replace crate 350 with 6

[domeier]

I'll rephrase that original post that was closed so it's not a classified:

I'm thinking about replacing my Crate HO Deluxe 350 with a 250 or 292 straight 6 for better fuel economy. A couple of you are saying I probably won't see better gas mileage with the 6, but now lookie here - I'm only getting about 12 mpg with my current setup. It runs absolutely perfect, so it's not a tuning issue, it just gets awful mileage.

71 Short stepside, 4WD, 330hp 350, TH350, NP205, 15" skinnies. Don't know what the gear ratios are, but they're low. Spins the engine up pretty high at 60 MPH.

You think if I swapped to a 6, I'd still see 12 MPG?

[stomper]

I think so. The 6's I've had weren't as great on gas as you'd expect 2 fewer cylinders to be. I'd say you're not doing too bad with 12mpg in a 4x4 with '...the engine up pretty high at 60...' I probably get 10 mpg or less at 70 mph in my K5 with 33's and 3.73's

Determine your differential gear ratio or get a good RPM reading at speed and post for us with tire size. That will help us help you.

Assuming the 350 is tuned and everything is up to snuff there... if you want to increase your mileage I'd look at a gear change or maybe a 700r 4spd tranny swap.

[PanelDeland]

Yep.I get about 14 in my Panel with a 250,3 OTT,and 3:73 rear.These trucks are heavy,not very tech and old.If you really want fuel milage from a vintage vehicle you will need to spend some buks.The first strike against your milage is the 4X4 because of all the extra moving parts,drag and weight of the 4 WD.To get descent milage i.e. 20ish, you would need to swap in the entire driveline from a later vehicle that gets that kind of milage and weighs about the same.You might help some by going to an overdrive unit.IIRC lots of 4 wheelers use a unit called the "Atlas" though some go with a gear vendors unit.Either will be pricey and probably take years to payback unless you put tons of miles on the truck.

[oldgold70c10]

A lot of it will depend on the overall setup of your rig. Such as what size tires your running, 2 or 4 wheel drive, rear diff ratio, transmission. Some say a smaller engine gets better milage because it just sips feul and your less likely to want to put your foot in it, while others say a bigger engine gets better mileage because it doesn't have to work so hard just driving normal, but you can put your foot in it more and loose economy.

Have you considered doing a cam swap on the crate 350 to a more stock(ish) cam?

[cdowns]

if you really reving the engine you have now at speed you'll b doin the same with a six cyl the only difference would b that the six will blowup alot sooner than the 350 8 cyl will

[domeier]

Dangit. I think maybe I'll be forced to buy a 2WD C-10. And then of course, I'll need to keep my 4WD for when global warming kicks in and it starts snowing here, and I should probably get me a wrecker, and a C-20, and a...

I gotta stop reading this site. You guys are putting too many ideas in my head.

[MARKDTN]

You'd be better off adding fuel injection and/or changing to an overdrive transmission than changing the engine to a 6 cyl IMHO. On my '83 K20 the stock 350 q-jet/700R4 got 14 mpg city. I changed the engine to a stone stock Tuned Port 350 from a Corvette. I now get 17+ city and over 20 on the road. A lighter or 2wd truck should get better than that with the overdrive.

[68gmsee]

My input:

As for economy on a six... There is a one mile per gallon difference on my 68 GMC with 3 speed manual and 3.54 rear end, vs the old worn out 69 Chevy with 350/350 and 3.08 rear axle: 15-16 vs. 14-15 mpg. Not much gain with the 250.

I'm keeping the 250 in the 68 GMC strictly because the truck is almost stock (p.s and p.b.) and for the novelty of it. Otherwise, I'd find a newer 350 and rebuild it.

[eurospec1]

IF you saved any money on gas buy doing this swap, it would probably take years to recoup the cost of the swap in the very small if any savings you would get, doesn't make sense to me

[GASoline71]

Trucks and MPG are not really a match made in heaven. Even new trucks get crappy gas mileage.

12mpg with a 350 in a 4X4 is pretty darn good.

Gary

[The Youngen]

If your looking for mpg, consider a 6.2. Thats what they were designed to be good at.

[brad_man_72]

ls swap. I get 15+mpg pulling my boat 70mph. More money but you might actually see a gain in mpg, hp, and relaibility.
12mpg with 4x4 doesn't sound bad at all. My 4.3 s10 4x4 usually got around 16mpg with fuel injetion and an overdrive....

[treveiger]

You'd be better at finding your rear end gear so we can see how high it is
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[truckster]

The six isn't going to save you gas. Maybe if you drive really, really conservatively, you might increase your MPG a little. More likely is that you'd miss the power of your 350 and be on the throttle all the time, and maybe even get worse mileage.

What kind of driving do you do? Is it mostly city, highway, or a combination? Load or no load? Any towing? A rear gear swap may be your best bet, but how and where you drive will help determine that.

