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 Post subject: Re: Parasitic reduction: Pro/Con
PostPosted: Mon May 07, 2012 4:46 pm 
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DTAS54 wrote:
It is almost intrinsic to the idea of this pulley discussion that we are indeed turning the alternator much faster than we need to at the rpm we race. Remember, at idle these alternators can generate enough current to run AC/radio/electric fans blah blah blah.



Yes they can, but my query is how much power/torque does it take to drive them at say 1000rpm generating 50A, and how much at 10,000rpm ?


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 Post subject: Re: Parasitic reduction: Pro/Con
PostPosted: Mon May 07, 2012 6:29 pm 
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Location: Montreal, Canada
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The formula to determine HP is HP= Torque X RPM/5252.
5252 is a constant. Search google and you'll get the story of where it comes from.
If 5252 is a constant and the torque required to turn the alt to get the 50amps is also constant (same electrical load, just different RPM, right?) then it goes without saying that increasing speed will increase the HP demand.
Lets try this then; let's imagine it takes 5ft/lbs to turn the alternator to generate 50 amps. Hypothetical number, just for shits and giggles. Then at 1000RPM we get:
5ft/lbs X 1000RPM/5252= 0.95HP
And at 10000RPM we get:
5ft/lbs X 10000RPM/5252= 9.5HP.
It takes 10X more HP to spin the same alternator pushing the same 50 amps it would seem... Very simplified way to see it but you get the picture. It probably takes more than 10X because of drag but no need to make this anymore complicated.
If I made a mistake in the math I encourage ANYONE to cerrect me as it's been a while since I last played with this :lol:.


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 Post subject: Re: Parasitic reduction: Pro/Con
PostPosted: Tue May 08, 2012 1:43 am 
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Joined: Wed Jul 27, 2011 5:30 am
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Location: New Zealand
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per revolution it is generating less current though. I think you are looking too deeply into it, though there may be some weight in it. I'd rather cut weight out of my car, and reduce gearing for reliability more than parasitic loss. I am enjoying this thread though, definitely :)\

I feel changing belt types at the same time as changing gearing will net a greater gain than just pulley sizing, from what I can gather from those posting here - I'd be interested in info on different belt options.

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 Post subject: Re: Parasitic reduction: Pro/Con
PostPosted: Tue May 08, 2012 4:37 am 
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Simon P. wrote:
Slightly off topic but does anybody remember how Mazda got in sooo much trouble when they falsely advertised the (then new) RX8 as having 250hp when in fact you couldn't dyno one above the 230ish mark?! . Maybe they disconnected all the belts and ran the engine stripped as mentioned by VR6Turbo...


Must have been that and a factory fresh unit with no lip seal wear at all :lol:

They were never quoted as 250hp in Europe. Only 220hp IIRC. I've never seen the fascination with rotarys tbh. I applaud Mazda's skill and perserverence in getting it to work but honestly, it sounds like a vacuum cleaner and is horribly lacking in torque below 6000rpm and it absoutely drinks fuel!

Anyway, sorry for going OT!


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 Post subject: Re: Parasitic reduction: Pro/Con
PostPosted: Tue May 08, 2012 10:03 pm 
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VR6Turbo wrote:
Must have been that and a factory fresh unit with no lip seal wear at all :lol:

They were never quoted as 250hp in Europe. Only 220hp IIRC. I've never seen the fascination with rotarys tbh. I applaud Mazda's skill and perserverence in getting it to work but honestly, it sounds like a vacuum cleaner and is horribly lacking in torque below 6000rpm and it absoutely drinks fuel!

Anyway, sorry for going OT!


Mazda NA got caught over-rating the HP output in some print adds. People bought the car and obviously started modifying them and going to the dyno only to find out they barely got 180RWHP out of them when stock... Made a "big" hoopla on this side of the pond for a short while. Suddenly all jap cars had misleading HP ratings :lol:
Your comments about the rotary sadden me though :( Yes, the RX8 was a heavier than the RX7, undepowered 4 seater with an identity crisis. But I'm from the RX7 crowed initially, and will always be a rotary fanatic. 3 moving parts, very compact and light, revs freely and responds so well to boost, there's just so many reasons rotarys were banned in most racing series.
My race car is a FC chassis, Haltech run heavily boosted Cosmo 13B powered toy (sorry DTA, so much support from Haltech for these...). If I could only find the time to put Humpty Dumpty back together though... ;)
End of off topic Mazda stuff from me.


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 Post subject: Re: Parasitic reduction: Pro/Con
PostPosted: Wed May 09, 2012 3:39 am 
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Simon P. wrote:
My race car is a FC chassis, Haltech run heavily boosted Cosmo 13B powered toy (sorry DTA, so much support from Haltech for these...).


Did you see this on DTA's homepage? - http://www.dtafast.co.uk/news/latest_news.shtml#Mazda

Horses for courses really. We all have our favourite engines :D

And that concludes my OT Mazda chat also!


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