Making a Low rpm Triple 350 cfm carb setup

xctasy

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For the longest time, I have been interested in doing a triple carb engine.


Because of certian software and hardware restrictions on the stock EFI unit on my X-FLOW engine, I've decided to lay it asside for a while.



I've found some calculations on ideal venturi sizes for independent runner intakes. I've been looking at Jacks six carb set-up for many years, but have recently come into a cash crop of 350 cfm Holley carbs.

Most of us here see only IDF or DCOE Webers as options, but I'm looking at a set of 350 cfm Holleys with tiny 30 mm venturis (1.1875") and looking at mating them with the current tractor tech EFI cam I have in my 250 six.

I've been studying this graph,

DV_Carb_Sizing_II.jpg



and done some rearranging to this. It produces the carb size in mm's via the LN function at 6, 8 and 10 000 rpm, so its out of range for the 3500 to 5000 rpm our stock sixes often do.

VenturiSizeInmmforIndepentRunnerCar.jpg




I want to see what happens at 4000 rpm with a very restrictive carb on an engine which normally doesn't like to rev much. I'm predicting the low end torque and 4000 rpm power will be excellent with these carbs. Time will tell.


I'm sorting out an adaptor plate for my stock intake, and decided to make it so I could run a stock 2-bbl and then hook up two outer 350 carbs when I got my gear sorted out. I have to certify this car, so I'm thinking it might as well have all the good stuff on it!
 
8) not a bad idea deano, but dont you think that three rochester 2bbls might be better as they are smaller carbs?
 
Affirmative on the 2CG Rochesters, much smaller, better carb. Horizontal split line, cast iron throttle body, nice smlaa plateform like the early Stromberg 97's, same as 500 cfm Holley throttle shaft arangement, plus a history of running on multicarb intakes. I've got one off a 327 Chevy in my basement!

Over ruling those pluses in a Huge plus for the Holley.

A) Easy access to the 4500 Dominator idle circuit used on independent runner drag racing carbs.
B) The 4150 series float bowls go right on and are supper compact, muche better than the 4160 style item from the 4-bbls. In the old days of dual quads, 4150 float bowls were a little more common, but they are still there!

If you score the 1976 to 1988 Economaster 350 I've seen used on the ME 221 or as an aftermarket carb from ebay, they came standard with early 4150 fuel bowl. It can point in towards the rocker cover on either a 200 log or any X-flow. That means the fuel stays close to the centre of the engine.


As for size, a 200 with triple 350 cfm 7448's or 9117's will be perfect for about 4500 rpm or so.


The 250 with triple 350's will be down to 3800 to 4000 rpm peak, and any independent runner intake will fail to tune properly past that level. You'll either get a great mid range and lousy top end. The cam has to be a low duration item.

If you used 500 cfm carbs, a 250 would run fine to 5500 rpm with power at 4800 rpm.

To run carbs as indopendent runners, the power valve is blocked off that the other circuits tuned to suit.


Just like on the Screamin Eagle side draft 2300 series carbs. They ran the 2300 Holley with a hybrid three bolt throttle body, and it runs a 58 main jet and 35 mm venturis on a 42 mm carb. Aftermarket 2-bbls are going to 1.58" ( 40 mm) and replaceable venturis. If the carb is sized right for inertail ramming, it can be done with a stock Holley carb if the 40 year old Weber carb sizing chart is honoured!

H-Dcarb.jpg


holleyTech01.jpg
 
As you said, time will tell.

This is interesting information, and i'll spit some out so you can get an idea of what I'm seeing:

200: At 6k, 39mm venturi. At 8k, 48mm venturi. At 10, 54~mm venturi.
250 At 6k, 42mm venturi. At 8k, a 53mm venturi.

It seems that low RPM's are more sensitive to venturi sizes. However, with a 30mm venturi, that might be a tad bit too restrictive for a 250. If that's all there is for options, then I can understand.

I'd personally take the 6k plotline, subtract about 6mm for 5k 200 or 8mm for 5k 250. I'm considering using multiple carbs despite FI, but that is mostly a far-fetched idea.
 
Triple 350 cfm carbs will work just fine on a 200 cube engine, and all you have to do is grab a stock log head to do it.

If I can resolve a calibration, then our dreams of cheap 350 carbs on 200's and maybee even triple CFI systems may become a reality. A stockish engine, and a lot of good networking from all of us will make this thing work, I'm certain.

3V2300_1.jpg


3Vcfi_1.jpg



Yeah, its been something I've wanted to experiment with. There is an EFI intake which can be used to mount three 2300 carbs. I've found heaps of little 9117 Holleys in NZ, they are real common, the only emission era 2300 with dashpot, air vent and econmaster venturis. I can buy three carbs for the price of one fuel regulator!

