8HP70 and 4.0?

bbaldwin237 · Jul 9, 2026

I haven't seen this talked about here yet. American Iron says they are nearing completion of a swap kit to put the 8HP70 behind a 4.0. They mention that it is shorter than a 42RLE and will include a standalone transmission controller. An 8-speed behind a 4.0 could be fun and make the engine feel more powerful and controlled. Of course, you're adding in a lot of complexity to a simple machine, but should be interesting to follow along.

https://www.facebook.com/share/185vMGUb4Y/

lBasket · Jul 9, 2026

@Wildman you could do a 4.0 swap!

TheBoogieman · Jul 9, 2026

Weekend-is-not-over-yet-do-you-understand (2017_11_20 00_38_12 UTC) (2).webp

Wildman · Jul 9, 2026

bbaldwin237 said:
I haven't seen this talked about here yet. American Iron says they are nearing completion of a swap kit to put the 8HP70 behind a 4.0. They mention that it is shorter than a 42RLE and will include a standalone transmission controller. An 8-speed behind a 4.0 could be fun and make the engine feel more powerful and controlled. Of course, you're adding in a lot of complexity to a simple machine, but should be interesting to follow along.

View attachment 698806
https://www.facebook.com/share/185vMGUb4Y/

I've got not skin in the game but IMO I think it's an interesting idea... And if you look at the gear ratios of the 8HP70 I think it might help..

lBasket said:
@Wildman you could do a 4.0 swap!

That would be like putting a carburetor on my Hemi....

ColoJeep · Jul 9, 2026

Wildman said:
I've got not skin in the game but IMO I think it's an interesting idea... And if you look at the gear ratios of the 8HP70 I think it might help..

That would be like putting a carburetor on my Hemi....

Come on , twin AFB's for the win !

AOSJ · Jul 9, 2026

from a quick web search it appears you could tell people "i got it from a Phantom" if you really wanted to

TJuser · Jul 9, 2026

This is a sweet idea IMO. Someone swapped the 8HP90 into a 7.3 powerstroke and made a kit, they're an awesome transmission. 10 speeds are a little much, I think the 8 speeds are just right

maybe @bohuggabee can get 55mpg now with the added gears

bbaldwin237 · Jul 9, 2026

Wildman said:
I've got not skin in the game but IMO I think it's an interesting idea... And if you look at the gear ratios of the 8HP70 I think it might help..

That's my thought. It would be a great alternative for some engine swaps because you would add a ton of "drivability" for those that aren't looking for high horsepower. Now, think about the 8spd behind a stroker! I'm very interested in following along with this development.

speeding_infraction · Jul 9, 2026

Obviously any trans swap will be expensive, but these are likely going to be north of $5k. The controller alone is 1500, plus transmission, converter, two adapters, crossmember etc. but, it would be sweet. If the 4l60 ever blows in my XJ, there is a very high chance that a zf8 transmission will take its place

D's Jeep Wrld · Jul 9, 2026

bbaldwin237 said:
I haven't seen this talked about here yet. American Iron says they are nearing completion of a swap kit to put the 8HP70 behind a 4.0. They mention that it is shorter than a 42RLE and will include a standalone transmission controller. An 8-speed behind a 4.0 could be fun and make the engine feel more powerful and controlled. Of course, you're adding in a lot of complexity to a simple machine, but should be interesting to follow along.

View attachment 698806
https://www.facebook.com/share/185vMGUb4Y/

Edit- Damn autocorrect made it look like I can't write in English

I mentioned it on my build thread
But im happy as hell it's happened,Domiworks has had an adapter plate

Can't wait to do it on my Lj,That's the reason I went with an Ax15 coming from the 42rle slushbox... But yeah I wanted to do an 8hp swap but no one had an adapter plate as of an year ago,I offered money but all I got was "yeah we got the scans/or the idea to do it..." so An ax15 it was

https://www.domi-works.com/products/amc-v8-to-dodge-8hp-70-75-90-95-adapter-kit

TheBoogieman · Jul 9, 2026

TJuser said:
maybe @bohuggabee can get 55mpg now with the added gears

He gets that mileage from an alternator fan made into a mini turbo.

bbaldwin237 · Jul 9, 2026

speeding_infraction said:
Obviously any trans swap will be expensive, but these are likely going to be north of $5k. The controller alone is 1500, plus transmission, converter, two adapters, crossmember etc. but, it would be sweet. If the 4l60 ever blows in my XJ, there is a very high chance that a zf8 transmission will take its place

There should be a ton of the transmissions in junkyards. They're online for around $700-800 which isn't bad.

