I have a 350 TPI engine in my 86 Iroc (not factory engine), rebuilt 700r4 Automatic transmission with overdrive, and the only things that have been changed are as follows:

1.6 roller rockers and new springs

bosch MAF (previous owner did that)

Edelbrock headers (previous owner did that)

3 inch pipes with a cut out

and the normal K&N's.


I am getting right at 12 MPG with maybe 20 miles on the freeway every week total. Mostly around town driving, and I dont' drive with a heavy foot.

I don't smell gas at anytime, and I am sure there are no leaks anywhere in the tank, because it sat in the parking lot for a month with 1/2 a tank, I come back and it still has 1/2 a tank. I wonder if my Gas needle is off tho, because when it tells me that it's almost empty, I can only fit about 10 gallons of gas in the tank. How large should my tank be?

I DO NOT know when the last time anything was replaced under the hood, all the recipts were stolen out of the car the week after I got it. I suspect that it's time to start replacing stuff.

Is 12 MPG even remotly normal for a 350 TPI engine? If not, what should I look at replacing? I don't know anything about sparkplugs/wires/cap/rotor/whatever else, but I have heard that this is a good place to start? Any suggestions as to what brands I should use?

Should I switch to synthetic oil? I need to change my oil here in a few weeks, so I might do that. I have heard it's good for a little MPG gain.


Thanks so much!

 

Well I would start off with the thing you mentioned (rotor, plugs, distrib. cap, wires). Also check the timing. That can affect gas mileage. Also might want to throw in a new O2 sensor if you think the one you have may be old.

As far a choices, Accel seems to be the choice of a lot of guys. But GM stuff should work just fine for your set up.

As far as your gas mileage on a 350TPI, 12 mpg sounds a little low for city driving. Iwould expect 16-17 range for city and in the 23-24 area for highway. But I have never owned a 350. So I may be a bit optomistic. I get 20 combined on my '87 305TPI but it has some mods on it. Otherwise I would expect a few more out of it.

One thing that will be a big factor in mileage is a cam. Do you know if the previous owner put an aftermarket cam in it? Also how about an aftermarket chip?

Those could also be suspect in your low mileage.

Bill

 

the previous owner told me that the engine internals were not modified, the 350 is just a "normal" chevy 350, but he did say it had a 350 chip, not the factory 305 chip.

02 sensor, and check the timing, I'll add them to my list as well :-) thanks!

 

350 TPI Crate engine with Auto. 12.5/13 MPG stop and go trafic and 21/23 MPG on the highway. Just a little less then my old 305 TPI. No one said a Camaro/Firebird with a 350 was going to be a economy car.

 

That's true. The Camaro economy car was the LO3. And I own one.

 

the tank should be a little under 16 gal. you are checking mileage by filling all the way up, driving, and counting how many gallons it takes to fill up again after going X number of miles, right? i assume you are, but i've heard of people trying to measure mileage from the gague, and it's just not accurate enough. (just checking to make sure this is a car problem, and not a measurement problem)

 

Gas tank capacity on most 3rd gen cars(except 1le cars) is 15.5 gals.
With our mostly inaccurate gas gauges, when on empty it will still have almost 3.5 gals left so the fuel pump will not over heat.

 

patrickm00: yes, thats how I'm doing it, counting the miles I go, and dividing that by the gallons.

So on empty, I should have about 4-5 gallons left? nice! (though I will not test it out!!)

 

I must have one of the rare gauges that reads correct, when it gets to empty you had better be at a gas station!!! No reserve at all.

 

Consider yourself lucky for getting approx. 12mpg because I get approx. 8-9mpg. On the highway I get about 18mpg which is about what I should get. DJP87Z28 is correct our cars had 15.5 gal tanks and were not know to be fuel efficient. I hope to get my city mileage to at least 13mpg with a few changes under the hood like new injectors, jet-hot coated headers and new TPIS manifold.

Oh by the way I have a 355 with T-5 and never run the A/C either.

 

Must be nice. I got 15mph on the interstate on the way up to thirdgen nationals. I had a few stupid problems I have since fixed. Not sure what I get now.

This how I measure MPG. First I go fill the tank up and right my milage down since my odometer doesnt work. Then I drive it until I need gas again. When I fill it up I right the milage down and the amount of gas it takes to fill it up. Substract the milages, then divide by the # of gallons of gas to added.

Easiest way I can think of when your gauge wont read below 1/2 tank and your odometer doesnt work. POS GM gauges

 

That's about how I do it as well. You only got 15mpg on the highway? At what speeds were you crusing at?