3" cross-over

[buddy]

It's only 4 cylinders worth of exhaust going through it.

If you think 4" pipe is good for 8 cylinders, consider 4" pipe has a cross sectional area of 12.6 square inches

2.25" pipe has cross sectional area of 4 square inches.

3" pipe has 7 square inches.

If looking for half the flow capabilty, just realize that you dont just drop the diameter of pipe in half.

2.25" would be appropriate for carrying 4 cylinders into a combined 8 cylinders into 3" pipe, but it is less than a third the flow capablility of 4" pipe.

So although 2.25" might make sense in this instance its not just because it only carries 4 cylinders of exhaust.

Now, in fluid dynamics, the restrictiing of the pipe doesnt mean that it negates all of the flow benefit of the previous pipe. The more area/volume behind a restriction, the more force it will exert on the restriction and the faster it can push out of the restriction. So if back pressure is 10 pounds per square inch, then 4 sqaure inches (of 2.25" pipe) of 10psi is 40 pounds, whereas 5 sqaure inches (of 2.5" pipe) is 50 pound. Once pressurized, it could speed up exhaust through the passenger manifold when it brings it back into a smaller pipe. Although, still a detriment to spool time because it will take longer to pressurize.

 

[3500GMC]

3" would be a total PITA to bolt up. But you should try it. :thumbsup:

 

[buddy]

I am not recommending a 3" crossover, I just think the 2.5" that are readily available may be worth getting over the 2.25" ones. I don't think its worth the extra hassle to customize one, unless taking out of the passenger manifold and into a separated turbo mount, like using HMMWV headers and combine them into a turbo mount.

Also, I will never use Felpro donut gaskets again, because they have an even smaller ID.

 

[turbonator]

I am not recommending a 3" crossover, I just think the 2.5" that are readily available may be worth getting over the 2.25" ones. I don't think its worth the extra hassle to customize one, unless taking out of the passenger manifold and into a separated turbo mount, like using HMMWV headers and combine them into a turbo mount.

Also, I will never use Felpro donut gaskets again, because they have an even smaller ID.

we used the hummer headers when we built the centre mount system on the pugg...... the up pipes were 2.5", and let me tell you guys, that 351 vgt spools instantly on there.... but the hummer headers are poorly built, we dissmantled them and rebuilt them.....

 

[randomid25]

If you think 4" pipe is good for 8 cylinders, consider 4" pipe has a cross sectional area of 12.6 square inches

2.25" pipe has cross sectional area of 4 square inches.

3" pipe has 7 square inches.

If looking for half the flow capabilty, just realize that you dont just drop the diameter of pipe in half.

2.25" would be appropriate for carrying 4 cylinders into a combined 8 cylinders into 3" pipe, but it is less than a third the flow capablility of 4" pipe.

So although 2.25" might make sense in this instance its not just because it only carries 4 cylinders of exhaust.

Now, in fluid dynamics, the restrictiing of the pipe doesnt mean that it negates all of the flow benefit of the previous pipe. The more area/volume behind a restriction, the more force it will exert on the restriction and the faster it can push out of the restriction. So if back pressure is 10 pounds per square inch, then 4 sqaure inches (of 2.25" pipe) of 10psi is 40 pounds, whereas 5 sqaure inches (of 2.5" pipe) is 50 pound. Once pressurized, it could speed up exhaust through the passenger manifold when it brings it back into a smaller pipe. Although, still a detriment to spool time because it will take longer to pressurize.

You said exactly what i was thinking, only more eloquently.