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Old 03-31-2007, 09:37 AM
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MPG SBC build

I need some advice for a rebuild, here's some background:

Over the past couple of years I have been building a little 'truck' to play with. Out with the old in-line 4, in with a SBC. Even stock low compression versions are going to get much higher torque and Hp numbers while getting similar mileage or better than my old 4. Thus, I have virtually no interest in more Hp when building this 350. What I am interested in is the best fuel economy possible.

The vehicle is not terribly heavy, ~4200 lbs loaded but has the aerodynamics of a brick *****house. It's got a New Process 435 four speed, coupled to a t-case (high 0.871:1 high range) and 4.10 R&P. With this combination my cruise RPM will be between 2400 and 2600 depending on tire choice. Most likely closer to 2500. It needs to run on 87 octane too. I've considered everything from a 283 build to a LT1. I'm pretty much open so long as it's a GM V8 (the mounts are done to fit a SBC). Also, it should idle smoothly and have good bottom end torque. This motor will hardly ever see more than 4000 RPM. Advice?

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Old 03-31-2007, 10:17 AM
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Lotsa work, but the LS1 gets 30+ mpg at 300+ hp in a 3000-lb vette. Its all in the heads which makes LS1s really shine in that department.

LT1 and Vortec are also good choices. Those two share the same very efficient port and chamber designs which help keep mileage up. Typical old-school chevy heads would max out at around 8.9:1 on the cheap gas and require around 38* ignition for best efficiency. Vortecs can run 9.5 or more on cheap gas and only require about 30-32* lead for best running.

When you say the words torque, mileage, and SBC in the same sentence, you're basically asking for the vortec recipe. Vortec gets you either factory EFI or easy carb use with any one of the aftermarket Vortec intakes. LT1 gets you factory EFI, but you only have one or two choices for carbed intake. For all intents and purposes concerning mileage and power output, consider the LT1 and Vortec the same engine, just more carb and intake choices with Vortec.

If you go LT1, get the iron-headed version. The iron heads flow better than the aluminum. Many times people blame the camaro/vette LT1's higher power ratings on the fact that they use the aluminum heads, but its not the case. They use a different cam and higher compression for more power, but the iron heads are the winner. In fact, the Vortec head came about BECAUSE of the iron LT1. The vortec port is a copy of the LT1 iron port.

When choosing a cam, select one with very little overlap. Resist the urge to go huge LSA, instead go for short duration. The factory GM Impala SS/Fleetwood/Roadmaster cam is a nice little torquer that specs out to 191/196 on a 111 LSA. A nice upgrade is the Comp Cams 304 grind for LT1s that specs out to 210/220 at 114 LSA.

Last edited by curtis73; 03-31-2007 at 10:25 AM.
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Old 03-31-2007, 11:02 AM
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I would go with a 400 SBC with about 9:1 compression and a tight RV cam with high ratio rockers, a performer manifold with EGR and long tube headers if you can. It won't be the abcolute best for milage but it will still be decent and give you the torque a truck that heavy needs.
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Old 03-31-2007, 01:20 PM
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Curtis73,
Generally Iím leaning toward the Vortec style heads on my 350 short block. They are pretty hard to beat in terms of performance vs price from what is out there to read on the net. The other possible option looks like the Edelbrock E-tec. Are they Edelbrockís copy of the Vortec chamber?

The cam you suggest from the Impala SS sounds terrific. Actually, it looks really close to the High Energy 240H (191/196-111 vs 192/200-108). One would think the GM version would cost more than the Comp Cams unit, but I could be wrong. Either way, I havenít seen a SBC (in my wonderings) with such short duration and as long a LSA as the Impala SS. Thatís pretty impressive. Should be a real torque monster! Weíll see shortlyÖ Iím phoning for prices right now.

All the Vortec heads are roller tip standard, are they not? Is that a huge consideration in economy? In other words, should I even consider a flat tappet hydraulic setup for the longevity and mileage advantage of the rollers?

