how to get better mpg in a 350?

[k3werra]

How would you make a 350 v8 with mild hp (320-350hp) get better mpg? from what ive read changing to FI and manual tranny (5sp/6sp). what else have you guys with 350's done to make your setup yield better mpg?

[BRAAP]

How would you make a 350 v8 with mild hp (320-350hp) get better mpg? from what ive read changing to FI and manual tranny (5sp/6sp). what else have you guys with 350's done to make your setup yield better mpg?

 

 

Most guys on here will tell you that they didn’t install a V-8 into their Z cars to get good mileage. Good fuel mileage is what their Nissan Sentra or TDI Jetta is for.

 

If you are building the engine and still acquiring parts, plan for EFI and taller gearing as you already noted. Also, keep the induction, cam, and exhaust fairly mild. Not too mild or the v-8 conversion becomes a worthless exercise in spending lots of money and time.

 

Now after you get the V-8 in the car, to improve its freeway mileage,. , Tune.. Tune... tune... Lots of tuning... higher tire pressures, narrower tires, lower ride height, close off the grill opening.

[Challenger]

Now after you get the V-8 in the car, to improve its freeway mileage,. , Tune.. Tune... tune... Lots of tuning... higher tire pressures, narrower tires, lower ride height, close off the grill opening.

 

Other then having a mild engine and tall gearing the problem with mpg on the Z is the aerodynamics. Im guessing improving the aerodynamics will improve mileage quite a bit?

[k3werra]

im still thinking about the swap. my uncle could help me with the swap, so im researching now about it. yah i know putting a v8 kind of defeats the purpose of mpg. but id like to get it atleast 20-25 mpg bc of gas prices with moderate power.

[A. G. Olphart]

im still thinking about the swap. my uncle could help me with the swap, so im researching now about it. yah i know putting a v8 kind of defeats the purpose of mpg. but id like to get it atleast 20-25 mpg bc of gas prices with moderate power.

 

Shouldn't be a problem... overdrive trans with 3.54 or the 3.7 limited slip rear.

 

Why not swap in a 280 turbo as you don't plan to go hog wild? (Not sure how the milages compare).

 

 

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[280zwitha383]

An overdrive transmission of any kind with fuel injection should get you into the low 20's. Some guys are doing low to mid 20's with carbed 350's and 383's with the double over-drive of the T56 transmissions. LS1 and probably a FI 350 with a T56 should get you into the high 20's I would think. One of the things that I think is somewhat of a folly in what most people (everyone I've seen) on here does is keep 3.54, 3.90 and even 4.11 gears. With the torque of a V8, such gears are hardly necessary in a street driven vehicle with 3 gears BEFORE you reach 1:1. Corvettes that come with the LS1/T56 have 2.73 gears (not sure if they all do), they weigh a little more than a 280z or 280zx and a considerable bit more than 240z's and can get low 30mpg on the highway. That is just my opinion but is something to think about.

 

Edit: Umm yeah, there aren't a whole lot of options for diffs lower numerically than a 3.54 so that might be a reason.

[getZ]

If mileage is that big a concern, put an LS v-8 instead of standard small block chevy. With the prices of all the after market stuff dropping, it hard to go to the venerable sbc anymore. I look at the power the LS based engines are making along with idle quality and I don't know if I would build another standard small block.

 

As an additional plus you can get all the ac stuff with the engine as well. I installed a vintage air setup and it added up to almost another grand, of course there are cheaper ways to do it, but mine was a complete kit that other members had installed so it was easy.

[Savage42]

One of the things that I think is somewhat of a folly in what most people (everyone I've seen) on here does is keep 3.54, 3.90 and even 4.11 gears. With the torque of a V8, such gears are hardly necessary in a street driven vehicle with 3 gears BEFORE you reach 1:1. Corvettes that come with the LS1/T56 have 2.73 gears (not sure if they all do), they weigh a little more than a 280z or 280zx and a considerable bit more than 240z's and can get low 30mpg on the highway. That is just my opinion but is something to think about.

 

I've always thought the same thing. The standard ratio for a C5 Vette with an auto is 2.73 and optional 3.15 and all 6 speed cars (including the Z06) come with 3.42. Being a 280Z is about 500lbs lighter than a Vette (3250 lbs for coupe & 3118 lbs for Z06), going with gears close to OEM would work well, as the factory does goes for be the best "all around" setup. I'll try the 3.36 LSD with the LS6/4L60E combo first and see how it works. Should have enough poo to light them up, anyway.

 

Of course, tire diameter also comes into play. I bet most have tires shorter than the 275/40-18s or 295/35-18s on back of a Vette, too.

[Pete84]

Where can you get a 3.36 LSD?

[Pyro]

Here is a little 350 chevy mpg story.

 

Recently helped a friend to a V8 conversion in his 77 280. He brought a complete engine with a new 670 holley carb, performer intake, and we installed a mild flat tappet cam (210/216, 114 lsa), and some old style 64cc double hump heads that had been slightly modified (2.02/1.60 and ported to 165cc runners). Exhaust system is block huggers headers with two 2.5 pipes into one 3" with a dynomax super turbo. Compression came out to be 9.5:1.

 

We also installed a TH350 trans with a 2200 stall and a 2.87 LSD dana 36 vette diff.

