MikeSammy

I have a 86 pontiac firebird with a 305 4bbl carb and was wondering if anyone had a step by step process for tuning a 305 to get 26 mpg since i know from asking previous members and finding out that someone had a tpi 350 that got 26 mpg. So, since I have a carb I was thinking that everything should even itself out.

built91Z28

Not sure of any tuning methods but I dont think you can directly compare your car to theirs. Just becuase they were able to get a certain MPG doesnt mean you will be able to. Plus a TPI 305 is a fuel injected motor. They normally get better mileage than a carb. Driving habits probably have the biggest impact on mileage anyways.

MikeSammy

Well mine is a 305 so its smaller than a 350 so in my manenr of thought I think that the little advantage that the tpi has would even it self out.

also, nice engine, what size engine is that 350 or 383 also about how many mpg do you get with that beast!
wait sorry I noticed that you stated it wasa 357 how exactly is that possible. did you get a 350 block bored .040 over to get that?

built91Z28

I didnt notice that you wrote it was a 350 TPI. My camaro originally had a L98 (350 TPI) when my brother owned it before me. He got nowhere near 26 MPG. He got maybe 16 or so city. If someone really got 26 MPG out of one there could be a million variables in that equation. For example, how long did he drive for to average out 26 MPG? Was it ALL highway and was the car a stick shift? What rear gears did it have? Was he heading south out of the mountains where most of the roads are downhill? There are just too many variable to be able to say he got 26 MPG.

For your car though, the best place to start is just to do a basic tune up if you havent recently. Check your timing, pull the plugs and see what they look like. Sometimes a smaller motor doesnt mean better mileage. For instance, my daily driver is a 04 Grand Am with the 2.2. I dont get that good of mileage becuase the thing is so under powered I have to go to 50% throttle to get going at a decent rate. I think I would get better mileage with a V6.

As for my Camaro, thanks for the compliment. You are right that it is a .040" over 350. As far as mileage goes I have no idea. Its not a daily driver at all. In fact in the 3 years I've had it driveable I have barely drove it. I've only put 430 miles or so on it. I do know once I drove from my house to my work and back to my house (18 miles total) on $5 of gas. Gas was around $2.60 then I believe. I used every last drop of the $5 though. I also kept my foot out of it. That was when I had the 3 speed auto and 3500 stall. Now that I have a 6 speed manual it I might be able to get 15 MPG or so city. Which isnt bad for a near 500 horse carbed motor. If I got WOT for an 1/8 mile or so the gas guage will drop down some. So at WOT I might be getting 1 or 2 MPG

camaronewbie

My 350 is nothing fancy, no hp monster, and I get 17mpg. Not sure 26 is even possible with TPI unless it's all downhill highway with a good tailwind and bump-drafting a semi! My neighbor has a 350 TPI and he says in town he barely gets 15, his is all stock, in perfect tune, and gets driven very little.

If it's a stock Quadrajet carb, they are really tunable and supposed to get the best mileage. If it's an aftermarket carb, forget it, 26 will never happen without a serious hill and a push!

Rolling Thunder

Yea you cant really compare to a TPI motor. TPI motors could get up into the mid 20s on the highway for MPGs but thats kinda deceptive because they didnt do nearly that well in the city. My 305 TPI motor on average gets around 20 mpgs highway and city combined so its still good but not mid 20s good. You can get better MPGs out of your current motor but it wont come from simply tuning and any mods you would do to improve the gas mileage would likely never pay for themselves in savings. I think the best you would be able to do reasonably is make sure your car is in a good state of repair and properly maintained. Driven nicely they shouldnt do too bad with gas mileage. If you are doing that bad odds are somethings wrong.

RED_DRAGON_85

just to shed some light on a typical 350 tpi mileage.
i get 10-15 city and 20-26 highway.
the weird thing is that i get better mileage when i drive slightly agressively in the city. seems like if i do my accelerating then go into coast mode, i do better than accelerating much more slowly.

best mileage on the highway is around 70-72 mph

RED86Z28

Assuming you have the stock carb there is nothing you can do to it for increased mileage except insure that it is operating properly. If you are running an aftermarket one you can get a wide band O2 gauge and make changes to get it to run optimally. I doubt you will reach 26mpg on the car no matter what you do. If you can do 20 on the highway then you are doing really well.

ntomsheck

iTrader: (1)

Haha yes 26mpg highway is entirely possible, I'm pretty sure he's referring to me. I think 'grimreaper' gets even better (about 29mpg) on an L98. If the car is stock, that's the problem
The injectors are crap, after they're replaced then one must dive into the computer. My torque converter, for example was set to lock up at 55% throttle (or higher) at 60mph. Well, I happen to be around 8 or 9% throttle at a steady 60mph. In essence, my TC never locked, and I've got a feeling more of our vehicles are like that. The 26mpg was actually combining about 15 miles of city driving, and 80 miles of highway driving. On the highway I stayed around 75/80mph.

