im fabbing up a custom v mount and am in the final planning stages. i've been searching whats the best piping size on this forum and there is no definitive answer(which i assumed would be the case), but also all the info is super old.

i have stock ports with an aspec 35r with a t4 1.06 so its 2.5" and i have the greddy elbow which is 2.75". i have no intercooler as of yet so i can go either way really.

ive seen cfm numbers for both but i'd like to hear if there is any practical wisdom i.e. experience floating around to see if there is indeed a difference.

i'm sure most will think there wont be much difference but under pressure, i know there is quite a difference in cfm as reflected by the numbers, but is that better? i want a responsive setup with good topend as well which is why i went 35r and a 1.06 but i know this is only part of a properly built single turbo system.

ive read larger piping increases midrange power with no ill effects on the top end but what about the low end? am i going to sacrifice a little spool?

thanks in advance all!

 

I run 2.5 all the way around with my 35r. I did a bit of research and saw no gains for larger diameter piping. People think bigger is better, but seems not be the case in all scenarios. Especially with a medium framed turbo like we have. My peak power of 455 was @ 7500 rpm. The turbo wanted more on the top end. I do have a heavy street port though. 1.06 hotside as well. I would have loved to crank it up to 21 or 22 psi to see the layout , but ran out of fuel. Gonna get some Id 20000's soon and try it out. 28+psi and giant turbo I would bump it up...

 

just to be clear thats 2.5 od correct?

 

Volumetric flow rate should be corrected for temperature and pressure, so this value will be same, no matter boost pressure. Engine displacement and VE% is what dictates volumetric flow and if the elbow has 2.75″ diameter, there is no reason to go bigger.

To small piping would cause slight pressure drop at peak flow, nothing what couldn't be solved by slightly higher boost, to large piping (large volume) will cause lag and transient response will suffer.

When you look at "real" turbo race cars or turbo F1, you will see that pressure piping is tiny for power levels they are producing...

 

I was thinking of going the M Spec replica that RX7Store sells but the pipping is 3". Inlet/Outlet for the IC is 3". That too big?

 

yes

 

I've driven G's car on low boost (13ish psi) and it's a beast, pulls very hard up top.... the tach needle just swings from 6500 to 8000 like nothing I've ever seen. I may be biased, but I don't think his 2.5" IC piping is holding him back

 

well i did some measuring of pipes i had lying around from other fab jobs and the stock 2.75" being plastic measures roughly 2 9/16" at the largest whereas the 2.5" aluminum is about 2 3/8", so its roughly 1/8" difference in inside diameter. probably not a huge deal either way now that i measured these pipes.

are there any opinions as to the larger pipe having a better midrange?

thanks for all your opinions based on experience. i appreciate it.