What's the thinking behind an asymmetric five blade radiator fan (eg 68 Camaro)?

[Matt Bacon]

Triggered by building a '68 Camaro with a 302.... why is the cooling fan "3-blades-gap-2-blades-gap"? Does it do something to balance the engine? Or provide a cycle of a bit more and then a bit less cooling? The kit parts are clearly a replica of the real thing, rather than just short-shot, so I'd love to know what the engineering reason for it is... I think I've only ever seen it on this Camaro and a Corvette. Everything else I've built has had evenly-distributed blades (admittedly with anything between 4 and 12 of them...) which I assume makes for smoother operation and consistent stress/wear on the bearings...

best,

M.

[webestang]

I think it has to do with noise reduction. 

[Mark]

2X on noise reduction.  I once had a pair of snow tires that had an uneven tread pattern, likewise for noise reduction.  Radial snows might have been quieter, but these were bias-ply, bought as radial tires were just ramping up in popularity here.

[Matt Bacon]

I can believe it… but does an asymmetric fan really make that much difference to the “noise” when it’s hanging off the front of a 302ci VB with an exhaust system that is consciously not working hard to hush up the “thrumble” as you wind it up?

I would have thought too that a bit of fine tuning of the shape of the fan cowl and the distance between the fan and back face of the radiator would have solved any resonance issues as quickly as chopping a couple of blades out of the fan…

best,
M.

[Rodent]

52 minutes ago, Matt Bacon said:

I can believe it… but does an asymmetric fan really make that much difference to the “noise” when it’s hanging off the front of a 302ci VB with an exhaust system that is consciously not working hard to hush up the “thrumble” as you wind it up?

I would have thought too that a bit of fine tuning of the shape of the fan cowl and the distance between the fan and back face of the radiator would have solved any resonance issues as quickly as chopping a couple of blades out of the fan…

best,
M.

I was thinking the same thing. I don't know if you have ever heard a solid lifter American smallblock run, but I would think that fan noise would be about the last thing they would be worried about (unless it wasn't noise but harmonics).

[Fat Brian]

I've seen that same style of fan in other Chevy kits with other engines, it was probably just an off the shelf part for the time that got installed on most vehicles. 

[Ace-Garageguy]

Evenly spaced blades can actually get to howl like an air raid siren in extreme cases.

 

[Quiet Eric]

1 hour ago, Matt Bacon said:

I can believe it… but does an asymmetric fan really make that much difference to the “noise” when it’s hanging off the front of a 302ci VB with an exhaust system that is consciously not working hard to hush up the “thrumble” as you wind it up?

I would have thought too that a bit of fine tuning of the shape of the fan cowl and the distance between the fan and back face of the radiator would have solved any resonance issues as quickly as chopping a couple of blades out of the fan…

best,
M.

The fan on the front of a Z28 was the same fan on any other V8 camaro. And the noise from an evenly space fan blade on the highway with an engine at 3500rpm...is very obnoxious. So finding a fix for that was pretty important to many drivers. They also did not remove any blades from the fan, they are just clocked a little differently and done so in a way that the fan is still balanced.

[peteski]

On 8/29/2025 at 6:00 PM, Mark said:

2X on noise reduction.  I once had a pair of snow tires that had an uneven tread pattern, likewise for noise reduction.  Radial snows might have been quieter, but these were bias-ply, bought as radial tires were just ramping up in popularity here.

In my experience pretty much every road tire out there has uneven tread pattern to prevent the whine or resonant oscillations.  The variation might be small but if noticeable if you look carefully.

[Russell C]

... ya learn something every day. Since the old Monogram Quickslilver kit had that asymmetric fan, I've updated my rebuild/mod project accordingly.

[bytownshaker]

3 x on noise reduction, before electric cooling fan becoming the norm we would have vehicles come in the shop with loud humming noise the increase with speed. Most of he time we found seized declutching fan units. Replaced the part noise gone.

[Carmak]

This is currently a hot topic in drone development. Drone makers are transitioning to asymmetric blade designs to reduce that horrible whine they make.

I watched a great video on this and they credit GM with some on the initial work on this topic. 

[Ace-Garageguy]

20 hours ago, Carmak said:

This is currently a hot topic in drone development. Drone makers are transitioning to asymmetric blade designs to reduce that horrible whine they make...

Noise abatement in anything aviation-related, particularly military, is an ongoing subject of development.

Much of the noise coming from aircraft with propellers or rotors comes from them.

Anyone familiar with the whomp-whomp-whomp of Vietnam-era UH-1 Hueys knows you (or the other side) could hear them coming long before you could see them.

That was one reason the old Huey's two-bladed rotor system was replaced with a 4-bladed rotor on the UH-1Y.

Back to the original topic: similar to the staggered fan blade spacing on car engines, "modulated blade spacing" on aircraft, including drones, works by varying the spacing between rotor blades, so the rotor's acoustic energy is spread over a wider range of frequencies. This makes the overall sound less intense and more difficult to distinguish from background noise.

Edited September 3 by Ace-Garageguy