1-bbl per two cyl carb (TriOffy i6 & Twin Edelbrock i4's)

[xctasy]

Found “Fran” Hernandez most famous phot the other day...,

In relation to the idea that its not about total airflow...

That got me thinking...

Year ago, Australlian Group C, Group A and then AVESCO racer Larry Perkins let Group A racers know a little secret. That his 4-bbl Quadrajet 304 cube racer made 460 hp at 6500 rpm, and it beets 462 hp at 6700 rpm. That got me thinking that true induction index of performance or efficency is when you make the maximum amount of power with the least revs for a given capacity. Many have used this before, but its "capacity, multiply it by revs at maximum power, then divide it by maximum power"

You get an Aspirations Index. The lower the number, the more efficent the engine is. In imperial, its (Cubic Inch Displacement*RPM) divided by flywheel net BHP. In metrics its (cc3 displacement*rpm)/kw.

There would be few that argue that on a revs per minute, per cubic inch basis, the old one throttle per two cylinder NASCAR 355 V8's make just about more power than just about anything gasoline powered without nitrous, a turbo, blower, or rocket fuel. 850 Bhp at 900 RPM from 358-cubic-inches is 3790. If you make 750 bhp net at 16000 rpm from 146 cubic inches, thats an Aspirations Index of 3114. I'm sure if a NASCAR had quad cams, air valves and fully interated spark and engine managment, it'd do 3000 Aspirations index.

One of the most effiecnt engines is the 109 mph, 1593 pound 1959 Saab 93 XP, 90 bhp at 6500 RPM from 46 cubic inches, an Aspirations Ratio of 3300. It is two stroke, so its not really apples verses apples. But it is a 2-bbl carb driving a 3 cylinder engine.

https://www.youtube.com/watch?feature=p ... _lXG2q4xGw


So both the 390 cfm 4-bbl carbed NASCAR V8s and old LeMans 2 strokes are using the same total cfm of carburation at 1.5"Hg , and getting class winning performance without even F1 style port on port induction. One flowing over 650 hp, the other 90 hp. The big engine has 8 times the capacity, and about 8 times the power with the same size venturis.

And that got me thinking...wouldn't a well fettled 3-bbl I6 do just as well?

I mean, a 2-bbl In line six cylinder TC3000 Argie racer does 390 hp at 9500 rpm from just 183 cubes, an apiration index of only 4457..

A properly optimized area carb barrel might give another 25% more power.


That got me thinking..


Doesn't this Triple Solex 34 W2 manifold throttle body , which supports three 1.14" venuris, just look like it could take the top of three Carter YFA's co joined?.




Four stud base, just perfect for three YFA Carter 1-bbl carbs


If some ones Two Stroke Saab 96 is missing a triple carb. I've got 'em...

 

[MustangSix]

Deano, let me toss another twist into that Pinto manifold setup. By pairing 1-2 and 3-4 you have effectively siamesed the two intakes. If you look at the firing order, you find that the two intake events overlap. The end result is a charge robbing situation where one cylinder on each pair is at a VE disadvantage. A good explanation is here: http://www.starchak.ca/efi/siamese.htm

On 4-bangers with separate ports a better solutions is this:

Another other potential way to fix the overlap issue is with a crossplane crank ala the Yamaha R1.

On an inline six, there is no overlap issue, thus dual and triple carb setups work very well with only a balance tube usually fitted to help smooth out idle. Offy makes a dual carb setup like that for the Mopar slant six. Often these are modded to use dual Weber downdrafts.

 

[xctasy]

There is some interesting fuel injection for one throttle per two cylinders as test by David Vizard in 1988. It showed the same thing.


Those set ups are really what a restrictor plate Nascar 4-BBL was....two siamese 2-bbl fours! The 2-bbl six cylinder engine is therefore the odd man out, with extra pulses, extra length to transmit air and fuel to, and the same holds ture but more so. This is why I really am convinced that a single 3 bbl inliner is much more sensible for power than a triple 3-bbl.

