Be sure to check some reviews. I have seen quite some situation where avionics where fried up while using switch mode power supplies. Be carefull with them. One advantage is that on your aircraft you will also have the battery switched on when using ground power, so you have a large buffer.
I don´t compare anything. But please look at your numbers with your eye of a technician : There is no logic in these numbers when you have a motor twice as strong as the original, a reduction gear, that would produce 4 times the torque than without one. Yet your numbers quote only 10 feet torque instead of just 11 with the old setup. It just doesn´t work out. The mistake must be somewhere, not my business.
VicMZ heavy weight version – direct drive, 2240 Watt / 11.1 Ft/Lbs load
MZ light weight version – 4 to 1 gear reduction and PM, 4200 Watt / 10.0 Ft/Lbs load
The torque rating is on the output side of the gearing, so this says the output rpm at rated terminal voltage is 4200/2240 × 11.1/10.1 = 2.06 times faster. Given that the torque output is close to the same, and engine cranking torque is more or less constant with rpm, the engine will spin about twice as fast when being cranked by the starter. Given 4 to 1 gearing the electric motor will therefore spin about eight times as fast relative to the heavy weight version. Amp draw will be about 1.8 times higher, the power ratio.
Torque is the product of force and distance, so the units are lb-ft or N-m for those who prefer them.
It gets really interesting when the electric motor makes a few thousand times more power and simultaneously spins several times faster than the engine after it starts 
25A is likely to be marginal though, especially if you want to use it at Annuals and operate the flaps, the pitot, etc. And I know you don’t have a landing gear pump.
Regarding the high speed starter, I can show you a video of my starting and you will see why it draws 2x the current
So better order the 50 Amp version for € 130 more? What’s the critical part, to not destroy anything?
Critical part?
Oh, sorry. I just read that Jesse said that i should be careful not to fry the avionics. So i was wondering what i can do to avoid that, or what to look out for in the device, etc.
Well, obviously, if the power unit blows up and goes high-voltage (say 50V) it will blow up most of your avionics.
So you take a risk.
I have built two of these (do a search here; I put up the schematics and all – we also had a recent thread under “APU”, IIRC) and put in two overvoltage protection circuits, each of which would independently cut the output, at about 35V. The last one I built was 12/24V switchable (actually 14.0/28.0) and the overvoltage protection tracks that by +3V. No expense was spared. Even the box was IP67 sealed. I don’t think the Red Box products are sealed.
The “starting packs” are just two car batteries in series, plus a charger, and they cannot generate more than about 28V (chemistry limited) so they are safe, but they won’t run the plane for many hours. The “non starting” Red Box products are AFAIK just a switch mode PSU in a big box, and I don’t know how they are made. You can get some awfully cheap and nasty chinese switch mode power supplies… for example I paid 600 quid for the Lambda module I use in these; you can get the “same” for 150 quid. OTOH those Red Box things might be made really well. They are certainly priced on the Stella beer principle (“reassuringly expensive”).
But in any case the more over-specced something is, the longer it will last and the more reliable it will be. I am building a new top-spec fanless PC now (for Justine) and it will probably draw about 50W (probably a lot less). I had the option of a 300W or a 500W power supply (also fanless). So I went for the 500W one… and your avionics are worth maybe 100K?
My TB20 draws 25A with everything on except the landing gear. So I built the power units to do 40A. That is also a handy size for the switch mode PSU module.
… thank you for the clarification. I will check with Redbox. 100 K is a good estimate, and I bet the avionics warranty would not cover that one …
EDIT: I just checked the brochure for the RBPS25 and it says:
FEATURES
• Direct connection to aircraft or land vehicle. • Rugged steel case.
• Fan cooled.
• User switchable between 14V and 28V.
PROTECTION
• Overload.
• Over Voltage.
• Over Current.
• Short Circuit.
Safety & EMC (power modules) • EN60950-1: 2006
• Emission – EN55022-B
• IEC 335-2-29
Peter wrote:
So I went for the 500W one…
Doesn’t make sense to me. The most difficult part of designing a switch mode power supply is the very low load case. So by severely overrating the switch mode PSU, you will get higher loss, your PSU will run hotter, which is detrimental to MTBF, and have a greater risk of overvoltage at very light load.
