My (second) air pump died so I bought a used one to replace it. I was surprised to find that the replacement air pump had a much smaller pulley than mine - so the original belt would not work.

It seems that automatic FD air pumps have a smaller pulley than manual ones (my car is manual) and thus require a shorter belt to compensate (5PK-1310 for an automatic vs 5PK-1355 for a manual). The connector and the air pump body itself appear identical.

I did some searching but could not find any discussion as to why they would be different. I presume the replacement automatic air pump will work fine for emission test purposes?

 

maybe the automatic one spins faster and flows more air under certain conditions? The automatic one probably has a different ECU programming to account for that (clutching of the pump, switching inside the air control valve). It may not matter but it's hard to say without making a science project out of it.

 

The JDM autos have the same pump as manuals for all years. I cannot imagine the US spec cars are any different. Do you have a picture of the pump you have fitted? My guess would be it isn't an FD pump.

 

I have seen the size difference but haven't really seen why there's a size difference or what models got what air pump.

Dale

 

US automatic cars had a smaller pulley on the airpump, hence the different belt size. I believe it had something to do with moving more air for an inconsequential difference in emissions. You can swap the pumps without issues providing you have the right size belt.

 

Thanks for the replies. For reference, here are the Denso part number decals attached to each pump. (These are not the Mazda part numbers for the air pumps.)

The automatic is marked N3A4 and the manual is marked N3A3.

There is a Banzai Racing page listing the belt sizes I mentioned above (5PK-1310 for an automatic, 5PK-1355 for a manual).

 

There's probably a max RPM rating on the air pump.
Given the auto's lower engine redline, they probably sized the pulley to give it a little more emissions passing power.

My manual car passed CA smog with an auto air pump.

 

I was one of the first to actually have a single turbo engine with a working air pump that past emissions testing in Texas with a stock air pump.
It did required reinstalling a metallic cat and re-tuning for the test. This also included hotter plugs.
After installing an auto air pump, it was even easier as only had to put the cat back on.

 

i should add that the pulley will swap over between the two