Q420 camshaft question

[2cams70]

Craven is right about the different shape of the curve for piston height versus crank degrees before and after piston TDC. But the error introduced by the traditional timing method to determine piston TDC due to this is only 0.05 degrees on a twin cam and not worth worrying about.

The bigger issue is that the piston TDC changes with offset and while most OEM pistons have an offset (1mm in the Lotus case) many replacement pistons do not. So you need to check the offet in any pistons you have and locate piston TDC and potentially you will need to remark the crank pully if going from standard OEM pistons to replacement forged pistons as this will change the piston TDC versus the crank by about 4 degrees

cheers
Rohan

If piston pin offset is of no consequence when setting up the cams it should follow that it also is of no consequence to the correct position of the timing mark on the pulley going from a zero pin offset piston to an offset pin one or vice versa?

[rgh0]

TDC of the piston shifts by 4 degrees versus the crank from a no offset piston to a offset piston. . Which direction it changes depends on what type of pistons it was marked for in the first place. If an orginal pulley I presume it was marked for TDC for the offset piston but I would need to actually check to confirm that.

cheers
Rohan

[2cams70]

That doesn’t make sense Rohan. You can’t on the one hand say that pin offset doesn’t significantly affect the TDC point for cam timing but does significantly affect the pulley marking for TDC (by 4 degrees). It’s either TDC or it isn’t TDC.

[Craven]

My take on this is Do we need TDC of the piston or TDC of the crankshaft, both for valve and ignition timing?
Twincam has a low offset pistons some have an offset as high as 14mm yes one four, in both advance of the centre and after the centre pistons.

[nmauduit]

That doesn’t make sense Rohan. You can’t on the one hand say that pin offset doesn’t significantly affect the TDC point for cam timing but does significantly affect the pulley marking for TDC (by 4 degrees). It’s either TDC or it isn’t TDC.

iirc the first query was about the amount of error resulting from non symetrical motion induced by offset when using the (rather widespread) x_before - x_after measurement to find out TDC (i.e. from x_before moving differently than x_after when offsetting the pin). You are now referring to the first order of TDC moving when offsetting the pin, not of the error in assessing TDC (which would be a second order variable).

TDC relates to piston position, it is the piston that set the reference for cam timing, not the crank.

[2cams70]

That doesn’t make sense Rohan. You can’t on the one hand say that pin offset doesn’t significantly affect the TDC point for cam timing but does significantly affect the pulley marking for TDC (by 4 degrees). It’s either TDC or it isn’t TDC.

iirc the first query was about the amount of error resulting from non symetrical motion induced by offset when using the (rather widespread) x_before - x_after measurement to find out TDC (i.e. from x_before moving differently than x_after when offsetting the pin). You are now referring to the first order of TDC moving when offsetting the pin, not of the error in assessing TDC (which would be a second order variable).

TDC relates to piston position, it is the piston that set the reference for cam timing, not the crank.

I'm not sure what you are trying to say.

[rgh0]

Trying to put it as simply as possible.

1. The piston TDC ( i.e. its highest point in the bore) moves by about 4 degrees with an offset piston pin versus the crank.

The photo below compares and orginal Twin cam OEM pulley on the bottom with an aftermarket pulley on top. The OEM pulley has TDC from the factory which would have been for the original offset pistons. The after market pulley is marked with TDC for after market zero offset pistons. I did this mark by the normal method of measuring before and after TDC with a dial guage at the same height and selecting the middle position on the degree wheel for TDC. You can see the 4 degrees difference.

2. The asymmetery in the piston height versus crankshaft degrees curves before and after the calculated piston TDC is very small for the 1mm offset OEM pistons used in the Twincam. Per my calculated data above the asymmetry is a total of 0.1 degrees in the 4 to 5mm below TDC range. Thus the error versus true piston TDC using the usual dial gauge and midpoint method is only 0.05 degrees out and can be ignored for the offset pistons.

