5.7 vs 6.4 towing

[GsRAM]

Short answer: A 6.4 powered 2500 or 3500 is what he should be looking for. 3500 if he can find one at a reasonable price, 2500 would also be ok. If squat is an issue he could install rear air bags to offset that. The 2500 frame and suspension is very similar to 3500 SRW trucks. 3500 has leafs, 2500 has coils, but they are large and robust. Good luck.

 

[sixback]

The more important option would be 4:10 gears vs 5.7/6.4. The 5.7 uses variable timing while the 6.4 uses variable displacement. I have a HD with 5.7 and 4:10 gears. I don't think I've ever had to rev that thing over 3K yet. Whatever, you think you lose with a 5.7 is more than made up for with 4:10s. And a limited slip rear end can be a fun bonus too.

 

[SouthTexan]

The more important option would be 4:10 gears vs 5.7/6.4. The 5.7 uses variable timing while the 6.4 uses variable displacement. I have a HD with 5.7 and 4:10 gears. I don't think I've ever had to rev that thing over 3K yet. Whatever, you think you lose with a 5.7 is more than made up for with 4:10s. And a limited slip rear end can be a fun bonus too.

I think you are 100% correct here. The 6.4L only has 27 more hp which is barely felt in the seat of your pants. Having 4.10 gearing would multiply the torque of the 5.7L to surpass the torque going to the wheels of a 3.73 geared 6.4L. Of course having 4.10 gearing in the 6.4L would have the most out of the two gassers.

 

[ramffml]

I think you are 100% correct here. The 6.4L only has 27 more hp which is barely felt in the seat of your pants. Having 4.10 gearing would multiply the torque of the 5.7L to surpass the torque going to the wheels of a 3.73 geared 6.4L. Of course having 4.10 gearing in the 6.4L would have the most out of the two gassers.

I disagree. You guys are looking at peak hp/torque numbers only, which are only the numbers you get at one specific RPM and at WOT.

Power under the curve is far more important, and the 6.4 is tuned to make more power at every RPM.

The rear gears only helps you for that first 1 or 2 shifts. On the highway the transmission will adjust to put your engines power where it needs it, with either rear gear. Unless you're towing in first, or in 8th, the rear gear differences are offset by the tranmission gear.

This is the magic of gears and torque multiplication. It can be done either in the transmission (downshift) or in the rear end, but the end result is "the same".

So if I had to choose one or the other, I'd pick 6.4 over the 5.7 every time vs 5.7 with your choice of rear end. You will notice that increase in torque/hp in every gear with the 6.4 whereas with the rear gear you only notice it immediately off the line where the transmission has no room to downshift any further.

 

[SouthTexan]

I disagree. You guys are looking at peak hp/torque numbers only, which are only the numbers you get at one specific RPM and at WOT.

Power under the curve is far more important, and the 6.4 is tuned to make more power at every RPM.

The rear gears only helps you for that first 1 or 2 shifts. On the highway the transmission will adjust to put your engines power where it needs it, with either rear gear. Unless you're towing in first, or in 8th, the rear gear differences are offset by the tranmission gear.

This is the magic of gears and torque multiplication. It can be done either in the transmission (downshift) or in the rear end, but the end result is "the same".

So if I had to choose one or the other, I'd pick 6.4 over the 5.7 every time vs 5.7 with your choice of rear end. You will notice that increase in torque/hp in every gear with the 6.4 whereas with the rear gear you only notice it immediately off the line where the transmission has no room to downshift any further.

Gears continue to play huge roll in how well your truck tows even after first and second. When towing, there is a constant changing force that is pulling you back and you need enough torque overcome that force along with the horsepower to go a certain speed. As you go up a hill, then that force pulling you back becomes even greater. With grater torque multiplication, you would require less engine load. Case in point, my truck in 6th pulling 14k put more load on the engine requiring the engine to do more work than doing the same thing in 5th.

Also, since a 4.10 cause you to rev about 150-400 rpm higher depending on the gear, some of that horsepower difference is negated. For example, at 65 mph in a hard pull, a 5.7L with 4.10s is at 3,717 rpm in third while a 6.4L with 3.73 is at 3,382 rpm. That extra 335 rpm will negate some of the horsepower difference since more rpm means more horsepower.

