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Date: Fri, 8 May 2009 17:00:11 -0600
From: Bob White <bob@bob-white.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Pulses Per Rev and the TACH: [FlyRotary] Re: John
 Downings Tach
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Hi Ed,

"if you are just
tapping one spark plug you would get 1 spark per 360 deg."

I'm not quite convinced of that.  Every piston fires once every 720
deg.  Compression/(fire plug)/power stroke is 360, then
exhaust/intake is a second 360.   If I sample one plug I will see one
spark for 720.  From Jeff's explanation, the tach mentioned samples
two spark plug wires for a 4 cylinder 4 stroke (although I think this
is 1 pulse per 360 rather than 2).  For a two stroke, only one plug is
sampled, so still 1 pulse per 360.  At least I think that's what I
understood Jeff to be saying.   On a 4/4 engine, the tach's that work
with a coil and distributor will see two pulses per 360 as two
cylinders will fire in one 360 and the other two in the next 360.

If I still have this wrong, then I need more help! :)

Bob W.

On Fri, 8 May 2009 16:32:20 -0400
"Ed Anderson" <eanderson@carolina.rr.com> wrote:

> Hi Bob,
> 
> A four cylinder four stroke fires all four cylinders in 720 deg or two
> cylinders each 360 deg (as you know).  Therefore, normally if you are just
> tapping one spark plug you would get 1 spark per 360 deg.  However, most
> (but not all)  tachs expect two pulses per 360 deg (frequently referred to
> as 2 PPR (pulses per revolution -360 deg).  So if your tach is expecting two
> per rev and is only getting one pulse per rev, then the period between
> pulses is twice as long as what it is expecting and therefore it will
> generally indicate 1/2 of the actually rpm (if its only get one pulse per
> rev and is set up for 2 PPR).  Now on some tachs you can set the PPR from 1
> to a large number, but for most of the ones we use its generally 2, 3 or 4
> for 4, 6 and 8 cylinder engines.
> 
> A two stroke two cylinder fires both cylinders each 360 deg instead of 720
> of the four stroke - therefore you get 2 PPR for the two cylinder two stroke
> exactly like you get for 4 cylinders on the four stroke for 360 deg.  So a 2
> cylinder 2 stroke and a 4 cylinder 4 stroke will read the same RPM.  At
> least that is how I recall it works.
> 
> Ed
> 
> Ed Anderson
> 
> Rv-6A N494BW Rotary Powered
> 
> Matthews, NC
> 
> eanderson@carolina.rr.com
> 
> http://www.andersonee.com
> 
> http://www.dmack.net/mazda/index.html
> 
> http://www.flyrotary.com/
> 
> http://members.cox.net/rogersda/rotary/configs.htm#N494BW
> 
> http://www.rotaryaviation.com/Rotorhead%20Truth.htm
> 
> -----Original Message-----
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Bob White
> Sent: Friday, May 08, 2009 12:18 PM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: John Downings Tach
> 
> Hi Jeff,
> 
> Thanks for the tip.  There is something I don't understand about the
> specs for this unit.  It's good for 4 cylinder 4 stroke or 2 cylinder 2
> stroke.  If I use the spark plug wire for the pickup, a 4 stroke
> (piston) engine will see one spark per two rev.  Why does it matter how
> many pistons there are?  If it's a wasted spark system, then one spark
> per rev. just like a two stroke piston engine.  There must be something
> basic I don't understand.  What sort of mod did it need for the V6?
> 
> BTW, what is the maximum rpm reading for that unit?
> 
> Thanks,
> Bob W.
> 
> On Fri, 8 May 2009 07:48:47 -0700
> Jeff Whaley <jwhaley@datacast.com> wrote:
> 
> > I like the belt and suspenders!!
> > I am using a UMA digital tachometer 2 1/4" from Aircraft Spruce that was
> originally designed for 4 cylinder engines using an electrical impulse
> pickup from spark plug leads. I modified it to mate with the Ford V6; I have
> modified it again to connect to the EC2 driving the R1 PRI injector. The
> rotary firing system is similar to a 4 cylinder so if this tach is still
> available (Part # 10-24885) it would work off spark plug impulse couplers.
> > Jeff
> > 
> > -----Original Message-----
> > From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Bob White
> > Sent: Friday, May 08, 2009 12:14 AM
> > To: Rotary motors in aircraft
> > Subject: [FlyRotary] John Downings Tach
> > 
> > John is using a standard tach that is normally connected to a
> distributor/coil ignition system.  The straight 12V pulse used for the
> > LS1 coils wouldn't activate it.  I built an interface for him that
> involved taking that pulse as an input and generating a shorter constant
> pulse width output.  I modified the tach to trigger on the lower voltage
> signal, and adjusted the scale factor to read correctly with one pulse/rev.
> As John reported, it worked for about 0.8 hours, then quit.  He is sending
> everything back to me so that I can find out where the problem is.  I would
> like to find a tach solution that John can use without modifying anything.
> The parameters are he has a 2 1/16 inch mounting hole, the tach needs to
> read correctly with 1 pulse/rev (would be the same as a two cylinder piston
> engine with coil and distributor), should trigger directly on a 12V pulse.
> > 
> > I've found tachs on the web that mention working with distributorless or
> electronic systems, but none of them were the 2 1/16 inch type.  I found a
> converter, the MSD 8913, but on the Summit Racing catalog it specifically
> says it won't work with the LS-1 system.  It work by sensing the current
> pulses to the 12V power lead on the coil.  I don't know why it wouldn't work
> with an LS-1 coil, but it doesn't seem like a good investment when the LS-1
> is specifically mentioned as not working.
> > 
> > Can anyone recommend a tach solution that John can use with his LS-1 Coil
> rotary engine combination?
> > 
> > On another topic, John has the only system I've heard of that has the
> capability to run on either EFI or a carburettor.  John posted a picture of
> his Tailwind a few days ago.  It's a very good picture but considering the
> redundancy of the EFI/carb system, I felt that the picture was lacking.  I
> submit this edited version as being more indicative of John's desire for
> uncompromised safety.
> > 
> > Bob W.
> > 
> > --
> > N93BD - Rotary Powered BD-4 - http://www.bob-white.com
> > 3.8 Hours Total Time and holding
> > Cables for your rotary installation - http://roblinstores.com/
> > 
> > --
> > Homepage:  http://www.flyrotary.com/
> > Archive and UnSub:
> http://mail.lancaironline.net:81/lists/flyrotary/List.html
> 
> 


-- 
N93BD - Rotary Powered BD-4 - http://www.bob-white.com
3.8 Hours Total Time and holding
Cables for your rotary installation - http://roblinstores.com/