2bbl or 4bbl carb on 250 2v engine.

[Stumps]

I've narrowed down the carb selection to two carburetors for my 250 2V build in my 74 XB Futura and I'd like some opinions.

I'm thinking of either going for an Autolite 2100 with 1.23 venturi (356cfm) on the factory 2 Manifold

Or

A 4100 with 1.08 (480 cfm) venturi on an Aussiespeed manifold.

Obviously the 4100 is a fair bit more expensive as I have to purchase both the manifold and carb (unless I use a 2bbl to 4bbl adapter, not sure how well that would work) but it would yield more overall power and I suspect better economy at light throttle settings given how much smaller the primaries are.

The 2100 would be easier to mount as I already have all the brackets and manifold to suit and cheaper as I only have to buy the carb (I have a rebuilt 1.14 venturi already).

I am chasing 220-250 bhp from the engine.
Which is a 30 thou over 250 with a 2v head with oversized valves fitted, a 214@50 thou speco cam (may change to a 224@50 thou crow cam) comp ratio is 9.5:1. The ignition will be a rebuilt points dizzy with a MSD ultra 6AL plus CDi module. The exhaust is pacemaker header into a dual 2.25inch system.
The trans is a WC T5 and the diff is a 3.27:1 lsd.

Is 220-250bhp doable or am I dreaming?

What would give better results the 2 barrel or 4 barrel?

 

[Frank]

The 4 barrel is much bigger, as the CFM formula used for 4V carbs is not the same as 1V and 2V. CFM can not be directly compared between 1/2 vs 4.
Using the 1/2 barrel measuring standard , a 4 barrel with 480cfm would be rated @ 677cfm. Reversing this, using the 4 barrel rating standard method, the 1.21 carb @ 353cfm would be rated at 250cfm.

677 over 353= 1.92. The 4100 is 92% larger than the 1.21 carb.

353 cfm will not starve for air on a 250" engine @ 80% VE until 6175 RPM. 677 cfm will not be fully used until 11,850 rpm.

 

[Stumps]

Yeah I get that 2 and 4 barrels are measured differently, which is what makes things confusing.

I know 4bbl's are measured at 1.5 inches and the 2bbl is at 3 inches but how do you determine what is the correct measurement for the engine?

289 V8's are often factory fitted with 4100's or bigger and they work very well, a 250 is only 39 cubes smaller so carb size wouldn't be that much different using the usual formula's

My understand of Vac secondary carbs is the engine will only open the secondaries as far as the available vacuum will allow so lets say on an engine that can only flow 350 cfm then using the 4100 as the example the front venturi are 240ish cfm which would provide better down low response compared to the 350cfm 2 barrel. but when the throttle is opened right up then the secondaries will only open 110 cfm or so...that is my understanding of how they work and I have seen this in practice with our 2v 302 and 351's Clevo V8's here in Australia in Falcons getting better cruise economy and low down response when changing from the factory 2 barrels to a 4 barrel which has smaller primaries such as a 600cfm Holley or something similar.

I could be way off here and maybe 6 cylinders are measured differently.

very happy to be corrected on this.

 

[Frank]

Yeah I get that 2 and 4 barrels are measured differently, which is what makes things confusing.

I know 4bbl's are measured at 1.5 inches and the 2bbl is at 3 inches but how do you determine what is the correct measurement for the engine?

289 V8's are often factory fitted with 4100's or bigger and they work very well, a 250 is only 39 cubes smaller so carb size wouldn't be that much different using the usual formula's

My understand of Vac secondary carbs is the engine will only open the secondaries as far as the available vacuum will allow so lets say on an engine that can only flow 350 cfm then using the 4100 as the example the front venturi are 240ish cfm which would provide better down low response compared to the 350cfm 2 barrel. but when the throttle is opened right up then the secondaries will only open 110 cfm or so...that is my understanding of how they work and I have seen this in practice with our 2v 302 and 351's Clevo V8's here in Australia in Falcons getting better cruise economy and low down response when changing from the factory 2 barrels to a 4 barrel which has smaller primaries such as a 600cfm Holley or something similar.

