Cold Air Intake 2.5" or 2.75"?

ASSISTANCE Hybrids & Advanced Aftermarket Engine Performance
1

I replaced my old cheap ebay cold air intake with another one (Spyder CP-402) which has a CARB #, so I don’t have to keep swapping it out with my stock one each time I get smogged. However, I noticed the pipe diameter is smaller at only 2.5" compared to the cheap no name ebay one. Unfortunately, I can’t gauge the performance difference because I swapped it at the same time I did my B18C motor swap, and I sold my old CAI. I noticed the AEM 21-402 intake model is 2.75", so perhaps that is the ideal diameter? (I bought the Spyder brand because it was a lot cheaper than the AEM one, although now I’m having second thoughts).

So the question I have is 2.75" better than 2.5"? I would think 2.75" is better for more airflow, however I read somewhere a smaller diameter would give greater velocity to the airflow. Does anybody have a definitive answer to this? If 2.75" is better then I may just go back to a cheap ebay one and use my old CARB sticker from the Spyder intake.

2

Both pipes are bigger than your throttle body, assuming it is stock. Your TB and intake mani will be bottle-necks… So either one will yield basically the same gains.

As a bolt on, gains are fairly minimal.

It’s like upgrading your header yet having stock cat/exhaust… Not a big gain because you’re still getting bottlenecks.

3

Everything I’m seeing and hearing from people that tune a lot is that bigger is better. But you’d have to dyno it to know for sure. It sucks that there isn’t a 3" CAI for the DA. Personally I think custom is the only way to go, but if all you have available is the 2.5" or 2.75" then I’d pick the 2.75" for sure. But if you can’t tell the difference, then who cares?