Fuel Mileage upgrades for a 91 Non intercooled

[Pascalray]

Hello All,



I have been lurking around here for years now. Have owned 3 dodge cummins powered trucks, 91, 2001, and am on a 2004. 5 now. I still have the 91 and love the way the power comes on in it even with 220,000 miles on it. I have spent hours reading the various MPG threads but still have some questions.



I am building a cherry 91 Jeep Grand Wagoneer with a 91 6BT non-intercooled cummins engine I bought. It is a true 60K motor, exceptionally well maintained, and in excellent condition. I'll be running early Dodge Dana 61/71 axles with the 3. 07 gearing. Built NV4500 with south bend clutch and a NP205.



I am familiar with the 6BT to full size Jeep swap so no issues there as I have seen it done first hand in an incredible FSJ M715.



I am building this Wagoneer for a daily driver with fuel economy being the number 1 goal. I chose the 91 NI engine because I like the torque curve and think it has the best potential for fuel economy out of the 3 generations of 6BT's. I considered the 4BT but the noise/vibration issues killed the idea plus I plan on towing with this vehicle.



I do not need more power with this engine, I just want to tune it to max efficiency. I am willing to sacrifice some power for that if the gains warrant it. I also do not mind spending money on it if the gains warrant it even if those gains won't pay for themselves in fuel economy.



Questions please:



1. With the 3. 07 gearing and NV4500 with skinny 33" tires and as little lift as possible, I should be running about 1620 rpms at 70. I expect to only be using 5th for highway speeds. I know that this is in the bottom of the torque band, but do you think it will be efficienct or will I be lugging the engine? I know that for a short time Dodge offered the 3. 07 with the Getrag 5 speed and have read posts from people happy with this setup. I also have a set of Dodge 3. 54 axles just incase the 3. 07's won't work for me but I would only like to install the axles once.



2. I have a wastgated 12cm/HX35 for it and feel this is perfect my my non-hot rodding DD useage. I think the cam profile of the 91 NI engine is perfect for what I am doing but am open to opinions on this. I possibly might make some gains with different injectors like the Lucas Marine but would like some opinions.



3. I'll be doing the 3200 gov spring and old smokey fuel pin mods. Any better ideas?



4. I'll tweak the pump to max efficiency and will probably have to back off on it some with bigger injectors if I go that route. I'll go BHAF and 4" exhast. Any other ideas?



I do not expect to get the money back in fuel savings on this project. It is just a hobby and I am enjoying the challenge to see what I can get out of it in terms of pure fuel economy. I would be tickled pink if I could get 25 mpg out of this thing but think 20 mpg should be easily attainable based on my experience with the Dodge trucks. My aerodynamics are probably worse than a Dodge truck though.



I have not seen a thread for pure fuel economy upgrades, even at the sacrifice of power. Any and all ideas would be welcomed and please don't flame me if this has been covered before. BTW, this is one of the best forums I know of.



Thanks

 

[MMiller]

Anything you do to a diesel(within reason) that makes it run better, can increase fuel economy, if driven sensably. I think that with the 5 spd/3. 07s you will still use 5th more then you think, even with the 33" tires.



A 91 jeep, isn't that pretty small jeep for full size axles and 6bt? Or is that a full size jeep?



Since you have it apart, I'd gasket match the exhaust manifold and turbo charger, and clean up the ports on the head. Air in-Air out, probably work at getting cool air into the air cleaner, not just air from the engine compartment. I think all in all, you are on the right track. Street tread tires as apposed to mud tires will net you better economy. I don't know weather I'd mess with the injectors, with the exception of cleaning and adjusting them.



Michael

 

[Pascalray]

A 91 jeep, isn't that pretty small jeep for full size axles and 6bt? Or is that a full size jeep? Michael

Thanks, it is a full size Jeep Grand Wagoneer. here is a pic of it right after I bought it:





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[KOG]

The Grand is the full size Jeep, B5. 9 fits fine as Pascalray said.

I think that what you're planning to do is right on target. It's unlikely that you'll be lugging the engine with the 3. 07 given the light weight of the Jeep (relative to what a 5. 9 will pull). You're probably going to want a lot of sound deadening as part of this project and will need more front spring, of course. Aside from the fact that this thing will still drive like a solid front axle Jeep, it should make a fine conversion. The Dana axles will be plenty strong enough for this application. I just don't see any problem with this. If you can find room for an intercooler, that can improve engine efficiency and thus mileage. But I'd be surprised if you can squeeze one into the Jeep.

