MPG with Chevy 350 and 700R4? (1 Viewer)

[Joey b 18c]

What MPG should I expect out of this combo? I feel like after adjusting the timing a bit it's gone downhill... I did a ~400 mile all highway trip and averaged 9, city is more like 6.

This is the intake manifold: Edelbrock 2701: Performer EPS Intake Manifold for Small Block Chevy | JEGS

Not sure what gearing is in the rear. There is a screw somewhere to adjust air/fuel, correct? Any other suggestions? Obviously wasn't expecting great MPG but this seems way too low.

 

[Dynosoar]

My 350 TPI (Tuned Port Injection) 700R4 combo yeilds 13-15 on the Highway and 10-12 in town.

Dyno

 

[Joey b 18c]

Thanks for the response...seems like I should be getting 12 ish on the highway at maybe 8-9 in town then.

 

[Elbert]

yes you should be getting better than 9 for sure. Is the truck in 4th gear at hwy speed.....assume you have the timing and carb setup right. Are you running old school 5.7 with HEI distributor and 4 barrel holley carb or something similar.

 

[Joey b 18c]

Elbert, yes the truck is on 4th, it's an old school 5.7 with HEI and the carb is in the link above, I believe there is a screw to adjust the A/F ratio. I will take a look at it next weekend and hopefully bump the MPGs. Just trying to think if there is anything else I could be missing...I'm not putting my foot into it, in fact the last week in the city I've really tried to baby it to see the max amount I'm getting now.

 

[LAMBCRUSHER]

never got better than 13 MPG in my old camaro with a 4 bolt, cam, and fresh 58cc heads on 92 octane...and that was highway with a tailwind cruising at 65...one of the reasons I won't go small block in a TLC...my straight 6ers average about 12...of course, they are not fast, and I don't thread the needle with either of them...one of the reasons I traded the old 'maro for the cruisers...you just can't thread the needle...at least, not at speeds high enough to result in unexpected sudden death...

 

[kbill45]

Back in the 70s there was a Holly intake and 4barrel carb combination to replace a 2 barrel that gave awesome results in both power and mpg. The intake was the Holley Economizer but I don't remember the carb size. You might check with a Holley dealer to see if they have an updated combo.

Yeah! Yeah! That's 40 years ago and I am 84..........

 

[cruisergear]

Chevy 350, Edelbrock manifold, Edelbrock 750cfm carb, hooker headers, 700r4. With 33" BFG K02.

I average 12.5 mpg/tank and I do about 75% highway, 25% other.

(That's using mileage data from a gps, since the odometer is not perfectly accurate after adding larger tires)

I plan to do a FiTech TBI conversion in the next few months to get my mpg's up a bit higher, among other benefits.

 

[inkpot]

Does seem a bit low on mpg. What kind of speeds were you trying to run? What Holly are you running? BAck in the good old days with my FJ 55, I had a worn out 78 sedan 350 with a Holly 450 4 barrel on it. I got 12-14 hiway with it as long as I stayed around 65 mph. It would run good and handle well at 80, but the mpg dropped to around 8. Your intake sounds like it should be OK for getting good gas mileage, but depending on which carb and other things, mpg can vary greatly. Generally speaking, the smaller the primary intake bores are, the better mpg you will get. My 450 card had 7/8" primaries, and when it died of old age, I replaced it with an Edlebrock 500 4 barrel that had 1' primaries. Regardless of how much I played with it swapping jets and metering rods, I could not match the hiway mpg performance of the 450 due to the larger primariy bores caused the air to move slower and therefore poorer breakdown of the fuel particles. John

 

[Joey b 18c]

Thanks for all the responses! I was pushing it on the highway, probably going 75 mph (2400 ish rpm) so that could be part of the problem there.

The FiTech TBI conversion sounds interesting, benefits vs the TPI that Dynosoar is running? I'd be really happy to get 10 in the city and 13-14 on the highway.

A few pics that might help (1st pic is old, I ripped out the MSD, it was total junk)

 

[Elbert]

old school plain holley 600 vaccum seondaires would be fine., or whatever that model offroad card they make. don't need a 750 on stock engine....I think something 600 or a little smaller would be good.

I ran a performer intake and holley 600 back a long time ago on a stock type 350, ran fine. With stock setup GM HEI distirubtor is good too.

I would not chase MPG , just get it running good and be done.

 

[Joey b 18c]

^ it runs good, just seems to be a bit rich at the moment. I might eventually look into FiTech TBI conversion if there isn't a downside to it (besides price). Might be best just to hold off and swap something else in it way down the road.

 

[Joey b 18c]

The carb is a Holley Demon 625, seems like the right size for the 350. The motor feels strong but is definitely running rich. The Holley guys suggested I buy their calibration kit ($80) to adjust the jets and rods. I just ordered the kit and will let you guys know if it helped in the next couple of weeks.

 

[Joey b 18c]

update. We just replaced the rods (up one size, 62,54 I think) to start and highway MPG went from 9 to just over 12, car seems to be running smoother and idling better after some adjustments as well. Happy with the result, so I doubt I'll mess with the jets. Still haven't put a ton of city miles on, but it's better for sure.

 

[gregnash]

Other benefit of the FiTech system, from my reading, is that it is a self-learning program. So basically, as you "break-in" the system it will learn and adjust as necessary. There was an article that they linked where they put the system (street pro I believe) on an old chevy with the 292ci 6er. The engine was known to be tired and they did not touch anything but putting the FiTech system on it, apparently they went from about 10mpg average with the carb setup to 14mpg with the FiTech system. I have been thinking about building up a 5.7L SBC with the FiTech system to throw in BeBe, do Vortec heads and what not, nothing crazy but enough to keep her running nice and have decent power. Currently my Fuelly states that I average 9.4mpg with mixed highway/city (75/25) and can get 13.5mpg on all highway.

 

[cruisergear]

We're installing the FiTech on our 350 as we speak. Should be driving it tomorrow.

 

[gregnash]

We're installing the FiTech on our 350 as we speak. Should be driving it tomorrow.

Sweet... will definitely have to post up some videos, pics and impressions.

 

[cruisergear]

Gor sure, i'll post a bunch here: BIG RED - Our 1984 FJ60
But will post some in this thread about mpg's when i know.

 

[lcwizard]

^ it runs good, just seems to be a bit rich at the moment. I might eventually look into FiTech TBI conversion if there isn't a downside to it (besides price). Might be best just to hold off and swap something else in it way down the road.

You can just buy a tbi motor of CL and swap parts over. TBI harnesses are the cheapest of the FI harnesses. I bought a whole used motor for 350.00 the last time I converted one of mine. I just picked up an LT1 with a 700r and factory harness for 300.00.
You'll need a simple adapter to put the TBI to your manifold. Even a marginal TBI will get 14~15 highway in a 60. Run an in cab fuel pressure gauge. It's the best diagnostic tool for the money. I've had TBI's in my cruisers off and on since 1990. 90% of my issues have been fuel deliver related like fuel filters or pumps. Spark delivery is next. Wires , rotors, module, etc

 

[cruisergear]

We've been scratching our heads on this FiTech install. Just couldn't get the Air Conditioner idle adjustment to work. Even after setting it to the exact specs recommended by the FiTech customer service. We ended up finding out that our FiTech unit had a defect so FiTech offered to replace it with an upgraded unit at no charge. So That's the silver lining I suppose. Getting the color touch-screen and timing control of the upgraded unit.

We hope to have everything squared away with the new FiTech unit in the next couple days. Wish us luck!