300 HP, 17 MPG and a carb.

[lehiguy]

So this weekend was the acid test. I Recently put in an NV4500, an NP241 T-case and a semi-floater 14-bolt six lug axle with disk brakes. I replaced the plugs in the 350, rebuilt the distributor and did Moab over the weekend. To my surprise I was showing 16.7 MPG on two tanks of gas and 15.8 on the the third turning 35" tires with 4.56 gears and a SOA conversion riding on OME springs. Aerodynamic she isn't. The trip through Beef Basin and the Needles District, Canyonlands was 13 MPG. The new crawl ratio is about 70:1 which made for a whole new experience in the rough stuff. Overall, I'm thrilled to death with the performance of these gear boxes and the milage the rig is getting which is largely due to the overdrive ratio of the NV4500 and the reduced parasitic power loss of the NP241 T-case. All this with a well-tuned Rochester doing the mixing chores. Later this year she gets TBI. I'm looking forward to that!

 

[Isotel]

i would not change a dam thing if i were getting 17 mpg in that beast! , but TBI is Killer on trail!

my Honda only gets 20 around town

 

[Downey]

I'll bet you've got an engine that has everything selected for h.p., which in turn is m.p.g.. Too many times guys start mixing torque stuff with h.p. stuff and end up with disappointment. A good little 300 h.p. engine with good running carb (normally a rariety) should be getting up in the high teens---TBI, even better!!!!

 

[Nemesis Gnat]

so this is slightly on topic... and may seem like a dumb question... but why are carbs frowned upon so much?

i know some basic things (starting, flooding off road, etc), but anything else?

 

[CruisinFJ60]

so this is slightly on topic... and may seem like a dumb question... but why are carbs frowned upon so much?

i know some basic things (starting, flooding off road, etc), but anything else?

and sometimes fuel miledge. but no you pretty much got it.

in an extreme off camber situation a carb will sputter if you dont keep your foot in it and even then sometimes they die (like when you put your truck on its side :whoops then they dont wanna start back up ect ect ect. With TBI or fuel injection it will run upside down (you wouldnt have any oil pressure but it would run so long as the fuel pump is getting fuel) if you stalled it on a big nasty hill you could get it started back up.

 

[Nemesis Gnat]

thanks!

 

[lehiguy]

Carb stuff

so this is slightly on topic... and may seem like a dumb question... but why are carbs frowned upon so much?

i know some basic things (starting, flooding off road, etc), but anything else?

There are issues with carbs. They cannot compensate for changes in temperature or extreme changes in altitude although later versions of the Rochester did do that and flooding is always a concern in off-road situations. I have run Holleys, Edelbrocks/Caters, Webers and Rochesters in addition to all the motorcycle carb tuning I did back in my racing days. For a 2F I would go with nothing but a Weber as the Holleys have too many issues with flooding no matter how much money you spend on various doodads intented to fix the design shortcomings. Trust me on that one. Same with the Edelbrock/Carter carbs as well as Holleys on V8s. Too much hassel to make them work in extreme angles. The Weber and the Rochester have a similar float design; small, central with the float hinging on the front so that as upward angle increases, fuel flow is shut off so you don't flood. In fact, I have never had a flooding issue on either a Weber or a Rochester unless its going DOWN something absurdly steep, and that only happened once. Tunability on a Weber or Rochester is fabulous if you learn it by the numbers, meaning caculating the metering area of the jet minus the rod area for the Rochester, (the Weber uses separate jets for the two circuits much like a motorcycle carb). Then you make changes in percentages. The lean cruise mixture is separately tunable from the low vaccuum or power circuit and you can change when that comes into play. Overall, a Rochester for a V8 is so amazingly tunable that it should indeed be the closest thing to fuel injection you can get when done correctly. To be fair, Edelbrock/Carters can be tuned in the same way but seem to lack the ability to make changes as minutely as the Rochester can and you would still have driveability issues off road. Mine stayed on for about a month before I tossed it.

 

[Nemesis Gnat]

thanks lehiguy, you really seem to know your stuff

i also sent you a pm with a couple more questions

thanks again

 

[bridges]

..... Later this year she gets TBI. I'm looking forward to that!

Are you going to post a thread on this conversion?

I'm just beginning to kick this idea around. I have not done a lot of research yet, but the efficiency of the TBI is appealing over my carb'd 350. I installed a 92 c1500 engine to replace a busted '87 targetmaster crate engine. Because there was no intake on the engine I purchased I went back to the carb set up at the time. (My first engine install - needed simple).

I'm getting about 15 mpg average now mated to stock 4spd. Depends mostly on my driving style for a given tank. High as 16.5, low as 13.

 

[lehiguy]

Are you going to post a thread on this conversion?

I'm just beginning to kick this idea around. I have not done a lot of research yet, but the efficiency of the TBI is appealing over my carb'd 350. I installed a 92 c1500 engine to replace a busted '87 targetmaster crate engine. Because there was no intake on the engine I purchased I went back to the carb set up at the time. (My first engine install - needed simple).

I'm getting about 15 mpg average now mated to stock 4spd. Depends mostly on my driving style for a given tank. High as 16.5, low as 13.

That's not bad!