Small letter 289 Cobra 2x4 intake and carbs value

[pbf777]

Here's the thing though Scott. Running smaller carbs on a t/a manifold is just going to run the engine lean at some point, if not at every data point.

     Not to be stickler for details but, your description of anticipated lean event is not fully accurate, although commonly stated, and my intention is solely to aid.       

     It's probably better to understand that the carburetor first and greatest function is that of an air-valve, controlling the total sum of atmosphere being allowed to enter into the engine's cylinder(s), and remember that the volume is constant, within the pumping efficiency of the device, only the density changes with the throttling effect, so often this sum is defined in weight measurements.  And this is the only effect one has as one operates the throttle, with the exception of the accelerator pump function on the transitional throttle motion.

     The responsibility of fuel application should be considered as a secondary function of the carburetor, as it is a reaction to the first.  It is the air motion thru the carburetors' venturi(s) which syphons the fuel from the reservoir within the fuel bowl(s), perhaps also understood as being forced out into the created lower pressure within the venturi by the higher pressure of the nominal atmosphere acting upon the fuel within the bowl(s). 

     So, in the example of the carburetor(s) being "too small", what most often actually happens is that as the engine capacity requirements encroach upon the the carburetors' throttle capacity one encounters the effect of throttling, the density is reduced (and so is the fueling requirement) with a pressure reduction experienced within the induction system, even though the volume passing the venturi(s) is still increasing with R.P.M., and with this increase in the pressure differential the fuel is more effectively syphoned, and in reality the performance drop-off from carburetor restriction will be yes, the lack of available air delivery, but the greater offender will be a fueling mixture going to rich!         
     
      As an example, perhaps, one recalls the old adage in drag racing of tuning a rich mixture for best E.T., and a lean mixture for M.P.H.?  And that kinda sums it up short & sweet!       

     Scott.

[jerry merrill]

I have a 67 mustang with a 347 and it has the T/A intake with the 390 cfm carbs and I can't say enough about how great it runs and has a decent idle even with a big comp roller cam. With the AFR 185 heads it certainly has 400+ hp and does not bog or hesitate at all, very satisfied with it. Just to mention The carbs are backwards as on big blocks.

[shelbydoug]

Here's the thing though Scott. Running smaller carbs on a t/a manifold is just going to run the engine lean at some point, if not at every data point.

     Not to be stickler for details but, your description of anticipated lean event is not fully accurate, although commonly stated, and my intention is solely to aid.       

     It's probably better to understand that the carburetor first and greatest function is that of an air-valve, controlling the total sum of atmosphere being allowed to enter into the engine's cylinder(s), and remember that the volume is constant, within the pumping efficiency of the device, only the density changes with the throttling effect, so often this sum is defined in weight measurements.  And this is the only effect one has as one operates the throttle, with the exception of the accelerator pump function on the transitional throttle motion.

     The responsibility of fuel application should be considered as a secondary function of the carburetor, as it is a reaction to the first.  It is the air motion thru the carburetors' venturi(s) which syphons the fuel from the reservoir within the fuel bowl(s), perhaps also understood as being forced out into the created lower pressure within the venturi by the higher pressure of the nominal atmosphere acting upon the fuel within the bowl(s). 

     So, in the example of the carburetor(s) being "too small", what most often actually happens is that as the engine capacity requirements encroach upon the the carburetors' throttle capacity one encounters the effect of throttling, the density is reduced (and so is the fueling requirement) with a pressure reduction experienced within the induction system, even though the volume passing the venturi(s) is still increasing with R.P.M., and with this increase in the pressure differential the fuel is more effectively syphoned, and in reality the performance drop-off from carburetor restriction will be yes, the lack of available air delivery, but the greater offender will be a fueling mixture going to rich!         
     
      As an example, perhaps, one recalls the old adage in drag racing of tuning a rich mixture for best E.T., and a lean mixture for M.P.H.?  And that kinda sums it up short & sweet!       

     Scott.

Semantics.

A 450 cfm carb with 75 jets tells me something about fuel delivery and the timing of it in specific rpm ranges. When compared to a 750 running 65 jets on the exact same engine.

Most likely the engine demand for fuel is being restricted by too small of a venturi in the case of the 450.

The runners on the T/A out of the box are 250cfm. The high rise, 220 worked. That right there explains a lot.

The flow of the heads at 297 at .500 complicates things in a nice way, but the flow at mid lift is better then the race heads of the day were.



Horsepower is also a function of engine torque delivered in a faster, i.e., shorter time. The shorter the time, the higher the number.

The amount of time to deliver the fuel to the engine is really what is being tuned here.



The variables cannot be tuned infinitely and instantaneously. Two 715 carbs may be too large. Two 390 carbs may be too small BUT they may not be.

They both could be perfect in a certain rpm range.

It depends on the time period each is allowed to deliver. The 390 needs more time. The 715s don't.



Gearing here is also significant. There are 2v carb'd 351 station wagons running 11 seconds.

It all depends on what you get and when.



The tuning can stand as it is now. I learned something with the Webers a long time ago. After a period of time they seem to be flat and lacking. Pull them off and go back to the single 4v and wow! You just gave away conservatively 100hp! Now they don't seem so flat do they? 

Ultimately it depends on the person deciding. What's good? What's not?



In this case, my case, only actual testing can illuminate a choice of performance characteristics, that is unless there is a dyno that can simulate a run available. An actual and variable loading.

Then the best combination can be selected  and maybe for each segment of the run.



That generally can not be done with an analog system (carburetors) but is possible with a digital system, electronic fuel injection which can plot each rpm.

Of course I did see that done by Bob Ingles with carb'd BB Chevy Pro Stock engines in the late '80s but that was the early days of that type of tuning to non multi-billionaire types that I had any introduction to. So I suppose my statement isn't completely true just not easy or cheap to accomplish.



It is said that Ford actually built a computerized dyno like that in 65 or 66 to run the MKII 427 GT40's through a LeMans cycle (simulated race) to determine the best operation of the engines for the greatest probability of success and dependability.

Mine is the school of hard knocks method. The grey hair at this point counts for something? Lots and lots of grey hair. You just need to keep it short enough to keep out of your eyes so it doesn't blind you to what is going on. Not so easy sometimes?

[pbf777]

Most likely the engine demand for fuel is being restricted by too small of a venturi in the case of the 450.

     Remember, the engines' demand for fuel, within a relatively narrow ratio range, is commensurate with the volume of air.            

     Scott.   

[TA Coupe]

I was looking for an old post and ran across this one and thought I would show a couple of pictures of the dual dominator on 1 of my engines. I was just setting it up so I could take a few measurements.  Also a couple of pictures of a dual dominator set up in a car and also on an engine stand.

          Roy

[TA Coupe]

A couple more

[shelbydoug]

I do realize that the dual Dominators is intended to run with somewhat of a radical cam profile but how "streetable" is that set up?