BBC fuel mileage .... Carb, TBI, MPI, or swap for an LS?

[Affende]

84 3+3 K3500 454 sm465

Currently getting 6.8 mpg and its running right on the ragged edge of way too lean. Edlebrock 1404 500 cfm.

i have a friend rebuilding an edlebrock 750 (came with the truck).

So my questions is ... i need some more gas mileage. I dont drive on the interstate that much, everything is in town or county highway but the mpg seems pretty low, even for a heavy tired 454.

Can i expect to get double digits out of this motor? Would i be better off swapping over to TBI ($1000), MPI ($2200) or going for a newer LS swap?

Thanks for the input!

 

[4WDKC]

with alot of stop and go you wont get much better. You could try to reduce stop and go traffic, slow down before lights when red trying to avoid coming to a complete stop and having to restart. my guess is the engine started life as low compression low powered smog motor which means it needs a minimum of new heads and cam to up the compression and torque.

 

[vkh]

I found my biggest improvement with timing. Set your initial timing around 10-12°, factory spec is way lazy for smog (think factory spec for mine was 4°). When's the last time you did a general tune up? A new cap and rotor, air filter, sparkplugs, sparkplug wires, and checking that everything is functioning right can make a huge difference.

 

[4WDKC]

I found my biggest improvement with timing. Set your initial timing around 10-12°, factory spec is way lazy for smog (think factory spec for mine was 4°). When's the last time you did a general tune up? A new cap and rotor, air filter, sparkplugs, sparkplug wires, and checking that everything is functioning right can make a huge difference.

I wouldnt even be surprised to find out gm retarded the cam on these like they did other engines.

 

[Affende]

I found my biggest improvement with timing. Set your initial timing around 10-12°, factory spec is way lazy for smog (think factory spec for mine was 4°). When's the last time you did a general tune up? A new cap and rotor, air filter, sparkplugs, sparkplug wires, and checking that everything is functioning right can make a huge difference.

i have a tune up ready to go into it when i get the new carb put on.

I did go through and fix a bunch of vacuum leaks ... i will check the timing when Im in there doing the carb next week.

 

[Affende]

with alot of stop and go you wont get much better. You could try to reduce stop and go traffic, slow down before lights when red trying to avoid coming to a complete stop and having to restart. my guess is the engine started life as low compression low powered smog motor which means it needs a minimum of new heads and cam to up the compression and torque.

heads and cam will be in the works this summer hopefully ... i just dont want to dump money into it if i cant get double digits out of it some way some how.

smog will come off when the head and cam work is done ... ill do headers at the same time.

 

[shiftpro]

Needs more compression and a squish zone that actually squishes.

 

[CSFJ]

heads and cam will be in the works this summer hopefully ... i just dont want to dump money into it if i cant get double digits out of it some way some how.

smog will come off when the head and cam work is done ... ill do headers at the same time.

With that big of a square, and being 4x4, and a big block, in city driving, I'd be surprised if you could regularly see double digit mileage without a downhill run and a tailwind. I know some of the guys on here have gotten into double digits, regularly with a 2wd on their tow rigs, but a crew 4x4 in stop and go just doesn't help the cause.

 

[Affende]

With that big of a square, and being 4x4, and a big block, in city driving, I'd be surprised if you could regularly see double digit mileage without a downhill run and a tailwind. I know some of the guys on here have gotten into double digits, regularly with a 2wd on their tow rigs, but a crew 4x4 in stop and go just doesn't help the cause.

im not terribly worried about double digits in the city ... city driving is probably 30% of my time, the rest is county roads at 55 or 60. Starting in august, though, i will have 50 miles of interstate driving each way every day. I really need to hit double digits for that, hoping for 12 or so.

 

[CSFJ]

im not terribly worried about double digits in the city ... city driving is probably 30% of my time, the rest is county roads at 55 or 60. Starting in august, though, i will have 50 miles of interstate driving each way every day. I really need to hit double digits for that, hoping for 12 or so.

Without an overdrive, I just don't see 12s. What axle ratio are you running?

 

[bucket]

Wow, yours is choked down with a 500 Eddy too?!

In the recent cold weather, I've been getting around 8 mpg. In the summer I get around 10 out if it. That's with 4.56 gears and 36's, along with driving through a couple very small towns and the rest at 55-60.

There are some differences though. I'm assuming you are running radials where I'm running 6 bias plies. I've also got a th400 and the 454 is fresh.

 

[Affende]

Without an overdrive, I just don't see 12s. What axle ratio are you running?

I honestly dont know. I had the covers off this summer but i didnt think to check. Im running 235/85/16 IIRC

 

[Affende]

Wow, yours is choked down with a 500 Eddy too?!

In the recent cold weather, I've been getting around 8 mpg. In the summer I get around 10 out if it. That's with 4.56 gears and 36's, along with driving through a couple very small towns and the rest at 55-60.

There are some differences though. I'm assuming you are running radials where I'm running 6 bias plies. I've also got a th400 and the 454 is fresh.

yeah ... i think the PO put it on there thinking it would help with the altitude (6000 ft) ... which is backwards lol. All of my calculations say a 500 cfm on a 454 maxes out around 3400 rpm.

 

[bucket]

yeah ... i think the PO put it on there thinking it would help with the altitude (6000 ft) ... which is backwards lol. All of my calculations say a 500 cfm on a 454 maxes out around 3400 rpm.

I don't have a tach, but that's probably about right. If yours is anything like mine, it chokes the engine when you want it to run hard, yet still runs rich at an idle somehow. It's also extremely sensitive to the quality of the fuel.

 

[Affende]

I don't have a tach, but that's probably about right. If yours is anything like mine, it chokes the engine when you want it to run hard, yet still runs rich at an idle somehow. It's also extremely sensitive to the quality of the fuel.

i have a protach taht the PO put into the bezel ... did a pretty good job too .. bu it seems to be reading high to me. I can run it out to 4500 rpm pretty quickly.

it does seem to run rich at idle ... and mine runs lean everywhere else (surges off idle and backfires if you dont roll into the throttle really easily) but i havent checked to see how fuel affects it yet.