Q420 camshaft question

[Craven]

Did you measure TDC by reading and noting the points on the degree wheel when the dial indicator starts to rise and when it starts to fall? The reference point for TDC is half way between the two. There is a dwell period when the piston reaches the top of the bore so it's easy to miss true TDC if you don't do the procedure correctly.

Simple, Crank shaft will be at TDC when the piston is lifted to it’s max height on the up stroke, that is when engine turned in operating direction.

[Andy8421]

Did you measure TDC by reading and noting the points on the degree wheel when the dial indicator starts to rise and when it starts to fall? The reference point for TDC is half way between the two. There is a dwell period when the piston reaches the top of the bore so it's easy to miss true TDC if you don't do the procedure correctly.

Simple, Crank shaft will be at TDC when the piston is lifted to it’s max height on the up stroke, that is when engine turned in operating direction.

At, TDC the instantaneous change in the position of the piston relative to angular rotation of the crank is zero. In practical terms, this means the piston doesn't move for small movements in the crank making an accurate location of TDC difficult to find. All the timing books I have read suggest that taking two measurements of the crank angle with the piston the same distance down the bore either side of TDC and finding the midpoint is the most accurate way to establish TDC.

[Craven]

No one is considering a dynamic situation here, there is a fixed distance between the crank journal and piston crown, when the crank TDC is reached the piston will have reached it’s max height in the bore. A piston with an offset pin does not follow the normal sinusoidal motion at the change of direction point.

[rgh0]

I can understand how the geometry is not symetrical around TDC for an offet pin piston. I have never done the calculations does anyone know how significant it is before I pull out my calculator.

cheers
Rohan

[2cams70]

Please pull your calculator out Rohan

[Andy8421]

No one is considering a dynamic situation here, there is a fixed distance between the crank journal and piston crown, when the crank TDC is reached the piston will have reached it’s max height in the bore. A piston with an offset pin does not follow the normal sinusoidal motion at the change of direction point.

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.

[Craven]

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 KiB) Downloaded 105 times

[2cams70]

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.

[rideaway]

Hi,

Rideaway - have you now resolved your issue? You've been quiet for quite a while.

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.

Regards
Stéphane

[ill_will]

Good to hear you resolved the issue Stephane.

On the asymmetric TDC issue:

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.)

[MACCA.GLM]

Hi Stephan

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

This post will await your result

Regards

G

[2cams70]

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.

[rgh0]

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.

The formating gets screwed up sorry

Θ (deg) Distance down Bore Apparent angle versus measured TDC
actual crank versus no offset piston (less 0.4 degrees from crank angle)
25.00 (BTDC) 4.48 24.60
20.00 (BTDC) 2.88 19.60
15.00 (BTDC) 1.61 14.60
10.00 (BTDC) 0.70 9.60
9.00 (BTDC) 0.56 8.60
8.00 (BTDC) 0.44 7.60
7.00 (BTDC) 0.33 6.60
6.00 (BTDC) 0.24 5.60
5.00 (BTDC) 0.16 4.60
4.00 (BTDC) 0.10 3.60
3.00 (BTDC) 0.05 2.60
2.00 (BTDC) 0.019 1.60
0.70 (BTDC) 0.001 0.30
0.20 TDC 0.001 -0.20
-0.30 (ATDC) 0.005 -0.70
-1.10 (ATDC) 0.019 -1.50
-2.10 (ATDC) 0.05 -2.50
-3.10 (ATDC) 0.10 -3.50
-4.10 (ATDC) 0.16 -4.50
-5.10 (ATDC) 0.24 -5.50
-6.10 (ATDC) 0.33 -6.50
-7.10 (ATDC) 0.44 -7.50
-8.10 (ATDC) 0.56 -8.50
-9.10 (ATDC) 0.70 -9.50
-14.10 (ATDC) 1.61 -14.50
-19.10 (ATDC) 2.88 -19.50
-24.10 (ATDC) 4.48 -24.50



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

cheers
Rohan

[2cams70]

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.

The formating gets screwed up sorry

Θ (deg) Distance down Bore Apparent angle versus measured TDC
actual crank versus no offset piston (less 0.4 degrees from crank angle)
25.00 (BTDC) 4.48 24.60
20.00 (BTDC) 2.88 19.60
15.00 (BTDC) 1.61 14.60
10.00 (BTDC) 0.70 9.60
9.00 (BTDC) 0.56 8.60
8.00 (BTDC) 0.44 7.60
7.00 (BTDC) 0.33 6.60
6.00 (BTDC) 0.24 5.60
5.00 (BTDC) 0.16 4.60
4.00 (BTDC) 0.10 3.60
3.00 (BTDC) 0.05 2.60
2.00 (BTDC) 0.019 1.60
0.70 (BTDC) 0.001 0.30
0.20 TDC 0.001 -0.20
-0.30 (ATDC) 0.005 -0.70
-1.10 (ATDC) 0.019 -1.50
-2.10 (ATDC) 0.05 -2.50
-3.10 (ATDC) 0.10 -3.50
-4.10 (ATDC) 0.16 -4.50
-5.10 (ATDC) 0.24 -5.50
-6.10 (ATDC) 0.33 -6.50
-7.10 (ATDC) 0.44 -7.50
-8.10 (ATDC) 0.56 -8.50
-9.10 (ATDC) 0.70 -9.50
-14.10 (ATDC) 1.61 -14.50
-19.10 (ATDC) 2.88 -19.50
-24.10 (ATDC) 4.48 -24.50



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

cheers
Rohan

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

[rgh0]

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