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GM 250 Inline six..


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  #1  
Old 02-26-2012, 01:53 AM
Grease Monkey Grease Monkey is offline
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Default GM 250 Inline six..

I got a new project truck, a 1967 One ton....

What options do have for getting better mileage/ performance from an inline 6 250ci engine? I know performance parts are not as widely available as the V blocks...

I'm not really a GM guy, this is my first real project with something from GM
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Old 02-26-2012, 10:36 AM
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Default Re: GM 250 Inline six..

No inexpensive tricks other than just get tuned up to specs, they are normally pretty econimical as built. If your after mega performace, swap it out and drop in a cummings diesel or some other such beast, not cheap. F.J.W.
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Old 02-26-2012, 11:09 AM
Zac C Zac C is offline
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Default Re: GM 250 Inline six..

My first vehicle had a I6 in it, those straight 6's really have some useful torque, I never realized how good it was until I had a V8 and kept stalling it trying to go slow off road.
I think that if you want to use less gas you will just have to avoid driving it... Don't forget you are driving a vehicle with all the aerodynamics of a brick
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Old 02-26-2012, 11:31 AM
Grease Monkey Grease Monkey is offline
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Default Re: GM 250 Inline six..

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Originally Posted by Zac C View Post
My first vehicle had a I6 in it, those straight 6's really have some useful torque, I never realized how good it was until I had a V8 and kept stalling it trying to go slow off road.
I think that if you want to use less gas you will just have to avoid driving it... Don't forget you are driving a vehicle with all the aerodynamics of a brick


Well.... I want it more as a hobby service truck, for tinkering around at museum and hauling the Farmall to Show's and picking up large parts and so on...

This isn't going to be my daily driver or anything like that...
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Old 02-26-2012, 11:41 AM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: GM 250 Inline six..

The 1966 250-6 I had, had a few modifications! First, I had the top of the intake manifold milled, after I plugged the original carb mount bolt holes with cut off bolts. Then I ground out the throat of the manifold to accept the bore of a 1966 California built Chevy 283 V-8 2 barrel carb. (yes, by then California already had airpollution crap goilg on) I marked the new bolt hole locations, drilled and tapped them and mounted the replacement carb. The carb used stock jets, fuel screw closest to the valve cover was set 1/8th turn leaner than the away setting (to stop center rich condition like you get with the single barrel carb), the throttle hooked up as is. I had this engine in a truck, and got 18 to 19 MPG! The engine also served (in different vehicles) until it had over 300,000 miles on it, when it broke an oil sender and damaged the bearings.

For your truck, if you don't replace the carb: Set points to factory spec. With the points open to the greatest position, try to push and pull the rotor toward and away from the points rubbing block, and see if the gap changes. if the points move more than 5 orn 10 thousandths, the distributer bushings are worn, and timing will be erratic, and will affect milage as well as timing. If you get a rebuilt distributer, check that too - I have got ones in worse shape than the ones I removed! Set timing to factory spec. THEN using a timing light, advance the timing 2 degrees, and take it up the steepest hill you can find, after the engine is good and hot. See if you get detonation (knocking). Repeat this test, until the engine knocks, and then back off 2 degrees. After setting to most advanced position, try to start engine, both hot and cold. If it starts OK, you are good. If any trouble (kick back or not smooth cranking), then you will have to retard timing to correct the problem. On my engine, I had no starting issues. You may have to retard back to factory spec in order to pass emisions, if they check them. Once the advance is set, you can mark the distributer, so it can be returned to the same spot, if you have to change the timing. You might have to reset the timing, depending on where you get your gas, quality and alky content varies, even when purchased at the same place. If you set 4 degrees back from knock point(unless close to factory spec), then there should be no issues, but power and milage will not be at peak values, Good luck!
Andrew
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Old 02-26-2012, 01:04 PM
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Ken Majeski Ken Majeski is offline
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Default Re: GM 250 Inline six..

I had a 63 Impala with a 230 I six in it and 3 speed on the tree. It was geared high and would deliver anywhere from 20 to 22 MPG at freeway speeds.

Later I had a 65 3/4 ton with a 292 six in it. Good low end torque but could not pass a Gas station

Then I had a 67 1/2 ton with a 250 in it... Not much torque or MPG

Any small block V8 (283,305,327, 350 ect) will bolt right to your bell housing and factory mounts can be had. Thats the best way to get performance out of the 250
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Old 02-26-2012, 01:28 PM
Grease Monkey Grease Monkey is offline
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Default Re: GM 250 Inline six..

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Originally Posted by FWurth View Post
No inexpensive tricks other than just get tuned up to specs, they are normally pretty econimical as built. If your after mega performace, swap it out and drop in a cummings diesel or some other such beast, not cheap. F.J.W.
Not with my budget for this project....
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Old 02-26-2012, 03:39 PM
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Default Re: GM 250 Inline six..

A 250 in a tonner? I would think it would have a 292. A 250 has motor mounts that are even from side to side. On a 292 one mount is offset a couple of inches.