[bigbadchev72]

What carb are you running? A good q-jet will get you way better mileage than a edelbrock will. I got 15.3mpg in my blazer most of that was on the highway and was not counting the train run. Wife followed me up to camp and all the way home, I used the mileage off her rig to calculate. I was running a mild 350 with q-jet, th350/205 and 4.56s with 38s.

Its possible, but for around town driving do a 6.2 swap, it will get better mileage than either and will have more power than the 6, plus there are not alot of diesel conversions in our rigs.

[domeier]

All interesting replies. Thanks a lot. To answer your questions:

1) I don't know what the rear diff gear ratio is. Any way to find out without popping the cover and counting teeth?

2) I don't have a tach, so don't know what rpm's are at cruise. Just know they're up there. Sounds to me like 3k at 60mph.

3) Tires are small. P235/75R15

4) Mostly city driving, with a little bit of freeway. No 4WD use whatsoever. Bums me out to think about hauling that 5 ton transfer case around and never getting to use it for anything.

5) I have an Edelbrock 4106. Q-Jets scare me. They look so complicated compared to the 4106! You really think a Q-jet would steal me an mpg or two? That would be an easy project.

About the 700R4: Is that a direct swap for the TH350, and how about a ballpark estimate for a good used unit?

And hell yeah - I would love to have an LS1. I had a '98 Z28,and that was flat out THE absolute very best engine I've ever owned in any vehicle, land, sea or air. I got 29 mpg in the Camaro at 75 mph, and it still ran like a watch when I sold it at 220,000 miles. Amazing, incredible engine. I love looking at the pictures of all you guys who put em in your trucks.

Hmm... The more I think about this, the more I'm starting to think I should probably sell my truck and buy a 2WD. Even if I put in a 700R4, q-jet and higher gears, I'm still going to be lugging around that lonely old transfer case and front diff., and it's too much time, work and money for me to change this truck over to 2WD.

[hugger6933]

iknow this is apples to oranges but I had a s-10 extra cab I put a 350 tbi 700into . I also had a guy burn me a chipfor the truck. With the 4.3 the best was around 19 [the 4.3's didnot get the mileage a lot thought they would] I drove to Little Rock to have a chip burned for the v-8, I got 19.6 on the way there and 26.6 on the way back with just a chip change. now that was driving 75 on the interstate with the a/c on. Yes the larger engine in my case got better because it did not have to work as hard

[domeier]

My dad has a 4.3 S10. Lousy gas mileage, and it's a nightmare to wrench. Took me about two hours just to change the plugs - one of them is damn near impossible because it butts up against the steering column... but I guess you already know all about that! I imagine a 350 must be tight in there, but probably no worse than the 4.3.

[ChevLoRay]

Edelbrock 1405 is what was on my engine when I got my '69. It ran rich; gas washed past the rings and diluted the oil in the crankcase. I switched to an Edelbrock 1901 Q-Jet...okay it's a 750CFM Q-Jet...and I saw better performance and had to put a heat shield on my starter solenoid because the engine ran leaner (and hotter). I don't see where the 1400-series AFB-style carbs have an edge over the Q-Jet.

But, if my 4.3 takes a dump in my '96, I'm gonna give some way-serious consideration to an LS-engine. It's logical for a later model. But, for your truck, I'd look at an OD gearbox and take what you get as a result. For the money, even with what you guys are paying for gas in CA, I just don't see an economical plus to switching to a straight six.

[eurospec1]

Originally Posted by domeier

All interesting replies. Thanks a lot. To answer your questions:

1) I don't know what the rear diff gear ratio is. Any way to find out without popping the cover and counting teeth?

2) I don't have a tach, so don't know what rpm's are at cruise. Just know they're up there. Sounds to me like 3k at 60mph.

3) Tires are small. P235/75R15

4) Mostly city driving, with a little bit of freeway. No 4WD use whatsoever. Bums me out to think about hauling that 5 ton transfer case around and never getting to use it for anything.

5) I have an Edelbrock 4106. Q-Jets scare me. They look so complicated compared to the 4106! You really think a Q-jet would steal me an mpg or two? That would be an easy project.

About the 700R4: Is that a direct swap for the TH350, and how about a ballpark estimate for a good used unit?

And hell yeah - I would love to have an LS1. I had a '98 Z28,and that was flat out THE absolute very best engine I've ever owned in any vehicle, land, sea or air. I got 29 mpg in the Camaro at 75 mph, and it still ran like a watch when I sold it at 220,000 miles. Amazing, incredible engine. I love looking at the pictures of all you guys who put em in your trucks.

Hmm... The more I think about this, the more I'm starting to think I should probably sell my truck and buy a 2WD. Even if I put in a 700R4, q-jet and higher gears, I'm still going to be lugging around that lonely old transfer case and front diff., and it's too much time, work and money for me to change this truck over to 2WD.

