Recommended 4bbl carb for stock 318
I've done some searching and there seems to be a lot opinions, but they all seem to be leaning towards max power. And for the most part using newer or large older carbs but I've been trying to fine a happy medium with near stock parts.
Before the 2bbl went back on there were long tube headers and an 850cfm double pump holley, ridiculous for what's there and what I'm looking for which brings me to now.
So the 2BBL runs well that I put on as a temporary carb until I worked out this 4bbl idea.
And now the only reason I'm looking to upgrade is I've also got that 4bbl intake and was wondering what would be best for my situation.
Currently the car is a 73' Duster Twister with a stock 318 with stock exhaust manifolds and stock 2bbl intake manifold. The only changes that have been made are a dual 2.5'' dual exhaust and a high flow air filter housing, with deleted vac lines and intake heater from the exhaust manifold. I'm not entirely sure what the 4bbl intake manifold is from (340 or 360) but it has the same stampings in the same places for firing order, cylinder numbers and rotation.
What I am looking for is low cost, better fuel economy, and extra power is always good. I'm not sure which size and model would be best, It was suggested that I use a 350cfm but said nothing else about it, another said to avoid Carter yet a third said to avoid Holley? So which is it? People say to avoid either one? My initial thought was a 450cfm Carter
I don't know for sure, and that's why I'm looking for some help. So what I am looking for is low cost, better fuel economy, and extra power?
Thanks
Chad
An alternative to post #7:
is to gear it up, and a bit more TC.
Once you have a 4bbl on it, you are gonna use it, and if you can't tune it properly, it's gonna drink gas. With stock 2.76 type gears and a stock 1800ish TC, it ain't gonna wake up much on the bottom..... unless you can spin both tires a good ways out.
2) If it doesn't spin
then the carb is gonna make a buncha noise but won't actually start pulling until the engine can actually use the cfm.Which is gonna be about 3000rpm when the primaries start to run out. You cannot make the carb jam 600cfm into the engine. The cam and induction system together with the cfm and rpm control how much air is gonna go into the engine. So if you have a 300 cfm primary, it's good to about 3000rpm, probably more. You can double the carb size and the engine will not make 1 horsepower more than it did on the primaries. And the worst part is 3000 rpm is about 34 mph with 2.76s and a 904.
3) If it does spin just one, it will wear out the spider gears, and eventually blow up.
4) If it spins both but only for 1or 2 car-lengths, your back at sub 3000 rpm and then you are back to paragraph two.
So the absolute most more power that a 4bbl will get you is maybe 12 hp, say 15 with dual exhaust. And this won't happen until maybe 3800/4000 rpm,and 44 mph.
Ok now at 1800rpm, your approximate stall speed, let's say your 318 can muster 140 ftlbs. Your transmission and rear gear will multiply this to 140x2.45x2.76=
947 ftlbs, into the rear axles. It doesn't matter if you have a 4bbl or not, 947 is all it's gonna make. And 947 will not spin two tires not even skinny ones.
But if you put a 3.23 in there, then you will get 140x2.45x3.23=1108 ftlbs, or ~17% more. And still on the primaries, but now,3000rpm is 29mph.
Ok but say you also put a 2400 TC in there and now your 318 spools up to say 240 ftlbs. Ok so now, on the start line, she is gonna put down; 240x2.45x3.23=
1900ftlbs into the rear axle, still on the primaries if it doesn't spin. This is an increase of 100%.......... Hyup double the ftlbs, and still on the primaries. Your cruise rpm will increase from 2250 to 2600, or the same difference as your rear gear increase, in percent. With a 2400TC your fuel mileage will only decrease by about one half the rpm difference in per cent (or about 8.5% loss) which maths out to about 2mpgs. Or you can slow down to 2250 and break even, not factoring in wind resistance.
But wait! 1900ftlbs is enough torque to spin both rear tires now, so be sure to install a SureGrip.
If you leave the 2.76s in there and go with a 2600TC, Maybe your 2bbl teener can make 260ftlbs, and the math comes to 260x2.45x2.76=
1758 into the axles on the start line, still enough to spin both tires. Your cruise rpm at zero-slip would be 2250, but I can't say how much fuel will get burned up in the 2600TC loafing.
But, there's still the 2.94 rear gear that will cruise at 65=2374@zero slip, so now the 2400TC is bang-on for cruising 65. Back to 240 ftlbs@2400, the math looks like; 240x2.45x2.94=
1730 ftlbs, still enough to spin two tires and an increase in starting line torque of 82.5% from original . And it's still a 2bbl.
A 4bbl will not touch this performance increase.In fact the secondaries may not pull in 8 horsepower until 3000rpm which would be an increase of 14ftlbs. But say you got both tires spinning and whacked the secondaries open and hit 3800 at WOT, right there, two feet from the starting line; now you get the full 15hp from the secondaries.....which is 21ftlbs;yippee!! But say your teener actually somehow manages to make an extra 25hp at 3800, on the secondaries, what's that worth? 34 ftlbs.
But say you put a cam in there too, and now you make 50 horsepower more but now at 4400 rpm; what's that worth? 60ftlbs at 4400. How about at 3800? not a whole bunch less. These are the peak numbers, not the stall numbers.But don't try this with 2.76s and the 1800TC cuz it will be a total dog off the line, unless you simultaneously increase the compression.
You can see how none of these torque numbers; 14/21/34/or60, hold a candle to gears and especially TC.
The TC can increase the stall-torque, from say 20% to say 100% depending on the numbers. If your engine is capable of generating 280ftlbs at 3800, but only makes 140 at 1800, that is a difference of 100%; so just pick what you need; enough to spin but not so much as to totally ruin your fuel mileage.
From 2.76 to 2.94 to 3.23 to 3.55 are all increasing in about 9.5% steps, so your starting-line torque will increase by the percent difference, and you can add them. Going one size at a time and working off 140 ftlbs@1800, the torque increases would math out to 13.3/26.6/40 ftlbs
The engine torque numbers I quoted are for illustrative purposes only and may bear no resemblance to your engine..... but the % torque increases are relative and so will still be pretty close.
Seriously, I personally can't wait for the power increase to begin in earnest until past 30mph with 2.76s, so the 4bbl is mostly good for passing, unless you hold it in gear to like 4500, so I hope your teener is up to the task.
My recommendation remains TC and gears, to get the power right on the start-line.
If you decide to make it a 4bbl, do yourself a favor. Do some 2bbl first-gear time trials from a dead stop to say 4000 rpm. Then after the 4bbl is on, repeat and compare.
