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Old 11-26-2008, 09:44 PM   #21 (permalink)
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the weight and vacuum do come into play. And as the motor gets older so goes the carb. I just got done having a nice dinner and "adult beverages" at the favorite watering hole and got in the Toothbrush Special and it started right up and ran as nice as new with 215,000 miles on the clock. That injected motor would never run that good for so long if it were carbed. Especially from the drunk I bought it from.

So those car choices are out Christ, any others?

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Last edited by guudasitgets; 11-26-2008 at 09:57 PM..
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Old 12-01-2008, 08:37 AM   #22 (permalink)
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Originally Posted by tehsocks View Post
It is a 4bbl Carb'd engine.
I have always heard that with a 4bbl your most efficient acceleration is achieved with your primaries wide open but with the secondaries closed. Depending on the model of your carburetor this may be easy to do or you may have to practice. Some carb designs (vacuum secondaries) can be tuned to adjust the timing of secondary activation.

While I haven't ever tried (never had a 4bbl carbed vehicle of my own to play with), this makes sense in theory. During steady-state operation engines run reliably on mixtures that are stoichiometric or slightly rich. The primary jets should be tuned at that point for reliable ping-free operation. The secondaries should only open much when significant load is placed on the engine and maximum power is demanded. The secondaries should therefore be jetted larger to achieve an overall very rich mixture (12:1 or likely richer) to prevent ping and to smooth acceleration effects with a constant throttle input angle.

So assuming that you haven't re-jetted your carb your most efficient acceleration will occur when accelerating as hard as you can without opening your secondary throttle valve. If you wish to try your hand at re-jetting you could lean out your secondary jets if you can restrain yourself from demanding more power than the engine could deliver ping-free. It may also benefit such an old carb to have a rebuild as the operation of the secondary throttle valve is usually dependent on springs that weaken with age.

Also pay attention to your ignition advance, especially if it is vacuum dependent. Tuning the combination of carburetor and distributor can give you excellent performance. My uncle used to be an independent auto mechanic before the prevalence of EFI and tuned his 1970 Mercury with a 429 to get 29 mpg highway with a new distributor and freshly rebuilt carb.
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Old 12-01-2008, 01:09 PM   #23 (permalink)
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Couldn't he just block the secondaries? Or even put a choke-cable block on them, so that when he's just driving around, they're blocked, but when he needs the power, pull the cable and it releases the block... something like that.

JW, cuz alot of the guys that I run with don't use accelerator jets (proper term?) they just use the primary jet in the carb... the other side is blocked off.
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Old 12-01-2008, 01:49 PM   #24 (permalink)
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Originally Posted by guudasitgets View Post
I had carb versions of those for fleet use, my brother still has his fuel injected escort theres about a 2 to 3mpg better fuel burn, but the later escorts had 1800. Carbs are not as efficient has EFI, that goes without saying. If were baseing this on what you got from your car, we have no way to varify it. The data is out there on the escort take it or leave it but there are plenty of them to compare. So that car choice is out. Got any other guesses?
I belive the harley lineup took a milage hit when they went to EFI, they also can no longer idle at that signeture loping potato-potato....
though, my bike has an older brother with the same engine only carburated, and I belive there are a couple MPG's to be gained from the EFI version, depending on how you ride that is, I think the older ones might be able to get a bit more with extreme ecodriving VS the EFI's.
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Old 12-13-2008, 04:52 PM   #25 (permalink)
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Originally Posted by tehsocks View Post
Hey, i have an 85 camaro (v8 5.0L 305ci) NOT an economical car by any means but its fun to see what it can do.

It can do consistent 30's on the highways and thats good. but thats aside the point.

I recently installed a vacuum gauge (hooked upto manifold vacuum), and my understanding is that when i have most vacuum im using the least amount of gas, correct?

Well I find that when im idling i have 22, and when i put my car into OD and start driving around, when i accelerate my vacuum goes all the way down to 14-16ish. and its horrible. But if i put it in 1st, rev to 3500rpm, and cruise around in 2nd at 3000-4000rpm. I have 22-24 vacuum.

And when i let off the accelerator in 2 or 1 my vacuum goes off the scale high which i also assume is good. But this doesnt make sense to me. I thought OD was the most economical thing you could do. But thats not what im getting from my gauge.

Ive tried doing this for about 100km's on this tank and i seem to be getting my normal gas mileage, so it doesnt seem to be using any more, and doesnt seem to be using any less.

But am i correct, is this how i read teh vacuum gauge, and does this seem odd to any of you? Ive never heard of revving high to get good mileage before.

(redline is 5000rpm)
What I'm hearing,and this has been touched on,is that in low gear,the engine sees little load,so it can operate at higher rpm with little throttle opening,hence the higher vacuum.--------The vacuum gauge,as an economy device was intended for carbureted cars to help you keep out of the power-valves and prevent secondary throttles from opening.With the primaries nearly wide-open,the volumetric efficiency is good,mixing is good,and high turbulence in the intake runners(also good).All good for mpg.

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