Option 5: Buy cheap cash good gas mileage car/pickup to drive around and save your C10 for when you want to drive it and don't need to worry about gas.

[truckster]

If you don't ever drive off-road and don't have any intention of going off-road, you might be better off finding a 2wd that suits your purposes. It's just basic physics that the added weight and complexity of the 4wd are going to require more fuel to push them down the road.

[406 Q-ship]

Originally Posted by domeier

All interesting replies. Thanks a lot. To answer your questions:

1) I don't know what the rear diff gear ratio is. Any way to find out without popping the cover and counting teeth?

2) I don't have a tach, so don't know what rpm's are at cruise. Just know they're up there. Sounds to me like 3k at 60mph.

3) Tires are small. P235/75R15

4) Mostly city driving, with a little bit of freeway. No 4WD use whatsoever. Bums me out to think about hauling that 5 ton transfer case around and never getting to use it for anything.

5) I have an Edelbrock 4106. Q-Jets scare me. They look so complicated compared to the 4106! You really think a Q-jet would steal me an mpg or two? That would be an easy project.

About the 700R4: Is that a direct swap for the TH350, and how about a ballpark estimate for a good used unit?

And hell yeah - I would love to have an LS1. I had a '98 Z28,and that was flat out THE absolute very best engine I've ever owned in any vehicle, land, sea or air. I got 29 mpg in the Camaro at 75 mph, and it still ran like a watch when I sold it at 220,000 miles. Amazing, incredible engine. I love looking at the pictures of all you guys who put em in your trucks.

Hmm... The more I think about this, the more I'm starting to think I should probably sell my truck and buy a 2WD. Even if I put in a 700R4, q-jet and higher gears, I'm still going to be lugging around that lonely old transfer case and front diff., and it's too much time, work and money for me to change this truck over to 2WD.

Your getting 12 MPG with mostly stop and go driving in a 5000 lbs 4x4 truck......and your complaining? A 250 is going to be working even harder to get all that to speed, so city driving the mileage will be the same if not worse.

Mileage is all about tune and how (and where) you drive. Sounds like your tune is pretty close at 12 MPG in the city, so learn to get used to the slow lane and press the throttle like there is an egg between your foot and the accelerator. A vacuum gauge is a good investment in getting better MPG to get the best driving style. Drive the truck so that you get the highest vacuum reading all the time and mileage will get a little better. If your looking for much better mileage, get a fuel sipping daily driver, way cheaper than an engine change.

I have a GMPP 330hp/350 crate engine in my 1968 Chevelle with a TKO600 and 3.31 gears on 26 inch tall tires. The car has a 1400 edelbrock with a Perfomer RPM airgap, it pulled down 21 MPG @ 75 MPH with a fuel leak! Around town I am getting around 14/15 MPG, now I don't drive this car with the idea of mileage.

[domeier]

Sorry, I meant 12mpg highway. I'm afraid to even check what it gets in the city. I used to live in the sticks, so the 4WD was good to have, but now it's just a cute little knob for Prius-driving passengers to point at and ask, "what does that thing do?"

The economics aren't all that bad when one reminds oneself that it's not financed and doesn't require full coverage insurance. No smog check is a big plus, too. I'd just like a few more mpg's, and was thinking a six would do it. I'm really appreciative of all the knowledge on this site to let me know I've been barking up the wrong tree. I hate barking up the wrong tree. For once in my life, I'd like to bark up the RIGHT tree.

I'm about 80% committed to selling it and buying a stock 2WD long bed. 15-18 mpg I could live with. Of course... if I was to do that, it probably guarantees that my next job would transfer me right back to the sticks...

[406 Q-ship]

Originally Posted by domeier

Sorry, I meant 12mpg highway. I'm afraid to even check what it gets in the city. I used to live in the sticks, so the 4WD was good to have, but now it's just a cute little knob for Prius-driving passengers to point at and ask, "what does that thing do?"

The economics aren't all that bad when one reminds oneself that it's not financed and doesn't require full coverage insurance. No smog check is a big plus, too. I'd just like a few more mpg's, and was thinking a six would do it. I'm really appreciative of all the knowledge on this site to let me know I've been barking up the wrong tree. I hate barking up the wrong tree. For once in my life, I'd like to bark up the RIGHT tree.

I'm about 80% committed to selling it and buying a stock 2WD long bed. 15-18 mpg I could live with. Of course... if I was to do that, it probably guarantees that my next job would transfer me right back to the sticks...

My 67 C10 SWB 350/TH350/3.08 gets about 13-14 in city driving and maybe 15 on the highway (don't do that very often in So Cal), so a 2wd is easier to get better MPG but not a for sure deal. My 1974 C10 SWB 350/700R4/3.42 used to pull about 14 city and 17 highway. So overdrive is definatly your friend.