Holleys hate reversion (fuel standoff and fuel spit back happens under low vaccum), so even running three very small carbs is likely to be very difficult.

The off idle performance is hampered because the carbs see a third of the original vac reading at any given engine speed, accelerator movement and load. When a really big Holley is placed on a small engine, you can always restrict the power valves and rejet it, even if its not sized perfectly. The off idle performance can be improved by reworking the thottl blade transition circuit, and the pump and cams can be fiddled about with to get a good result, aka wsa111.

With the IR runner set-up, and even using a some smaller carb venturi, it will be hard work. The carbs have to be perfectly seamless in there fuel delivery curve from 500 to 5000 rpm, from 0 degrees throttle angle to 80 degrees, from 10 inches Hg of vac to 0.5.

The trick is to use some old, ancient Holley tricks.

Some Holley carbs have it all sorted for an independent runner intake. One is the old #6214 1050 cfm 4500 series Dominator. There are a range of newer ones and some 850 cfm 4150 carbs that have the mods I'm looking for.Such as the long, thin booster venturi to respond to tiny vaccum changes and one other thing....an intermediate circuit metering block which bolts right on to a 2300 with a few mods. And the velocity stacks used on many 4500 and the rare 6425 650 cfm 2-bbl.

650cfmPN6245_2300series.jpg


My friend drag races a water injected Pontiac Formula, and about three years ago he said that he'd like to experiement with pluse tuning at very low rpm (His 455 only revs to 4800 rpm, and he's mad keen on running a special quad carb set-up at some stage).

Since then, I've looked at trying to build power without increasing the rev range. The carbs are optimised, the cam is optimised, and the engine runs high compression and maybee an anti detonation additive.

The other plus is that when the 2300 carbs are mounted on the stock intake, you can then cut off the back flange like on the old Tri Power Holleys on Six pack Mopars and Shelbys and Vettes.

Each runner can be isolated with ease, and the whole carb can be turned inwards to the rocker cover. Then there is heaps of space for other heavy duty objects (supercharger!) to fit close. It's really amazing that the 5 1/8" air cleaner boss can have 2 inches cut off if space is tight. Then you can package it anywhere when engine bays are tight. A good option for running a Tripple 500cfm 300 I6 in a Falcon or Mustang without having to cut up the spring towers.
See how compact a Holley carb can become?

R1653HolleyMiddleCarb.jpg
.

That's a lot of little jobs that I'm sure I can do.

The other spinoff is that there are now some really great air cleaners which allow you to run ram tubes inside the carb, and still have as good airflow as a having no air cleaner at all.

I have been working on a single 2300 carb on my X-flow engine, with this air cleaner

KNHA-4503_forHondaCFR450RandCFR250R.jpg


and a few other tricks. Once I've got that sorted out, I send it to a buddy in Texas, and then start on the tuning three carbs.


The 1980 250 x-flow Engine runs the stock EFI 264 degree cam, alloy head, 350 cfm carb, welded intake, Pertronix 1865 electronic ignition, a stock 1989 AOD, and stock exhast with no headers.
 
This a bit off topic and probably really out there, but I wonder if that Harley carb would work on my 170. The only advantage I see is to be different. But nonetheless, would it work do you think?
 
Three Hog Screemin Eagle carbs would fit, but would cost about 600 bucks each, and you'd have to make a huge plate to separate the extractos from the float bowl, or fuel would boil!


One would work okay. 600 bucks worth of individuality!
 
X - you know what might be interesting are the split Dominators the US Pro Stockers use. I'll see if I can dig up a link, as these are basically a 4500 split in half to center over the runners a bit better.

IIRC the Pro Stocker 4Vs are running 1250 or so cfm, so a 3-2 setup would wind up at ~1000cfm on an IR intake.

Just some food for thought...these are obviously in the "not cheap" category.
 
In a true IR system, you may find that triple 350's may be too small. Each half of that carb has to feed a cylinder on each intake gulp and for that small period of time may not have the flow capacity to meet demand.

However, there shouldn't be any real challenge as far as the carbs themselves. In most V8 applications with a dual plane manifold, each side of the carb acts independently. The same holds true when moounted in an IR configuration, though you will have to account for jetting and power valve operation differently.
 
I myself have been looking at a triple carb setup. I have a 300 and am putting 3 weber 32/36's or holley 5200's (same carb) on it. The main reason is...
1. The smaller venturi will keep a lot of the low end torque.
2. The weber is a progressive carb, So it gives fuel mileage benefits.
3. Easy to find, any 70's 4 cyl w/ a 2 barrel.
4. CHEAP! I found them for pinto's for $30

The Weber is easily adaptable, I know it doesn't have high rpm potential, but it is a 300 Torque Beast. It wasn't built for Rpm, but low end. My turbo setup and cam and head (on the way) are all based around this philosophy. Smaller carbs work well as long as you keep their flow restrictions in mind when building your motor around them.
 