 

[JLEONARD]

With my 91 non I/C, 5 spd Getrag, 3. 54 differential I get 22/24 mpg on a daily basis... the variation depends on how hard/fast I am running the truck. I don't baby it that's for sure.

I have gotten 25 + on trips, esp if I stay under 65 mph.

Subtract 2 mpg for winter fuel... . and I'm not even sure the 10 cement blocks I'm hauling around now for winter traction hurts that number.



Nothing I have added/modified has changed the basic mileage. The difference is directly proportional to how heavy my right foot is, how many tailgaters I smoke out, etc.



And I'm not sure anything can be less aerodynamic than one of these old bricks. :-laf:-laf



Best of luck in your project.

 

[Pascalray]

Thanks.



In terms of the mecahnical swap, I have all of the details worked out including the suspension, motor trans mount, wiring, plumbing, etc. FSJ's have been my hobby for 30+ years and I have done other diesel conversions, caddy, oldmobile, and SBC's/BBC's in them in the past. I am very familiar with them.



I appreciate your advice so far. I have always been pleased with the stock performance on my Dodge trucks and have never done anything to mod them except a K&N filter. When it comes to modding these things, I am in the dark. I have heard a 16cm exhaust housing will net better mileage over the 12 cm, I have heard bigger injectors will net better mileage, I have heard certain cams will net better mileage, etc. etc etc. I have heard gearing makes the biggest impact. Most of this is from the manufacturer's which I am sure are biased. For sure the cold air intake and bigger exhaust make sense to me. But I have also heard that a gen 1 IP delivers optimum fuel efficiency at 1800 rpms but hear real world experiences which tell me they deliver excellent economy starting at 1500 rpms. Are there and accurate stock 91 NI dyno charts flowting around anywhere? Fuel charts for our pumps? Any other useful data?



I think what it is all going to come down to is perfectly matching the IP, turbo, injectors, intake/cam, exhaust, etc. etc. and this is an area I need help in from real world experiences.

 

[JLEONARD]

I think what it is all going to come down to is perfectly matching the IP, turbo, injectors, intake/cam, exhaust, etc. etc. and this is an area I need help in from real world experiences.

Personnaly, I don't think you can be that fine as far as dialing it in. Flow charts, etc are all good info but what it really comes down to are things like aerodynamics, vehicle weight, actual horsepower required to maintain a certain speed, etc.

Diesels use fuel based on actual horsepower used, not on any particular rpm so forget that angle. You probably want to be mid range in rpm, but that's a big window. I don't believe you will be able to be as precise as you are thinking.

What I think is wise would be do do some mild to moderate power upgrades, monitor boost and egt to make sure you are in the ballpark, and enjoy the vehicle.

 

[therabbittree]

I run 307s and 5 spd in my truck and its really nice combo etc. . I use 1st gear and 5th gear for highway speeds. . my mileage isn't too great 15-17 etc i haven't been able to figure out a good rpm yet . . then I gave up because i think my tires were eating teh mpgs up. . I have 285s truxus mud terrains. . pretty aggressive and wide. . i am looking into a 19. 5 swap for tall and skinny . otr something else. . but at teh rate im going it will be 2 or 3 years ro get new tires ha. . these ones have been on teh truck 3 years and still 75 % tread...

I don't drive it too much. .

thanks

Deo

 

[NIsaacs]

If maximum fuel milage is your most important goal, I would leave the motor completely stock. Gear and drive it 55-60 about 1900 rpm. Maybe change the turbine housing to a #16 or even a #14 and back the fuel screw out some from stock. You will get 26 mpg+.