Inline sixes are prevalent in south america. Turboing one is comon down there. Ethanol feul is used there also. High octane so detonation is not a problem.

Headers would be the first thing for mileage and power.

307 v8 pistons and/or 194 head will raise compression.

Offenhauser and Clifford 6=8 both have multi carb and 4 barrel intakes. More power but not feul economy. Cams are avalible.

The changeover to a small block v8 from a 250 is easy enough. there are another set of bolt holes in the frame rails to relocate the motor mount brackets for a v8.

Big block v8 had different frame brackets from the factory.
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Old 02-26-2012, 04:56 PM
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Default Re: GM 250 Inline six..

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Originally Posted by Andrew Mackey View Post
... Then I ground out the throat of the manifold to accept the bore of a 1966 California built Chevy 283 V-8 2 barrel carb. ...I marked the new bolt hole locations, drilled and tapped them and mounted the replacement carb. The carb used stock jets, fuel screw closest to the valve cover was set 1/8th turn leaner than the away setting (to stop center rich condition like you get with the single barrel carb)...

Andrew
Entertaining story, but unless you had a Dual Plenum Intake manifold (with one side feeding 3 cylinders & the other side feeding the other 3 through seperate runners), setting one jet leaner than the other doesn't make a damn bit of difference on the 'center rich' condition. It does help to fine tune the overall fuel mixture a tad leaner for the slight increase in milage, but with a single (open) plenum, you could have adjusted the outer one an 1/8th turn leaner and gotten the exact same results.

Other than doing lots of 'fine tuning' to get the engine operating at peak performance & efficiency, the best way to increase both performance AND ecconomy is to swap in a later Overdrive transmission which gives you nicely spaced gear changes, and usually a lower 1st & 2nd gear to get you moving, along with the overdrive to drop the engine RPM by roughly 30% (give or take depending on your choice of trans) for better milage.

If you do change (or modify) the intake and change the carb., I would go with either a progressive two barrel or a vaccuum secondary progressive four barrel as the primaries are smaller for better fuel economy as long as you keep your foot out of the throttle and don't open the secondaries.

A good header will help, and then if you want to dig in a little deeper & do a cam change, a good low end Torquer cam will give you even more bottom end lugging power to get you moving with less throttle opening, and sip a little less fuel when up to speed than a more radical cam would. Straight Sixes are good for torque.... Do what you can to accentuate the low end torque more so than the top end horsepower (which is just a number on a calculator, since you messure torque and calculate HP based on torque & RPM), and keep the RPM low to boost fuel economy.
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Old 02-27-2012, 02:57 PM
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Default Re: GM 250 Inline six..

The best thing to do is leave the motor alone I have this engine in my 68 c10 1/2 ton it will run forever no mater what happens. I once was moving from Pahrump NV to Gallup NM hauling a 16 foot flat bed trailer loaded I went over the Hoover dam and was almost to Kingman AZ when it sounded like I dropped a valve when it actually blew the head gasket out between #5 and #6 piston on the left side there is no water hole right there so it wasn't leaking. I drove the rest of the way like that when I got to Winslow AZ my mom was driving my dads 60 corvair when the ring and pinion stripped out. I got a tow rope and tied it to the back of the trailer and towed the car the rest of the way fully loaded trailer and car in tow made it there no problem it pulled and pulled. I put a new head on it and gasket and it ran for another 7 years hauling with no problems I am in the middle of restoring it right now if you want better fuel mileage then put an overdrive trans in it. If you have the manual use a nv4500 if you have the auto like I have then use a 700r4 not to bad to swap ether in I had the th350 auto with 3.73 gears and got 8 mpg no matter what
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Old 02-27-2012, 03:24 PM
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Exclamation Re: GM 250 Inline six..

It did make a difference with the mixture screws on the 250. The center draw is right under the carb, setting the outer screw lean would not affect the center 2 cylinders, like the inner one. The truck it was in was a 1950 IHC L-120 with 410 rear and 750-16 split rims (3/4 ton chassis). The engine and trans were mounted on a custom mount, and the drive shaft was altered to mate to the 3 speed manual trans. A Hurst short throw floor shifter completed the set up The 250 with the 2 barrel sure outdid the 240 with a column shift, that was original equipment!

Grease Monkey, As for your truck, setting the timing up is the cheapest mod you can do to get better milage. Swopping cams for a light use truck is not really going to help, unless you are going to put a lot of milage on it. For casual use, I would leave the engine stock. Check the distributer, like I said earlier, that will be the biggest robber of milage if it is worn out. There is no cam chain in that engine, cam drive is gear to gear. Set idle mixture screw to have the engine run best, and you should be all set. Make sure to have a new airfilter, a dirty one will eat gas. Also, make sure to replace the crankcase PCV valve. If it sticks, the engine won't idle well. Make sure the choke opens all the way, once the engine warms up. I replaced the one on my 250 with a manual, as once the engine fired up, it didn't need it.
Andrew
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Old 02-27-2012, 04:20 PM
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Default Re: GM 250 Inline six..