In the past, I have felt more of a difference when towing from a gear change, that I did with a tuner that added only 25 hp.

 

[ramffml]

Gears continue to play huge roll in how well your truck tows even after first and second.

Rear gear ratio is multiplied with the transmission gear ratio to get your final torque multiplication. You can change either the transmission gear (upshift/downshift) or change the rear end (3.73 to 4.10) and have pretty much mathematically equivalent values. I haven't done the math for this specific scenario, but I've done that quite a bit with the 5.7 and 3.21 vs the 3.92, and there are pretty much 5 identical gear ratios in that case.

When towing, there is a constant changing force that is pulling you back and you need enough torque overcome that force along with the horsepower to go a certain speed. As you go up a hill, then that force pulling you back becomes even greater. With grater torque multiplication, you would require less engine load. Case in point, my truck in 6th pulling 14k put more load on the engine requiring the engine to do more work than doing the same thing in 5th.

This is true.

Also, since a 4.10 cause you to rev about 150-400 rpm higher depending on the gear, some of that horsepower difference is negated. For example, at 65 mph in a hard pull, a 5.7L with 4.10s is at 3,717 rpm in third while a 6.4L with 3.73 is at 3,382 rpm. That extra 335 rpm will negate some of the horsepower difference since more rpm means more horsepower.

You can't pick just one RPM/speed and draw a conclusion. Because when you pick another speed, the 6.4 might have just downshifted and is putting out 4300 RPMs vs the 5.7 "struggling" at a lower RPM.

In the past, I have felt more of a difference when towing from a gear change, that I did with a tuner that added only 25 hp.

There will be a difference off the line when doing a gear change, that's it. There is nothing special about a rear gear over and above a transmission gear; they work together to form your final torque multiplication and as long as the transmission can downshift you don't need to increase the rear gear ratio. From a stop, at that point the transmission is in first and has nowhere left to go. So at that point, the rear gear is the major differentiator. If you're on the highway passing a truck or going up a hill, RPMs are RPMs, doesn't matter if your transmission or rear gear got you to those RPMs.

 

[SouthTexan]

You can't pick just one RPM/speed and draw a conclusion. Because when you pick another speed, the 6.4 might have just downshifted and is putting out 4300 RPMs vs the 5.7 "struggling" at a lower RPM.

Here are the speeds and gears that will get you to 4,300 rpm along with the rpm the 4.10 5.7L would be at.

1st gear(if TC was able to lock) at 35 mph: 6.4L 3.73 is at 4,309 rpm and the 5.7L 4.10 is at 4,737 rpm which is 428 rpm higher.

2nd gear at 61 mph: 6.4L 3.73 is at 4,270 rpm and the 5.7L 4.10 is at 4,694 rpm which is 424 rpm higher.

3rd gear 80 mph(which is way to fast when towing): 6.4L 3.73 is at 4,299 rpm and the 5.7L 4.10 is at 4,725 rpm which is 426 rpm higher.

After this gear, you are going well past the governor of the truck to get to 4,300 rpm with a locked torque converter that is sending 100% of the horsepower to the ground. So in ever case of a 3.73 6.4L being at 4,300 rpm, a 4.10 5.7L is over 400 rpm higher which means some of that horsepower difference is a moot point.

In a passing situation like you are saying? Yes, it is mostly about horsepower. However, in a towing situation of going up a grade, it is a combination of torque and horsepower since the forces pulling you back are increasing and 4.10 5.7L will be putting more torque to the wheels while a 3.73 6.4L will be putting slightly more horsepower.

 

[ramffml]

Here are the speeds and gears that will get you to 4,300 rpm along with the rpm the 4.10 5.7L would be at.

1st gear(if TC was able to lock) at 35 mph: 6.4L 3.73 is at 4,309 rpm and the 5.7L 4.10 is at 4,737 rpm which is 428 rpm higher.

2nd gear at 61 mph: 6.4L 3.73 is at 4,270 rpm and the 5.7L 4.10 is at 4,694 rpm which is 424 rpm higher.

3rd gear 80 mph(which is way to fast when towing): 6.4L 3.73 is at 4,299 rpm and the 5.7L 4.10 is at 4,725 rpm which is 426 rpm higher.