So letís start my theoretical build. Assume the 350 bottom end checks out and will be fine with a .030 over flat top piston. Is zero deck too tight with the small combustion chamber of a Vortec head? Or if a person chooses the E-tec 170ís should you leave the pistons and deck height stock to avoid too high CR? If my math is correct, the E-tec with stock pistons, etc. should be around 9.7 Ė 9.8:1. (0.025 deck, 0.041 head gasket, I guessed at the piston relief volume @ 5cc). Too high for 87 pump gas with this style of combustion chamber?

A friend that builds race engines also told me to go with gapless top rings, period. Any disadvantage?

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Old 03-31-2007, 01:42 PM
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Get a 4.3L Vortec V6 out of a '96 pickup. Lots of power/torque and better mileage than a V8. Sits on standard SBC V8 mounts. Its a very good truck package for economy/towing/daily driving.
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Old 03-31-2007, 01:48 PM
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I put a 99 Vortec 5.7 with reprogrammed computer in a 55 Chev- smooth, good gas mileage, surprisingly good power- was real cheap from the bone-yard. Bill
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Old 03-31-2007, 02:11 PM
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If power is not a concern I would go with a smaller cube small block with high efficiency heads, cam as mentioned and definately EFI. You have to have EFI with closed loop control(O2 sensor) to get high mileage numbers.

While a 305 gets passed over alot, they will do well in the mileage department. If you went with a 334 stroker using a 383 crank in a 305, it would help in the grunt department and still keep cubes down. Vortec heads would also work well on this setup.
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Old 03-31-2007, 02:57 PM
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Quote:
Originally Posted by ap72
I would go with a 400 SBC with about 9:1 compression and a tight RV cam with high ratio rockers, a performer manifold with EGR and long tube headers if you can. It won't be the abcolute best for milage but it will still be decent and give you the torque a truck that heavy needs.
ap72, I completely forgot about EGR. That is something I will have to remember. EGR makes economy while cruising, doesn't it? As for the 400 or other size engines, I'll look into it after I find out the condition of my 350 bottom end. After all, the whole point of building a motor for economy is to save money. This motor is already in the truck! It was cheap though...

I didn't think the weight was that much. 'Loaded' it's probably going to be ~4200 lbs. The kerb weight with a full tank of gas and me sitting in the cab will be closer to 3400 lbs. The loaded weight is because I tend to carry lots of stuff around on trips(parts, camping gear, recovery stuff). Maybe I should get more efficient at cleaning out stuff before I leave the house! Prior to changing out the entire driveline, it was only rated at 4675 lbs gross. The parts all came from vehicles with a minimum gross of 6000 lbs... so they should be able to take a mild V8 hauling around so little!
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Old 03-31-2007, 03:02 PM
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Quote:
Originally Posted by F-BIRD'88
Get a 4.3L Vortec V6 out of a '96 pickup. Lots of power/torque and better mileage than a V8. Sits on standard SBC V8 mounts. Its a very good truck package for economy/towing/daily driving.
F-Bird, That -WAS- the plan when I first started. Somehow along the way I was talked into going straight to the 350. It just seemed to make sense. The guys I talked to were getting the same mileage with V8's as the 4.3 and had more power/torque to spare. Now that all the mounts are done I would like to stick with the 8 (engine mounts are no problem since they match. The fore/aft situation is a problem because the t-case mounts are set on the back and there is no room ahead of the fan)

BTW- what is the motor mount relationship compared to the V8 on the 4.3? I don't know what the measurements are on the 350 off hand, but assuming center line on the mounts are 'X' from the front face of the block on a 350, what are same measurements onthe 4.3?

Good suggestion though!

Last edited by hillman88; 03-31-2007 at 03:13 PM.
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Old 03-31-2007, 03:07 PM
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Quote:
Originally Posted by hotroddr
I put a 99 Vortec 5.7 with reprogrammed computer in a 55 Chev- smooth, good gas mileage, surprisingly good power- was real cheap from the bone-yard. Bill
Hotroddr- That's something I am looking into right now. The serious advantage of this route is speed. A little more time setting up, but I should be able to find a complete running motor and drop it in. That -should- be faster than rebuilding my 350 with the benefits of the computer control.

