Yet you keep posting away.

Yawn

Yo people,

It makes a lot of sense to further elaborate as to why the fitting of a 4-barrel carburettor will unleash these ponnies hidden in your motor, & why your intake manifold choice will further enhance / alter the power-curve characteristics of your engine.

CARBURETTOR CHOICES

There are so many carburettor/manifold combinations to choose from, that it's quite easy to get confused, & just as easy to make a mistake, whereby you'll lose good drive-ability manners, if-not horsepower...Most of the manufacturers out there nearly always offered some-sort of performance package that usually involved the fitting of a factory carb & manifold set-up, that almost always had the "bugs" ironed-out, provided a "legal-tender" set-up, & unleashed the missing horsepower & torque. These should be the first to consider, if only because they work-well, & they're legal !

Alternatively, there are many & varied 4-barrel designs to choose from, & experience has shown me that a "spread-bore" design will always offer great mileage & drive-ability, whilst releasing all those horses when needed. They do this by having small primary barrels, that allows the carb to see the induction pulse from each cylinder as a much-stronger-negative-pressure-signal that will always provide better fuel atomization, and therefore better throttle-response with no-holes or dips. It should be remembered that the carburettor is actually a fuel-supply-pump whose job is to provide the correct amount of atomized fuel for the engines needs at all RPMs where possible. It must do its best to convert a liquid into a near gas-state to get the most energy out of the fuel supplied.

At a certain point in the RPM range, the primary side of the carburettor will reach a "full-saturation", & will no-longer be able to provide the air & fuel demands of the engine. At this point, the larger secondary side of a progressive 4-barrel carburettor will start supplying the engine with more air & fuel to meet these new demands. The beauty of the progressive type of 4-barrel carb is that the actuation of the secondary side is almost always controlled by the engines demands, usually by the means of an air-flap ( like a choke-valve ) above the secondary-side venturies, so that only the correct amount of fuel & air is delivered as per engine demands, regardless of poor throttle control. These set-points are also adjustable that will allow you to tune it to your own engines needs.

Surprisingly maybe to some people, a great carburettor to fit onto many differing engines ( & sizes ) is the Rochester Quadrajet. It's capable of flowing up-to 730-750 CFM, depending on year-model, & will work very well on engines of a capacity anywhere from 250 inches & up-to say 350 inches, when a " Duel-plane" intake manifold is used. Typically the primary barrels will have 34mm diameter, & the secondaries will have a massive 54mm diameter barrels. Because of this carbs operating principal, even if it's too-big for the job, it will only supply what's needed to your motor when correctly ( & relatively easily ) set-up. Nothing exceeds like excess LOL !

The general rule to follow when deciding on a carb for your engine, especially when using a "duel-plane" manifold, is to use a carb who's flow-rate capability is 1.8 - 2.3 times the cubic inch capacity of the engine your using. eg. Using a 302 Windsor ? Then 302 x 2.3 = 694.6. This would indicate that a carb of around 650-700 CFM will do the job just nicely ! If you've fitted a cam that makes the engine produce more power higher in the RPM-range, then this carb has an extra 50 CFM of air/fuel flow to provide your engine with...

DUAL PLANE MANIFOLDS

Put very simply, the "dual-plane" intake manifold basically divides your V-8 engines intake tract into 2-halves...Or if you rather, into 2 x 4-cylinder engines...The reasons for doing this were to provide relatively-even fuel distribution to all of your cylinders, as development tests proved many-varied air / fuel distribution problems at different operating RPMs occurred when using a single-plane design. This occurrence happens because of firing orders, & hence intake-valve openings of some cylinders that are side-by-side to each other, causing one of the two cylinders to be starved of air & fuel, at different operating RPMs. Also, sometimes two cylinders will be drawing the mixture on the same cylinder-bank one-after-the-other, causing the same issues when the air/fuel mixture is needed to be yanked back-across to the other bank. I won't be more specific since different makes of engines will have numbered their cylinders differently to one-another, & therefore have a different "firing-order", yet the physical pattern of this "induction-flow" will be near identical, if not the same.

Possibly the most important thing to realize here is that with the "dual-plane" design, each of your engines cylinders "sees" only 1/2 of your carburettors flow capacity, since each cylinder is only exposed to two of the 4-barrels. This obviously enhances the negative-pressure-induction-pulses from your cylinder, & aids greatly to fuel distribution plus atomization, but will almost always have a limit as to how-far your engine will freely rev-out to. This type of manifold has other unavoidable hurdes in that each intake runner is a different length, & almost always meets into a "T" intersection where the air/fuel then gets redirected into cylinders on opposite banks. On a more positive & realistic note, should you find that you only rev your engine out to say 4,500 - 5,000 RPM, then this type of manifold design will be exactly what you need !

