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Ernest,
I've attached a couple of plots of MAP and RPM data collected from my 13B engine stand which has a belt driven centrifugal supercharger installed. The system is set up to maintain the pressure in the duct between the supercharger and the throttle body at about 3" Hg above that in the intake plenum. Without the supercharger, the plots are very similar except that the highest map is 23' Hg (7200 ft altitude on the ground here) with a corresponding highest RPM.
When you have your system all set up, I would expect your data would look similar to mine.
Steve Boese
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From: Rotary motors in aircraft [flyrotary@lancaironline.net] On Behalf Of Ernest Christley [echristley@nc.rr.com]
Sent: Friday, June 17, 2011 9:45 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Blower does work
On 06/17/2011 06:10 PM, Ed Anderson wrote:
> Hi Ernest,
>
> I think Tracy's question might have it roots in the following.
>
> First, the manifold pressure really did not change much with rpm.
> Typically you would see an engine under load go from lets say 12" Hg
> at idle to 29.8" Hg at WOT which in your units would be from approx
> 40KPA to 100 KPA at WOT. Without the blower, it looks like most of the
> orange was below 60 KPA although I think I see a bit up around 65
> KPA. So that would indicate approx 13" Hg at idle to around 19.0 "HG
> at your highest - hardly what you would expect to see if the engine
> was loaded and turning those rpm.
>
> So that indicates your blower is feeding a small throttle opening and
> an engine that is not processing too many CFM of through put. The
> blower appears to be producing approx 15 KPA above the orange or an
> increase of approx 4.5 " Hg. However, the question is with less
> restriction (WOT) would that pressure profile increase due to higher
> rpms or drop due to less restriction?
>
I'll be working on getting more load this weekend, but in the meantime I
probably should call the previous post a false alarm.
This even I went set out to run a test that would be more methodical
than the previous "grab whatever is in the logs" method. I set the
engine up in three situations: with the blower charging the intake,
with the air filter turned around to pull ambient air, and with no air
filter at all. I attached a picture showing how I turned the filter
around. (It should also be apparent why my front rotor is getting
cheated as far as air supply goes. The air just goes rushing past it.)
For each configuration, I made a run, trying to hold the throttle at
every 500 RPM on the way up for 5 seconds (1500rpm, 2000rpm, 2500rpm,
etc), then did the same on the way back down. I made a datalog for when
the engine was warming up, with the blower active, with the filter turn
around, with no filter (filterless), and again with the blower in
action. That graph is attached. Basically, this shows the same MAP for
every RPM, except for the filterless run, which has the MAP jumping all
over the place. I don't know what the deal was with the data I got last
night, but given the strict control and short time frame under which it
was taken, I trust this data much more.
Upon further reflection, I think I'm may be looking at the wrong thing.
Given the constraints of such a light load, the RPM is going to follow
whatever the MAP is. Is that a "No duh!" statement? Looking at the
graphs, I think what would be more telling is RPM vs throttle opening,
or MAP vs throttle opening. What say you all?
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