Max RPM on Ram 6.4

[Dean2]

So according to all the specs I can find, max RPM for the truck version of the 6.4 is 5,800, and the specs say it is "Limited" at that RPM. Normal shift when I let the 8 speed transmission do its thing is 5500 to 5600 RPM. Does anyone know exactly how the engine RPM is limited. So for instance, if you put the gear limiter in 3rd and reach 5,800 rpm, will it force an up shift, or just cut power so the engine won't over rev.

I presume you guys that race these things will have run into this before. Thanks for the help.,

 

[Lsujker]

Intervention Methods:

• Fuel Cut-Off: The primary method for the 6.4L HEMI, where the ECM shuts off fuel injectors to select cylinders when RPM hits the limit. This leans out combustion, halting acceleration without dumping unburnt fuel into the exhaust, protecting the catalytic converter. Fuel cut is standard in Ram trucks for smoother operation and less exhaust wear.
  
  
• Ignition Cut (Secondary): Less common but used in some scenarios, the ECM may disable spark to certain cylinders, especially in performance tunes. This can cause popping sounds from unburnt fuel, less ideal for stock Rams with cats.
  
  
• Throttle Modulation: The 6.4L’s drive-by-wire throttle allows the ECM to reduce air intake if RPM approaches the limit, especially in soft-cut scenarios, ensuring a gradual power drop.
  
  
• Soft vs. Hard Cut: Ram typically uses a soft limiter in stock settings, gradually cutting fuel or spark around 5,800 RPM to avoid abrupt jolts, ideal for heavy-duty tasks like towing (up to 17,730 lbs). Aftermarket tunes (e.g., DiabloSport) can switch to a hard cut, stopping combustion instantly at the set RPM, preferred for racing but harsher on the drivetrain.

 

[Dean2]

Intervention Methods:

• Fuel Cut-Off: The primary method for the 6.4L HEMI, where the ECM shuts off fuel injectors to select cylinders when RPM hits the limit. This leans out combustion, halting acceleration without dumping unburnt fuel into the exhaust, protecting the catalytic converter. Fuel cut is standard in Ram trucks for smoother operation and less exhaust wear.
  
  
• Ignition Cut (Secondary): Less common but used in some scenarios, the ECM may disable spark to certain cylinders, especially in performance tunes. This can cause popping sounds from unburnt fuel, less ideal for stock Rams with cats.
  
  
• Throttle Modulation: The 6.4L’s drive-by-wire throttle allows the ECM to reduce air intake if RPM approaches the limit, especially in soft-cut scenarios, ensuring a gradual power drop.
  
  
• Soft vs. Hard Cut: Ram typically uses a soft limiter in stock settings, gradually cutting fuel or spark around 5,800 RPM to avoid abrupt jolts, ideal for heavy-duty tasks like towing (up to 17,730 lbs). Aftermarket tunes (e.g., DiabloSport) can switch to a hard cut, stopping combustion instantly at the set RPM, preferred for racing but harsher on the drivetrain.

Very cool, great explanation. I Googled the hell out of that this morning and could not find a thing. Maybe my Google Fu is weak this morning.

Thanks a bunch.

 

[Lsujker]

Very cool, great explanation. I Googled the hell out of that this morning and could not find a thing. Maybe my Google Fu is weak this morning.

Thanks a bunch.

You can thank Grok. AI is fantastic.

 

[rzr6-4]

You can thank Grok. AI is fantastic.

Lol, sometimes. I've made myself look bad before here trusting info that after getting called out I went back to verify, and was completely wrong.

 

[Lsujker]

Lol, sometimes. I've made myself look bad before here trusting info that after getting called out I went back to verify, and was completely wrong.

For sure! This was only a snip-it to the full response. later down kept saying, make sure you replace the oil every 6K miles with 5w-20... made me question myself.

 

[Wild one]

So according to all the specs I can find, max RPM for the truck version of the 6.4 is 5,800, and the specs say it is "Limited" at that RPM. Normal shift when I let the 8 speed transmission do its thing is 5500 to 5600 RPM. Does anyone know exactly how the engine RPM is limited. So for instance, if you put the gear limiter in 3rd and reach 5,800 rpm, will it force an up shift, or just cut power so the engine won't over rev.

I presume you guys that race these things will have run into this before. Thanks for the help.,

It should just bounce off the limiter,and won't upshift past the gear selected via the ERS Dean. The 1320's with the 6.4 shift at roughly 6400/6500 in drag mode,and they're basically the same bottem end as far as rods and crank goes,bit differant pistons and compression though.The truck 6.4's have better pistons then the car 6.4's have,and better heads.
Swap out the valves and springs in the truck heads,and you basically have Hellcat heads with a naturally aspirated intake manifold bolt pattern.
If you're gonna boost a stock bottem end naturally aspirated Hemi,the truck 6.4 is the one to use,as it's 10:1 compression ratio is more boost friendly then the cars 10.9:1 and the 5.7's 10.5:1 compression ratios are,and out of the 3 engines has the most robust piston.
Plus all truck 6.4's use the BGE block,and come through with forged cranks,but then again all 6.4's use a forged crank,while the 5.7's use a cast crank.The car 6.4's didn't get the BGE block until the 2018 model year.
The standard shift rpm if you let the transmission do it's thing is 5600 in the trucks with the 8 speed,whether it's a 1500 or 2500,if you use the buttons and run it up higher,you have be very quick with the shift,as it'll hit the limiter damn near instantly

 

[Dean2]

Thanks Wild. Appreciate the in depth info. I gather hitting the rev limiter does no real damage, just arrests further acceleration to keep the revs in the 5800 rpm limit. If the Trans shifts at 5600 there would seem to be very little advantage to trying to use the limiter switchs to get the last 200 rpm. If you missed the shift by even a hair I would think that would totally pooch the run.

 

[Lsujker]

Thanks Wild. Appreciate the in depth info. I gather hitting the rev limiter does no real damage, just arrests further acceleration to keep the revs in the 5800 rpm limit. If the Trans shifts at 5600 there would seem to be very little advantage to trying to use the limiter switchs to get the last 200 rpm. If you missed the shift by even a hair I would think that would totally pooch the run.

Curious what you are trying to do exactly. Just general info or are you racing this pooch?

 

[Wild one]

Thanks Wild. Appreciate the in depth info. I gather hitting the rev limiter does no real damage, just arrests further acceleration to keep the revs in the 5800 rpm limit. If the Trans shifts at 5600 there would seem to be very little advantage to trying to use the limiter switchs to get the last 200 rpm. If you missed the shift by even a hair I would think that would totally pooch the run.

I used to launch both the Challengers in 3rd,and let the transmission do the first couple shifts,as even with paddles,you couldn't shift them fast enough in the lower gears to not bounce off the limiter,after 3rd i'd take over the shifting on the cars,as by then you have enough time to actually make the shift yourself.
The truck was tuned to shift at roughly 6100/6200 on the 1-2 shift and 6400 on the 2-3 shift,after that it was set to shift at 6600,as even the computor could hardly make the first couple of shifts fast enough to keep it off the limiter if we raised the 1-2 shift to 6400 and the 2-3 shift to 6500,and we had the limiter set at 7,000 rpm

 

[Dean2]

Curious what you are trying to do exactly. Just general info or are you racing this pooch?

No plans to race it. Just trying to figure out the gear limiter as I use it a lot in the mountains and with weight on the back it runs up pretty high in the rev range holding the load back on steep downhills. I also want to understand what happens if I take off from a stop with the limiter in second or 3rd if I am not paying attention and the motor hits the rev limit before I realise I have the limit set too low.