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Date: Fri, 20 Aug 2004 11:50:11 -0400
From: Mike Robert <pmrobert@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: EM-2 MAP readings
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Russell Duffy wrote:

> There is likely some loss in the long TB barrel- I think it is about 4 
> ½” long. My barrels are 44 mm dia. What are yours?
>
> I had measured them to be about 1-5/8, which is probably 42 mm. Each 
> barrel feeds a single runner that's also 1-5/8" ID. That runner later 
> splits into two to feed the two ports of the rotor. I could put a 
> small plenum between the TB, and the runners, to allow each runner to 
> pull from both barrels, but I can't see how that will do much for me.
>
> You can see my plot of MAP vs RPM at 
> http://members.cox.net/alg3/Dynamometer%20test%20report.htm
> and scroll to near the bottom. It went all the way down to 25” at 7000 
> rpm. I’d be interested in what TWM has to say.
>
> Thanks for the link Al. I've tested this with nothing ahead of the TB, 
> so I can't blame it on any upstream restriction. I looked back at some 
> old numbers, which were with a larger 44 mm TWM TB, and I was only 
> getting 27 inches or so MAP at WOT and low altitude. That makes me 
> think, even more, that the low MAP numbers are a result of the port 
> placement of the TB. I doubt I'll be able to call TWM today, and I 
> further doubt they'll be willing to admit to any problem if there is 
> any, but I'll let you know what they say.
> Rusty (off to Panama City)

OK, here's some data from a ground flyer (car guy) - I use 50mm Weber 
Alpha TBs on my 12A. The TB ports have routinely reported several kPa 
less than MAP taps (which are teed together) placed closer to the engine 
in the manifold itself. I've been told that the TB MAP ports are mainly 
to be used for balancing the idle vacuum and not to be used for EFI MAP 
sensor purposes. My personal experience upholds this theory/line of 
reasoning. I'm thinking that the TB ports are affected by 
laminar/nonlaminar/turbulent flow in the TB bores influenced by throttle 
plate position/presence/etc. At certain RPM ranges, I get MAP readings 
in excess of atmospheric - precisely when the intake is REALLY howling; 
no more than 102 kPa in any case. I'm at sea level, my MAP sensor 
usually reads 98 kPa ambient. I'm sure the excess kPa is either due to 
intake tract resonance or artifact due to MAP sensor port 
placement/interaction with lots of mixture flowing.

For your consideration,

-Mike