I replaced the clown-car 151R battery with a more reliable 51R (Everstart, Walmart, $150).

Here's where it gets crazy: I used to here the VTC start-up rattle maybe once or twice a week (2015, 64,000 miles). Now though I've only heard it twice in 6 weeks. I'm thinking the oil pressure builds up quicker because the starter spins faster (Mobil1 5W-30, as always). I'm not declaring victory, but I can't think of anything else because nothing overwise has changed.

 

In the 2015 aren't you supposed to be using 0W20 ?
And yes, I can believe that with the more powerful battery oil flow to the upper area of the engine is faster which might reduce VTC rattle.

 

Flexing the battery cables and cleaning battery terminal corrosion (which happens when swapping any battery) could have some positive effects outside of the battery swap. I'm not defending it - I also think the 151R is undersized. Maybe it was the smallest available at the time, but my CRX came with a group 51 and its only substantial electrical accessories are the HVAC blower [30A fuse], halogen headlights [4x 10A], and rear defroster [20A].

That said, and for the sake of science-y argument, could folks test their engine grounding? Apologies if I'm thread-jacking. Data points with and without VTC rattle, with 151R or 51R would all be helpful, just post what you have - Year and trim, mileage, battery (brand and age would be nice), rattle/no rattle, measured voltage.
Test procedure is to use a multimeter to measure DC voltage from the engine block to the negative battery terminal while the engine is cranking over. These points are already electrically connected by the ground cables, so there is "zero risk" to your car. In quotes because some people do craziness. Keep the test wires away from moving parts and you'll be fine. Alligator clips for your test leads will simplify hookup greatly. You don't need an expensive multimeter for this (mentally fortify yourself if you google mine) though a min/max function will help greatly - without it you will need to prop up the meter so you can see it while cranking the engine, and catch the highest value it displays (maybe record it with a phone?).
Anyway: Set meter up to measure volts DC (line with dashes under it, negative lead in the COM port, positive in the V/diode/capacitance/anything-but-amps port). Connect meter negative to the battery terminal clamp, meter positive to the cylinder head lug. Meter should read zero or very close to it:


Start the min/max function if your meter has it. Crank the engine. Note that before cranking the engine, your meter will register changes in the car's power usage from things like opening the door and pressing the brake pedal - this is normal, though the value from cranking the engine will be much larger. Turn off the car, check your measurement.

2020 EX, 78k miles, original (about 5 years old) Johnson Controls 151R, no VTC rattle, 0.424V peak:

 

Yes, 0W-20 is recommended, but not required. I use a -30 because to help with the GDI fuel dilution.

 

I upgraded my battery and found the same result! Also found that when my car is parked slanted nose down it will rattle but with nose up doesn't rattle. Also fill the oil to the top of the stick also reduces or eliminates the rattle.

 

Your VTC issues have nothing to do with battery voltage. That is an internal engine problem that doesn't connect to the electrical in any way.

Same goes for parking your car up or downhill. Complete nonsense.

It's an internal oiling issue cause by poorly designed mechanical parts. lol

 

VTC is ECM and hydraulically controlled. Electrical power is needed, oil pressure is needed. I was trying to get people to rule out the electrical side by testing the ground path during the highest current-draw period of operation - engine cranking. I suppose testing how low the battery voltage drops would be wise as well, but standard battery-health testers do that.

Parking the car on a slope might help retain oil in the VTC passages/actuator rather than draining back to the sump. "Might" because I haven't torn down one of these engines to trace the passages. Oil pressure pushes a bad VTC actuator from a free-floating position into a controlled position, ending the rattle. Air needs to be pushed out before any meaningful force can be applied by the oil. The actuator's faulty return spring should be holding it in a certain position when there's no pressure, but it's faulty.

 

If that was the case than the fix-all from Honda would be a simple new battery replacement. Which it isn't... It's a poorly designed internal mechanical spring issue.

 

OP isn't claiming a fix, they're claiming improvement. Look up "mitigate".

 

Two experiences vs no experience... interesting. Anyways, to add to your experience- I've noticed in cold weather, my VTC rattle has mitigated also. It's almost hardly there- I would assume due to "vacuum" hold on where the oil is vs warm weather. Almost 192k.

I've been looking at getting the battery upgraded as I have not even a 2 year old small version which if I leave the low beams on for even 10 mins - voltage drop is so bad it is literally impossible to start... Sigh... Wonder why they came up with this "idea". (I'm using a Advance 'Gold' battery mind you). Anyways, thanks for sharing. This might be the final nail in the coffin for upgrading. Cheers!

 

For folks with post-refresh Fits (2018+), Honda changed the battery tray to be 151R-exclusive. You'll want one of the 2015-2017 trays for the 51R conversion: part number 31521-T5A-000.

 

More like misinformed...

 

Ah that explains the two different types of trays. However you don't need to get the larger tray for the larger 51R battery - you can simply trim the edge ridge to modify the smaller tray to accommodate the larger battery as many have. Not ideal but it works.

 

It won't retain acid leakage, which is 90% of the reason you have a plastic battery tray in the first place. The other 10% is to reduce abrasion (paint removal leading to rust) on the support frame under the tray - The tray has lugs on it that keep it from sliding around on the battery support.

 

I have never had any problem with acid leakage. A couple times a year I use a damp rag to wipe off the top of my battery and even so I rarely see any sign of any acid on the battery top. My battery and tray are firmly fastened down and don't slide around (although in an accident they might).

 

Try mixing up a solution of water and baking soda, and spray it around the base of the battery. If there's acid, it will fizz just like a vinegar/baking soda reaction. If there's no acid, the baking soda won't hurt anything.

 

Yeah I don't see acid spillage to be a problem. Back in the day with screw caps and regular water filling, maybe but not with today's so-called maintenance batteries.

Anyway, here's a pic of what has to be hacked off (key for size reference):

 

At $40 on Amazon - cutting the small one is definitely ideal, lol. Shoot I'll just paint cardboard at that price 🤣

 

Or $17 on ebay. LOTS of gouging to be found on Amazon these days. I recently bought an Ancel scan tool to use with my GK - $270 on ebay, $400 on Amazon.

 

Wtf... Welp.. Ebay it is then. Thanks for the heads up!