250 i6 fuel economy w/o sacrificing HP

[markallen]

right now im running a stock 250 with a T-5 tranny and a 3:07 rearend (getting about 15 mpg) im thinking about keeping the 6 and hopping it up a little instead of changing to a 350. i was wondering if anybody had any suggestions as to hopping it up with out going below 12 or 13 mpg

[geezer#99]

You're looking on the wrong end. Your mpg is suffering cause of the O/D and 3.07 gear.
You'd get better mpg with more gear like 3.73 or 4.10. Less throttle opening equals more mpg.
However nothing wrong with some hop-ups. Headers and a 4 bbl intake with small 390 holley will help.
With your gearing a 350 would probably get better mpg though.

[Fantom]

I keep telling you to keep the I-6......Add 4 bbl, headers & chrome. Maybe a small cam. It's just gas!!!!

(did you figure out who I was yet?)

[PanelDeland]

You could also look around the BB.Someone has done a FI system on a 6.IIRC he used one designed for a Jeep but it worked well and his milage and performance are both improved.The 3:73 gears,FI and a header shoukld make for good milage and keep most street 350's on their toes.

[MARKDTN]

Originally Posted by PanelDeland

You could also look around the BB.Someone has done a FI system on a 6.IIRC he used one designed for a Jeep but it worked well and his milage and performance are both improved.The 3:73 gears,FI and a header shoukld make for good milage and keep most street 350's on their toes.

Howell makes a FI system for a straight 6 jeep that could work with a Chevy 6.

http://howellefi.com/jeep-tbi-products.html

[markallen]

haha yeah i fiqured out who you were fantom

[markallen]

Originally Posted by geezer#99

You're looking on the wrong end. Your mpg is suffering cause of the O/D and 3.07 gear.
You'd get better mpg with more gear like 3.73 or 4.10. Less throttle opening equals more mpg.
However nothing wrong with some hop-ups. Headers and a 4 bbl intake with small 390 holley will help.
With your gearing a 350 would probably get better mpg though.

i dont understand why i would get better mileage with lower gears like 3.73 or 4.10 i thought higher gears = more mileage

[longbed67]

Get yourself a wideband and look at your #'s the 3.07 is to low of a ratio for the h- power and torque curve of a stock 250, thus you must put more foot into the throttle, thus richening the mix and killing fuel economy. I drove through the mountains with a 3.73 and a stock 250 and got 17+ all day long on my trip back from washington. If you want to keep the 3.07 you can but do a cam change, headers, clifford intake, holley 4bbl carb (get it tuned properly), and an hei ignition and you will have about 75-100 more hp and ftlbs torque and you have better fuel econ. visit www.12bolt.com, tom has a lot of info and products at very good prices that will help you out on the 250.

[68gmsee]

I agree with going sbc. Since your truck is not completely original I'd go with a 350 and keep the rear end as it is. You'd have quite a bit more power and it would be alot cheaper than upgrading the 250.

I converted my 69 to a 350/350TH, swapped the rear end for a 3.08 and my mpg was about 16 on the highway. I was getting about the same or a little less with the original 250 but the 350 gave me quite a bit more power.

I keep the 250 in my 68 GMC because it's totally stock except for power brakes and power steering and I just cruise in it every now and then.

[longbed67]

if you are going to do a swap... why not do something that these trucks had in them like a 327 with a good compression your making the same power as a 350 if not better and your not doing a run of the mill swap...

[markallen]

what rearend gears would give the most mileage after i get headers and a small cam?

[longbed67]

If you do keep the 250 L6 motor AND do the power upgrades, I would keep the 3.07 I am currently in the process of turbocharging my motor and doing a LOT of head work and am going down to 2.76 gears to make up for not having an overdrive trans. In doing the power upgrades you will have the power to keep out of the throttle and improve fuel econ. but with more power comes the want to get on it to show off...

[FL71C10]

I would think the 5 speed transmission would help make up for the lower rear end gear ratio.

Originally Posted by longbed67

Get yourself a wideband and look at your #'s the 3.07 is to low of a ratio for the h- power and torque curve of a stock 250, thus you must put more foot into the throttle, thus richening the mix and killing fuel economy. I drove through the mountains with a 3.73 and a stock 250 and got 17+ all day long on my trip back from washington. If you want to keep the 3.07 you can but do a cam change, headers, clifford intake, holley 4bbl carb (get it tuned properly), and an hei ignition and you will have about 75-100 more hp and ftlbs torque and you have better fuel econ. visit www.12bolt.com, tom has a lot of info and products at very good prices that will help you out on the 250.

[faribran]

my truck used to have the 250.... i have since upgraded to a SBC....... more power and better gas mileage..... all around more user friendly..

