Angle BTDC - ignition setting

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Radovan Ch
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Angle BTDC - ignition setting

Postby Radovan Ch » Fri Jul 03, 2020 8:26 am

Hello all,

I am new in the forum.

I have one question, for Ariel NH 350, what is the angle (on degree wheel) when ignition is set to piston stroke BTDC in 1/2" (12,7 mm) (with lever set to fully advanced)? It should be around 38 degrees.

Thanks in advance for any feedback.

Radovan

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rschurr
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Re: Angle BTDC - ignition setting

Postby rschurr » Fri Jul 03, 2020 2:35 pm

May be helpful, the example says:

If you have an stroke of 80mm and you have 7mm BTDC your angle is 30°

Kolbenweg und Kurbelwinkel.jpg


Regards Rudy
1923 James Model 12 500ccm v-twin -- 1960 Chevrolet Corvette C1
1926 Douglas EW 350ccm flat-twin -- 1967 Chevrolet Corvette C2
1936 Motosacoche 500ccm single --- 1979 Chevrolet Corvette C3
1948 BSA A7 500ccm parallel twin -- 1955 Ariel Square Four MKII :!:

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cmfalco
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Re: Angle BTDC - ignition setting

Postby cmfalco » Sat Jul 04, 2020 5:03 pm

That chart is incorrect because it does not take into account the length of the connecting rod.

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Re: Angle BTDC - ignition setting

Postby john.bebb » Sun Jul 05, 2020 5:53 am

Hey guys - resolution to your issues is simple trigonometry of arcs...
You don't even need to do the maths... (you just gotta know and when to divide or multiply by 2)

https://handymath.com/cgi-bin/arc18.cgi

Enter into "Radius of Arc" (set units @ mm) 85 (i.e. Stroke spec. of NH 350)
& enter into "Height of Arc" (set units @ in) 0.5 (i.e Radovan's 1/2 inch)
Press "Calculate"
Result gives "Angle Subtended by Arc" (set units @ deg) = 63.45 degrees
The Angle in question is HALF of the Subtended = 31.725 degrees

To obtain the distance BTDC for the implied set 38 degrees: -
Enter 85 (units = mm) (stroke of NH 350) into "Radius of Arc"
& enter 2 x 38 = 76 into "Angle Subtended by Arc" (set units @ deg)
& set Units "Height of Arc" @ in/mm as desired.
Press "Calculate"
Result gives distance of 0.71" or 18.02mm BTDC to give 38 degrees advance at specified 85mm Stroke

Personally, I tweak my ignition timing by ear & feel...

Cheers, John

Please note; - the reason we have to multiply or divide by 2 - is that the Arc Calculator as linked considers the "Arc" to cover BOTH sides of TDC.

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cmfalco
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Re: Angle BTDC - ignition setting

Postby cmfalco » Sun Jul 05, 2020 6:40 am

No, again, it is simple trigonometry, but it is more complicated than a simple arc. The length of the connecting rod is required.

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Re: Angle BTDC - ignition setting

Postby nevhunter » Sun Jul 05, 2020 8:09 am

It is but so is the crank "throw".I/2 stroke It's another side of the right angled triangle. Nev

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Re: Angle BTDC - ignition setting

Postby david.anderson » Sun Jul 05, 2020 10:15 am

there is a calculator in the members area which includes the rod length.
http://www.arielownersmcc.co.uk/members ... ulator.htm
A change in rod length can have a considerable effect. A longer rod results in more dwell time around TDC, ie the piston travels slower and as such the advance must be retarded. The longer rod also has the effect of advancing the ignition slightly. The calculator indicates that 1/2" advance on the NH is about 41.5 degrees advance.
David

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rschurr
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Re: Angle BTDC - ignition setting

Postby rschurr » Sun Jul 05, 2020 10:36 am

cmfalco and David are right, but the (simplified) table is intended for homemade measurements, where the error for the usual rod to stroke ratio is very small (about 1,5 mm if you double the rod length). For example the chart shows 40,5 ° for the data set as David calculated.

But for the purist the exact formula is:

kolbenbewegung exakt.png


Rudy
1923 James Model 12 500ccm v-twin -- 1960 Chevrolet Corvette C1
1926 Douglas EW 350ccm flat-twin -- 1967 Chevrolet Corvette C2
1936 Motosacoche 500ccm single --- 1979 Chevrolet Corvette C3
1948 BSA A7 500ccm parallel twin -- 1955 Ariel Square Four MKII :!:

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Re: Angle BTDC - ignition setting

Postby jj.palmer » Sun Jul 05, 2020 1:39 pm

An accurate method, if you have the time.
Not knowing the con. rod length I have assumed 200 mm, if this method is used please ensure that the correct length is substituted.

John P.
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cmfalco
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Re: Angle BTDC - ignition setting

Postby cmfalco » Sun Jul 05, 2020 8:14 pm

rschurr wrote:cmfalco and David are right, but the (simplified) table is intended for homemade measurements,
My question is, why would you knowingly do something incorrectly when there are on-line calculators that make it just as easy to do it correctly? It's not a matter of "homemade" vs. and accurate measurements. I carried a "timing stick" on the Cannonball two years ago, and a "homemade" measurement made with it came in very handy on the side of the road when the points plate slipped. The two notches that sit on the fin show how far the piston moves for a 3-deg. difference in timing.

For what it's worth, my advice is, if you're going to do this, spend a few minutes to find the length of the connecting rod and use an on-line calculator to find an accurate value rather than use a graph that you know ahead of time is inaccurate, to determine a value to use that forever will be inaccurate.
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