Who reprogrammed the computer for you? The little I've heard about the newer engines in non-stock vehicles is that there seem to be endless issues getting the ECU to settle into its new home. That might have been strictly newer engines though.
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Old 03-31-2007, 03:22 PM
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Quote:
Originally Posted by TurboS10
While a 305 gets passed over alot, they will do well in the mileage department. If you went with a 334 stroker using a 383 crank in a 305, it would help in the grunt department and still keep cubes down. Vortec heads would also work well on this setup.
Turbo-
That's a different approach that didn't cross my mind. After the 4.3 idea, I was thinking small displacement V8... 283 actually. So the idea with your stroked 305 is the age old 'long stroke = torque' mantra? There isn't an 'issue' with the longer stroke (I keep accidentally typing stork!), but do they last as long or longer than the standard stroke motor due to side loading? I'm thinking of a situation like the 250/292 straight 6. Great torque engines that run like a top, but a friend that builds them says they don't last as long between rebuilds due to the side loading (oval bores). Don't take my comments and questions as any kind of slight on your ideas either. Any knowledge I have about this stuff is all heresay since I have no first hand experience with any of it. I have alot to learn so keep the ideas and insights coming!!!
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Old 03-31-2007, 03:26 PM
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I believe the engine mount pads are further forward on a V6 block. The trans bell housing position is the same as V8.
you can swap a 4.3 V6 directly onto the V8 Mounts and attach the bellhousing without moveing it.
I've drove the 4.3vortec V6 in a full size pickup. It feels like a V8. Has lots of torque. The over all fuel mileage is going to be better. Reguardless of what your V8 friends say.
I would swap in the whole deal EFI ,air box/filter and all. And try to emulate the stock exhaust system's approxamate back pressure. (Use the stock muffler and cat from the 96 truck.) That way it will run fine without messing with the Computer calibration. If your overall bias is for fuel mileage I think this is the better choice. I doubt you'll be disapointed with this motors overall power. Like I said, it feels like a V8 more than a 6.
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Old 03-31-2007, 03:39 PM
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Quote:
Originally Posted by F-BIRD'88
you can swap a 4.3 V6 directly onto the V8 Mounts and attach the bellhousing without moveing it.
Now THAT is interesting. If the distance from the belhousing face to the mount is the same I could drop in the 4.3 without many issues. It would also give more space up front... aprox 4.5" for electric fans or whatever I like.


Quote:
Originally Posted by F-BIRD'88
I've drove the 4.3vortec V6 in a full size pickup. It feels like a V8. Has lots of torque. The over all fuel mileage is going to be better. Reguardless of what your V8 friends say.
I would swap in the whole deal EFI ,air box/filter and all. And try to emulate the stock exhaust system's approxamate back pressure. (Use the stock muffler and cat from the 96 truck.) That way it will run fine without messing with the Computer calibration. If your overall bias is for fuel mileage I think this is the better choice. I doubt you'll be disapointed with this motors overall power. Like I said, it feels like a V8 more than a 6.

I'll start looking around for one of these... just for the heck of it!
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Old 03-31-2007, 03:44 PM
How fast is fast enough?
 
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I have a 4.3L along with a mess of other enignes and it definitely feels like a V8 when you buy gas, but otherwise just a V6. In stock form it gets pretty miserable gas milage for a engine of that size- but just like a V8 that can be crutched. I would go with the 335 stroker idea.
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Old 03-31-2007, 04:54 PM
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Quote:
Originally Posted by ap72
I have a 4.3L along with a mess of other enignes and it definitely feels like a V8 when you buy gas, but otherwise just a V6. In stock form it gets pretty miserable gas milage for a engine of that size- but just like a V8 that can be crutched. I would go with the 335 stroker idea.
AP72-
So your experience with the 4.3L hasn't been as stellar I take it?

_
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