I'm ramping up the "legacy" 250 and currently running it on the single center 7448 for initial testing. I've been trying to find a way to physically fit three 2BBls on an Offy 3X1 adapter but it is very tight. The progressive 5200's are not much smaller. Langdon's Stovebolt sells new Carter-Weber carbs used in multi carb apps that are physically smaller than the Holleys' . I don't know their CFM or pedigree but I believe they are not progressive.

ENGINERADFROMFRTRT.jpg


Powerband 8)
 
Lovely stuff, Powerband.

There is a weird thing when you go from pure isolated runner (like a Triple DCOE 40, 42 or 45 set-up) to a simesed set like on a 180 degree intake on a V6 or I6. Apparently, you end up not needing a lot of venturi. So a 250 reving to make power at 5500 rpm needs DCOE 45's with 40 mm venturis, but if the 2-bbl carb joins into one runner, then you can make do with A 32/36 with 26/27 venturis. There is a loss of power, but there is no pulse tuning, no fuel standoff, and the engine is less sensitive to jetting requirements than with an isolated runner set-up.

Three 5200, 5205, 5210 or 6500 Holley Webers should fit okay if the outer ones don't have an electric or water heated choke. Problem is

a)height and
b)the need to run three Stovebolt adaptors.
c)And the increase in loading when the carbs go to the secondary circuit, thats three times the peak load.

Three DGES 38's would be better, as they open all at once, and you could just reduce the idle jets and go conservative on the power valve and mains.

I'm looking at isolated runner at very low revs. The better Triple Weber D-type Jags and DB5 Astons and even Charger R/T E38's have a magically refined low speed amble when in high gear, despite big cams over 280 degrees. So some kind of spin off from the Offy style progressive linkage, with an ability to run all carbs below 1200 rpm, but just the centre from 1200 to 2500 rpm, and then back on all three from 2500 to 4500 rpm should work really well.
 
You lost me a couple of times. You said IR then you said run on the center carb until sorted then add the others.

I'm looking at isolated runner at very low revs. The better Triple Weber D-type Jags and DB5 Astons and even Charger R/T E38's have a magically refined low speed amble when in high gear, despite big cams over 280 degrees. So some kind of spin off from the Offy style progressive linkage, with an ability to run all carbs below 1200 rpm, but just the centre from 1200 to 2500 rpm, and then back on all three from 2500 to 4500 rpm should work really well.

So, you are trying to build a progressive/IR intake?

If the Webers are capable of running in IR configuration, then they will be easier to tune with a plenum?

I have also considered an IR setup and the Holleys are priced right. If the blower and turbo guys can reference the power valve to boost, couldn't we keep the power enrichment and activate it some other way than manifold vac?
 
351wF-250":3meey6h2 said:
I myself have been looking at a triple carb setup. I have a 300 and am putting 3 weber 32/36's or holley 5200's (same carb) on it. The main reason is...
1. The smaller venturi will keep a lot of the low end torque.
2. The weber is a progressive carb, So it gives fuel mileage benefits.
3. Easy to find, any 70's 4 cyl w/ a 2 barrel.
4. CHEAP! I found them for pinto's for $30

The Weber is easily adaptable, I know it doesn't have high rpm potential, but it is a 300 Torque Beast. It wasn't built for Rpm, but low end. My turbo setup and cam and head (on the way) are all based around this philosophy. Smaller carbs work well as long as you keep their flow restrictions in mind when building your motor around them.

This make sense to me, mpg, $$$, etc. Keep us updated on this project. Sounds simple, affordable and useful.
 
Stubby":2h0wqrca said:
You lost me a couple of times. You said IR then you said run on the center carb until sorted then add the others.

I'm looking at isolated runner at very low revs. The better Triple Weber D-type Jags and DB5 Astons and even Charger R/T E38's have a magically refined low speed amble when in high gear, despite big cams over 280 degrees. So some kind of spin off from the Offy style progressive linkage, with an ability to run all carbs below 1200 rpm, but just the centre from 1200 to 2500 rpm, and then back on all three from 2500 to 4500 rpm should work really well.

So, you are trying to build a progressive/IR intake?

If the Webers are capable of running in IR configuration, then they will be easier to tune with a plenum?

I have also considered an IR setup and the Holleys are priced right. If the blower and turbo guys can reference the power valve to boost, couldn't we keep the power enrichment and activate it some other way than manifold vac?


