3.8 engine mods
Forum Tech Advisor
Joined: Jan 2008
Posts: 1,208
Likes: 11
From: Billings, MT
If you look at the graph and chart, the torque increases are steep until about 2500 rpm. The HP increases continue to be very steep all the way up to 4500 rpm.
At a very minimum, you want to be running 2500 rpm at 70 mph. Tuned, that puts you at 94 RWHP and 198 RWTQ.
At 3000 rpm, you still get a pretty big increase to 114 RWHP, but the RWTQ results only increase to 199.
If you are going to run 35s with a 6 speed, I'd recommend 4.56s if you have a light rig and/or live/drive at sea level in flat terrain. If your rig is heavier, you live well above sea level, or drive hilly or mountainous terrain, I'd recommend 4.88s with 35s. If your actual tire size mounted is 34", that puts you at 2600-2800 rpm at 70 mph.
Given the non-aerodynamic design of a wrangler--particularly a lifted one--more power is desired. If you want good fuel economy, you need to run light and at slower speeds, with stock tires and little or no gear.
With 4.88s and 35s and a 6 speed (07-11), if you drive 60 mph, you should be able to get close to or more than 20 mpg. However, at 80 mph, you aren't going to get more than about 13-14 mph.
Those Japanese engines you referred to with 300 HP don't produce that amount of power below 3000 rpm--more like over 5000 rpm.
At a very minimum, you want to be running 2500 rpm at 70 mph. Tuned, that puts you at 94 RWHP and 198 RWTQ.
At 3000 rpm, you still get a pretty big increase to 114 RWHP, but the RWTQ results only increase to 199.
If you are going to run 35s with a 6 speed, I'd recommend 4.56s if you have a light rig and/or live/drive at sea level in flat terrain. If your rig is heavier, you live well above sea level, or drive hilly or mountainous terrain, I'd recommend 4.88s with 35s. If your actual tire size mounted is 34", that puts you at 2600-2800 rpm at 70 mph.
Given the non-aerodynamic design of a wrangler--particularly a lifted one--more power is desired. If you want good fuel economy, you need to run light and at slower speeds, with stock tires and little or no gear.
With 4.88s and 35s and a 6 speed (07-11), if you drive 60 mph, you should be able to get close to or more than 20 mpg. However, at 80 mph, you aren't going to get more than about 13-14 mph.
Those Japanese engines you referred to with 300 HP don't produce that amount of power below 3000 rpm--more like over 5000 rpm.
Thread Starter
JK Newbie
Joined: Apr 2014
Posts: 64
Likes: 0
From: Olathe, KS
If you look at the graph and chart, the torque increases are steep until about 2500 rpm. The HP increases continue to be very steep all the way up to 4500 rpm.
At a very minimum, you want to be running 2500 rpm at 70 mph. Tuned, that puts you at 94 RWHP and 198 RWTQ.
At 3000 rpm, you still get a pretty big increase to 114 RWHP, but the RWTQ results only increase to 199.
If you are going to run 35s with a 6 speed, I'd recommend 4.56s if you have a light rig and/or live/drive at sea level in flat terrain. If your rig is heavier, you live well above sea level, or drive hilly or mountainous terrain, I'd recommend 4.88s with 35s. If your actual tire size mounted is 34", that puts you at 2600-2800 rpm at 70 mph.
Given the non-aerodynamic design of a wrangler--particularly a lifted one--more power is desired. If you want good fuel economy, you need to run light and at slower speeds, with stock tires and little or no gear.
With 4.88s and 35s and a 6 speed (07-11), if you drive 60 mph, you should be able to get close to or more than 20 mpg. However, at 80 mph, you aren't going to get more than about 13-14 mph.
Those Japanese engines you referred to with 300 HP don't produce that amount of power below 3000 rpm--more like over 5000 rpm.
At a very minimum, you want to be running 2500 rpm at 70 mph. Tuned, that puts you at 94 RWHP and 198 RWTQ.
At 3000 rpm, you still get a pretty big increase to 114 RWHP, but the RWTQ results only increase to 199.
If you are going to run 35s with a 6 speed, I'd recommend 4.56s if you have a light rig and/or live/drive at sea level in flat terrain. If your rig is heavier, you live well above sea level, or drive hilly or mountainous terrain, I'd recommend 4.88s with 35s. If your actual tire size mounted is 34", that puts you at 2600-2800 rpm at 70 mph.
Given the non-aerodynamic design of a wrangler--particularly a lifted one--more power is desired. If you want good fuel economy, you need to run light and at slower speeds, with stock tires and little or no gear.
With 4.88s and 35s and a 6 speed (07-11), if you drive 60 mph, you should be able to get close to or more than 20 mpg. However, at 80 mph, you aren't going to get more than about 13-14 mph.
Those Japanese engines you referred to with 300 HP don't produce that amount of power below 3000 rpm--more like over 5000 rpm.
That being answered... Now my question is, will a 4.56 gearing out of a '06 TJ (Dana 30 and 44) work with my JK?
Last edited by SteveLNew; May 3, 2014 at 12:26 PM.
JK Super Freak
Joined: Apr 2008
Posts: 1,267
Likes: 2
From: Thunder Bay, Ontario
The problem is not modding the engine with pistons and cam, or big bore kits. The problem is finding someone that can program it to run properly and make use of all that potential. There is no doubt in my mind that a good mechanic and programmer could pull 400hp out of this motor. But is there a market for it when you can just as easily do a Hemi or LS transplant.
