MG1 rpm ??

I think it's the other way around. Bob said he is using bigger tires, not smaller. So for each revolution the car moves more than it knows. You are registering less miles than the actual miles.

 

Yeah, good question, not at all a senior moment

I agree with your assessment that you'll have a shorter moment arm, not sure about the "more linear force" though...my physics are not that good

My guts tells me that you'll have a faster 0-60mph time with smaller wheels. The car will be under maximum load all the time, and under that particular conditions the shorter the better.

Just don't forget to have a portable GPS or some other device to let you know the actual speed while clocking, since the speedo with smaller tires will reach 60 mph reading before you actually get there.

 

Someone doing a search and landing on this thread may find that link has gone dead. Here was the original, at the Wayback Machine:

TOYOTA: Company > Technology

That gave the MG1 RPM limits for Gen 1 and Gen 2 (6500 and 10000, respectively). Those can also be found on p. TH-26 of the 2004 New Car Features manual. It did not give the corresponding limit for Gen 3.

Corresponding stats for the Gen 3 (P410), Prius c (P510), and Gen 4 (P610) transaxles and others can be found here:

https://openinverter.org/wiki/Category:Toyota

where it says the Gen 3 MG1 top RPM is 13500 (like MG2), and Gen 4 17000 (again like MG2).

Interestingly, MG1's power capacity seemed to be higher in Gen 3 (33 and 42 kW, repectively, according to ORNL), but just 23 kW in Gen 4.

 

Too late to edit the above, but in the last paragraph I meant "higher in Gen 2 and Gen 3" ... and also "respectively"

 

That's interesting; thanks! I haven't known the speed limit for MG1. That limit of 13,500 RPM is a long way from the top speed it's allowed to spin backward during coasting with engine stopped, only about -5500 RPM. The reason for that might have more to do with electrical voltage it generates than with centrifugal stresses.

The 42 kw limit for Gen 3 seemingly wouldn't allow the claimed peak power engine speed of 5200 RPM below about 67 mph.

 

Hmm. The way it looks to me ... the published engine specs are 73 kW at 5200 RPM and 142 Nm at 4000 RPM.

Eyeballing this graph, I'd say the torque's gone down from the 142 Nm peak at 4000 to maybe 130 at 5200.



To deliver 73 engine kW with no more than 42 kW through MG1, 31 kW would have to follow the mechanical path.

The torque at the PSD ring is always 78/108 of the engine's torque, so at 130 Nm engine torque, that would be 94 Nm at the ring.

[Edit: wait, I've done this wrong ... fixed]
For that to be 31 kW, the ring has to do 3149 RPM.

Rather than looking up final drive ratios and tire revs per mile, I'll say in a recent test run my ring did 1643 RPM at 36 MPH.

So 3149 ring RPM would be just under 69 MPH in my Gen 3.

Looks like I agree.

Makes me wonder what they're thinking with even lower MG1 power limits reported for Gen 4. Slightly higher [edit: nope, see below] final drive ratio there, but not a huge change. Haven't calculated how fast you'd have to go there to use full engine power with only 23 kW at MG1.
[/edit]

[Another edit]
I just watched Prof. Kelly's P610 deep dive, which is probably where those figures on openinverter.org came from, and when he gave the 23 kW power limit for MG1, he specifically said that was for Prime. But I never saw where he gave any different figure for non-Prime. So I wonder if there's a difference there.

There are differences in the Prime and non-Prime final drive ratios. On openinverter.org, those are given as 3.947 and 3.476, respectively, but I see from the video that those numbers only reflect the teeth in the differential ring and its drive gear. If you also figure in the 53:65 ratio between the counter driven gear and the PSD ring (the ring is the one with more teeth!), the way I would do it, that comes out 3.218 and 2.834 respectively. So the ratio is not higher than Gen 3's, in fact it's a teensy bit lower for Prime, and considerably lower for non-Prime.

So it seems like the road speed to use full engine power with only 23 kW at MG1 would be strikingly high.
[/another edit]

I was reading an old post recently where someone explained that as the limit remaining unchanged in the firmware from Gen 1 days when MG1 maxed out at 6500 RPM.

It seems like an odd thing for Toyota to overlook, but I guess I don't have any better story.

 

Interesting approach.
By scaling a full-screen version of that graph, I came up with 133.1 Nm at 5200. Or about 134, based on the claimed 73 kw@5200.
There are 3.2685 (=(55/54)×(58/22)) ring gear revolutions per wheel revolution in Gen 3.

 

My notes have the same 55/54 for the PSD ring to counter driven gear part, but 77/24 for the counter-to-differential part, working out to ~ 3.268. I got those numbers from the attachment here:

Gen III MG2 RPM vs MPH | PriusChat

I just used the observed ratio from my test run last week because it was faster than looking that up plus trying to find the revs per mile of three year old Altimax tires.

 

You are correct, 77/22 is the final drive ratio. I obviously was looking at the correct pair of tooth counts when I calculated the same 3.2685... overall ring-to-wheel ratio you got, but was looking at a different pair of tooth counts (specifically the MG2 reduction) by mistake when I tried to enter those numbers in post #28. Not sure how such blunders happen. Sorry![/QUOTE]

 

I had done pretty much exactly the same kind of thing in the [edit: wait, I've done this wrong...] part in #27. Don't know how it happens, but it does.