Apologies for not being clear, I was using ‘instantaneous’ in a mathematical context.
The slope of a sinusoid is zero at the peak and the trough, maximum halfway between the two. In real terms, the piston moves by differing amounts for the same angular movement of the crankshaft depending on where the piston is in the bore. Halfway down the bore it moves the most per degree of crankshaft rotation, at the top and bottom of the stroke it hardly moves at all. With the piston right at the very top, for very small moves of the crankshaft, the piston doesn’t move (the ‘instantaneous’ bit from a maths perspective).
This lack of movement of the piston relative to the crankshaft makes it hard to locate TDC accurately. It is more accurate to choose a point where the piston is moving more quickly relative to crankshaft rotation (say 30mm down from the top of the bore), note the angular position of the crank when the piston passes this point on the way up, and again on the way down, then TDC will be halfway between these two points.
Yes that right, for a piston with a pin on axis but with an offset pin this is not the case. In the motorcycle world TDC is given as Max piston height, reason the end of the crankshaft is normally inaccessible. If you can run a Excel speadsheet I did this many moons ago for the 500cc Norton may be of interest.Timing 2 for NOC.xls (17 KB)
Without meaning to offend some of you guys would make it out to be a complex process getting out of your seat and going to the toilet. This thread reminds me of when I was a kid at the annual family Christmas do asking a seemingly simple question and all of the grumpy old uncles trying to compete with each other with what they know and me never getting an answer to the question asked.
Rideaway - have you now resolved your issue? You’ve been quiet for quite a while.
By the way I now include myself in the “grumpy old uncle” class so for all the other “grumpy old uncles” (you know who you are) please don’t take it personally.
In fact, I didn’t say anything more because I was so ashamed! :
The timing wasn’t right, when I reassembled it using plasticine to measure the clearance, the angular disc wasn’t fixed correctly and it shifted. I didn’t check with the TDC marker on the crankshaft pulley.
Everything is back in order, the clearance is sufficient. This weekend, I’m going to reassemble the cylinder head for good and do the final timing. I’m going to do a few tests on the TDC measurement to see what the different opinions are.
Assuming the effect is significant (i.e. introduces an error of more than 0.5 deg for a road engine, say) I would think it should be possible to use the standard “20 thou before and after” technique and then rather than taking the centre point between the two degree wheel readings, apply a correction based on stroke, rod length and pin offset, and the stroke offset you use (the 20 thou figure above.) Easy enough to do in a spreadsheet perhaps.
In practical terms this would mean, if your 20 thou point on the upstroke is at 100 Deg and the 20 thou on the downstroke is at 120 deg, you could calculate TDC is at 100 + 0.42 * 20 deg, rather than the uncorrected value of 110 deg (0.42 I’ve just made up, this would be the number outputted by the spreadsheet.)
This is interesting for me as I have for a while been thinking of doing this job with 420 lift cams on my standard sprint. if there are no more complex issues machining etc etc and you get good results I will do it.
For around a grand it seems like a good power improvement
Miles in his book has a very good section on how to set up the cams correctly. Suggest you read it and not get too sidetracked with what could possibly be red herrings.
Well I pulled out the formula and set up an excel spreadsheet.
The first column is the actual crank angle.
The second column is the distance down the bore of the piston, A piston with no offset has 0.000mm distance down the bore at crank TDC of 0 degrees. A Twin cam piston with 1.0mm offset moves the actual piston TDC to 0.4 degrees before crank TDC and the piston is 0.001 mm down the bore versus a no offset piston.
The third column is the measured angle at selected distances down the bore versus the actual TDC of the offset piston.
This data needs careful interpretation as with a piston offset both the shape of the curve before and after the piston TDC is different but also the location of piston TDC versus crank pin TDC is different. When actually measuring the distance down the bore of equal amount before and after these effects largely cancel each other out. In the Lotus Twincam example. The error when setting the timing by measuing the crank angle at either side of TDC with pistons an equal distance down the bore and setting TDC at the centre point is only 0.05 degrees.
In any case most non OEM forged pistons dont have any offset. So i suspect all of this is largely hypothetical for the QED forged pistons that started this discussion.
An interesting exercise as 39 degree here today and to hot to work in the garage
Rohan I think Craven’s opinion is the fact that piston pin offset means that the middle of the dwell period when the piston is at TDC is not symmetrical means that the traditional methods for determining TDC for cam timing purposes is not correct. Do you agree with that opinion? OEM pistons for all Kent series engines (not just LTC) have offset pins. The purpose of this is ensure a smoother transfer of loading from one side of the piston to the other at the transition point when the piston changes direction. Pistons without offset are more prone to piston slapping noise.
If Craven’s opinion is correct than the question then becomes what is the correct method for determining TDC on an engine with offset pin pistons
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
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
