98 4.0 to smaller engine

[Karcastic]

Hello,

I have another post in the b series specific forum but thought I'd investigate here also for more input.

I have a 98 b4000 rwd auto trans that I want to put a smaller engine in. I've read how easy a 2.9 to 4.0 is but I'd like to do the opposite, go small. Not sure what my options are really, any help/input is greatly appreciated.

Thanks everyone

 

[James86]

Um, why?

 

[RonD]

4.0l to 2.9l might get you 2mpg better, might being the key word.

4.0l has enough torque to run lower ratio axle gears, which lowers RPM at a preset speed, so if you do alot of highway driving MPG would be improved.

Look on the drivers door edge for a large sticker, it will have the current axle ratio on it, like 3.55, 3.73 or 4.10
Example:
If current 3.73 axle was doing 55mph at 2,000rpm in 5th gear
Then 3.08 axle would be doing 65mph at 2,000rpm in 5th gear

You do lose the quick acceleration BUT, you lose that with a smaller engine as well

 

[stateranger]

If you wanted to go down to a 2.9 you would have switch back to Obd-1(lots of work). If went to a 3.0 you would need the ecu+harness and transmission.

 

[Karcastic]

4.0l to 2.9l might get you 2mpg better, might being the key word.

4.0l has enough torque to run lower ratio axle gears, which lowers RPM at a preset speed, so if you do alot of highway driving MPG would be improved.

Look on the drivers door edge for a large sticker, it will have the current axle ratio on it, like 3.55, 3.73 or 4.10
Example:
If current 3.73 axle was doing 55mph at 2,000rpm in 5th gear
Then 3.08 axle would be doing 65mph at 2,000rpm in 5th gear

You do lose the quick acceleration BUT, you lose that with a smaller engine as well

Thanks Stateranger and RonD for your responses.

RonD, so is my understanding correct, smaller axle ratio on my 4.0 might get me close enought to the most feul efficient engine made for these trucks? With a smaller axle ratio, would smaller tires be better also? I have factory 14 inch rims and 15 inch winter rims. I do a lot of highway driving around southern Ontario Canada. The stop and go traffic once in/around Toronto is daily consistent also, I wouldnt gun it in stop and go traffic anyway or at highway speeds so I am ok with loosing off the start line power.
Thanks

 

[Karcastic]

If you wanted to go down to a 2.9 you would have switch back to Obd-1(lots of work). If went to a 3.0 you would need the ecu+harness and transmission.

I thought there was a b2300 model in 98 too. Was that a 2.3? Whould that be a direct swap just curious now for replacement insurance now as I see most people on here agree that fuel economy doesn't change much over the different engines. In this case ill probably work on bettering my 4.0 mileage. I'd reduce axle ratio, tire size, weight and maybe upgrade air flow/exhaust. Not sure about other mods. Electric fan would be smart also from my research on here.
Thanks

 

[RonD]

Actually taller tires reduce RPM at preset speed, so would give better MPG on highway.
Notice I said "taller" not larger, "larger" tires weigh more so would reduce MPG.
Thinner tires have less friction so less rolling resistance, also less grip when cornering and in the rain, although hydroplaning on wider tires is an issue as well.

4.0l and 2.3l/2.5l are different blocks so engine and trans have to be swapped along with motor mounts, computer and wiring.
Better to sell the 4.0l truck and buy a 2.3l truck.

Exhaust changes, headers, are done to change the power band, the RPM range when engine has the most torque, factory exhaust is tuned for mid-band power, most headers go for lower power band, so take top-end power and move it down to lower RPMs.
It would not increase MPG unless you were an aggressive driver and the lower power band allowed you to accelerate faster with less gas pedal, but the difference would be marginal and probably non-existent if you drove that way, lol.

 

[Karcastic]

Actually taller tires reduce RPM at preset speed, so would give better MPG on highway.
Notice I said "taller" not larger, "larger" tires weigh more so would reduce MPG.
Thinner tires have less friction so less rolling resistance, also less grip when cornering and in the rain, although hydroplaning on wider tires is an issue as well.

4.0l and 2.3l/2.5l are different blocks so engine and trans have to be swapped along with motor mounts, computer and wiring.
Better to sell the 4.0l truck and buy a 2.3l truck

Ok, thanks RonD

I guess I could post a possible trade ad on this site to see if anyone would be interested in upgrading. I'm at 330k, there's a minor po303 so I change that spark twice a year, needs paint. Only things that have went on it is the fuel regulator, alt and batt obviously and rear fender flares. Not sure how much money I'll be into the switch after. I really thought a weekend with my mechanic and a low mileage cheap donor would be my best route. I can find lots of rust buckets in Ontario with low k parts. No ins bull, reg bull, tax bull, weekday bull, emission/safety bull, ontario rust bucket bull and hidden bull. My truck is in amazing condition. Only been in Ontario for two years so I guess I could get a fair price/swap for her. I'll be looking to change trucks as I work on my current mileage.

 

[cp2295]

Sell it and buy a 4 banger with 3.08s.