 

He was getting 8 mpg in the city and 12 on the highway. We tune the hell out of the carb and nothing helped. Leaner jets or richer jets, bigger power valves or smaller, heavier or lighter secondary springs didn't seem to effect mpg. We also made sure the ignition timing was setup right (15 initial, 38 total mechanical, and another 12 from the vacuum advance for light load (high vacuum). Plugs looked good and the engine ran fine but mpg sucked.

 

Switched to a Q-jet and now he gets 12 city and 18 highway. That is a 50% increase with no other changes. And the power is the same. And his city driving is fairly spirited and highway mpg is at 80 mph with ac on. Q-jets rock! Ugly as hell but engineering art work.

[Z-8]

:)If you do a quick search, I'm sure you can find info on the 3.36 LSD. It's been talked about a lot.

[getZ]

I don't know how true this is but the newer style vortec heads off late model trucks are supposed to be more efficient so that would translate into better mileage. The down side is they require a different manifold as well just for a vortec style setup.

[Guest I Love Late 90s Wrestling]

Either get a Honda or drive more reasonable

[rudypoochris]

I don't have a Chevy 350, but in my 4.0L explorer I just don't let the RPMs exceed 1000 until top gear (this is FI though, a carb might not meter well at under 1000rpm). Really helps alot on the mileage. I floor it below 1000 and get decent acceleration with little consumption. Went from around 18-19mpg combined to 23 combined. I can hit 24 or 25 if I really try. It foulds up your chambers a bit from what I hear though, so blow em out every once in a while. And yes, taller gears help.

[that240guy]

My 260z with the 3.545 ratio, T56, and LS1 engine gets 30-33 mpg on the highway. I get high teens around town. My car is light though. 2350 lbs. A stock weight Z prolly would still get almost 30 on the highway.

[280zwitha383]

I don't know how true this is but the newer style vortec heads off late model trucks are supposed to be more efficient so that would translate into better mileage. The down side is they require a different manifold as well just for a vortec style setup.

 

If you keep the fuel injection sure, if not I doubt you'll see a difference especially if you put a different than stock cam (which wouldn't make a lot of sense not to). Ask me how I know....

[getZ]

Here's a link to give you an idea of what is possible with some of the older v-8's. Not completely apples to apples comparison, but it gives some perspective. In a nutshell, as the technology got older the mileage got worse. Like anybody needed a website to figure that out.

 

http://www.fueleconomy.gov/feg/findacar.htm

 

If an LS engine out of a corevette or camaro is out of reach, you might consider a 6.0 liter out of a truck or cadillac as an option, but you won't have a 6 speed for an option.

 

I think my old 2000 silverado did pretty well around 20 mpg with an automatic, headers and cat back, but it was a much heavier car then a Z. When you kept your foot out of it, the engine would run about 1200 - 1500 rpm.

 

By the way I'm in the east valley if you want to see a small block chevy fitted in a 72 z car with ac to boot.

[k3werra]

thanks for the replys guys. ill keep doing research and getting advice. ive aimed my hp goals lower to 300hp with 25 mpg in mind. a lsx swap seems to be the easiet route for this. but i want to go through the hassle of doing the 350 swap since that would be a little easier for me to do with the resource and time i have. if only gas were back to $1 range/gallon life would be so much easier....

[280zwitha383]

Switched to a Q-jet and now he gets 12 city and 18 highway. That is a 50% increase with no other changes. And the power is the same. And his city driving is fairly spirited and highway mpg is at 80 mph with ac on. Q-jets rock! Ugly as hell but engineering art work.

 

Depending on what Quadrajet he got I would agree that with light driving you could get decent gas mileage similar maybe to a 2-barrel. However, the primaries are HUGE on those things and they're rated at 700++ cfm. If you were to do the same "spirited" driving as with the 670, you should get worse gas mileage.

 

...ive aimed my hp goals lower to 300hp with 25 mpg in mind. a lsx swap seems to be the easiet route for this. but i want to go through the hassle of doing the 350 swap since that would be a little easier for me to do with the resource and time i have. if only gas were back to $1 range/gallon life would be so much easier....

 

If you want to go much bigger than that in the future (with fuel injection) the LSx engines are probably for you. If you're content with a 350hp hurdle you can get a 350 vortec for probably atleast 1/2 the price of an LS1 and either get a 4l60/700r4/200-4r/T56 and maybe even T-5 for your overdrive. They come with a 4l60(e) and you could buy both at once. The "hurdle" comes from the intake and the low flowing (19lbs/hr) spider injector of the vortec engine. There are of course solutions but they're $$ just like anything else. I think it would be an excellent first engine with room to grow. They make aftermarket FI intakes, a different fuel injector, and better heads. If you believe the articles, vortec heads with machined heads for bigger lift and a healthy cam will break 400hp pretty easily. That is with a carburetor but if the intake and injection issues were addressed it should be as doable. Another thing that is a big plus for these engines is that they come stock with a roller cam. Roller cams are your friend

[A. G. Olphart]

Depending on what Quadrajet he got I would agree that with light driving you could get decent gas mileage similar maybe to a 2-barrel. However, the primaries are HUGE on those things and they're rated at 700++ cfm. If you were to do the same "spirited" driving as with the 670, you should get worse gas mileage.

 

 

Sorry, but you got it exactly backwards: The Quadrajet primaries are tiny, the secondaries are huge. They make great mileage carbs... that is why GM gave them the tiny primaries. (Tuning them requires a bit of knowledge).

 

We need to keep the quality of technical info at HybridZ above reproach. If a person isn't dead sure of his (or her) facts, they shouldn't be posted.

 

 

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