Old question but..MPG and building a motor?? Tbi to Carb etc.

[olllie]

Looking at upgrading my new purchase 1987 2wd, tbi 350 700r 3.42 rear,

Any input from would be great.

Im going with what should be the original engine. so I think its an L05. Still need to check the numbers. but was told its original... 392000km !!! still runs strong

Stock heads... I'm pretty sure tbi heads off 87's are not too bad, maybe I can find an old writeup on getting them to breath better. Use Mid length headers, I currently have an h pipe and dual out.

Small cam keeping the power low, maybe a "mild thumper" or another comp product. need to read up on cam numbers for this build still. Upgrade the springs to match the cam, rest of the valve train would be refreshed but stock. Bottom end would be gone over but basically stock, stock pan & pump etc. oversize if needed.

Dual plane Intake, 650cfm carb (Qjet?), nice hei (in cap coil) vac and mechanical advance, ditch the the computer. I would use a lock up box for the 700r and use a fuel regulator with the current pumps.
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I did start a mpg log on my truck and I'm seeing about 12.6mpg (us gallon) playing with gap and advance. The best I've seen is 15.8mpg. I do drive hard at times but its in short bursts seem that the truck always smells rich at the pipes but it does have 390k and the valve seals are not doing well.

So big question is with the above build, would my mpg go down or up?? and what kind up power could i expect? guessing on a good day, I now maybe see 150hp (crank) Im just looking to make it a nice "strong" daily driver. But 150hp is not cutting it.

I know tbi can make 300hp but Im not a big fan of early injection and obd1.

[WuchaGot]

I just bought a '87 v20 4WD last week. Rebuilt the TBI in 45 min, WAY easier than rebuilding carbs. I do like carbs but far as injection is concerned its a piece of cake to work on the TBI. OBD1 I do not care for.

[olllie]

I agree that tbi is not a nightmare to work on,

but i would like to build more power than i currently have. Im sure building a tbi motor to make the same power as a carb motor will cost more cash?

and ill need to deal with chips and limited cam choice. I have no clue what to do...

...Or put 450hp down and say forget the budget and mpg! only live once.

[Goldfinger]

If you want mpg look to your rear end it has a extremely large impact. my little 305 od transmission and high gears of 2.73 can get me in the low 20's highway and the mid 20's down the interstate. acceleration would take a drastic hit.

[bnoon]

TBI heads are junk without serious work (about 2-3 hours per head to get them ported professionally and still won't flow more or maybe even as well as other stock heads). Get a set of the 098 casting 350 TPI heads, if you want a compression bump the 58cc 416 casting 305 HO heads (from carb and TPI 305 F-bodies), Vortec 350 castings, GM Performance large port Vortecs, or maybe the TPI Corvette heads (again 58cc chambers).

For good MPG and ease of TBI tuning, you need to look for a cam with larger lobe separation in the 114 to 116 range and not too much duration (in the low to mid 220's at .050, maybe even high teens). Keeping the TBI is a tuning restriction but can be done. The thirdgen.org forums has an active tuning section and several members have worked with magazines like Car Craft back when they did their 305 build and tuning back in the early 2000's. That article is still online at Car Crafts website last I looked. They did stock 350 Vortec heads cut to bump compression back up on their little 305. Cam choice is critical if you're a tuning newbie as just about anything other than stock will piss off the stock tune and make your ride a rich blundering doofus under 3000 RPM.

Turbo City used to be the place for modified TBI units and injector spacers. Might search for them.

OBD-1 is VERY nice for tuning for MPG actually because of a little thing called Highway Mode that was disabled in OBD2 for GM's in the states. In OBD1 tuning you can set your highway mode enter speed, exit speed, AFR, duty cycle (too keep you from going lean too long and burning up), and other things. I had my '86 Trans Am 305 TPI putting down over 300 horse at the wheels and pulling 26 MPG interstate on a non-ECU friendly Summit 1104 camshaft with similar tuning. It can be done!

[bnoon]

I forgot about the carb route you're leaning towards. Staying carb and wanting MPG, you'll want a Q-jet for sure as the small primaries will sip gas.

Several modifications to those carbs can be made as well, such as roughing up the bottom edge of the secondary butterfles where the discharge tubes spray on them to increase fuel atomization, cutting transition slots, modifying the vacuum pod to control the opening of the secondaries, swapping metering rods, etc. You may also want richer primary jets with a modified engine if you want more throttle response, or stick with the lean jets if you're more concerned with part throttle fuel sipping for MPG.

750 CFM quads are most popular and there are some 800's on old Buick/Olds/Pontiacs. For my stock rebuilt quad, I went to mountaintink on ebay. I've used his rebuilds in other projects and kept him as my fav seller list for years. I have one of his carbs ready to go on in the next few weeks.

[olllie]

thanks for all the info "bnoon" I guess this post is in the wrong forum..

I am totally disappointed that the 87 heads are crap though..

I was under the impression they didnt suck that bad.. lol.. maybe vortec is the way to go with head choice.

[olllie]

Only been driving the truck for a month now, and spend about an hour every night reading on this board and others. Im a toyota guy so these trucks are pretty much new to me.

[bnoon]

Vortec heads will give you more useable low-mid range and have raised tumble ports for better MPG as well, so they might be your best choice given your goal.