34248979_258_amc_triple_2300_Holley.jpg


34248941.jpg


daves-engine.jpg



The intake manifold has three carbs mounted like the ones above.

At idle, they run all three, independent runner. The log has two flapper valves isolating the three carbs.

Then above curb (kerb) idle (1200 rpm), the flapper valves open up, and the outer carbs drop out of the circuit. Above 2500, its back to independent runner, triple carbs, with the flappers shut.

In this manner, the centre carb runs most of the time, runner high vaccum. Under low vac conditions, it can run richer.

'How' the two flappers operate to convert it from log to isolated is subject to patent!
 
Summary of my purpose.

1. I'm trying to bring Port on port, isolated runner 'pulse tuning' right down to our stock 252 , 256 and 264 cams used in 200 and 250 I6 Fords, and fill out the torque power and economy curves as much as possible with stock Holley 2300 series 350 or 500 cfm parts.

2. I'm defining ideal Holley carb sizes by use on existing 200 and 250 engines, and defining new so-called Y3.5, Y4 and Y4.5 values for 3500, 4000 and 4500 rpm based on the time honoured Weber Isolated runner venturi chart

Background absolutes.

1. The ideal engine capacity for 2300 series 350 and 500 cfm, or split 4500 1050 or 1150 Holleys on I6 engines is a natural log (Ln) function. The Ln function varies due to rpm at maximum power.

As listed above, the venturi sizes in mm's are listed

at Y6 (6 000 rpm) is 13.9 Ln(x)-48.9,
at Y8 (8 000 rpm) is 17.1 Ln(x)-59.6
and Y10 (10 000 rpm) is 17.9 Ln(x)-58.9

From http://forums.hybridz.org/showthread.php?p=680623, I followed a link, and found the formulae for 5000 rpm via use of a Weber Jetting program used for 1200 to 2400 cc 4-cyl engines. It confirms the Y6 and Y8 equations. For Y5, the formula is

Y5, (5000 rpm) is 12.5 Ln(x)-46.8

The carb tuning looks to be so sensitive that a venturi even a mm or 40 thou too big or too small can vary the power rpm by 200 rpm.

I'm aiming for a Y4 (4000 rpm) or even Y3.5 (3500rpm) target.

I am now summarizing that a 250 Falcon with a three #4412 500 carbs needs a cam which provides peak power at 5000 rpm.

A 200 Falcon six with three #7448 or 9117 350 cfm carbs needs a cam whch provides power at below 5000 rpm, possibly 4800 rpm.

A 250 Falcon six with three #7448 or 9117 350 cfm carbs should only need a cam which provides power to 3500 rpm to run in an optimized manner.

2. Since I have an engine block that can be built as a 200, 221, or 250, and a block which can take any Ford head, and any number of cams, I can test any engine configuration.

3. The time hounoured standard jetting of isolated runner engines with no power valve, is always 1/25 th of the venturi diameter. So a 350 will like a 48 jet, a 500 will like a 56 jet. Since one carb is working for only a third of an engine , a power valve may need to be two stage and the PVCR's have to be reduced from 53 or 62 thou down to perhaps 16 to 19 thou. Any of the commonly available two stage power valves will cover for the difference from open loop (11.5:1 , wide open throttle ) to close loop (14.7:1, cruise) conditions. This will allow an effective enrichment of 2 or 3 jet sizes whenever vaccum drops, but will do so in stages. With some science applied, a triple carb engine should lean cruise like an EFI champion.
 
Fascinating 3- 2bbl set up. Wonder how cylinder to cylinder filling is kept constant when running on just the center 2-bbl?
 
Back in the late 1940 s maybe early 50s dual carbs were real popular. Someone figured out if you took your single one barrell carb reworked the manifold and put another carb on you could out run the guy down the street that only had one carb. Then the 60s came. A little LSD, some legendary muscle cars and even manafactures started throwing on multiple carbs. Dodge came up with as many as 6. Some racing engines had 8. 1 per hole. Why not 2 or 3 carbs per hole? Then nascar said wait this is stupid we are going to start limiting carbs, and holley stepped up. If you cant put more on- then make one carb bigger. This is acttually profound. Why would you want to bolt on 9 lawn mower carbs equaling maybe 300 cfm when you could put on you 4bbl equaling 1200 cfm. Why all the work to try to size and syncronise those carbs for what return? someday they are going to need repair. Do you want to repair 9 small carbs or 1 big one? Back in the day when nothing else was avaliable it made sence but today its at least borderline stupid. Put on the modern carb that flows for your enginge. Anything else is just a lot of extra time money and work, only to dump unburned gas out the tailpipe. 1 engine 1 carb kiss lol
 
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