I'm about to order the Melling oil pump from Jegs, but what's the difference between these two pumps?

689-M55 - SB-Chevy Stock Replacement

689-M55A - SB-Chevy Z28 Hi-Output

Does the high output mean high volume or high pressure or both?

Thanks,

Jody J.

 

I think it means hi volume but not high pressure. I'm not sure on that though.

 

Can someone clarify this?

M55:
Chevrolet: 200-262 (3.3L-4.3L) V6, 262-409 V8, standard volume, steel, cast, OEM replacement oil pump

Don't waste time installing an old worn-out oil pump when you can get this brand new Melling oil pump at such a low cost. This pump is manufactured from high quality steel with all new components and is individually hand turned and pressure tested to guarantee it'll perform as good as OEM.
$15.95


M55A:
Chevrolet: 302, 350, and 409 OEM performance engines, standard volume, steel, cast, OEM replacement oil pump

Don't waste time installing an old worn-out oil pump when you can get this brand new Melling oil pump at such a low cost. This pump is manufactured from high quality steel with all new components and is individually hand turned and pressure tested to guarantee it'll perform as good as OEM.
$29.95

I noticed the M55A is the ones that More Performance sells:
I'd like to know the difference too.