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/
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