Shunt Mod Questions

[Yoshi Moshi]

I have a RTX 3090 TUF. I have been looking into possibly using a shunt mod to get more performance and had some questions.

 

1) I understand how you can draw double the power, but isn't the 8 pin pcie power cable only rated up to 150 W? I understand that the graphics card would think it's drawing half the power it really is, but isn't drawing more than 150 W from a single 8 pin power connector dangerous? Like certain fan headers on motherboards are only rated to a certain amount of current. Plugging in a fan that draws more current could cause great overheating.

 

2) Is it only suggested to shunt mod the 8 pin power connectors and not the PCIE 16x slot that is connected to the motherboard, because it is only rated to certain wattage?

 

3) Has anyone tried reselling a shunt mod GPU? Does having this modification make it harder or easier to resell your card? Or do you end up just removing the extra resistor to get it back to stock?

[Letgomyleghoe.]

8 minutes ago, Yoshi Moshi said:

1) I understand how you can draw double the power, but isn't the 8 pin pcie power cable only rated up to 150 W? I understand that the graphics card would think it's drawing half the power it really is, but isn't drawing more than 150 W from a single 8 pin power connector dangerous? Like certain fan headers on motherboards are only rated to a certain amount of current. Plugging in a fan that draws more current could cause great overheating.

 

Yes this is the official spec but you also have to consider that the official ddr4 spec is something like 2666? I don't know it doesn't really matter. The point is you can go out of spec and be just fine until you start drawing something over like half a kilowatt. 

11 minutes ago, Yoshi Moshi said:

2) Is it only suggested to shunt mod the 8 pin power connectors and not the PCIE 16x slot that is connected to the motherboard, because it is only rated to certain wattage?

 

Derbuar did a shunt mode on everything and his conclusion was that there is no point to doing anything other than the 8 pins thats why people suggest just doing it just on the 8 pins. 

12 minutes ago, Yoshi Moshi said:

3) Has anyone tried reselling a shunt mod GPU? Does having this modification make it harder or easier to resell your card? Or do you end up just removing the extra resistor to get it back to stock?

Depends on the person. 

Also suggestion before doing a shunt mod consider flashing a bios with a bigger power budget. 

[Jurrunio]

1. Then guess why many PSUs have 2 PCIe 8pin on a single cable

 

2. Yes, tolerances on the PCie slot is way smaller than the PCIe cable

 

3. A reason people use to talk price down

 

and really all could be solved by flashing a bios with higher power limit

[Yoshi Moshi]

Right so the official spec is 150 W per 8 pin power connector. When you draw more power through the connector, you don't have to worry about the conductors from your power supply to the 8 pin female connector on the card overheating? I don't understand how it's ok to draw more power than spec? I'm confused how the power supply even allows it.

 

It's a power supply that is limited by the power rating of the power supply, right? And it allocates certain amount of power to each type of connection that you can draw up to, and I thought the 8 pin connectors were limited to 150 W. I guess I just don't understand how you can go over spec without overheating? To much current going through to thin of conductors.

 

When the spec says 150 W is the limit, it's not true? I don't understand.

[Letgomyleghoe.]

1 minute ago, Yoshi Moshi said:

150 W is the limit

It doesn't say its the limit. Its the recommended spec. Now disclaimer I don't know how psus work but heres my best guess. A psu has 3 rails 12v, 5v, and 3.3v. On the 12v rail you have all the pcie and eps connectors. However much current your 12v rail is asked of the psu will delivery. It doesn't care how much current each connector is pulling. Their is no limit saying that connector x is limited to x amount of current.

[Yoshi Moshi]

5 minutes ago, Gundar said:

It doesn't say its the limit. Its the recommended spec. Now disclaimer I don't know how psus work but heres my best guess. A psu has 3 rails 12v, 5v, and 3.3v. On the 12v rail you have all the pcie and eps connectors. However much current your 12v rail is asked of the psu will delivery. It doesn't care how much current each connector is pulling. Their is no limit saying that connector x is limited to x amount of current.