SINGLE PLANE MANIFOLDS

When all-out power & revs beyond the say 5,00 RPM threshold are desired, there is no better choice ! This manifold is designed to have "relatively" straight, direct & equal length runners, & the problems associated with fuel distribution are considered secondary when compared to the out-right performance gains...Especially since they abate a little when the engine is at full & stable operating temperature, & continued / sustained high RPM use is evident. The "bugs" can be "tuned-out", if not ironed-out completely by various methods, often by extending the the length of the intake-runner of the cylinders that fire one-after-the-other & when side-by-side on the same cylinder bank...Carburettor selection is much more difficult to generalize here since each cylinder sees all 4-barrells of the carburettor, hence the full flow capacity potential is available to each cylinder. As a general guide though, for say a 5.0 ltr capacity engine that will comfortably rev to say 6,000 - 6,500 RPM, a progressive 4-barrel carb of between 500-600 CFM would probably provide reasonable drivability through-out the entire rev-range, without sacrificing top-end power, but it all comes down to a lot of variables, not least of which is valve over-lap ( cam ) & your own specific requirements. You will however drink more fuel & have poorer running when the engine is cold, but you can still quite successfully use a set-up like this on the streets !

Cheers,

Rastus

PS There's nothing quite like the induction-roar heard on a V-8 with a 4-barrel carb at WOT !!! Even SELLC will say it's just as nice as a CISE 560 at WOT, if not better LOL !

Rex thinks digital ignition, coil over plug, direct injection and 4 camshafts is the way to go.

But in terms of bang for your buck aka lower cost horsepower I like the Edelbrock carbs, which are basiclly re-branded Weber's.

For whatever reason, Holly carbs like to crap out often plus I never was a big fan of Chrysler which was the only OE installer of Holly carbs that I am aware of. Quadra-Jets are okay, but the feedback carbs were a pain in the ass, and even then all the QJ's are a pain in the ass to rebuild, however I will give the Rochesters this ---- they were pretty fucking tuff and packed a good punch for factory GM vehicles. Then again I bet if you built a shit ton of any of them it would soon be childs play, but that's my point... The Edelbrock / Weber carbs were the ones that could handle a backfire and several years of abuse with no need to mess with it.

But that's just me. Carbs were on there way out when I was on my way in.

Hi PogoPossum,

Thanks for your insight & quality post ! Yeah, I agree that there's nothing like the sound of the larger V-8's at WOT running with long-period cams, & I think it's a shame that the younger generation of people are left with the modern EFI engines with arguably very limited tuning options, & very expensive ones at that ! This whole thread was pretty vacant of posts for a long time, & it seemed to me to be a door to open to the not-so-distant past, & to remind people that once upon a time there was quite a few and many varied tuning options for all makes of engines, V-8's or otherwise. You could have fun & experiment with things even with a limited budget, & make your own unique combination of "Go-Fast-Parts" & have fun with your friends.

Anyhow, you're correct in saying that there are a lot of elements to consider when choosing an intake-manifold, particularly a single-plane design, & I was only trying to show that not so long ago, you had a smorgas-board of options to choose from, single-plane manifolds being one of them, & that the power out-puts of these engines still provide comparable & even superior out-puts to even the modern-day engines, all-be-it without modern-day refinements etc.

Anyhow, the point for me is that you can still have an older car that is more than capable of delivering the goods, & have plenty of money in the bank to play with things, plus have the potential to out-perform some modern cars for a fraction of the cost. The only problems would be the law & how legal-tender your modifications hold up...

Cheers,

Rastus

Hey PogoPossum,

I'll bet you've been very-careful with your 2nd-3rd gear-shifts ever since ! I think you're right in suggesting finding a "pre-emission" compliant vehicle, not so much because you "can" modify them to a certain extent, but simply because the overall performance of even a standard engine was surprisingly pretty-good ! Even the factory performance options were more than subtle improvements over standard, & they combined a full-warranty to go with this option ! For me it's not so-much that the performance of todays cars is less ( finally after 25+ years they actually go quite well again ! ), but they lack "personality"...Who knows, in 20 years time I'll probably be driving something that's "new" today, & saying the same thing LOL !

Still, let's all hope that they keep finding the "crude" to keep piston-power going strong, because I think it a sad day should we all be forced to drive electric cars ! Can you imagine them trying sling us a $100:00 dollar-bill to re-charge the battery at the service station ?...

Cheers,

Rastus

PS One of the main reasons why I bought my little 1973 3.5ltr SLC was knowing that by the end of the 1980's, it actually produced about 30 more horse-power than the 4.5ltr...And also because it happily revs to 6,300rpm in standard trim with no-mods. Combine that with to-days ever increasing fuel costs, the 3.5 "should" be reasonably easy to afford to run for the next ?- many years... Around 600ks per tank of gas to me is better than 500ks per tank (4.5ltr), especially since it's $1:80 per litre for 98 octane fuel now down this way...That's about $6:80 US per gallon...