Methods to find TDC?

I wont chime in with any opinions or MSpaint diagrams lol. I recently checked the timing on my cams that had previously only been worked on by the dealer. I recorded all my work on videos and just watched the TDC one to recheck my experience.
I found that over a 3 degrees sample, the the amount the piston moves is minuscule, so a precision instrument is a must.
My digital gauge was giving my erratic readings(grrr) so I used my old faithful analogue gauge, and to my eyes it showed just a a dwell of 2 to 2.5 degrees. ( did each measurement numerous times)
Having knowledge of this debate, I recorded 2 sets of numbers when checking the cams, both with old belts and with new, and compared the readings using 0TDC vs -1TDC.

I found 3 of the 4 measurements(cyl 1 I & E, Cyl 8 I & E) showed perfect using "Brian Crall's" method, aka the ferrari method and only 1 of the 4, was 1/2 degree off.
I know that isn't prof, But it was enough for me to use "Brian Crall's" when i installed the new belts

 

In my defense, it was laminated and glued to a curricular saw blade.

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So Kenny, it's not exactly clear but are you say you didn't have to change the pin locations on the cam pulleys?

 

I used a coat hanger as well. My degree wheel showed the valve actions specific to the 355 and was much larger then my older summit racing one Image Unavailable, Please Login

I moved the pin a few times on the one that was off, essentially settled with the 1/2 degree with happened to be between the two different TDC methods. After i was all done dave posted that the it is possible to get closer since it makes a vernier scale... Info that would have been usefull a few days prior lol

 

Does that mean it's a good time to mention that dwell is longer at BDC than TDC?

 

I would actually agree with you on that James. The motion of the piston is less sensitive to rotation of the crank near BDC than it is a TDC.

 

Cool, looks like I'll be using a dead stop for my next service

 

I like the videos Dave, looking forward to trying this myself. There -should- be .5 deg difference in methods though agreed it doesn't matter either way in terms of final result.

What I'm having trouble with is if you can measure 1deg dwell at tdc how would you arrive at the exact same crank deg tdc using the indicator from both cw rotation and ccw rotation?

Would seem to me the 1 deg dwell has disappeared if that is the case.

 

Still think we may be running into stack up variance when visually finding quarter degrees chasing a half degree.

Looking forward to Johns explanation, until then I like this post

 

So based on this we would see a0.6 difference in degrees??

 

Or would it be 0.3 degrees differance

 

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This Johk's presentation is absolutely correct. The dwell shown (to the left of the TDC) is when rotating CCW. When rotating CW, the dwell will be the same but only on the right of the TDC. So, whichever direction you rotate the crankshaft, it is the full dwell (not half) and it only changes sides but the TDC remains in the same position, as shown. The method using piston stop will therefore give you correct TDC (same as when using a dial gauge) and it is probably more accurate as, with a dial gauge, you will probably slightly overshoot or undershoot the TDC. On the other hand, how important is it really to find the perfect TDC when, after that, you insert the cam pulley dowel into the "aligned" holes that will not be perfectly aligned but nearest to alignment and the timing will have to move a bit.

 

The methods are the same because it is exactly as I said in post 18 https://www.ferrarichat.com/forum/posts/145738062/. The dwell is a result of bearing clearances, not geometry. Any oil film gets squeezed out around the contact point between journal and rod bearing, (and the same with wrist pin clearances). Oil film can not support a compressive stress (or a tensile stress). And when you consider the area of the contact point ( a smaller cylinder inside a larger one would theoretically have an infinitesimally narrow contact) even the relatively small load due to piston friction in the cylinder is enough to put the squeeze on things, similar to this:
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I'm good with agreeing to disagree. Having disassembled engines that have sat for decades and there is still oil on the bearings. No way you are "squeezing out" all the oil with zero load.

Either way, would seem to be a matter of geometry if the dwell is as previously mentioned easily changed by rod/stroke ratio. If it was only due to squeezing oil out of the bearings measured dwell would be the same for every engine.

http://blog.appliedspeed.com/blog/2016/2/24/rod-stroke-dwell-rod-angle-piston-speed-calculator


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Yes, like a vernier scale but the switch hole-to-hole is in steps which are "X" degrees so, if it happens that you are in-between two "vernier" positions, you will have to chose one or the other pair of holes (very slightly advanced or retarded cam timing from the "perfect"). It is interesting that the Jaguar V8 engine (AJ26/AJ27, quad cams), on which I am about to set the cam timing, has no "vernier" holes but the position of the cam sprockets can be continuously varied. The sprockets engage snugly onto the cams (no key-ways, smooth and no tapper) and they are just clamped onto the cam with a large centre bolt with a very thick washer when in correct position.

 

The load is not zero as there is friction between the piston and the cylinder. Even with this small load, the oil will be squeezed out and/or move around the journal to the opposite side if you just give it sufficient time. And there is quite a bit of time for the oil movement when you are turning the crankshaft very slowly approaching the TDC (unless, of course, you are working at -20C).

The measured dwell will depend on the crankshaft stroke - shorter stroke will produce smaller dwell.

 

Sorry, I withdraw my statement about smaller dwell with shorter stroke as I am not sure. It might be the same.

 

Dwell (as measured with the tools here) changes with rod/stroke ratio so different stroke or rod length. Would be easier to see if I found a calculator that went out another tenth or 2.

I'm not saying that rod bearing and wristpin clearance has -zero- effect on this, what I'm saying is it just doesn't squeeze -all- of the oil out by turning the engine around a few times only load being ring/piston drag in the bore. If that's the case these things would be metal to metal on every start up.

Completely agree that we are big time splitting hairs on all of this. Could always get fully adjustable cam gears from Toda if you can't get where you want to be with the original gears.

As for dwell being -all- after tdc, I am finally understanding that aspect of it though have not tried it myself. Everything I've been talking about here was based on Dave's measurements.

To me, reading the above seems you are landing at the same wheel degree and finding the same dwell area on the wheel.

I don't think Dave would have posted the above quote if he found the 1deg dwell on the other side of TDC change depending on direction of crankshaft rotation but maybe I am interpreting this incorrectly.