a high compression engine is going to grenade the whole thing. Almost guaranteed.

Long story short: American V8's of that era were pisspots. Tolerances were loose and materials were shoddy. On top of that, they were thrown together by relatively unskilled labor in a hurry. Some of that can be overcome by competant machining, which you have already accomplished. But we're still talking about a pushrod V8 with only two valves per cylinder. And you want something with a good degree of reliability in addition to seat-of-the-pants performance.

I hope I'm not coming across as being too preachy. But I am concerned that if you go in the direction you've indicated, there will be some expensive lessons in store.

So, where should you go with your particular engine?

First, I'd go through the whole thing carefully to make sure the previous fellow's work was up to par. Then I'd look for a more agressive cam to increase lower and mid-range torque. Forget about the high end - the 305 probably shouldn't be taken above 5000 rpm anyway.

A set of aluminum heads will shave weight, as you've already indicated. And the biggest diameter header/exhaust system you can fit. Because if you're pumping 1000 cfm in, you've gotta help it expel them just as easily. Consider an X-pipe or crossover to get the advantages of scavaging. And have everything mandrel bent instead of bumpy and pinched. You want to minimize backpressure all the way back.

Beyond that, get a fully adjustable MSD ignition. That way, you can advance or retard the spark to fit your conditions to a greater degree than the HIE or via mechanical adjustment.

Since you're coming to the Nats, spend time looking through the swap meet and talking with the vendors for specific recommendations rather than thrashing on the car before you get here.

Preachy?? ? No way!! the engine was bought as a box of new bits and i assembled it myself, i agree an X pipe is high on the " to do list ".

Sorry i may have mislead you the engine is a " 4 bolt mains 350, not a 305 , the heads are however of a 305 (cast iron).

Yes i agree Turbos require lower compression etc to avoid detonation problems.

Theres seems so many parts for sale for the sbc, i just didnt want to waste money on "unknowns" .

i dont want to appear arrogant or rude, i have done engineering so i understand the fundementals of bhp , i was hoping for some proven knowledge.

TransAmer99, thankyou. and any help is exactly that help.

My bad... Kindly allow me to remove the egg from my face! Had I but reviewed your story under the July Bird-of-the-Month section, I'd have known that you were already running the 4-bolt 350. Apologies to all.

I don't think you could go wrong at all if you mimic the recipe Pete detailed above. But you'll be shopping for 93 octane or better from that point forward.

Egg on your face! no way mate, it was me i hadnt laid it out to clearly in my description.

Im still toying with the idea of the tunnel ram set up, but heads would be nice........ descions , descions....... lol.

i dont suppose anyone knows roughly how much a set of alloy heads weigh?

I was just trying out those two formulas and I get a completely different answer depending on which one I use. For a 455 with a 5500 red line and figuring 90% VE I get only 650 cfm. Way too small for that engine. Using the second formula I get 800 - 900 cfm.

Heh heh heh! If it were me and I was doing a Pontiac mill, I'd vote for a Tri-Power setup.

In case you'd like to know the story of how and why the TA got a shaker in 1970: After the debut of the TA in 1969, with it's functional ram air scoops, Pontiac planned to go racing in the SCCA Trans Am series. They had even developed a special 305 c.i. engine (essentially a destroked 400) for the job and outfitted it with two carbs. In order to clear the hood, they needed to cut a hole and to keep them covered, they designed the scoop attached to the engine itself instead of the hood like everyone else up til that time.

The shaker scoop faced backwards following the engineering theory that the air turbulance at the base of the windshield would create a high-pressure zone forcing more air into the scoop than if it were facing forward and subject to laminar flow disruption. Plus the shorter path than from following ductwork near the front of the car would ensure a fresher, cooler, denser charge at the carburater.

The theory of the high pressure zone didn't pan out, however, due in part to 'rollout' to the sides by the turbulance and the distance from the base of the windshield back to the scoop. But the design had been cast.

Ulitmately, GM refused to allow Pontiac to homalgamate (2500 units available to the market, per SCCA rules) the 305 for production and the effort died. Nonetheless, 25 engines were completed and delivered to 'back door' racing teams like Jerry Titus & Terry Godsall.





The pics above show replicas of the original racercars of the Titus/Godsall team. For more info, go to:

http://www.historictransam.com/homepage.htm

Nows that GOOD INFO!!

cheers mate, and no i didnt know how the scoop came about!

Those racing birds are just lovely, thankyou.

The aluminum Bowties I have for the Monza weigh 23.43 lbs each, complete except for rocker arms.

Larry, you're right. The first method seems to fall short. I prefer the 2nd method myself as a rough guide for carb size.

Thankyou Hotwings, im trying to work out my best options, while im across im going to try and grab as much stuff as my wife will let me !!

Shes already getting suspicious of me ..........

Oh lastly do you think your heads made a noticeable difference?

Sorry its 20 dumm questions time, but whats good on paper doesnt necessarily mean its good in the real world!

as for the formula to figure out the carbs- i dont think its complete

you didnt figure the new cam size ( lift duration overlap,and port work)
but its always crisper ,a lil lean- (no carbon fouling,no wet plugs)
my car ran great with 750 duel , but was rich as hell ,and flooded easy
also - you didnt consider the high flow intake manifold, you will want/need
not bustin ur chops, just thinking of these things- they all change the motors performance


??hmm top fuel is so rich on fuel it sprays out the pipes- nothing like compression