20 mpg or not?

[Firebirdjones]

Originally Posted by mrhino

Within the week or so, the wife & I will be starting the engine build for the crew cab project we have going on. Over the weekend, It brought up a interesting debate with a few buddies that I trust and respect for their engine performance knowledge. My Goal with this engine is to get 20 MPG. A couple said it could be done and a couple said it wouldn't happen. Everyone made their points and theories. The debate and conversation was educational and interesting so I thought Id ask it here and see what some of the thoughts are.

The truck is a 88 1 ton crew.
4:11 rear
305/70/16 tires
700r4 trans

the engine will be a .30 over 5.7
58cc 187 swirl port heads with a mild port job.
20cc dished pistons
vortec Roller Cam 191º/196º 0.414"/.428" lift on a 111º LSA.
1 5/8 headers

so do you think it will get 20 MPG?

It's really going to depend on alot more than just what you have listed there. Your ability to sharply tune the engine with a wideband as well as driving habits, the elevation you live at etc....are all going to play a role. All you can do is maximize your efforts and see what you get.

Those tires are just about 33" tall. Even with the .70 overdrive I'd like to see the rpm knocked down further for max highway fuel economy, and that would require knocking some of that gear out. With the 3.06 first gear the truck gets moving easily enough and doesn't need those 4.11's. A 3.73 would work nicely and drop the cruise rpm another 3-400, probably worth another mpg.

You going carb or fuel injection? Either will do the job well enough if you can tune the carb properly (again using a wideband). To maximize fuel injection you'll need tuning software with the wideband, and concentrate on lean cruise conditions with light throttle input. Lots of variables here and alot to be gained.

If I can get 14 mpg highway from a 502 in my 79 1 ton with no overdrive then I see no reason why you can't do better with a 350.
I've never maximized my combo as far as squeaking mpg out of it. Still has heavy 16.5 steel wheels, mechanical fan, and few other tweaks that can be addressed for max efficiency. Basically a sharply tuned quadrajet on a performer intake with a tweaked distributor, headers, 3" exhaust, cold air intake setup is all that's been tweaked on. Stock 3.73 gears with a stock 30" tire running a 400 turbo. Seems to work pretty good for what I use it for so it's left alone.
My blazer is probably the closest to your combo. But I've installed a 6.0 LS engine in front. I run the 4L60E (electronic version of your 700) and also use a 33" tire but I installed 3.73 gears for a mix of performance and economy. The 6.0 (364 cubes) is a stock 04 LQ4 down to the stock exhaust manifolds. I removed the mechanical fan and went dual electric to cut down on some drag. Custom 2 1/2" dual exhaust. I tuned it with a wideband, concentrating on lean cruise, aggressive DFCO tables, some throttle and timing table tweaking as well as trans table tweaking etc...This is a 4x4 still using the stock (heavy Hp robbing) 205 transfer case and the truck weighs 4,960 lbs. I get 17 mpg around town, and 22 mpg highway.

So yeah, 20+ is doable in these old boxy trucks with the right parts, but what I've done isn't cheap either.

If it were my truck (and you probably don't want to hear this ) and I were going for a fuel economy effort, I'd probably ditch the 355 idea, and grab a 5.3 LS to swap in. The electronic fuel injection alone allows more tuning options and more tweaking, it'll make more power too, and more likely to give you the fuel economy your after.