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Drive Pressure Cummins 6.7 & T4 vs T3 Turbo

Posted by Diesel Power Source on Sep 30, 2017

Here is an actual Question from a customer that was looking into our 3rd gen Swap kit for 6.7L 

Answer regarding the "Drive Pressure Cummins Myth on a 6.7 Cummins" & "T3 vs T4 Turbo"

Here are our turbo kits for 6.7 Cummins Turbo Kits.

Customer Question:

I have friends that have blown head gaskets because a T3 creates to much drive pressure and the  6.7 flows so much air that it spools a T4 as fast as a stock turbo which is a T4i. 

My diesel tech Friend said do not put a T3 on your 6.7 Cummins you are asking for problems.
Because it’s a 6.7. This is my first Cummins and I really don’t want to screw it up. Im just tired of listening to the T4/T3 thing. I guess I’m asking if this is true? About the drive pressure.
Our Answer:
Unfortunately, because there so much conflicting information, some of which is popular but wrong, it is very difficult to know what to believe. So here are the facts that we have gathered from very expensive high-tech testing equipment. 

Despite what many are saying these days, drive pressure doesn’t and cannot cause head gasket failure. Drive pressure is the exhaust coming out of the engine when the exhaust valves are open, and the cylinder pressure is released into the exhaust manifold. Even if drive pressure is 120 psi (which would be an insanely powerful truck) it’s far less then when the ignition stroke which gets into the 1000’s psi realm. You see it is boost pressure (Not Drive Pressure), engine compression ratio and timing that determines cylinder pressure, which is what the head gasket is holding (6.7 Cummins compression ratio is higher than 5.9). The good news is that unlike drive pressure, you can control your boost pressure and timing, which ultimately determines whether the head gasket holds or not. To be clear let me repeat, the drive pressure has nothing whatsoever to do with head gasket failure. Drive pressure causing head gasket failure is a myth that probably started because the new 6.7L trucks were blowing head gaskets earlier and people (so-called experts) assumed it was because of the higher driver pressure, but such an assumption is completely incorrect. The 6.7L engine runs a higher compression ratio than the 5.9L engine, so when boost pressure multiplies with the compression ratio it equates to high cylinder pressure. Drive pressure is usually ONLY 100 psi or less, which is not significant enough to cause head gasket failure. This is not a guess on our part, we have invested $100,000’s into testing technology, such as Computational Fluid Dynamic (CFD) software, that has proved this for us. Here is a logical way to look at it, if drive pressure caused head gasket failure, then every time someone applied an exhaust brake every head gasket would surely fail. The action of exhaust braking creates extremely high drive pressure (much higher than the drive pressures when the exhaust vales are open and cylinder pressure is release). 

As far as T3 vs T4 goes, we have actual CFD testing showing that our wastegate T3 .80/14 housing flows more with the wastegate shut than a T4 S300 turbine housing. The. Our wastegate can flow 22 lbs/minute more than that which makes our T3 housing actually flow the same as a T4 1.1 a/r housing. In other words, drive pressure on our is wastegated T3 turbine housing would be lower than a 0.9ar T4 housing and our housing would spool so much faster. Very similar to stock turbo on a 6.7 truck. 

Here’s a link to a video that talks about T3 vs T4:

Hope this helped!

Diesel Power Source, PMA

More FAQ: 

What causes 6.7 Cummins wastegate problems?

There is not a 6.7 Cummins Wastegate, you must be confusing it with the Variable vane.  If it's having problems, it's likely that the vane is sticking or the actuator is not working right.

What causes all of the 6.7 cummins turbo problems or 6.7 cummins vgt turbo problems ?

Mostly the actuator and vane sticking.  Run the exhaust brake during normal driving and it helps.