That's what I'm thinking. Do you the current limit on the conductors going from PSU to GPU? There's certainly a limit on that. I know that if you try to draw to much current through to thin of a wire, it will melt and overheat.

 

How do I get a BIOS with a larger power limit?

[FaxedForward]

3 minutes ago, Yoshi Moshi said:

Right so the official spec is 150 W per 8 pin power connector. When you draw more power through the connector, you don't have to worry about the conductors from your power supply to the 8 pin female connector on the card overheating? I don't understand how it's ok to draw more power than spec? I'm confused how the power supply even allows it.

 

It's a power supply that is limited by the power rating of the power supply, right? And it allocates certain amount of power to each type of connection that you can draw up to, and I thought the 8 pin connectors were limited to 150 W. I guess I just don't understand how you can go over spec without overheating? To much current going through to thin of conductors.

 

When the spec says 150 W is the limit, it's not true? I don't understand.

A spec is not the same thing as a hard limit. How do you think power supplies have been feeding 250-300W graphics cards with a single 8-pin at the PSU end that splits into two 8-pins at the GPU end?

 

The cables are not at any risk of overheating whatsoever unless we are talking about a very cheap power supply or an extreme power draw. The wire gauge is typically more than sufficient and they are not long enough to cause voltage drop issues.

 

As far as I know PSUs do not strictly allocate power to each type of connection like you are assuming, each connection can pull whatever it needs from the “pool” of power available.

[Letgomyleghoe.]

1 minute ago, Yoshi Moshi said:

That's what I'm thinking. Do you the current limit on the conductors going from PSU to GPU? There's certainly a limit on that. I know that if you try to draw to much current through to thin of a wire, it will melt and overheat.

 

How do I get a BIOS with a larger power limit?

I'm pretty sure ocp will trigger way before a wire will melt or overheat.

For a bios with an unlocked power limit check https://www.overclock.net/threads/official-nvidia-rtx-3090-owners-club.1753930/

right now their is not a lot of them that have confirmed compatibility with certain cards to be exact zero. But if the 2080 ti is anything to go off https://www.overclock.net/threads/official-nvidia-rtx-2080-ti-owners-club.1706276/#/topics/1706276?page=1 their will be a lot of them.

[Mark Kaine]

8 minutes ago, Yoshi Moshi said:

Do you the current limit on the conductors going from PSU to GPU? There's certainly a limit on that

PSU manufactures know what overclocking is they can't just put a hard limit therefore... it makes no sense.

 

And if it's a trash PSU it'll likely blow up, melt the motherboard or whatever, it's basically the fault of whoever used a trash PSU out of spec then...

 

That's not to say high end PSUs won't blow up, it's just less likely.

[Deli]

55 minutes ago, Yoshi Moshi said:

Right so the official spec is 150 W per 8 pin power connector. When you draw more power through the connector, you don't have to worry about the conductors from your power supply to the 8 pin female connector on the card overheating? I don't understand how it's ok to draw more power than spec? I'm confused how the power supply even allows it.

 

It's a power supply that is limited by the power rating of the power supply, right? And it allocates certain amount of power to each type of connection that you can draw up to, and I thought the 8 pin connectors were limited to 150 W. I guess I just don't understand how you can go over spec without overheating? To much current going through to thin of conductors.

 

When the spec says 150 W is the limit, it's not true? I don't understand.

Many PSUs have output 8-pin sockets can be used for both 8-pin CPU or 6/8-pin GPU. The PSU doesn't know nor care the power goes to CPU or GPU. And the power limit for 8-pin CPU according to specs is 384w. Although the pin-out for CPU and GPU cables are different. Out of the eight pins, only three are 12v for GPU, while four for CPU. So in theory, the GPU cable can carry 75% of the 384w = 288w, without overloading the wire or connector pins. If the GPU is connected by two 8-pin cables, plus the PCIe socket. It will be able to draw 600w+ without exceeding the limit of the cables.

 

Although if the wire gauge is too thin, like using 18awg, while drawing full power. There might be some voltage drop, enough to cause stability issue.