Strizzo · Jul 9, 2026

I'd be curious to see the dimensions compared to the current trans options. It would be pretty cool to have an 8 speed but the top ratio is even lower than the 42RLE, so you might still run into the same issues of having to run super short axle ratios to be able to use all the gears, which then also makes trouble with driveshaft speeds. Otherwise you're really going to end up with a 7-speed auto where 8th gear only happens on a long downhill stretch. Looking at the ratios, the 42RLE first gear is between second and third gears in the 8HP, so you'd get an improvement in crawl ratio without having to run lower gears, but you'd likely just be using 7th as your OD and staying out of 8th.

8HP75:

42RLE:

Definitely could be useful but maybe something like the ZF 6HP would fit even easier and get you the right number of gears and ratios, if it has the same bellhousing pattern and could be mated to a transfer case.
6HP:

bbaldwin237 · Jul 9, 2026

Strizzo said:
I'd be curious to see the dimensions compared to the current trans options. It would be pretty cool to have an 8 speed but the top ratio is even lower than the 42RLE, so you might still run into the same issues of having to run super short axle ratios to be able to use all the gears, which then also makes trouble with driveshaft speeds. Otherwise you're really going to end up with a 7-speed auto where 8th gear only happens on a long downhill stretch. Looking at the ratios, the 42RLE first gear is between second and third gears in the 8HP, so you'd get an improvement in crawl ratio without having to run lower gears, but you'd likely just be using 7th as your OD and staying out of 8th.

8HP75:View attachment 698833 42RLE: View attachment 698834

Definitely could be useful but maybe something like the ZF 6HP would fit even easier and get you the right number of gears and ratios, if it has the same bellhousing pattern and could be mated to a transfer case.
6HP:View attachment 698835

Even if you only ended up with 7 usable, that's better than 3. The engine would be able to maintain a more consistent RPM throughout and wouldn't have to downshift to 1.54(32RH) or 1.57 (42RLE) when going up a moderate hill. This would give a more consistent power feel and ultimately improved fuel economy, even if only slightly. And even though with the lockup torque converter you have an almost infinite low 1st, but look at that 4.71 1st gear.

ColoJeep · Jul 9, 2026

bbaldwin237 said:
Even if you only ended up with 7 usable, that's better than 3. The engine would be able to maintain a more consistent RPM throughout and wouldn't have to downshift to 1.54(32RH) or 1.57 (42RLE) when going up a moderate hill. This would give a more consistent power feel and ultimately improved fuel economy, even if only slightly. And even though with the lockup torque converter you have an almost infinite low 1st, but look at that 4.71 1st gear.

The gearing is much better , how reliable is the transmission and controller ?

Strizzo · Jul 9, 2026

ColoJeep said:
The gearing is much better , how reliable is the transmission and controller ?

The ZF 8HP is probably the best automatic transmission ever made. Very reliable and built to handle quite a bit of power. I didn't see where the controller was coming from.

I had a land rover LR4 with the 8HP and it was honestly perfect.

mrblaine · Jul 9, 2026

Strizzo said:
I'd be curious to see the dimensions compared to the current trans options. It would be pretty cool to have an 8 speed but the top ratio is even lower than the 42RLE, so you might still run into the same issues of having to run super short axle ratios to be able to use all the gears, which then also makes trouble with driveshaft speeds. Otherwise you're really going to end up with a 7-speed auto where 8th gear only happens on a long downhill stretch. Looking at the ratios, the 42RLE first gear is between second and third gears in the 8HP, so you'd get an improvement in crawl ratio without having to run lower gears, but you'd likely just be using 7th as your OD and staying out of 8th.

8HP75:View attachment 698833 42RLE: View attachment 698834

Definitely could be useful but maybe something like the ZF 6HP would fit even easier and get you the right number of gears and ratios, if it has the same bellhousing pattern and could be mated to a transfer case.
6HP:View attachment 698835

How long have you driven a correctly geared 42RLE?

Strizzo · Jul 9, 2026

mrblaine said:
How long have you driven a correctly geared 42RLE?

In your opinion, what is a correctly geared 42RLE?

mrblaine · Jul 9, 2026

Strizzo said:
In your opinion, what is a correctly geared 42RLE?

It isn't a matter of opinion. Anyone who has driven one would never make the statement that OD is only good for long downhill stretches of highway. So, either you have never driven a 42RLE for any length of time, of the ones you have driven were not geared properly.

So, that also means that the 8th gear in the 8HP is useful when the gearing is good.

BTW- this all has been discussed and the previous question and concerns were not addressed so maybe someone has some new info that will be useful. Have they figured out a good CAN Bus translator to toss into the controller install? If not, how do they plan to shift it properly in the TJ?

bbaldwin237 · Jul 9, 2026

mrblaine said:
BTW- this all has been discussed and the previous question and concerns were not addressed so maybe someone has some new info that will be useful. Have they figured out a good CAN Bus translator to toss into the controller install? If not, how do they plan to shift it properly in the TJ?

Only thing I've seen them reply so far was that it would be controlled by a standalone unit?

8HP70 and 4.0?

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