It's easy to tell a 292 from a 230 or 250. The block is Taller due to the increased stroke.
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Old 02-27-2012, 04:24 PM
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Default Re: GM 250 Inline six..

Doesn't really matter what you think you remember happening 40 some years ago, or even if you do remember it accurately that you know why it worked the way it did. Without a dual plenum to divert the fuel from one barrel differently than the other, it doesn't matter which side of the carb you lean out. Both barrels dumping into an open plenum will mix together in the airflow just the same. There are millions of carbs out there that for whatever reason one side seems to adjust better than the other whethers it's on a V8 or an inline engine. If you noticed more of a difference adjusting one side versus the other, it was more likely due to the factory manufacturing tolerances in the carb, or a plugged or partially plugged passage in it. Unless you had temperature probes attached to each cylinder to take readings, how do figure leaning out the whole engine somehow affected the center cylinders more than the outer ones. I've seen carbs over the years where you can screw one of the idle screws all the way in, or remove it altogether with No difference in the idle while adjusting the other side even just a little makes a huge difference.

There are other tricks that can be done, like adjusting the valves different on the outer cylinders tricking the engine into thinking it has a bigger cam on those cylinders for more airflow drawing in more fuel and balancing out the cylinders or doing a little light grinding on the individual distributer cap towers to alter the timing to each cylinder if you have a tune up machine with the cylinder cancel button so you can see the power drop off from each cylinder, but simply adjusting one side of the carb versus the other on an open plenum manifold makes no difference in how the air/fuel flows through the manifold.

As for setting the timing. Most engines will perform better with a quicker advance curve which you can adjust by changing the springs & weights. Advance kits are available at most "Speed Shops" with instructions for how to procede. Or you can check it out by removing one spring but leaving both weights in place. Be sure to put a small e-clip or c-clip in the groove that the spring sat in to keep the weight from flying off. Depending on the idle speed, it may or may not affect the timing At Idle, but it will give you more advance at part throttle for more power with the total advance still at factory settings. Yes you can advance the overall timing as Andrew suggested, but even if it doesn't start to ping or detinate under load, there is still a point where the power still start to drop off and you need to retard the timing a couple degrees again for peak performance. If there aren't any drag strips in your location to check it out at, you can find a long stretch of road and using a stop watch check your Zero to 60MPH times and keep adjusting for maximum performance which will also be peak efficiency. You may actually reach that long before you get to the point of detination, especially in an older low compression engine. There is a limit to how far you can advance the timing on any engine, and some can actually try to fire backwards if advanced too far even if they still hadn't gotten to the point of knocking. Maximum advance isn't always maximum power or efficiency.

And if you have the desire to do it, a cam change will make a huge difference in any engine in any vehicle whether it's a light duty truck, a daily driver grocery getter, a tow vehicle pulling a heavy trailer, or even a Taxi Cab. If you want to boost your low end pulling power and gain a little better milage at the same time, get an RV cam designed for motor homes. If there are no RV cams available for your particular engine, find one with similar lift & duration. It will boost your low end torque giving you more power, and with that extra power you won't need to step as hard on the gas pedal, increasing the fuel milage. If you pick a cam rated as having a 50 or 100hp increase on the top end, it will do so by sacrificing low end torque and cost you on overall drivability.
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Old 02-28-2012, 01:03 AM
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Default Re: GM 250 Inline six..

If you have a 1 ton it is most likley a 292. Good engine lots of pulling power. As for gas milage take the money you would spend fooling with it trying to squeeze 2 more miles out of it and put it in the tank and just use it when you need a ton truck.
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Old 02-28-2012, 10:45 PM
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Default Re: GM 250 Inline six..

I took a look at a 67 brochure. The 250 was std in the C30.

Make sure the tune up is right. Maybe find a splitter trans that is under/direct/OD and add that in the driveline. Good low restriction muffler, not a glasspack.

My dad bought a GMC 1/2 ton 292 new in 1969, Still in the family. I also have 1967 and 69 1/2 Ton Chevys with 250s for now.
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Old 02-28-2012, 11:25 PM
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Default Re: GM 250 Inline six..

We had a 69 and a 70 GMCs that I should have kept,they both were fully equipped with 350 motors/turbo hydramatic trans/and air/half ton. We got along with them very well, just a bit hard on fuel but easy to maintain. I decided to go with heavier units to pull the larger GN trailers/ Ford 3/4 ton with the Navistar 7.3, gave the GMCs to my relatives,they lasted a while, too bad they didn't keep them. F.J.W.
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Old 02-29-2012, 02:23 PM
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Default Re: GM 250 Inline six..

You can put a later model HEI distributor in. If your good you can convert over to fuel injection. Also you can put an overdrive transmission in it.
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Old 02-29-2012, 04:19 PM
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Default Re: GM 250 Inline six..

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you can convert over to fuel injection.
I'd vote for this one. I put together a fuel injection system from junkyard parts and put it on a '79 Jeep with an AMC 360. It went from 11-13 MPG to 14-17.5 MPG. The downside is if you break down you are less likely to be able to rig something to get back home.
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