After this gear, you are going well past the governor of the truck to get to 4,300 rpm with a locked torque converter that is sending 100% of the horsepower to the ground. So in ever case of a 3.73 6.4L being at 4,300 rpm, a 4.10 5.7L is over 400 rpm higher which means some of that horsepower difference is a moot point.

In a passing situation like you are saying? Yes, it is mostly about horsepower. However, in a towing situation of going up a grade, it is a combination of torque and horsepower since the forces pulling you back are increasing and 4.10 5.7L will be putting hundreds of lb-ft more torque to the wheels while a 3.73 6.4L will be putting slightly more horsepower.

I haven't double checked your math because I'm lazy and because it doesn't really do much for my argument.

You're still cherry picking mph and rpms. There will be speeds and rpms where the one truck shifts first, then the other one shifts, then the first one shifts etc; leapfrogging behaviour. There will be many times when the 5.7 has just upshifted and the 6.4 is probably at higher rpms.

It's more clear (to me anyway) when you compare 3.21 vs 3.92 in the 5.7:
6th gear for the 3.21 is a gear ratio of 1 (transmission) * 3.21 (rear) = 3.21
7th gear for the 3.92 is a gear ratio of 0.82 (transmission) * 3.92 (rear) = 3.214

Which means both trucks are putting down identical torque at the wheel in that scenario. They'd be at the same RPM, the same MPH, delivering the same torque, even though the rear end with the 3.92 is signifcantly deeper. That's because even though rear end is deeper in the 3.92, it's running a taller transmission gear, canceling out its advantage.

Which is my point, rear end is combined with the transmission to form your gear ratio. Going back to your cherry picked numbers, the 2n'd gear at 61 mph. Lets say both trucks are still accelerating past 61, what's going to happen? The 5.7 runs out of rpms and upshifts; at that point, the 6.4 probably still has room in 2nd gear so it's putting down a ton more torque after the 5.7 upshifts.

You're also not calculating torque at the wheel in any of this (which again, I'm too lazy to do). But run the numbers and I'm willing to bet that even in your 2nd gear scenario the 6.4 is putting down more torque even though RPMs are lower. Could be wrong there, but again doesn't change my point because trucks dont' hold a single speed, and your numbers are clearly picked to show where the 5.7 is sitting higher rpms above the 6.4 but that isn't the case across the entire range of speed from 0 to 70.

 

[SouthTexan]

I haven't double checked your math because I'm lazy and because it doesn't really do much for my argument.

You're still cherry picking mph and rpms. There will be speeds and rpms where the one truck shifts first, then the other one shifts, then the first one shifts etc; leapfrogging behaviour. There will be many times when the 5.7 has just upshifted and the 6.4 is probably at higher rpms.

It's more clear (to me anyway) when you compare 3.21 vs 3.92 in the 5.7:
6th gear for the 3.21 is a gear ratio of 1 (transmission) * 3.21 (rear) = 3.21
7th gear for the 3.92 is a gear ratio of 0.82 (transmission) * 3.92 (rear) = 3.214

Which means both trucks are putting down identical torque at the wheel in that scenario. They'd be at the same RPM, the same MPH, delivering the same torque, even though the rear end with the 3.92 is signifcantly deeper. That's because even though rear end is deeper in the 3.92, it's running a taller transmission gear, canceling out its advantage.

Which is my point, rear end is combined with the transmission to form your gear ratio. Going back to your cherry picked numbers, the 2n'd gear at 61 mph. Lets say both trucks are still accelerating past 61, what's going to happen? The 5.7 runs out of rpms and upshifts; at that point, the 6.4 probably still has room in 2nd gear so it's putting down a ton more torque after the 5.7 upshifts.

You're also not calculating torque at the wheel in any of this (which again, I'm too lazy to do). But run the numbers and I'm willing to bet that even in your 2nd gear scenario the 6.4 is putting down more torque even though RPMs are lower. Could be wrong there, but again doesn't change my point because trucks dont' hold a single speed, and your numbers are clearly picked to show where the 5.7 is sitting higher rpms above the 6.4 but that isn't the case across the entire range of speed from 0 to 70.

I didn't cherry pick anything. I used the 4,300 rpm you stated and plotted all of the speeds and gears that a 3.73 6.4L would reach 4,300 rpm along with what a 4.10 5.7L would be at the same speed and gear. To me, cherry picking data is basing a point by rpm instead of vehicle speed. We don't drive in the real world based on rpm. We drive based on vehicle speed hence the reason we have speed limits and not rpm limits on highways.

