Oil pump gears. Helical or Straight cut

[AC sports]

Rebuilding a few oil pumps this weekend for a batch of 3 Fiat twincam engines I'm starting on, and came across an interesting discovery. The later model oil pumps have 10 straight cut teeth per gear, along with a relief slot in the end of the gear, visible at 3'0 clock.
The earlier gear pumps have 9 helical teeth and no relief.
The housings I've modified to be interchangeable, but unfortunately I only have spare NOS helical gears.
Now the question is why did Fiat change gear types?
What's better suited to high rpm? Helical or straight?
I've heard many arguments for both....no harm in hearing opinions here either.
Both types run around 0.004' gear to wall clearance, end float I cut to 0.002' as they vary quite a bit.
I think the early 9 tooth type carry more oil between the teeth per revolution. Noise doesnt seem to be an issue with the straight type either.

[peejay]

Helical gears would put a thrust load somewhere in the system.

Slots like that are good for lubricating the shaft they ride in. The engines I play with have gerotor type pumps and the shaft is located by a bore in cast iron. An old racer who'd been building these engines literally since I'd been born advised to cut a small notch between the pressure side of the mounting face and that bore, for positive lubrication.

I haven't had a problem yet, but I'm sure I will someday, and when that comes I'll add that to my list of Weird Old-Guy Habits, along with never installing a used transmission without disassembling it and Loctiting all of the internal bolts.

[PackardV8]

FWIW, both work. Several obsolete engines, Studebaker V8s among them, were designed with helical gears and then later switched to straight gears.

Chevrolet V8s ran just fine with straight gears for more than fifty years and then Melling began offering a helical gear pump.

[Walter R. Malik]

Oil Pumps with helical gears will have less pulsing amplitude than straight cut gears and tend to not "ratchet" as much.
Output is very close to the same.

[BCjohnny]

The original design was Lampredi from the mid 60s, so may have incorporated some 'theoretical' ephemera .... ex Ferrari liking that sort of thing

The later design most likely came from a production rationalisation ...... maybe gear cutting capacity, they look hobbed or shaped, not sintered

I'm not sure I would be worried about any of it, unless experience proves otherwise ......