My background was with each of these engines, the A series and the Pinto SOHC I4. Each spent long hours under the foot of Mrs Xecute from 1988 to 1996 with SU HS6 1-3/4 and a variety of 2 bbl Holley carbs. Each produced exceptional fuel economy with sprightly performance in my 1098 and 1993 cc engines.

In 1970-80's dyno tests with siamese port A-series Mini 1275-1435 engines, just a 2-bbl 45DHLA/45DCOE, the sharing of two ports has no advantage if all else is equal to a single 1-3/4" carb at the 90 to 116hp level. Similar tests with the B series show no advantage with twin carbs at 115 hp levels.Consider this


the excellent 2-bbl Piper RDA throttle body fue injection system,

In A series situations, fine fuel is atomisation, with the above system lost a large amount of power with siamese ports. The findings were that fuel has to be atomised at a certain large droplet size to gain power. In everything else, fuel economy, transient response, tractorbility, any kind of measure, the fuel injection system was a delight. It principally failed to make power and torque compared to any of the same kinds of 1-3/4 inch 1-bbl or 2-bbl carb set ups in the 240 cfm per throttle sizes. A massive 18% loss of 15 hp and 500 rpm off the peak power level of 98 hp on an 85 cubic inch A series 1400 cc engine. The same thing happens in B series engines when you add a Ford Tempo TBI system, a very good almost 1-3/4" injection (1-9/16")system. You loose power and torque, but gain everything else.

Ford went to big single 1-9/16" electronic 1-BBL carb to replace the 2-bbl Holley Weber on the 2.3 OHC. The Tempo HSC carb got a 1-9/16" carb, and it worked too. The in line 3.3's and 4.9 F150 trucks got the same carb but suffered. It was not a bad step, it made economy and kept power, with an improvment in mpg and emissions...when the system's MCU or EECIV was working properly on the F150.

With the single DCOE 45 or DHLA 45, on the 4cyl Pinto 2000 engine with the Lynx intake above in your post Jack, it is inferior to an independent runner systems with twin 45's, but the torque is similar. The reason is that the pulse between cylinder induction doesn't create any degree of overlap to one throttle serving two cylinders. The advantage with these non independent runner, non port on port engines is that you don't have to run twice as many throttles to get the same low end performance. Any IRPOP system will show gains over a siamese, doubling the carbs and giving them a straight shot always does that, but a single 1.6875" carb doesn't eclipse a 2-bbl 1.6875" carb like the Holley 2-bbl 500 cfm carb at the 125 to 155 hp level in a good flowing head and intake like the SOHC can have.


It all comes back to a 6 cylinder setting.



With more induction pulses, you can see why throttle throttles per engine would replicate the same dynamics as the 4 cylinder. Normally, you add 50 to 100% more cylinders and carbs in a common runner, the duty cylce doesn't go up 50 to 100%. We found that with the 1900 cc Holden L4, which made 80 hp/100lb-ft, and the same 2-bbl Varajet carb, same engine with two cylinders added on to make it a 6 cylinder 2.85 liter engine resulted in a not 120 hp, but 100 hp, not 150 lb-ft but 137 lb-ft....the induction was only worked 10 to 20% harder despite a 50% increase in cylinders

Duty cycles for the carb don't have to go up for a siamese port, one bbl per 2 cylinder six if they are split. You have to size the carburation to suit the pulses.


And the suumary is, if you want to use a bare minimum of carburation to get the best result, the restrictor plate NASCAR was it, effectively close to a saimesed one throttle per two cylinders is it. Any more, and the carbs have to be huge to pulse tune, any less, and you may or may not get a power loss. It all depends on the manifold.

 

[MustangSix]

Exactly. That's why calculations for CFM go out the window when you implement multiple, independent carbs. It's not unusual to find well over 1200cfm of total carburetor capacity on relatively small engines with IR systems.

The IR solution is terrific once you work out the intricacies of the linkage to get equal movement at the throttle blades. And even better, get them all to return to idle the same.

Our Zetec has 38mm Keihins, probably around 1000 cfm measured conventionally. With a non-IR plenum manifold, the carb would have a quarter of that capacity.

BTW, that car is nearly complete and has been driven extensively this past year. See photo.

Anyway, just to pique your interest, I'm in the throes of tinkering with another inline six: