Question on fuel consumption - Team Camaro Tech
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post #1 of 9 (permalink) Old May 19th, 15, 11:58 PM Thread Starter
Aleksi
 
Join Date: Feb 2015
Location: Finland
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Question on fuel consumption

Hi guys,

been driving my '68 for about a week now and i'm a little concerned about the fuel consumption. I know that these things a thirsty but man the fuel economy is lousy! Last weekend i drove the car for about 60 miles on a country road doing a pretty even 50mph the whole way and i ended up getting maybe 6mpg(!)

The guy i bought the car from knew even less about it than i did but what i know about the setup is that it's a standard early 70s chevy 350 with a holley double pumper and a single-plane intake as well as a TH350 w/ high stall trubine and a 1:3,75 (or close to that) rear end. It also has a pretty loud (=big) electric fuel pump installed.

So i guess i'd like your opinion on whether this is normal or is there something wrong with the setup. The engine seems to be running fine. One way or another i have to do something about this because the price of gas here in Finland is ridiculous, it's like 1,50 euros / liter which is over 6$/gallon
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post #2 of 9 (permalink) Old May 20th, 15, 03:22 AM
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kevin
 
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Re: Question on fuel consumption

I had a 77 chevy pickup with a 350 and a 4 speed. I only got 10-12 mpg.
have you pulled the spark plugs and looked at them for proper combustion?
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post #3 of 9 (permalink) Old May 20th, 15, 03:51 AM
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Scott
 
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Re: Question on fuel consumption

Sounds like a very similar setup to my current 69 when I took ownership of her. Mine was a 406 rather than a 350 and another difference was mine has the original M21 manual transmission but other than those two differences they sound identical. Mine had an MSD billet distributor without a vacuum advance canister and 6AL ignition box. Even when taking it easy I was only getting 6 mpg and when having a bit of fun, the mileage dropped even more.

I've made a number of changes and now have more than doubled the mileage and increased the performance as well, a win/win scenario. The first stage of changes which yeilded most of the mileage increase was the following; Replaced the double pumper mechanical secondaries carburetor with an electric choke vacuum secondary which made a huge difference right off. Next to go was the MSD distributor and 6AL box, I purchased a stock points style distributor with vacuum advance canister, went through it to get rid of all excess play and installed a Pertronix Ignitor III ignition module and Flamethrower III coil. I did recurve the mechanical advance on the distributor and replace the vacuum canister with a B28 low vacuum unit and add a vacuum advance limiter plate to get the ignition dialed in. I also tuned the carburetor with the aid of a wide band AFR data logger by re-jetting the carburetor, replacing the power valve, changed the accelerator pump squirters and vacuum secondary canister spring. The above changes got me the majority of my fuel economy.

One year later I initiated the second stage of changes with included replacing the mis-matched flat tappet hydraulic camshaft and 034 cast iron cylinder heads with a hydraulic retro roller camshaft setup from Chris Straub and a set of AFR 210 cylinder heads. I also at that time replaced the single plane Edelbrock Victor Jr. intake manifold with a dual plane Edelbrock RPM Airgap intake. This top end upgrade increased performance without any loss in mileage when cruising.

In my opinion though your biggest mileage killer is that mechanical secondaries double pumper carburetor. If you don't have a vacuum advance on your distributor you are losing mileage there as well. The high stall torque converter also impacts fuel economy but you may need it due to your camshaft?

69 Camaro Z/28 RS, original Azure Turquoise, M21 & 3.73 12-bolt posi. NOM 406ci, AFR 210 heads, Straub hyd. roller cam & Dynatech 1-3/4" > 1-7/8" headers, QuickFuel 750 Annular. Photos at:
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post #4 of 9 (permalink) Old May 20th, 15, 02:12 PM
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Re: Question on fuel consumption

A high-stall torque converter without a lockup clutch is going to give you pretty bad gas mileage; especially when putting around below the stall speed.

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #5 of 9 (permalink) Old May 20th, 15, 02:28 PM
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Brian
 
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Re: Question on fuel consumption

I think the consensus is with that setup you are lucky to be getting 6 MPH which doesn't help you much on face value. But you can start with the suggestions to change the carb to an fuel economy style as suggested as an immediate fix to get you cruising in the limited summer you enjoy over there, then in winter swap out the more fuel aggressive components for fuel economy with performance minded style if you can find them.


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post #6 of 9 (permalink) Old May 20th, 15, 06:06 PM
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Re: Question on fuel consumption

Your car should do much better than 6MPG. I took my car for a 40 mile drive in January and it got 10MPG. And mine is a 427SBC with 4:10 gears, 6,000 stall and a 1050CFM carb.

Your car is probably running too rich with retarded timing. What is your timing set at? Are you running vacuum advance?

