From 13.5 to 17 MPG with an Class B RV

[mpvue]

Originally Posted by rgathright

Whoa... carb's are notoriously hard to increase FE due to their "always on nature".

I suggest installing a 2bbl carb with an Edelbrock Performer RPM manifold and a restriction plate. Essentially copy what the NASCAR guys use. Then you are going to want to install miles of cavernous (think the size of a 1" water hose), steel braided transmission cooling lines. The trannies get real hot and reduce your efficiency on those motorhomes. I assume you have a 400 or 350 power glide in there right?

I don't see why you'd suggest a 2bbl; a properly sized 4bbl will be ruuning most of the time on its primaries, which are smaller than the openings on a 2bbl. then, under load, the secondaries open up. a vacuum secondary carb only opens the secondaries when the vacuum signal drops, and this point is adjustable w/ springs.
years ago, I was able to tune my Dad's 460ci F250 for a few extra mpgs by putting a heavier spring in the carb; IIRC, he went from 9mpg to around 12-13.
even better than a carb in this application, there are myriads of fuel injection kits for chevy V8s, from simple TBI setups to full blown laptop programmable rail injectors.
as for the transmission, there are stout overdrive units available as well as modern computer controlled 4 and 5 speed automatics.

[thatguitarguy]

Originally Posted by mpvue

I don't see why you'd suggest a 2bbl; a properly sized 4bbl will be ruuning most of the time on its primaries, which are smaller than the openings on a 2bbl. then, under load, the secondaries open up. a vacuum secondary carb only opens the secondaries when the vacuum signal drops, and this point is adjustable w/ springs.
years ago, I was able to tune my Dad's 460ci F250 for a few extra mpgs by putting a heavier spring in the carb; IIRC, he went from 9mpg to around 12-13.
even better than a carb in this application, there are myriads of fuel injection kits for chevy V8s, from simple TBI setups to full blown laptop programmable rail injectors.
as for the transmission, there are stout overdrive units available as well as modern computer controlled 4 and 5 speed automatics.

Do you have more info on the spring swap? Would this be similar on a Chevy? For me, this sounds like a better starting point than a full induction swap.

[mpvue]

Originally Posted by thatguitarguy

Do you have more info on the spring swap? Would this be similar on a Chevy? For me, this sounds like a better starting point than a full induction swap.

on holley carbs w/ vac sec, there is a spring kit available, real simple to swap out. for stuff like that, summit racing is your best bet for product and tech advice. I'm not familar enough w/ other carbs like rochesters or carter/edelbrock to know how they signal the secondary opening.

[thatguitarguy]

Originally Posted by mpvue

years ago, I was able to tune my Dad's 460ci F250 for a few extra mpgs by putting a heavier spring in the carb; IIRC, he went from 9mpg to around 12-13.

Increasing FE by 3mpg from 40 to 43mpg is worthwhile, but going from 9 to 12mpg is a 33% increase, and that's HUGE.

[mpvue]

Originally Posted by thatguitarguy

Increasing FE by 3mpg from 40 to 43mpg is worthwhile, but going from 9 to 12mpg is a 33% increase, and that's HUGE.

it's a trial and error thing; generally, for performance, you would tune the transition to secondary opening so that it couldn't be felt, you wanted it to be smooth. the 'pull' that some would feel means that they are opening too soon, hence a 'bog'.
conversely, you can have them open later. in my Dad's case, it seemed that even at part throttle cruising, the secondaries were partially open.
a vacuum gauge really helps when doing this.

[suspectnumber961]

Best bet? Buy a GPS and stay with the 2 lane highways...cruising at 45-50 mph. Use the GPS to find the shortest routes.

Moving all that air out of the way is where your gas is going?

Amazon.com: DeLorme Earthmate LT-40 GPS [OLD VERSION]: Software

[instarx]

To add to the easy aero improvements take that shade canopy off the side if you don't use it. Easier than adding a kammback.

[motojak]

I have the same Pathfinder as 5bikes. Learned the hard way not to advance the timing on a 454. Now have hypertectic pistons, Thorley double Y headers. Went from 4.5 to 6.5 mpg. Got to beat that. Anybody tried Megasquirt? Also need an accurate real time way to check fuel flow (usage) on a carb.

[comptiger5000]

AFAIK, the best you can get for real-time fuel flow on a carb is a marine fuel flow meter. They're not cheap though.

[tumnasgt]

Originally Posted by thatguitarguy

Increasing FE by 3mpg from 40 to 43mpg is worthwhile, but going from 9 to 12mpg is a 33% increase, and that's HUGE.

Yea, it's the silliness with MPG that means I prefer l/100km (or a miles/gallons equiv if you live in the US or UK).

26l/100km to 19l/100km shows how giant the change is from 9MPG(US) to 12MPG. Same reduction in fuel consumption as going from 17MPG to 34MPG.