"NICK"

 

[MMiller]

Thanks, it is a full size Jeep Grand Wagoneer. here is a pic of it right after I bought it:





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10-4, those full size Jeeps were not very popular in my area. The Grand Cherrokees on the other hand are a dime-a-dozen. Thats what was stuck in my head. My buddy did have a 79 Cherrokee Chief with a 360 and dual exhaust Too bad the quadra trac wouldn't allow for any smokey brake stands or burnouts. The 429 powered 78 Ford Bronco he replaced it with did though. Its amazing that we didn't kill anybody on Saturday nights when we were in high school. :-laf



I think that even in stock tune, with the rotary pump, you will still be able to knock down low 20's if you don't let the truck get too tall. With my gangly 93 in stock tune 20's were pretty regular, and still pull down high teens after a hard Saturday night uptown... ... ... I don't think that Jeep is going to be worse aero wise. Its probably a little narrower, and smoother looking then a first gen. I'll bet with everything in good working order 20's will be easy. Driving with the boost gauge under 10 psi and pyro under 800 nets best economy, no matter what configuration. If the pyro is high, then its lugging under the charger, and if its high boost, it means your trying to drive too fast!



Michael

 

[RWWinslow]

Below 55 mph fuel economy is governed by engine and drive line efficiency, and the tires in use (i. e. rolling resistance). From 55 mph to 65 mph, aerodynamics begins to play an ever increasing role. Above 65 mph, the engine horsepower is used to primarily overcome drag.



I have found the best fuel efficiency with the stock bosch injector - I have tried several combinations. Keep the air intake as smooth and straight forward as possible - however you will probably pick up some turbo whine and or flow whine on the intake. Maintaining valve clearances on these engines is critical for fuel economy. You might also consider some type of fuel air separator system (i. e, Pureflow Technologies, etc. ) - I have documented a 5% improvement in fuel consumption for stationary industrial engines using this type of equipment.

 

[Pascalray]

Great information and thanks.



I guess the trick is going to be seeing if the 3. 07's will give me enough power to cruise at 70 on the highway.

 

[dpuckett]

If the Jeep is a ton lighter than an average pickup, I'd say you'll do just fine. They were available in 93 pickups with stock power and 3. 07s with a manual.



I'd be a little cautious with the 12cm housing. They work fine in some trucks, but I havent had much luck with them and MPGs. I seem to gian a MPG or two when I go to a bigger housing. I have a 16 on mine now, and it spools just as fast as the 12, and crusie EGT is about the same, with a lot less boost (5psi vs 10-12).



Daniel

 

[abledog]

increasing fuel milage

If all else fails a stiff throttle return spring and a sore right foot may have some affect.

 

[therabbittree]

enough power?. . ? my truck is basically stock besides pods and the pump is screw driver adjusted. . it walks all over my friends 2nd gens and powerstrokes. . with the 3. 07s 100 plus on the highway is EASY... heck the 45-80 rush is great . . I had to turn my pump down otherwise the stock getrag clutch would slip doing the 45 50 mph to 80 pull. . the southbend fe should fix that heehe

Later

deo

 

[Forrest Nearing]

Pascal, bumping the injection timing a little should help with fuel economy.



one concept that people fail to grasp is that by loosening the AFC and bringing the fuel in sooner (even if it means having some visible smoke (note I said "some", not a coal train)) you can increase mileage...



why? how?



because our engines are (relatively) low compression engines that are designed to be turbocharged. burn efficiency is greater with boost present, so by getting into boost quicker, you're getting into efficiency quicker.



manufacturers have to meet EPA mandated emissions regs that we may not necessarily have to meet



so don't be affraid to have a little visible smoke, it's not necessarily "money exiting the tailpipe" as some would say

 

[dpuckett]

Forrest, I had noticed the same concept in some of my pickups. Not so much on others, though (like the current one).



DP

 

[Pascalray]

Thanks for your replys. I got sidetracked with work for a couple of years but I am back on the case now.

 

[RSchwarzli]

Welcome back to the case!



From my experience, I was sitting at 16 avg MPG. Then I changed the 727 to a getrag, turned up the pump by way of the fuel screw to the tune of 273/650, and I was then running 23 avg MPG. With a few more things like intake, e manifold, timing to 1. 79 MM lift, and exhaust, I seem to be at 25 although I cannot confirm this yet. Have not done enough testing, also with the change to winter fuel and extended warm up/idle time, I will have to wait till spring to give an accurate answer.

 

[ENafziger]

Thanks for your replys. I got sidetracked with work for a couple of years but I am back on the case now. <!-- google_ad_section_end --> <!-- / message -->

So what's the latest progress?

--Eric