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Date: Fri, 08 May 2009 19:31:07 -0500
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Pulses Per Rev and the TACH: [FlyRotary] Re:
 John Downings Tach
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The water is a bit muddy because of all the different ignition schemes.

On a 'normal' 4stroke engine, if you're using a 'wasted spark' type 
system, then there really is a spark for each 360 on each plug. The 
double ended coil fires both plugs; one is igniting the mixture for the 
power stroke and the other spark is simultaneous but 'wasted', firing 
into the exhaust gases near the end of the exhaust stroke.

Charlie

Bob White wrote:
> 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
>>
>>
>>     
>
>
>