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For development purposes, what difference does it make if the HP is before or after losses? For that matter, what difference does it make whether you get properly calibrated HP readings or not?

I don't bother measuring torque at all for my development. All I care about is whether any change makes more or less HP. A propeller (dummy board or otherwise) makes a fine test load. As long as the load is the same, the RPM alone will tell you whether you are going in the right direction. To refine your results, use fuel flow to verify and calibrate. At 14.7 : 1 mixture, the engine (almost any engine) burns about .5 lb/hr. From that you can calculate HP pretty accurately. A mixture monitor will tell you if you are at 14.7 : 1 very accurately.

Nothing wrong with building a dyno, but IMO, that effort would be better spent on the final project. Life's just too short :>)

Tracy

On Jan 10, 2008 9:28 AM, Thomas Jakits <rotary.thjakits@gmail.com> wrote:

I have to rethink this....

Always thought, if youput the whole package onthe dyno it measures "what you get", like a chassis dyno, what you get is rearwheel-power minus slip....

What I want is a dyno to do my own little experiments, mainly motorcycle engine oriented (BMW airhead).

I understand that with a generator I need the efficiency numbers for that specific gen.

On the other hand I am not really in need of exact numbers. It doesn't matter if I have 70hp or 71 or 72,5 as long as the numbers are consistent. The idea is to see a change in specification/modification show up on the dyno...

It has to be somehow lowcost!!

Thanks for the posts!

Thomas

PS: Keep it coming if ideas appear!

On Jan 7, 2008 9:36 AM, Ernest Christley <echristley@nc.rr.com> wrote:

Charlie England wrote:
> Hi Thomas,
>
> If I understand your question, I think the assumptions might be
> reversed. My understanding is that any dyno, whether water brake,
> prop-loaded (air brake), etc. that uses the scale/lever technique is
> actually measuring raw engine torque. If you want to measure the
> losses in the gearbox and/or prop, you would measure thrust, convert
> to HP & compare to HP calculated from torque/rpm.
> If you are proposing measuring the output of a generator, you would be
> ignoring the losses in the inefficiency of the generator. That would
> be the equivalent of measuring only thrust in the previous example.
>
> If I'm in error, hopefully someone more knowledgeable will jump in.
>
> Charlie
>
>
> Thomas Jakits wrote:
>> Ooh yes! Pleease!
>> thjakits@gmail.com <mailto: thjakits@gmail.com >
>>
>> Thank you very much!
>>
>> Question to a possible "DIY/cheapo"-dyno:
>> I saw various plans/models where one bolts up the
>> engine/PSRU/club-prop and measures power via rpm x momentum (via
>> bathroomscale/lever). This gives me the overall hp - incl any losses
>> from the PSRU and inefficiencies fomr the prop.
>> What a about a pure engine-dyno? Drive a waterpump/generator (where
>> do I get a cheap 300kw generator??) ???
>>
Maybe use a prop bolted directly to the output shaft. That gets around
the gearbox losses. A ducted fan would also be a little safer. Maybe
Perry can convert his old ducts and fans into a dyno business?

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