The previous owner installed a Holley carb on my 84 305 Firebird. He intended to use the car for racing whereas I'll be using it as a cruiser and don't intend on doing any racing. I was told that the Holley carb is giving me poor gas mileage and because I don't intend on racing I should use a Rochester carb to save gas. Will replacing the Holley carb with a standard carb increase my mileage?

 

Changing carb back to a Rochester not a good idea.Holley Carbs built to enable them to run efficient.Check out the holly carb web page and see what jetting is available.It might be the case that your carb is jetted for performance and not economy.Rochester wear over time and overhaul kits more expensive usually then holley kids.Just me thoughts

 

I agree. A carb needs to be tuned. If power valves, idle mixtures, and jets are not adjusted it will affect driveability. It's not like an EFI setup that will automatically adjust itself off an O2 sensor. Check out the how-to sticky for Holley's. It's a little harder to get a carb tuned in with just a vacuum gauge vs a wideband o2 but it's manageable. I got my old Speed Demon dialed in for idle and part throttles it was getting 16-18 MPG. On the dyno they only had to tweak the idle mixture screws about 1/8 turn, but WOT was a different story as throttleshaft leak was found on the dyno with the wideband. I would have had a very hard time finding something like that with a vacuum gauge without load.

 

It'll depend on your driving habits, but a spreadbore Rochester will tend to have slightly better mileage for normal street use than a squarebore Holley. Any carb needs to be tuned though, and a conservatively driven Holley's mileage would still beat an aggressively driven Qjet.

 

To tune a Holley to compete with a bolt-on q-jet, you'd need at least wideband O2 capability or a dyno tune. Either one of those will cost about the same as a decently-functioning q-jet.

But, first, let's establish what you have. What intake manifold? What distributor? What do you know about the rest of the system such as exhaust, cam, heads, gears? You didn't indicate what transmission you're running - if automatic, what is the converter stall?

All of these things affect efficiency. To change one part without considering the whole, expecting a huge improvement, is a pipe dream.

Consider this: If you have to change the carb, intake, distributor, cam, gears, and converter in order to improve economy, you can buy a lot of gas with what that would cost - even at today's gas prices.

 

i agree..............is the holley in its base form good fr mileage.no fricking way. can they be tuned,for better mileage,yes. can you do it? maybe but like it was said above,takes an o2 wideband which if you dont have youre screwed.

what is your mileage currently? heres why i ask. if its in the teens already forget doing anything. keep in mind what youre driving. youre not going to get great mileage regardless and should expect it. yes i know someone will chime in and say the got 345 mpg but theres always one. youre driving a v8 carburated car with 1980s technology weighing over 3500 lbs. its not a honda civic hybrid.

goign back to another post,if you have a holley know more than likely he swapped manifolds. if you really wanted to go to a qjet youll need a new manifold or a **** *** adaptor(which i wouldnt recommend). even if you get a cheap mainfold youll need 35for gasmet set, 2 gallons of antifreeze at about 10 a gallon..thats 55 bucks right there,not counting manifold and replacement carb.

theres other cheaper better way to improve mpg. lighter weight oils,even synthetics show improvements. do you honestly keep tires inflated. do you actually drive at or near thwe speed limit? big mpg changes from 55-65 and 70+.. you try lightening your car? you got junk it in you dont need? gear ratios can help,..

will a qjet get better mileage,id say yes over a hollet almost any day. how much more is subjective but either way not a big percentage. i got a qjet dyno'd at 500 hp and got 17 mpg on the highway with ac on at 65mph. i was floored considering what i have. lets say you get 10 mpg and drive say 7500 miles a year(below average but i doubt its a daily driver?) 7500/10=750 gallons x 4 a gal =3000 a year on fuel(ouch) now you crank it up to 12mpg. (20% increase). 7500/12=625x4=2500. a yearly savings of 500 bucks... what would you have spent to get that 20% increase?

mpg is a odd bird.its hard to accuratley figure out your mpg. sure you can try to fill a tank,drive,record mileage and refill and record gallons pumped but unless youre completely filling it up how do you know how much you ve burned? you ever fill up,nozzle pos but you keep pumping? every pump is different yet you cn pump 1-3 extra gallons..a big difference in figuring out how much fuel youve burned . and mpg is influnced by too many factors. rain, winds, day/night,humidity,uphill down hill? so to say you get better mileage here vs there is tough to say.

in a nut shell do you really need to improve mpg on a mid 80s firebird that will be cost effective for you?

