V6 penta to V8 5.7

[GapperGus]

Thanks for stopping to read.
I got F*cked on my truck, yes my own stupidity. I have a 15’ V6 4x4 with 120k on the dash. I still owe 20k. I traded my car at Covid prices with almost 6k Negative equity. So I’m stuck with this Thing for 4 more years. So buying a new truck is out of the question. So now I’ve come to the idea of swapping to a V8 once this sucker blows (inevitably). Tons of donor trucks in my area, and I’m pretty handy with a wrench and have some mechanically inclined people around me. I know I could just put the body on the frame of a truck and call it a day. But the donor will probably be messed up. So, I know with all the modules and stuff it’ll be a nightmare. But it’ll be cheaper than trading this thing for a V8 with more miles. Just wanna know what I’d be facing

 

[GeauxinUp]

I would ASSume at the minimum, you'd need donor engine, PCM, and wiring harness. Not sure about mating engine and trans-same flexplate?? What trans do you have? If not the 8-spd hp70, I'd swap that too.

 

[TradesmanGuy]

Sounds like a project that could take months, as it would be a learn as you go, tracking down very specific parts, and maybe even some custom pieces need fabricating.

Hopefully you have a well-equipped shop, plenty of time on your hands and plenty of friends to help with the big and awkward parts.

 

[kurek]

Swapping to a different engine that's already available in the same vehicle is almost never a good value. Enjoy your truck. Buy a used charger or something if you need a v8 fix.

 

[Yardbird]

What is wrong with the Pentastar that is making you want to go V-8?

If take off power is a problem, change the 3.21 rears to 3.92 (edited). As far as road rpms go, 3.21 in 7th gear is the same rpms as a 3.92 (edited) in 8th.

The Pentastar does not nave low end grunt. It's all higher rpm power. That's why 8th is useless in a 3.21 truck, and I never run my truck in full auto.

I lock 8th out always. Anytime I have tried to use 8th that isn't downhill, it shifted back to 7th at the first gust of wind or the truck sensed a small rise was ahead.

With the multitude of engine specific modules on these trucks, I don't know if a HEMI swap is even possible without huge trouble and expense.

If you don't need the power of a HEMI, and have 3.21 rears, just get in the habit of locking out 8th, or manually shift the gears as needed, which is what I do. Or do a 3.92 swap (edited).

Most every road I travel I know what gear I need for best performance and mileage. That's no aggravation for me as I drove trucks for 50 years, and have became used to matching gears to terrain.

That being said, my Pentastar is not used to pull anything except the wife and I, and an occasional trip to Lowes or HD. If I need power, I still have my old V-10 powered Dodge truck.

 

[Wild one]

What is wrong with the Pentastar that is making you want to go V-8?

If take off power is a problem, change the 3.21 rears to 3.55. As far as road rpms go, 3.21 in 7th gear is the same rpms as a 3.55 in 8th.

The Pentastar does not nave low end grunt. It's all higher rpm power. That's why 8th is useless in a 3.21 truck, and I never run my truck in full auto.

I lock 8th out always. Anytime I have tried to use 8th that isn't downhill, it shifted back to 7th at the first gust of wind or the truck sensed a small rise was ahead.

With the multitude of engine specific modules on these trucks, I don't know if a HEMI swap is even possible without huge trouble and expense.

If you don't need the power of a HEMI, and have 3.25 rears, just get in the habit of locking out 8th, or manually shift the gears as needed, which is what I do. Or do a 3.55, or even a 3.92 swap.

Most every road I travel I know what gear I need for best performance and mileage. That's no aggravation for me as I drove trucks for 50 years, and have became used to matching gears to terrain.

That being said, my Pentastar is not used to pull anything except the wife and I, and an occasional trip to Lowes or HD. If I need power, I still have my old V-10 powered Dodge truck.

3.21 and 3.92 cross over,not 3.55. 3.21's in 7th are virtually identical to 3.92's in 8th.

 

[Yardbird]

EDIT...The Folling Info is WRONG. I corrected it in another post below>

On my truck 3.21 in 7th is exactly the same rpm as 3.55 in 8th. I've run the numbers multiple times, but I could be wrong (EDIT..I was wrong). I have been once or twice in the past....

I don't think there is 25 rpm difference.

Even on flat I-95 in south Georgia and Florida, I can't pull 8th. Going fast enough to keep up with traffic the wind resistance is enough to force downshifts on a grade so slight you have to look hard to see it.

