Which one would be better for my purposes?

[Skunksmash]

I am looking at two replacement throttle bodies for my 1987 chevy TBI 5.7L R10 truck. (Automatic, 2WD) My only concern here is increasing fuel efficiency, making power is not my top priority. Which of these two do you think would better help me gain fuel efficiency? This one:

http://www.amazon.com/exec/obidos/ASIN/B000CMJN8Q/interactiveda8609-20

or this one:

http://marine-performance-parts.com/cfmtechportedthrottlebodyforgmtbi435057.aspx

... which is an original GM throttle body that has been bored out to 620 CFM (from 480CFM) for better airflow. The Holley brand throttle body is 670CFM. I know that there is an obvious price difference, but you have to take into account that the holley comes complete, whereas the CFM unit comes incomplete and you have to pull your injectors and whatnot off of your old one. That really isn't a problem for me but it explains the price difference. Those little injectors are about $80 each and there are two of them. So which one do you think would better serve to increase gas mileage? I do not know much about TBI so i figured i would come here and ask.

[VileZambonie]

So you're planning on replacing your TBI for better fuel efficiency only? You will find that it was money very poorly spent and your efficiency isn't going to go up noticeably at all. Any fuel metering device (electronic) is controlled by the computer. recalibration is your best bet.

[Captkaos]

Neither would server your purpose.
A Holley TBI unit would require a custom chip to work with your setup and the bigger Factory would also.  If you are after gas mileage, have a custom chip made for what you are hoping for. 

How much mileage are you after?  How much are you getting now?

[Skunksmash]

I am getting 14Mpg, and i think it can do better. Granted, the truck is old. I am just beginning the restoration process and it will be fairly slow. I will not be replacing the engine for a year or two, but i would like whatever upgrades i do to be transferable to the new one. The holley site says that their throttle body is 50 State legal because it improves gas mileage rather than decreasing it. I can only assume that it could do this by allowing more air in than the stock throttle body.

Also, wouldn't it just be canceled out anyway when the air reaches the smaller holes in the top of the intake manifold? Or are they larger? I have no idea how many CFM the manifold holes themselves are capable of flowing...

[Captkaos]

14 city or 14 higway?  How many miles on the motor?  What trans?  What gear ratio, what size tires? What size engine? 4x4 or 4x2?
My 305TBI 87 gets 17/22, that is about all you are going to see out of a 2WD truck
More air in = the need for more fuel.  Problem is you don't need all this extra flow unless you are building for power  A small TBI would be far better than a HUGE one.
It would be like swapping from a 2bbl to a 4bbl holley in hopes of getting better mileage.  Or going from a Q-jet (small primaries) to a Holley, (bigger primaries)
You WILL have to get a chip for the Holley to work on a stock setup. trust me.

[Skunksmash]

14 city or 14 higway?  How many miles on the motor?  What trans?  What gear ratio, what size tires? What size engine? 4x4 or 4x2?
My 305TBI 87 gets 17/22, that is about all you are going to see out of a 2WD truck
More air in = the need for more fuel.  Problem is you don't need all this extra flow unless you are building for power  A small TBI would be far better than a HUGE one.
It would be like swapping from a 2bbl to a 4bbl holley in hopes of getting better mileage.  Or going from a Q-jet (small primaries) to a Holley, (bigger primaries)
You WILL have to get a chip for the Holley to work on a stock setup. trust me.

Ok so stick with the stock setup. I have no idea what mileage is on the truck, i am the 3rd owner of it and the mileage says 45K, which could mean 145k, or 245K. It is getting 14MPG city. I would like to match your 17 city, 22 highway if i could. I mean hey 3 more MPG is money in my pocket lol. 700R4 trans, 5.7L engine, 2WD, longbed, single cab. I will look up the tire size and rear end gear ratio. But im sure they are just typical of one of these old trucks, it doesn't have any major modifications like being lifted way up with big tires and all that. Kinda makes me curious though... why would they say that you will see a slight increase in mileage with their TB if you are going to be seeing a decrease... maybe i will call em up and have em tell me lol

[VileZambonie]

Like Chris said more air means more fuel. I would look at your final drive ratio and if it's a 3.08 bump it up to a 3.42 or 3.73 if you're not doing a lot of highway.

[heyharris1]

not to steal the thread but here is some food for thought, i have a 1993 g20 conversion van, 350 engine 700r4 tranny with 3:42 gears on the highway the thing was just nasty, could not climb hills and gas was not that good. talk to some guys at a tranny shop and rear end specalist and this is what they said Quote" the problem is because your van is to heavy for the tranny rear end combo, you engine is working to hard to move the big beast, so they recommended to go to a 4:10 or 4:11" end quote. i dont remember which gear i chose, i think the 4:10 but it woke the beast up. climbs hills like there flat gound and with the overdrive tranny on the highway the engine now stays in its optimal powerband and the fuel mileage actually increased from about 14 mpg to about 17 mpg and on the highway its closer to 21 mpg which aint to bad for a 6000 + pound rig. anyways you may just wanna look at a gear change and keep it all stock.

[Skunksmash]

But wouldn't changing the gears to 4:10's actually decrease gas mileage while on the highway?

[werewolfx13]

Not if the engine is lugging when climbing hills/driving into a headwind. Now, a van, especially a conversion van, is going to need lower gears for the same task as a comparably equipped truck, simply because of the added wind resistance and weight (for that van, I'd guess more like 8000+ lbs, my grandfather had a g20 350/th400 former hotel shuttle that was a normal roof with all the back seats removed, and with half a tank that thing weighed nearly 9000 lbs, and was only a little bit longer than a normal van).

+1 on the gear change, 3.42s if you spend more than 50% of the time behind the wheel on the highway, 3.73s if more than 50% is spent in town.

[Captkaos]

the factory gearing on these was a poor choice.  3.42 are a good mileage increase on anything with factory 3.08's...

[Skunksmash]

Hmm... I have heard some people get a whining noise after doing this, although they did not do it correctly. You guys got any idea what a price might be on having the gears installed by someone? I mean even if it's just in your area, it would give me a ballpark idea at least. I doubt i am qualified to change the gears out. I don't think the gears themselves are all that expensive.

[Captkaos]

around here, $150 labor for the install if the axle is out of the truck.

[heyharris1]

i went through 6 states and it costed me 760 bucks, droped it off and picked it up for the whole job, new berrings and the gears
jim

[VileZambonie]

I would charge on the average between $700-$800 bucks for the whole job.