3. The engine and its cam timing and ignition timing only cares about the actual piston TDC so setting these based on the normal methods of determiing TDC for either offset or non offset pistons works. Just dont be fooled by what markings may exist on any crank pulley you are using as they may not match the pistons you are using.

cheers
Rohan

[2cams70]

Sorry but we're going to have to agree to disagree on this one Rohan. We have established that an offset piston pin does not affect the TDC position by a significant degree. It therefore should logically follow that it does not affect the position of the timing mark on the front pulley to any significant degree. I don't see it as being anything more complicated than that.

For reference I used the same spec. as Rideaway during my build. Same QED pistons and same Q420 cams. I set the cams up as per Miles Wilkins method. Position of TDC was determined using Miles Wilkins method. When I set the engine to TDC and refitted the front pulley I found that the pulley mark aligned perfectly with the TDC mark on the front cover. The pulley I used was an aftermarket QED pulley with the timing mark groove already machined. When I compared the original pulley with the QED one I could see that the timing marks were in exactly the same position.

I think the traditional MW method for determining TDC applies equally well regardless of whether the piston has offset pins or not. If it didn't there would be an alternative published procedure relating to our engines for that case.

[rgh0]

Do you know if the QED pistons have offset wrist pins. If they do that would explain your observations versus the orginal pulley marking?

regards
Rohan

[Andy8421]

Sorry but we're going to have to agree to disagree on this one Rohan. We have established that an offset piston pin does not affect the TDC position by a significant degree. It therefore should logically follow that it does not affect the position of the timing mark on the front pulley to any significant degree. I don't see it as being anything more complicated than that.

I am not sure your statement is correct. There are two issues at play here, and assuming Rohan's grasp of rotational geometry is correct, he has demonstrated two separate points.

1. Having an offset gudgeon pin moves the point at which TDC is achieved (in the case of a Twinc) by approximately 4 crankshaft degrees from the classical 'crank straight up' position of a non-offset gudgeon pin setup.

2. Using the 'same distance down the bore, before and after TDC' method is still an accurate method of finding the correct TDC in either the case of a central gudgeon pin or offset gudgeon pin as the shape of the curve of piston height vs crankshaft degrees is sufficiently symmetrical around TDC in the case of the offset gudgeon pin not to introduce significant errors.

[2cams70]

Thank you Andy841. Your explanation does actually make some sense. Although I didn’t measure them those QED pistons must have standard offsets. Thank you Craven also for raising a topic from which we can learn.

[Andy8421]

OK, so in the spirit of full disclosure, I have just tried some back of the envelope maths.

Assuming that the piston is at its maximum height when the centre of rotation of the crank, the crank pin and the gudgeon pin are all in straight line, it should be easy to approximate the change in angle of the crank required to move the gudgeon pin by a certain amount by considering the system as a right angle triangle.

With a conrod of 125.171mm and a stroke of 72.75mm, that gives a hypotenuse of 125.171 + 72.75/2 = 161.546mm

Assuming a 1mm offset, the 'opposite' side of the right angle triangle is 1, so as sin = opposite/hypotenuse, the angle will be:

angle = arcsin(opposite/hypotenuse) = arcsin(1/161.546) = 0.355 degrees

What am I doing wrong?

Edit: I got the conrod length wrong, I think I used conrods for a Kent block. The LTC conrods are 121.922mm, but that still gives an angle of 0.361 degrees, not the 4 degrees Rohan calculated.

[rgh0]

mmmm....... need to go back and recheck my spreadsheet. Maybe I got an order of magnitude wrong somewhere.

[2cams70]

Dammit..............and I was just about to hand out the grumpy uncle awards!!

[rgh0]

mmmm....... need to go back and recheck my spreadsheet. Maybe I got an order of magnitude wrong somewhere.

Andy
My calculations were correct and I actually said in my post above with the calculations the difference was 0.4 degrees in TDC. Somehow this went to 4 degrees in my mind in later posts

The measurements of my pulleys reinforced that 4 degrees in my mind but given the accuracy of TDC markings on the pulleys this difference is due more to marking and measuring errors on the pulleys not due to offset versus non offset pistons

So in the end the TDC shift versus the crank is small at 0.4 degrees and the error due to the asymmetric curve when setting the TDC by measuing distance down the bore is even smaller at 0.05 degrees for offset versus non offset pistons

Sorry for the confusion

cheers
Rohan