I could be way off here and maybe 6 cylinders are measured differently.

very happy to be corrected on this.

No correction, you've got the picture. There's no "correct measurement" for a specific engine. Manifold vacuum will increase from zero when (if) a carburetor begins to be a restriction to the incoming flow demand of the engine. So if an engine maxed out the 2150 and the intake read 1.5" of vacuum, that would be drawing 250cfm. (from the calculations in post #2). If the engine keeps increasing rpm and reached 3" of vacuum the same 2150 carb would be flowing 353cfm. Carbs don't suddenly shut off, the power continues to increase, but to a lesser degree with a smaller carb.

A possibly simpler way to get a mental baseline for deciding a carb is, how many cfm does the engine actually consume? Assuming 80% volumetric efficiency a 250 cu in 4 cycle engine consumes this:

1500 rpm: 87cfm
2000rpm: 116cfm
2500rpm: 145cfm
3000rpm: 173cfm
3500rpm: 202cfm
4000rpm: 231cfm
4500rpm: 260cfm
5000rpm: 289cfm
5500rpm: 318cfm

There is an additional factor I call the "gulp factor". Since piston engines are not a smooth inhaling rotary pump, the intake cycle is a relatively quick burst of inhaling, carbs that are larger than the engine's net air consumption make more power (usually) because there's extra flow capacity for those spikes of intake cycling. Intake manifold design plays a big part in the "gulp efficiency" as well. A larger plenum intake can use a smaller carburetor, since more air for the gulp is already present in the intake.

You pointed out a 289 having a 4100- true. But most factory carburetors had a cfm rating very nearly equal to the cubic inch of the engine. Smaller carbs get a better vacuum signal internally, so they have better cold-start, quick, sharp throttle response, and good economy. 280cfm 2V carbs made 310lbs of torque and 210HP on 307" V8's, for example. The 353cfm 2150 carb came factory on thousands of 351 and 400 cubic inch V8's.

None of this is advice, there's pros and cons to both carbs you're considering. Either one can be tuned to run very well and strong on your 250.

 

[Engine Fan]

Maybe the 4 barrel with a divider in the plenum?

 

[aussie7mains]

I've narrowed down the carb selection to two carburetors for my 250 2V build in my 74 XB Futura and I'd like some opinions.

I'm thinking of either going for an Autolite 2100 with 1.23 venturi (356cfm) on the factory 2 Manifold

Or

A 4100 with 1.08 (480 cfm) venturi on an Aussiespeed manifold.

Obviously the 4100 is a fair bit more expensive as I have to purchase both the manifold and carb (unless I use a 2bbl to 4bbl adapter, not sure how well that would work) but it would yield more overall power and I suspect better economy at light throttle settings given how much smaller the primaries are.

The 2100 would be easier to mount as I already have all the brackets and manifold to suit and cheaper as I only have to buy the carb (I have a rebuilt 1.14 venturi already).

I am chasing 220-250 bhp from the engine.
Which is a 30 thou over 250 with a 2v head with oversized valves fitted, a 214@50 thou speco cam (may change to a 224@50 thou crow cam) comp ratio is 9.5:1. The ignition will be a rebuilt points dizzy with a MSD ultra 6AL plus CDi module. The exhaust is pacemaker header into a dual 2.25inch system.
The trans is a WC T5 and the diff is a 3.27:1 lsd.

Is 220-250bhp doable or am I dreaming?

What would give better results the 2 barrel or 4 barrel?

Small 4 barrel around 500cfm will work on the Aussiespeed manifold, stick with the 214 cam for everyday driving, the 224 will be a bit rough. Run a Bosch Australian made ignition with its matching coil and 12v supply via a relay, MSD is expensive and unnecessary. Your exhaust sound perfect, Ive found a single 2.5 with trubo muffler to be best, but there isnt much difference, twin will be a bit noisier, and cost more. A divider welded into the middle exhaust port fit the headers better, but isnt essential. 9.5cr is just fine, filling the cylinders is more important than high compression. 220-250 is a bit high with this combo, 200 more real, but thats still comparable to a good 302. Do run an electric fan and ideally a electric water pump to reduce parasitic losses. Cold air works as a cheap power help.