Not another fuel economy thread (Thomas Chevy P30 / 350)

[cadillackid]

I got 5-7 in my Superior in the higher elevations (what the carb was tuned for).. once down near the ground I was about 4-5 or so..



I havent driven it a ton since i rebuilt the carb and fixed all the vacuum leaks but it seems like the gauge is going down much slower now since i did that...

I also re-geared so im guessing that has something to do with it..

[o1marc]

Originally Posted by cadillackid

I got 5-7 in my Superior in the higher elevations (what the carb was tuned for).. once down near the ground I was about 4-5 or so..



I havent driven it a ton since i rebuilt the carb and fixed all the vacuum leaks but it seems like the gauge is going down much slower now since i did that...

I also re-geared so im guessing that has something to do with it..

Gearing will definitely have an impact on MPG's. If you're turning fewer rpm's at the same speed, you are saving fuel.

[kidharris]

Originally Posted by cadillackid

I got 5-7 in my Superior in the higher elevations (what the carb was tuned for).. once down near the ground I was about 4-5 or so..

You got wings on that thing?

[yuletree]

Hi all, thanks for all the replies!


I switched back to 87 octane, seems to not have much of a problem.


I think my valve guides are worn. I do get some puffy smoke if I don't drive or start the bus often, but otherwise things have been fine.


My speedo is about 5mph faster than reality (according to some radar gun signs), and at 50(55) I'm clocking about 3000 rpms.


The carb, despite being rebuilt, does need love and I'm struggling to get it just right. I seem to be having some issues with the choke. Starts up fine somedays, adjusted the idle fuel circuit as best I can, but the settings the carb rebuilder set (3 turns, both sides) seems to have the best performance. Fast idle I can't get quite right...as a result, occasionally the bus stalls on cold start and I have to go in and fiddle with the choke plate to get it to maintain idle and warm up. Took me awhile to figure that one out


I've resigned myself to the mileage I have, but I am curious if the speedo is registering 5mph faster, if the odometer would run the mileage up faster as well. Would that make it appear that I have worse fuel economy than in reality?

[BarnYardCamp]

turning 3000 rpm @50 mph...sounds like you've got a pretty high rear end ratio.
Like some body else said, pull a couple spark plugs...if there is a lot of black carbon build up...that's a sign of too much fuel.
Pull the oil dip stick, can you smell gas in the oil? Some of the chevy 350 engines still used mechanical fuel pumps in the mid 80's. A lot of times, when they fail they will dump fuel into the oil, they are relatively cheap and easy to replace...2 bolts and 2 fuel line attachments.
You said you switched to 87 octane with out much problem, maybe somebody retarded the timing a bit to run cheaper fuel. Timing should definitely be checked.

[EastCoastCB]

Originally Posted by BarnYardCamp

turning 3000 rpm @50 mph...sounds like you've got a pretty high rear end ratio.
Like some body else said, pull a couple spark plugs...if there is a lot of black carbon build up...that's a sign of too much fuel.
Pull the oil dip stick, can you smell gas in the oil? Some of the chevy 350 engines still used mechanical fuel pumps in the mid 80's. A lot of times, when they fail they will dump fuel into the oil, they are relatively cheap and easy to replace...2 bolts and 2 fuel line attachments.
You said you switched to 87 octane with out much problem, maybe somebody retarded the timing a bit to run cheaper fuel. Timing should definitely be checked.

That would be a low ratio. Numerically high, but a low ratio.

[magnakansas]

hell so much to talk about.... um I will start with plugs, and timing..... You can have too much advance in the ignition timing and it can hurt over all power. The idea is add advance up to the point you start to "ping" or "detonate" under full throttle conditions..... The british method does not use a timing a light.... low engine rpm in third gear, floor it enough to keep it out of down shift condition.... from 2000 rpm on up... no pinging? add two degrees of timing and do it again..... repeat until you have pinging... back down two degrees and lock it...... now, if you can put it on a chassis dyno... see if that ignition timing makes best power/torque, under the curve. The most advanced timing can be too advanced for best power under the curve. what this means, is after driving and setting timing, go to the chassis dyno and do a couple of runs in third gear heavy throttle. full throttle and look at the power curves, then back timing off two degrees and check power curve again... it there more area under the curve or less? if there is more area under the curve.... back timing off two more degrees and run again. Keep doing this till you find best power.... but wait there is more

Spark plugs.... The bigger gap you can run without misfire or burning a hole in your ignition rotor the better your throttle response and mpg.... The down side is you will have to gap the plugs more often... you will see better mileage at .065 plug gap than you will with .035 but you burn through ignition rotors more often and you will have to set the gap on the plugs more often. I ran a NGK bran plug, with the center electrodes ground by hand to more of a pointy shape, and gapped to .065 Inches... I had to carry ignition rotor with me, and HEI module at all times. along with tools. Champion plugs also have soft center electrodes like the standard NGK plugs. Delco and Denso brands seem to have a harder material for the center electrode than the NGK or the champions. gets into theory of equations where the metal ions are a catalyst in the break up of long chain hydrocarbons during the combustion process.

There is a tension spring that holds the air flapper valves on the secondaries shut... you can adjust that tension to hold the secondaries shut until the engine is revved up more. Forces the primaries to run longer up the rpm range and helps to keep fuel atomized and does not bog down the air speed so much when the secondaries open.... I used to tune air fuel carb mix with secondaries locked shut... and worked playing with primary jets AFTER I set the float level. then when I got that good, I then worked with the needle holder an needles to set secondary fuel metering at higher engine rpm and load. I found the qjet to be very tuneable but I had box with a hundred parts or more for tuning them.

factory air cleaner with dual intake snorkals, the intake hoses face right at the front for as much cool /ram air as I could feed the engine. I think the air cleaner I used came from a 454 vette but I dont really remember where. Open air cleaners do not work as well in hot weather... that is... any temps above 60 degrees.

I blocked the exhaust passage under the intake... that might cause a problem with california emmissions..... Do you have to run EGR with your model year?

I finally got my 1970 350 engine with a three speed automatic to give me 15.5 miles to the gallon at 62 miles an hour regularly...... that is with 4.11 gears and 275/80 -16 tires....

Been so long since I did mpg work on my small block truck....... Oh dual exhausts,,,,, use the x pipe type cross overs...... 2.5 pipes minimum for standard camshaft 350 engine. bigger for bigger v8s or with hotter cams.

electric fans instead of mechanical fan, but often folks under estimate cooling needs and do not have enough fan.... safe is to stick with mechanical..

best fuel economy? slow down. I got 17mpg at 55 mph and 10mpg at 80mph

william