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post #7 of 9 (permalink) Old May 20th, 15, 10:44 PM
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Steps
 
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Re: Question on fuel consumption

Quote:
early 70s chevy 350 with a holley double pumper and a single-plane intake as well as a TH350 w/ high stall trubine and a 1:3,75
OK the high stall.. the engine is slipping even on the slightest rise or acceleration....
Bad economy.. which is why modern trans have 'lockup' functions and before that factory 1200 or 1600 rpm torque converters

Now the diff ratio... cruising down the road the engine needs a given fuel air ratio to go into each cylinder.. about 15:1...so basically its a matter of reducing the volume of the engine per mile... hence why modern trans now have 4 or 5 or even 8 speeds with over drives....and a very common rod mod in recent yrs...
The old trans final drive was 1:1 and before modern trans the factory went down to diff ratios around 2.7 and 25 :1 basically 60 mph becomes around 1900/ 2000 rpms
And with modern over drive trans as low as 1400...

Keep in mind that modern cars are ECU monitored and injection.. not carb.. the down side of a carb is it only works efficiently at a narrow pm band width.....so u have a double pumper... band width roughly 5000 + rpms....

Now the single plane manifold.. thats also designed for top end efficiency.. bad at bottom end... which is why factory inlet manifolds where duel plane

Basically a street track car running on the street... in say in that IF the engine has been dialled in correctly with AFR, IMAP, TPS using 21 st century data logger tools to establish correct tuning specs, you would find it far more economical AND faster.

I run a mild cam, 2.7 diff thru a T350 , 1200 rpm torque converter high compression to run 105 octane fuel thru a 450 cfm carb... around town we get around 30mpg open road far more....Then if going to a weekend meet at the track.. 45 mins 2 guys and a hoist swap the diff to 308 and torque converter to 2500.. to run 13.8 @93mph
Ideally , if had the money at the time would have thrown a 4 speed trans in
And yes the car has been dialed in with data logger.

keeping in mind ....You have brought this car... bottom line rule, when designing an engine/ running gear one desides what the car is to be used for 95% of the time....then matches all the 'go' components to do that....then dials the engine in for maximised results.
Basically same as buying a factory car... do u want a economical shopping basket, or a open road tourer, or to pull a boat, or a sports car...

My Spelling is not incorrect...it is creative

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post #8 of 9 (permalink) Old May 21st, 15, 01:03 AM Thread Starter
Aleksi
 
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Re: Question on fuel consumption

Thanks for your comments, these help a lot!

I bought the car realizing that the current powertrain setup was far from ideal given the type of use i had in mind. Still i was a bit surprised that the fuel economy was so poor. Maybe it's running a bit rich like Steve suggested - on top of everything else.

I was actually thinking what Brian suggested and do some first aid by switching the carb. Also i don't have a vacuum advance distributor installed atm. I think a new intake and turbine are also parts that i can get my hands on fairly easily, i'm not sure about the differential. Then again, that 95% of use will be easy cruising around town so i would imagine that the disadvantages of the current diff won't be so evident in that sort of use.

To complicate things i'd would like to keep/change the car as original as possible since we have this museum car registry in Finland which basically means that if you have an old enough car that is in "original" condition, you can get a "museum car" status on it, which means no annual vehicle taxes, no annual inspections and very cheap insurance. But that's another story.
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post #9 of 9 (permalink) Old May 21st, 15, 02:33 PM
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Steps
 
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Re: Question on fuel consumption

Quote:
I bought the car realizing that the current powertrain setup was far from ideal given the type of use i had in mind.
u picked up on that part of my post.....
I would expect around 12 mpg around town 15/ 16 mpg (British gal.. 4.54 L per gal) open road cruising (not knowing the cam but assuming it matches the manifold and carb...If well tuned..slightly more if dialed in.

If considering turning it into a fun, light up show off every now and again.. economical cruiser... 1st thing would be to drop on a dual plane manifold, 450 or 600 vac sec holley (depending on the rpms u want to hit for longer than a few seconds)
This brings the rpm power range down more into where u are working at.
Drop a 1800 stock type torque convertor in...
All simple bolt on stuff...
A lot more can be gained by changing to a more suitable milder cam to work between 2500 thru 5500...tend to drop to the next one down if selecting from catologes.
Consider dropping the headers and go for factory cast truck rams horns.
And dial the engine in ( a current thread touches on how to.. and use search function.. several very old threads talking ppl thru over a couple vweeks stage by stage)
Now u will find it will light up real easy with little rpms and substantial improvements to economy.. getting into the low 20 mpgs

Then last the final gear ratio... can either change to a 308 or slightly bigger.. 308 is a good all rounder when matched to the above power range
OR
A 700r trans..with your diff or a 308 to 355.. lower 1st and 2nd neck jerk and the long legs in OD cruising around 1500/ 1800 ( rem the volume of engine mixture per mile in my post above?)... and with the engine working more efficient at low rpms... FAR better economy sitting at lights intersections, and far more bottom end torque for low rpms cruise work.

The trans change would be the most involved and expensive of all the changes above.

My Spelling is not incorrect...it is creative

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