And yes I am calculating torque to the wheels. About a year ago I was debating on changing my 3.42 gears for 3.73 or 4.10 in my truck since I only use it for towing now so I made an excel spreadsheet that auto populates torque to the wheels and rpm if I input gear ratio, speed and tire size.

No matter what speed, or tire size I input, then 4.10 5.7L will always have more torque to the wheels and be at a higher rpm gear for gear than a 3.73 6.4L using their peak numbers. Of course these are peak and I don't have the torque at every rpm, but the same difference of 30 lb-ft runs throughout the whole power bands from the dyno's that I have seen of the two engines. Last one is my diesel when it was in stock form.

 

[ramffml]

I didn't cherry pick anything. I used the 4,300 rpm you stated and plotted all of the speeds and gears that a 3.73 6.4L would reach 4,300 rpm along with what a 4.10 5.7L would be at the same speed and gear. To me, cherry picking data is basing a point by rpm instead of vehicle speed. We don't drive in the real world based on rpm. We drive based on vehicle speed hence the reason we have speed limits and not rpm limits on highways.

The 4300 was something I probably through out there in passing. The problem with your argument is that you find a MPH where the 5.7 is revving 300 to 400 rpms above the 6.4, then make the claim that the 5.7 is putting out more power. No. Because are the two trucks accelerating? What happens when the 5.7 runs out of RPMs? It will upshift into second where the 6.4 is still on first. Who do you think is putting more power down at that point then?

That's why I'm saying you're cherry picking your data, because all your examples are speeds where the 5.7 just so happens to be revving above the 6.4.

Please share your spreadsheet. From a brief look at the pic it seems like you're using a constant value for torque (one constant per engine) which again is incorrect but I'm in a hurry today so I could just be reading it wrong...

 

[SouthTexan]

The 4300 was something I probably through out there in passing. The problem with your argument is that you find a MPH where the 5.7 is revving 300 to 400 rpms above the 6.4, then make the claim that the 5.7 is putting out more power.

No sir! I never said the 5.7L is making more power. In fact I said the opposite. This is a quote of what I said above.

However, in a towing situation of going up a grade, it is a combination of torque and horsepower since the forces pulling you back are increasing and 4.10 5.7L will be putting more torque to the wheels while a 3.73 6.4L will be putting slightly more horsepower.

--------------------------------

Because are the two trucks accelerating? What happens when the 5.7 runs out of RPMs? It will upshift into second where the 6.4 is still on first. Who do you think is putting more power down at that point then?

That's why I'm saying you're cherry picking your data, because all your examples are speeds where the 5.7 just so happens to be revving above the 6.4.

So if the rev limiter is 6,500 rpms and you had factory 33 inch tires and you were somehow able to lock the TC in first gear. These are the mph that this would happen...

48 mph

and 53 mph


Then in second gear at.....

85 mph


and 93 mph


After this, you would be going well past the speed limiter.

The rest(or 95-97%) of the speeds you will be going when towing, the 4.10 will be at a higher rpm. Again, that is if you can get the TC to lock in first which I don't think it does from the factory.


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Please share your spreadsheet. From a brief look at the pic it seems like you're using a constant value for torque (one constant per engine) which again is incorrect but I'm in a hurry today so I could just be reading it wrong...

This is why I stated......

Of course these are peak and I don't have the torque at every rpm, but the same difference of 30 lb-ft runs throughout the whole power bands from the dyno's that I have seen of the two engines.

Not sure you are reading what I am posting?

 

[ramffml]

Lol, I guess that's what I get for posting in a hurry. My bad, you did mention the constant torque.

I remain unconvinced, but to be perfectly honest you've got me thinking anyway and at the very least it's proof that I should have reserved forming a strong opinion without crunching the numbers.

In any case, I do want to continue this further probably next week some time as it's going to be an equally busy weekend for me. I will see what I can find for torque curves with real data, I'd like to get rid of that constant torque number as for start that is inaccurate. I have to believe the 6.4 is tuned differently than the 5.7, I've read that it was tuned to make more torque down low for truck duty, so it wouldn't surprise me if the torque curve is flatter compared to a steeper curve for the 5.7. If you have/find torque curves please post them here, will save me some google headaches!