 

There's a lot more to this than I thought. I can't answer all of your questions but this is what I can tell you. I have a Edelbrock manifold, three speed automatic tranny, and recently had the rear gears changed. I'm not sure of the gear ratio but these are supposed to better match with the three speed as the gears that were there were for the four speed. Not sure about the exhaust system name, I think it's a stainless steel Dynomax. I didn't put it on so I can't tell you the exhaust size.

 

see,even there. you have a 3 speed tranny.id have to assume t350 or possible t400. either way theyre bigger and by them selves use more gas to turn them which is one reason newer transmissions are much smaller. this is not even taking into consideration the rear gear ratio w/o an overdrive. its a losing battle my freind,so to speak.the cars werent built for mpg and unles youre riping the carb out and going with a efi set up the mpg wont be great regardless. fine tune it,get her running the best you can, keep the speed down and enjoy it...yes gas is not cheap but its not a honda either...

 

more info is needed. What CFM is the carb? Vacuum or mechanical secondaries? what series, 4150 or 4160? There are way too many variations of holley carbs to simply just ask if the holley is getting poor milage

 

Are you doing a lot of interstate driving? Because that 3 speed isn't helping (as far as the gas issue, anyways). Combine that with numerically steeper gears and you're going to have a gas guzzler by default. Like mentioned, there's a lot of elements involved into getting decent MPG, but those two in itself are icing on the cake.

 

i have a 4160 mechanical secondary with a semi built 350 nothing more then a mild cam from comp cylinders bored 60 over and crappy 882 heads with no flow. 700r4 tranny the older non electric one and 2.73 gears in the back. i consider myself lucky to be getting at least 10 mpg. i could probably do a bit better if i took the time to plug in a fluke to the o2 sensor i have plugged into the exhaus. but its cold right now and i dont feel like sitting outside to do that. you could lean it out a little but be careful with that to lean and your engine will start to get hot. your best bet is to search air fuel ratio chart versus o2 sensor output (im assuming you have one in there somewhere) connect a multimeter up to the sensor and let the car warm up and see where the volt are. once you get a base line you can adjust your carb to get the optimum voltage there for getting the best air fuel ratio you can get in the drive way in park and maybe in drive with the parking brake one. takes time do this but its really the only thing you can do other then go for a fuel injection kit with cost upwards of 2k and will take a longer time to install.

 

How about cost of new carb vs the improved mileage. At 1-2 mpg more, payback would take a long time. Just playing devils advocate.

 

Actually a TH350 is smaller than the TH700R4 and has less rotating mass and lighter over all than the 700R4. The advantage of the 700R4 is a lower first gear ratio for torque multiplication without a high (numerically) rear gear ratio and the overdrive gear which lowers the final gear ratio by ~0.70 which for a car with 2.73 gears lowers it to an equivalent ratio of ~1.91. Another example, a car with a 3.73 gear and 700R4 will have a final gear ratio close to a car with a 2.73 gear and a TH350.

Another thing to consider is that the OP might have the old Metric transmission which is lighter than the TH350 but also weaker due to the efforts to lighten it for better fuel economy.

My old 72rs Camaro had a 350 with a 2.73 open diff and a TH350 and I would get around 18mpg (highway)with a Holley 600 Vac. Sec. installed on it (was a 2bbl originally). That was as long as I would keep my foot out of the accelerator but getting stuck in stop and go, rush hour traffic and my mileage would sometimes get as low as 12mph. Put it this way, the first location I work at was only 15 miles from my house both right off the expressway. It took me, on average, the same amount of time and gas for that trip as it does now to travel 28 miles, both took 30-35 minutes to go either direction. To save gas I bought a 67 Camaro that had a 250, mono bbl carb motor with a Muncie 4 speed trans and 2.73 posi gears. Got it for 2K back in 1989 and I had a blast with that car.