It's possible to tune a carb to mid 20s, but that will have to be mostly highway. I've seen far too many old geezers with their chevelles, and impalas, belairs, cornets, etc, getting 23-25mpg on their way to shows.

MikeSammy

Yeah I was actually reffering to you. Would help explain to me how to tune the torque convertor for better mileage? Thanks Douchermann! Also, are injectors found only on fuel injected becuase I have a carbed engine

Atilla the Fun

I've done 25 mpg (repeatable by friends) in several Camaros with 350s, each with wildly different combinations of parts and gearing. I'd say the main things are: matching your cruise rpm to your cam, using a reman '80 LG4 carb, and getting the tune-up spot on to what your combination really wants, then driving nicely. I've spiked over 30 mpg with at least 2 different carb'd 350s this way, 25 is a very reachable average. A low compression 305 has to work harder, and even though a stock LG4 can do 1600 rpm at 65 mph, that doesn't mean 1600 is better than 1800.

ntomsheck

iTrader: (1)

No, carbs don't have injectors. The closest thing they have to them are jets, but the only similarity being that they introduce fuel. If you have a CC-qjet, you can adjust when your torque kicks in, kicks out, etc. If there's no computer whatsoever, then hopefully you have a stand-alone torque converter control device of sorts.

People always seem surprised to see V8's getting 20s and 30s as their MPG rating, but the fact of the matter is, the V8 doesn't have to work as hard as the little four cylinder counterparts.

I said it in another thread, but here it is again: Your engine is most efficient at it's peak torque. However, if your peak torque is 3 or 4k RPMs, that's not an ideal cruise speed. (Unless of course, you have some crazy radical gears)

Atilla the Fun

Make that a crazy radical cam. Even if you have a stock LG4 with TQ peak at 2400 rpm, that's still too much rpm at cruise. If you're running 4-series gears, well, the production T56, most '94.5-'02 versions, anyway, have a 0.50:1 sixth.
Cruising at torque peak was a late-'70s / early-'80s idea for 305-powered fullsize pickups.
the later the cam closes the intake valve, the higher the cruise rpm it wants. Equally important, the earlier the exhaust opening, the higher the cruise rpm. And cams with more overlap need more cruise rpm.
Higher dynamic compression at modest rpm also needs more gear to keep out of detonation.

MikeSammy

Ok thanks everyone. I guess Ill give my engine a good tuneup and ask a auto tech or someone to tune the torque convertor. thanks

RED86Z28

I did some looking around and many of these cars were factory rated for mid 20s from the factory. The 4cyl cars despite their lack of power were capable of around 30.
My experience has been low 20s are about it at while running at 75 without the ac on.

Rolling Thunder

Yea well its a complicated matter really. Fundamentally what it boils down to is on the highway it really dosnt take much power to just cruise. Because of this these v8s just simply cannot generate thoes lower hp ratings as efficiently as smaller less powerful motors. The key is to have and appropriately sized motor for an economy car and thats what they tried for. Not to big as it just generates waste and not too small as then it would be underpowered and fuel costs would again increase. The thing is what most people would consider "undersized" in many cases is actually fine for fuel economy even though it may feel sluggish to the point where your debating getting out and pushing lol.

Atilla the Fun

But then came the LS1, and it can average 31 mpg at a sustained 70 mph doing 1560 rpm.

treep78

Allow me to end this entire argument forever. For a stock Camaro the EPA has all the numbers, just go to FuelEconomy.gov and enter your year, engine and tranny in their car search and it will tell you your cars stock gas mileage. Though it should be noted that almost every third gen regardless of year, engine, and tranny gets about 16 city 25 highway, and that's with the new adjusted numbers.

Apeiron

EPA ratings are laboratory numbers and often have very little agreement with real-world mileage figures.

GanymedeElegy

iTrader: (1)

So From What I Read, It Would Be A Good Idea To Get A Mild Street Cam And Dyno This 5.7 To Find Its Peak Torque, Hopefully Around 2250 RPM And Set Up My Rear End To Cruise About 75 MPH (My Normal Cruise Speed) And I Should Hit Mid And Upper 20's In Mileage? The 5.7L I Got Is Out Of A 1990 Chevy G20 Van With 212,000 Miles, And The Engine Is Hella Clean, No Oil Anywhere, But The Tranny Leaks A Bit, Im Not Taking Its Tranny. Im Keeping My Stock 700R4 And Plan To Rebuild It. Would It Be A Good Idea To Raise The Compression Ratio A Bit?