Dreded 460 fuel mileage

Hey Fellas, for those who have been follwing my progress, you'd know just how much work I've done to the Cobra. Seems to be running real good these days( despite a small inconsistency in rpm at high rpm) she's quite thirsty. I've done a few trips and am only getting about 6 mpg. Should I expect better or is that about what it should be. I've recently increased my timing to 14° from 8° BTDC. My calculations are before timing increase. I tried 18° first, where I had no detonation, but a significant lost power at high rpm.
Also consider selling the new Holley and installing a throttle body like the Holley Sniper, or FiTech kit. Anyone running these and have a recommendation of which one is easy install, and tune. Also how much fuel mileage I would gain most importantly.

Gary, Bill Vose and Jim can shed some light as to what can realistically be expected regarding gas mileage realized with a 460 powered truck [carbureted and EFI].

Our vehicles have poor aerodynamics and speed can really impact fuel efficiency.

Sounds abt right with a Holley on it. From my understanding a different carb might help but Ive never had a Holley so I cant say.

Thank you it's a motorhome so there's nothing aerodynamic about a brick.

My F150 [still like a brick] absolutely "fell off the edge of the cliff" regarding gas mileage when speeds exceeded 62 mph. 60-62 mph was the "sweet spot".

My F250 w/a recently-built 460, headers, and a manual transmission with overdrive gets between 11 & 12 MPG if running at 65 MPH w/o a head wind.

And if I remember correctly that's not too far off what my '72 F250 w/a 390 topped with a Quadrajet and running a C6 got when running at that speed.  But when it was loaded down with our 9 1/2' self-contained camper we got from 7 - 9 MPG depending on the conditions, with 7 being with a head wind and 9 being under the best conditions.

So, I don't think your 6 MPG is that bad.  However, you didn't say at what speed.  And as has been said, speed kills - MPG.

Hey Cobra,

My wife and I bought a 1993 E350 Cutaway RV (31ft) last winter with a 460EFI and 4spd Auto/OD. It had a lot of new parts...EGR valve, IAC, plugs and wires and filters, etc...and it just got 8MPG pretty consistently driving at about 60-65MPH, sometimes less depending on the wind. It was probably more like 7.5MPG most of the time, and 8MPG on the good days. I did a lot of reading on MPG, and I think the general consensus was that an E350 RV with a 460 will NEVER see double digit MPG numbers, so 6-8 seems to be pretty common, with 9 MPG being the upper limit when all conditions are good and the hwy is flat with no headwinds. Probably not much help for you, but some extra data from a fellow member.

There's a reason they say that with a 460 it's all about smiles per gallon.
C6?
If so, I'd keep an eye out for a used gear vendors. Not cheap but will really help with the rpms at highway speeds.

Yes, Scott is right.  The C6 is a big part of your problem, for two reasons.  First, it doesn't have an overdrive, so your engine is spinning higher RPM than my engine or Bill's engine is at the same speed.  In my case, I have a manual with OD, in Bill's case he has the E4OD - with OD.  So if you were to put an overdrive unit, like the Gear Vendors that Scott suggested, on there it would drop your RPM and help the MPG.

The other reason is the torque converter is always slipping on a C6.  In other words, the engine is turning ~300 RPM more than the input shaft to the transmission because of the slippage in the converter.  And that is all wasted energy, which you won't get back by installing an overdrive unit.  It gets turned into heat, and you need to make sure you have a large enough tranny cooler to get rid of the heat.

Now for the question about EFI.  There are essentially two types of EFI: port injection and throttle body injection.  Port injection means that there are injectors for each cylinder and they are positioned near the intake valve so that the fuel goes as directly into the cylinder as is possible.  With a throttle body the injectors are where a carburetor would have been.

Bill has port injection on Darth and coupled with the E4OD he gets about the same MPG I get with Big Blue, albeit with a heavier truck due to it being a crewcab and dual rear wheel.  So port injection helps, and I'm hoping to pick up about 1 (maybe 2?) MPG when I go to port injection later this year.

But I don't believe that throttle body injection will give you as much.  While port injection tailors the fuel/air ratio for each cylinder, that isn't possible when you are injection in the center of the intake.  So the AFR cannot be as precise and the MPG isn't likely to be as good.

Having said that, unless you are going to drive the wheels off that thing it isn't likely that going for port injection can be cost beneficial.  It isn't an easy task.  I haven't completed mine so am not the best person to definitively say that, but I've done enough work on it to know.  But Bill is the one to answer the question as he has done it.

Thanks for the write up that kinda puts things in perspective.  Maybe I'll try to keep her under 65mph sounds like that's my best chance at better fuel mileage. A lot of times I find myself just going with the flow of traffic which could be upwards 70-80mph

The wind drag on that must be significant, with the air catching under the overhang.  I think you'll see a noticeable difference on the MPG at 65 vs 75 MPH.

But advancing the timing like you've done will also help.  However, you said "I've recently increased my timing to 14° from 8° BTDC. My calculations are before timing increase. I tried 18° first, where I had no detonation, but a significant lost power at high rpm."  And that worries me.  I've never seen a significant loss of power when setting the initial timing in the teens.  If anything, it is the opposite.  Which makes me wonder if your balancer has slipped.

Have you replaced the balancer?  If not, do you know if it is indicating accurately?  In other words, have you found TDC and checked to see that the balancer is pointing to TDC?

Good point Gary, I haven't replaced the balancer. I'll have to check that out. I did the timing in the RV storage yard, I let it warm up to operating temperature then made my adjustment to 18° it sounded real good, and had a nice snappy throttle response. Took off in it for a test drive and under load (accelerating onto the Freeway)it seemed a little sluggish and I felt I had to have my foot in the throttle more than usual yo maintain speed. So I took the next turn out adjusted to 14° and it helped a lot. Even did a little burn out merging onto the shoulder of the freeway.
*Side question how would I know if my valves need adjusted. I thought I heard a little tap coming of the throttle in park with the doghouse off.

If I replaced the C6 with an E40D would it just be plug and play, or would there be any modification to make such as, shift selector on the column, drive shaft length, torque converter? I think I read somewhere that the carb I bought isn't compatible with the OD tranny... I can't be sure though

Where you at Jim?? I always look forward to  your expertise.

Yes, check the marks on the balancer.  Do you have a tool with which to find TDC?  They are really a piston stop and you'll read the balancer a few degrees ahead of TDC and then a few degrees after, and if the middle is TDC then the balancer is correct.

Changing out to an E4OD is a major effort.  The tranny requires a computer to control it, and a standalone one of those, like from Baughman, runs ~$750.  Bill, and I when I get Dad's truck going, use the EEC-V ECU to do that as well as control the EFI.

Plus, the E4OD is much longer than a C6, so the crossmember has to be changed and the driveshaft shortened.  It is a lot of work and expense, and won't pay off for you for many, many miles.

Probably many more like it but this is a simple/fun site to rough guesstimate these things.

Update: a lot of other factors come in as well that the dollars shown on that site don't encompass. When reducing rpm you also have engine benefits and noise reduction. The latter for me personally is a huge benefit. As always, pros and cons.

Great information guys thank you. Looks like I'll just drive slower and plan my trips to make them short and sweet.... Kinda disappointing because I'd really like to cross country this beast.
Thanks again

On second thought what kind of chassis could I drop the living quarters on that would get better mileage...

Motor homes get good gas mileage when they are parked at the camp site!

That's for sure!