I could draft, but I don't want rock chips on front of my truck, and who knows when the person up front might need to brake hard.

 

[huntergreen]

It would easier and cheaper to put a turbo on the pentastar.

 

[Wild one]

On my truck 3.21 in 7th is exactly the same rpm as 3.55 in 8th. I've run the numbers multiple times, but I could be wrong. I have been once or twice in the past....

I don't think there is 25 rpm difference.

Even on flat I-95 in south Georgia and Florida, I can't pull 8th. Going fast enough to keep up with traffic the wind resistance is enough to force downshifts on a grade so slight you have to look hard to see it.

I could draft, but I don't want rock chips on front of my truck, and who knows when the person up front might need to brake hard.

If you use actual transmission overdrive ratio's multiplied by rear end ratio's 3.21's and 3.92's are the cross over gears. 3.55's are a bit of an oddball as they don't cross over with either 3.21's or 3.92's.
I could be wrong,but as far as i know the little 845E uses the same overdrive ratio's as the 8HP70 does,but then again i haven't really paid much attention to the little 8 speed to see if it uses the same overdrive ratio's as it's bigger brother uses.

 

[diymirage]

Seems to me the best option is to let the wife drive it and make sure the insurance is up to date

 

[Yardbird]

If you use actual transmission overdrive ratio's multiplied by rear end ratio's 3.21's and 3.92's are the cross over gears. 3.55's are a bit of an oddball as they don't cross over with either 3.21's or 3.92's.
I could be wrong,but as far as i know the little 845E uses the same overdrive ratio's as the 8HP70 does,but then again i haven't really paid much attention to the little 8 speed to see if it uses the same overdrive ratio's as it's bigger brother uses.

I went back and calculated the different gear ratios with the 3.21 - 3.55 - and 3.92 rears.

You are right. I can't remember anything these days.

My truck with a 7th gear ratio of .84
Rear axle ratio of 3.21
My tires are 611 revs per mile
2100 RPM
Equals a speed of 76 MPH

Same transmission in 8th gear is .67
Rear axle ratio of 3.92
Tires 611 revs per mile
2100 RPM
Equals a speed of 78 MPH.

So. I would need to go to 3.92s to be able to use all 8 gears anywhere but downhill.

Sorry for my flawed thinking. I know I figured this a few years ago, and I thought the magic number was 3.55. Glad I didn't spend money to change to that.

 

[HemiLonestar]

Just ride it out; run to fail (if it fails). If it does fail, replace with stock engine. Unlikely you want to mess with trying to get everything working correctly because it'll be a massive headache before it gets better. Pay it down then trade in on what you really want now that you've learned not to get the baby engine in something this heavy.

 

[jzacha]

I have the pentastar in a 14 ram and it’s been great. 3.55 gears. I have 35” tires and have towed a grand Cherokee across country (including the mountains) with it.

 

[TradesmanGuy]

I have the pentastar in a 14 ram and it’s been great. 3.55 gears. I have 35” tires and have towed a grand Cherokee across country (including the mountains) with it.

I've no complaints about my Pentastar. Granted, I'm not hauling gravel or a four horse trailer, up and down mountains, so my requirements might be insignificant compared to others. But for most I bet the Pentastar satisfies. Of course there is the mindset, if there is more, then I want it and NOW.

 

[Bob Horowski]

You owe 20k on a '15, you need financially inclined people around you, not mechanically inclined people.

 

[Grams]

The OP doesn’t read or care what is posted in this thread he started.

For 95.75% of all driving Any of Us do….. the little Pentastar is al lthat’s needed, and changing out the engine (and all the trouble that’ll be) is an expensive and silly mistake.

Changing the differential gears is simpler…but an even simpler (and FREE) solution is to use a lower gear by pushing that button on the gear-shifter.

 

[Yardbird]

The OP doesn’t read or care what is posted in this thread he started.

For 95.75% of all driving Any of Us do….. the little Pentastar is al lthat’s needed, and changing out the engine (and all the trouble that’ll be) is an expensive and silly mistake.

Changing the differential gears is simpler…but an even simpler (and FREE) solution is to use a lower gear by pushing that button on the gear-shifter.

I need to buy an extra gear button. Though not showing, I'm sure mine will have the numbers worn off in a few years. I have been using the button to limit what gear I want for the speed and terrain since the day I was driving it home from the dealer.

I have a gear for each speed limit and terrain.

I hate having the trans constantly shifting. I pick a gear for the speed I'm going.