Full lift timing method.

Road-race engines and ancillaries - general discussion
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4v6
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Full lift timing method.

Post by 4v6 » Fri Jun 01, 2018 7:22 pm

Hello all, bit of a daft question probably but I cannot seem to find an answer anywhere regarding it.

As I understand it, the full lift timing method consists of adding together the first two figures on your cam iming information card, such as 25 65 65 25, and then adding 180 degrees of crank rotation to give duration.

In the example above you would have 270 degrees of duration, and to find full lift position you would divide that by two then subtract the first number (inlet opening) to give the number of crank degrees (135) minus 25 = 110 atdc...if Ive got that right.

The question regards the subrtaction of that first number. Why?
Surely full lift would be half the duration?
What am I missing?
I have sent an email to a cam company but had nothing back hence posting it here.

Any help gratefully appreciated to satisfy my addled brain.
Tony Warren. GC #96.

TomLouwrier
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Re: Full lift timing method.

Post by TomLouwrier » Sat Jun 02, 2018 10:20 am

Hi Tony,

You are right that full lift happens at half the duration. That is: in cam degrees.
But since the lifting starts at 25' btdc crank degrees, the cam degrees have a 25' offset (which is cam timing by definition...).

So when you subtract the 25' you place full lift (cam timing) correctly related to crank timing. And that gives you full lift in crank degrees.

thank you!
Tom
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4v6
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Re: Full lift timing method.

Post by 4v6 » Sat Jun 02, 2018 6:59 pm

Thanks for your help on that Tom, a lot clearer now!
Best wishes, Tony.
Tony Warren. GC #96.

Guy Croft
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Re: Full lift timing method.

Post by Guy Croft » Mon Jun 04, 2018 7:11 pm

subtracting the lowest number gives you FL relative to TDC for inlet and ex cams rather than just the geometric angular centreline. ..

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Guy Croft, owner

4v6
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Re: Full lift timing method.

Post by 4v6 » Mon Jun 04, 2018 7:40 pm

Thank you Guy, now makes better sense to me.

Best wishes, Tony.
Tony Warren. GC #96.

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