[longbed67]

Never thought to ask is the 15 MPG in town or highway? if your going into 5th gear on the highway with the OD .73:1 gear your going to throttle harder than if you kept it in 4th with the straight drive 1:1 gear. If it is in town... Get a proper tune on the carb jetting and ignition timing, and stay out of the throttle...

[brad_man_72]

there's a reason everybody is doing ls swaps on these trucks. I get better mpg pulling my boat 70mph. You can buy alot of gas for what the swap is going to cost you...

[Ackattack]

You could put a TBI injection system on it.
How about an electric fan?

Try driving in 4th gear on the highway and see what that does to your mileage. That's free and might be worth a shot.

[R.L.]

What kind of gas mileage are you getting now and how does it preform street wise? Does it not keep up with traffic as is?

[Z1971]

I would keep the 250. Everybody (including me) has a SBC. Try a GM HEI and headers first. Tom Langdon at Stoveboltengineco.com will get you a rebuilt factory HEI with new cap and coil for $95. He also has rebuilt Holley Weber Progressive 2-Barrel Carburetors for $80. You will need a $15 adapter to fit the stock intake. Clifford has headers and their 4 barrel intake is better than the Offy. I would suggest a holley 390 and Clifford 270cam if you want to change your intake. Tom Lowe at 12bolt.com has some dyno runs with the 270 cam where he got 270hp from 250 inline.

The heads are the real weakness of the Chevy inline. Bolt-in lump ports really help the flow. Tom has everything even turbo manifolds. http://www.12bolt.com/home He also has several projects on Inliners International forum.

[ol_skool_chevy]

Originally Posted by geezer#99

You're looking on the wrong end. Your mpg is suffering cause of the O/D and 3.07 gear.
You'd get better mpg with more gear like 3.73 or 4.10. Less throttle opening equals more mpg.
However nothing wrong with some hop-ups. Headers and a 4 bbl intake with small 390 holley will help.
With your gearing a 350 would probably get better mpg though.

Sorry but I have to agree with geezer#99.
your Much better running the 3.72 gear with that t-5

[SOON2BLAZE]

Originally Posted by Z1971

I would keep the 250. Everybody (including me) has a SBC. Try a GM HEI and headers first. Tom Langdon at Stoveboltengineco.com will get you a rebuilt factory HEI with new cap and coil for $95. He also has rebuilt Holley Weber Progressive 2-Barrel Carburetors for $80. You will need a $15 adapter to fit the stock intake. Clifford has headers and their 4 barrel intake is better than the Offy. I would suggest a holley 390 and Clifford 270cam if you want to change your intake. Tom Lowe at 12bolt.com has some dyno runs with the 270 cam where he got 270hp from 250 inline.

The heads are the real weakness of the Chevy inline. Bolt-in lump ports really help the flow. Tom has everything even turbo manifolds. http://www.12bolt.com/home He also has several projects on Inliners International forum.

ditto and the 3.73 gears. Still alot cheaper than a sbc swap. I'm guessing the 3.07s are the stock 40 year olds.

[markallen]

a guy my dad works with said that if you put a performance cam in a 6 cylinder the motor will not hold up bc it is not made to do things like that with...is there any truth to this

[Palf70Step]

You will typically get your best MPG when your vehicle is running around it's peak torque RPM. That way at a steady RPM, your truck/car is working more efficiently. Sometimes a lower rearend gear (higher number) will get your better MPG. If most of your driving is highway, then you would probably do better mileage with a 3.42/3.73 rearend with the 250. If most of your driving is in town, then I would stay with the 3.07

As far as the engine not holding up to a cam, baloney. Now just just doing a cam and not looking at the rest of a 40 year old motor is asking for a problem.

Freeing up your exhaust flow with headers or a good split manifold, possible an intake and carb change will also probably help. Before I would do any engine mods, I would check my over condition of the engine first. How's the compression, carb settings, timing, etc... As mentioned a good HEI will help. BTW your tire size will effect your MPG also. Large low profile means more resistance, thus it takes more to push the vehicle.

Remember you are moving a LARGE brick. Try things you can afford, but keep the truck running. Don't lose the fun of driving it.

[68gmsee]

Originally Posted by markallen

a guy my dad works with said that if you put a performance cam in a 6 cylinder the motor will not hold up bc it is not made to do things like that with...is there any truth to this

Way back in the 50's and 60's there were quite a few of them in drag and speedway circle racing. I've seen some of these sixes out run V8's in the 1/4.

These engines can hold up pretty well if built right. Check this Youtube video. Chevy 250 on a dyno.

http://www.youtube.com/watch?v=5uVNQFkIZXc

[longbed67]

Depending on whether your tranny is a standard t5 with the .63 OD gear or the close ratio t5 with the .74 OD gear this is what your RPM bands look like with a 3.07 rear and 255/60R15 tires for a total tire height of 27.05" If it is the standard t5 the ~1300 rpm at highway speed would explain the low fuel econ. because you dont have enough power to push it down the road.