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TDC 2* ATDC
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norris
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PostPosted: Wed Jul 18, 2018 10:21 am    Post subject: TDC 2* ATDC Reply with quote

I decided to check my TDC via the piston stop method. Turns out that TDC is at 2* ATDC on my JayCee pulley. In trying to figure out why, I remembered that the adjustable cam gear is set @ 2*.
Can you guys confirm this reasoning?
Does this mean that if I want to set my total advance at 28* that I need to use the 26* mark on the pulley?

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Coyotemutt
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PostPosted: Wed Jul 18, 2018 11:25 am    Post subject: Re: TDC 2* ATDC Reply with quote

norris wrote:
I decided to check my TDC via the piston stop method. Turns out that TDC is at 2* ATDC on my JayCee pulley. In trying to figure out why, I remembered that the adjustable cam gear is set @ 2*.
Can you guys confirm this reasoning?
Does this mean that if I want to set my total advance at 28* that I need to use the 26* mark on the pulley?


Yep. If you're 100% sure the crank is slightly wrong anyway.

I do wonder how often TDC isn't where it should be on new crankshafts. The way my car responds to timing changes makes me think I have a crank that is wrong in the other direction.

I think I'll check that today
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Coyotemutt
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PostPosted: Wed Jul 18, 2018 11:40 am    Post subject: Re: TDC 2* ATDC Reply with quote

Yep! As expected, mine is about 5 degrees BTDC. Not sure if it's the pulley or the angle of the woodruf key. In any case, this is why you confirm TDC rather than take it on faith!
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Bugsy61
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PostPosted: Wed Jul 18, 2018 1:06 pm    Post subject: Re: TDC 2* ATDC Reply with quote

If you should decide to correct for cam to crank relationship, would be interesting to hear from both of you as to how much difference it makes in driveability of the car. Especially since one is retarded and one is advanced.
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Dale M.
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PostPosted: Wed Jul 18, 2018 1:15 pm    Post subject: Re: TDC 2* ATDC Reply with quote

Easiest thing to check would be pulley... IF pulley spot on, the crank is ground a bit off....

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Dale
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iowa vw
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PostPosted: Wed Jul 18, 2018 1:16 pm    Post subject: Re: TDC 2* ATDC Reply with quote

Is there any way to confirm the crank and/or cam is accurate at TDC as a short block rather than having to assembly the engine fully?
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Dale M.
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PostPosted: Wed Jul 18, 2018 1:24 pm    Post subject: Re: TDC 2* ATDC Reply with quote

IF you have #1 piston & cylinder in place and some sort of hold down on cylinder, the positive stop method on #1 will work...

This could also be used as "positive stop tool" (on short block)..

http://www2.cip1.com/ProductDetails.asp?ProductCode=ACC-C10-7090

Dale
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iowa vw
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PostPosted: Wed Jul 18, 2018 1:39 pm    Post subject: Re: TDC 2* ATDC Reply with quote

Dale M. wrote:
IF you have #1 piston & cylinder in place and some sort of hold down on cylinder, the positive stop method on #1 will work...

This could also be used as "positive stop tool" (on short block)..

http://www2.cip1.com/ProductDetails.asp?ProductCode=ACC-C10-7090

Dale


Ok that makes sense. Put on P/C #1 and hold down. Then put the pully on the see if TDC show on that to see if the pulley is incorrect. But what about determining if the cam is not on TDC? I must foggy in the mind
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mark tucker
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PostPosted: Wed Jul 18, 2018 1:59 pm    Post subject: Re: TDC 2* ATDC Reply with quote

the cam has nothing to do with tdc. the cam has nothing to do with the pully's marking. Ive seen assys out over 7 degrees. a little hear and a little there the same way and it can be a lot. thus the reason for degreeing when building.it's not just pully and crank or key. a decked case thats not decked square will change tdc too!!! also some pullys that go on hard may be shearing off some keyway material from not being lined up good thus throwing it off and some have a key way thats too wide. and some have keys that fit loose.... I try to only use oe vw keys,(one of the 2 or 3 oe parts I like) many of the new keys are powdered mattelllllll....I dont like them as they crumble. 2 degrees...I wood not worry about... just degree the cam where you want it..accounting for the 2 degrees..or not.you probably could never find any differance anyway running it on cahssie dyno.
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norris
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PostPosted: Wed Jul 18, 2018 2:46 pm    Post subject: Re: TDC 2* ATDC Reply with quote

Thanks Mark, you answered my question.....the cam being advanced or retarded has nothing to do with timing at the pulley.
And Dale.....my JayCee pulley is not built wrong. The center of the keyway is an exact 90* from the TDC mark on the pulley.
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iowa vw
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PostPosted: Wed Jul 18, 2018 2:51 pm    Post subject: Re: TDC 2* ATDC Reply with quote

I understand the pulley has nothing to do with the cam timing. Just wondering if you can determine if the gear or bolt machining are off on the cam relative to the crank being correct/incorrect without having to mock the entire engine up.
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