I just went through this article in Chevy Hi Performance http://www.chevyhiperformance.com/t...10ch_carburetor_tuning_gas_mileage/index.html It made a lot of sense to me but I am no way a carb expert. My Bigs 950HP cnc makes good power and drives very well but I have the exact symptoms at idle described in the article. I have never understood why a Holley carb cannot get the same gas millage as a q-jet. I understand that the small primary gives good part throttle response, but if you get the AFR's the same at a given rpm and load the millage should be the same. Comments and opinions please.
The reason that a Q-jet will have an advantage for gas mileage is that the smaller primaries require that the air has a higher *velocity* (i.e. same amount of air through a smaller opening). Velocity is important because it helps with mixing (i.e. homogeneous mixture) as well as droplet size (more turbulence = smaller droplets of fuel). Better mixing and smaller droplet size burn more efficiently and more completely in the combustion chamber. Picture a "very large" droplet of fuel in the combustion chamber. After ignition, the fuel on the outside of the droplet begins to burn first, and if the droplet is big enough then it won't finish burning by the time the combustion stroke is through. You'll have leftover fuel (the inside of the droplet) and leftover oxygen that never got to combust that fuel. So even though the amount of fuel entering the engine could be the same, the small primaries will help with efficiency for other reasons. (Not a technically perfect explanation but hopefully it will help) -Bob C.
If all of that is true then why does fuel injection work so good for milage? The injectors spray directly into the air stream just in front of the port. I think it could be argued that the fuel droplets are relativily large as they come out of the injector. Therefore the good milage comes from tight control of the fuel air mixture. Of course the next arguement is about different types of injectors and on and on.... Part of this articles arguement is you can use the adjustability of the Holley to better emulsify (sp?) the idle fuel feed. This could give the same effect as the smaller primay bores.
Okay, please explain why you would think this. And remember it's not only the average size of the droplets, but the uniformity of the sizes (distribution above/below the average). I'd be willing to bet that FI would win this hands-down any day, but you seem to be convinced otherwise. Yes, obviously FI does have very very tight control (especially for transitional events), however droplet size & uniformity are also crucial. -BC
Doug, nice find on that article. I've always wondered over the years how much work could be done to improve transitions between the circuits, but it hasn't been since recently that the wide band gages have been available for us to see the finer points of what's going on. Is it interesting enough to get me to dive in? Probably not at this point, since I have so many big things to work on that the little ones will have to wait! Devon
Bob, I am just not sure that droplet size is anywhere near as important as air fuel ratio. As far as injector droplet size it is just a guess on my part. My take (no experimental data here) injectors spray pure liquid gas into the air stream with only a short time to vaporize. It would depend on the injector, pressure and a bunch of other stuff to determine the fuel droplet size. Carburators leak a mixture of gas and air (emulsion) high into the air stream with a lot of time to mix. The difference due to air velocity should be minimal. In fact below 2000 rpm the throttle blades are barely cracked so what difference would throttle bore make? The point being with a Holley you can work on that emulsion and with the ability to adjust with air bleeds and the idle feed restriction you should be able to get a better fuel curve. The fuel curve is what the factory works with and thus we have more powerful motors with better economy. I do not disagree that droplet size is a factor but my feeling is it not the primary factor. Then again I could be totally full of reprocessed field grass. :idea2: Devon, It seems to me that should not be a lot of money to try this stuff out. Just some time and the ability to have a wideband meter.
Agreed, Doug - but finally having the car home again and doing some poking around, I see the stock valvtrain is once more suffering from the beating of the 308S camshaft, and I have these funky aluminum heads from TA/Gessler to deal with. Oh, the humanity! Devon
Why do you suppose that bigger cams have worse idles and worse gas mileage than smaller cams? -Bob C.
When I was talking to Denny Mannor this summer he said that the real killer for a big cam as far as millage is the overlap.
Wellllll, my experience has been that holleys get better milage than others.....I went from a qjet that had nothing wrong with it to a holley and picked up 3 miles per gal...... A carb is nothing but an atomizer.... that changes liquid to a vapor.... LIQUID GAS WILL NOT BURN,,,,UNDER ANY CIRCUMSTANCES....it must turn into a vapor first before it can burn.... a carb only does this....the fuel ratio is determined by jet size vs the amount of air coming in.....if a carb is not getting good milage and the electricals are good on the engine.... start looking for what is clogging up the system.... air bleeds, emulsion tubes, air cleaner element ,ect..... also look for fuel leaks into the engine.....every carb, no matter the manufacturer, is designed to be rebuilt every 25,000 miles and almost no body does that.... now days when the cat pee gas turns to snot it is more important than ever..... we used to do a ''major'' tune up every 25,000 miles.... this incuded new dist cap, rotor ,wires, points and condenser and taking apart the carb and ''boiling'' it in carb cleaner, blowing it out with compressed air and putting it back together and cking all adjustments....
That is exactly right (gee, how could I be qualified to disagree with Mr. Manner about anything car-related anyway). That said, the overlap creates a terrible signal to the carb, and reversion in the intake, thereby causing the symptoms that Bob is explaining. There are other factors like unburned fuel flowing through the combustion chamber and right out the exhaust (you know, that eye-watering gassy exhaust smell at idle), but the crummy idle and such is due to the overlap, and it also explains why a fuel injected engine can be tuned to run much smoother. You can "tell" the computer what the engine needs instead of relying on vacuum and airflow alone to decide what it "should" be wanting; and fuel atomozation going into the combustion chamber at low RPM can be so much better with fuel injectors. Oh, and if you have a decent sized cam, a 15:1 A/F will make your engine idle like crap. Like everything else with cars, its a balance of compromises, mileage, drivability, acceleration, horsepower. Good heads and exhaust are about the only no-compromise performance modifications I can think of.
