Methods to find TDC?

This makes sense, I didn't consider that. Seems like more gear adjustment is not necessary.

 

James, I have download the spread sheet you referenced. It's basically the same as mine but I figure you wouldn't believe mine. I have made two changes. First, I changed the increment for the crank angle to 0.01 degrees rather than 1 degree. Second, I reformatted the Piston Position cells to scientific format. Now, look at the piston position. even 0.01 degrees from TDC the piston is displace, and the position is changing. Next look at the Piston velocity. Obviously, even at 0.01 degrees it is not zero. How can the piston be stopped in the dwell if has velocity. The thing about this spread sheet is that it doesn't account for clearances (neither does mine) so it represents the purely geometric behavior in the limit of zero clearance. And, as shown, there is no dwell. The is no angular region around TDC where the piston stops and remains at rest as the crank continues to rotate. Clearances, and only clearances are responsible for the dwell. If there were no clearances the only dwell you would measure would be because of the limitation of the instruments used to measure the small displacements around TDC.

By the way, as wrist pin off set is introduced the piston stop method will becomes less accurate as the offset increase due to a lack of symmetry in piston motion about TDC. I will have to modify my spread sheet to account for this. I don't think the one you linked to accounts for in the way I expect.

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John I agree that if we bring it out another tenth dwell no longer exists! That's why I have mentioned "as measured with the tools here". As I understand it piston dwell is considered to be degrees of crankshaft rotation within .001 piston movement.

In texting with Dave last night I mentioned that all this makes perfect sense if there is no 1deg of dwell but, that is what is viewed with the tools and methods we are using even if that ceases to exist if measured with more precision.

Still, if you adjust rod/stroke ratio you can see differences in piston position vs crank angle change which is more than just a result of bearing/wrist pin clearance and would change "degrees of dwell" within .001 piston movement.

Good point about the offset wrist pin. Maybe we should use a bridge type piston stop with the head off that has a larger surface area that would square the piston in the bore rather than a small point the piston can rock on.

 

I still agree with that

https://www.ferrarichat.com/forum/posts/143491674/

 

One more comparison of piston stop vs dial indicator method. Just further shows there can be some variance in interpretation/result if using an indicator. Not so much going to happen with a piston stop.

https://www.ferrarichat.com/forum/posts/144460171/

 

I don't know how many holes in the 355 cam and sprocket but in the 550 it is possible it achieve an accuracy of .06 of a degree in setting the cam in relation to the crank.

This is because moving the sprocket in relation to the belt with the cam locked in place changes the starting angle of the sprocket/cam hole relationship so for every belt tooth the sprocket is moved a new and different vernier equation is established between the sprocket and the cam.

 

If so, 0.001 is just something some one made up. Arbitrary. Why 0.001? why not 0.005 or 0.0005, or 0.1? It has noting to do with TDC. If you define the dwell as where the piston move less the 0.00X from the TDC position dwell what would that tell you about TDC? I'm ok with calling that dwell, but it would then be unrelated to what we are discussing.

No argument that stroke and rod length effect things. It's geometry.

 

Yes, but they are all offset. Search and read up on it. It's mainly used for reasons as I mentioned last night, to reduce piston chatter in the cylinder, or noise. Also used to more align the thrust axis on the power stroke. Whether or not 355 pistons are offset I have no idea. But your TDC results suggest that either it isn't or it isn't very much.

 

Arbitrary yes. Why not .005 or .0005 or others? I don't know, maybe not repeatable at that level. Dave what value did you use to determine 1deg dwell? We never discussed a standard for this.

Did we find tdc by the last .0005? Why not .0005? Why are we visually splitting 1/4 degrees on a wheel with a pointer instead of using a rotary angle encoder?

Probably because the values that are commonly used are as accurate as they practically need to be.

If the above is true how is it unrelated to what we are discussing?

 

Agreed!

I wonder when the Ferrari method was developed. Maybe at a time before 4/5 valve heads where the spark plug (and therefore piston stop placement) was not centered on the piston? I guess taking up the slack against a stop is taking up the slack no matter where it lands but you would think it would be better centered above the wrist pin/connecting rod than offset to the side of connecting rod centerline.

Maybe it was an insurance policy so tech's didn't mistakenly turn the engine over by the starter or too hard with a stop in place and damage things.

 

Well you just need something to hold the stop right? (which is why you can use your leak down tool as well?)

 

Neat model. Did you gain the overlap by advancing the intake cam, retarding the exhaust cam or both? Just wondering if the results are influenced by IVC as well as overlap.

 

Dave, Put a laser pointer 90 degrees to the crank and a tape measure 10 ft away then measure the distance the laser dot moves. About 1 inch per 1/2 degree. (0.477465)
Or put the tape 9.5493" away and the laser dot will move 2 inches for every one degree of rotation.

 

I did this today for the fun of it. It is a comparison of how the piston moves in the limit of zero bearing clearance (dark blue curve) with how it moves with clearance (pink). It assumes rotation is continuous from 10 degrees before TDC (-10) to 10 after. I used 0.01 mm clearance. The blue curve (no clearance)is symmetric about TDC and there is no dwell, or region where the piston remains at a constant position as the angle varies. The ink curve, with clearance, shows a different behavior. First, on the approach to TDC (piston moving up) the piston position is lower at any given angle. This is because the closing the clearance gap is like having a slightly shorted con rid. Once TDC is reached, the piston remains at a constant position because the crank has to rotate to a position where the clearance is now opened at the top of the con rod and closed at the bottom (see my post #18). This defines the dwell in terms of the piston not moving over some crank angle. In this case about 1.75 degrees. Once the crank rotates past the dwell the piston start to move down. But notice it is at a higher position that the piston with no clearance. Again, this is because the con rod looks a little longer. If L is the length of the con rod between bearing centers (crank and wrist pin) and c is the bearing clearance, the on the way up the con rod look L -c on the up stroke and L + c on the down stroke.

Also note that if you define the dwell as the angle between which the piston moves no more than some small amount the dwell is symmetric for zero clearance but not symmetric when clearance is considered. For example. with zero clearance the zero clearance case cross the 175.5 position at -2 and 2 degrees. But with clearance it crosses 175.5 at about -1.7 degrees on the up stroke and 2.2 degrees on the down stroke.



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If they are, then in the 355 it can not be very much because if it was the piston stop method would give a different result than the dial gage method for finding TDC. The 355 is what matters.

 

GM pistons aren't Ferrari pistons.

 

That is neat. How are we reaching higher dwell numbers calculated with clearance than what Dave actually measured? Actual clearance/movement with oil smaller than .0004in?

This is with zero main bearing and wrist pin clearance as well?