Twincam Camshaft detail measurement

[rgh0]

I have always wanted to better understand the detail of various cam profiles that i have used over the years. Previously I have had a couple measured by my local cam grinder but he does not supply the full detail and I cant afford to pay and to keep going back to ask for it to be repeated with more detail

But now that I am semi retired I have time to explore doing it myself So I decide to build me own cam test rig. I cant afford a full automated rig with rotary and linear encoders and digital output to a PC, so I decide it build it based on manual measurement. I wanted the measurement to replicate the actual follower movement versus the cam lobe so I setup the measurement in a spare head I had laying around.

Key features of the setup were
1. The standard follower was held against the cam lobe by a light spring so the measurement taken is actual lobe lift at the follower without any clearance
2. The dial gauge was angled at 27 degrees to be parallel with the follower axis using a digital angle gauge
3. A 1 mm diameter 50mm extension pointer was used on a 20 mm travel dial gauge to fit down beside the cam lobe onto the follower and measure the full follower travel
4. A degree wheel was fitted to the front of the cam.

A couple of photos of the set up

I then proceeded to measure lift versus cam degrees for a standard sprint cam that I had in my spare parts collection. I made multiple measurements, the first at 5 degree intervals then a second at .5 mm intervals ( these agreed well) plus did specific checks at the standard measurement points of 0.01 inch / 0.25mm and 0.05 inch / 1,25mm. These are plotted on the graph below plus the calculated veloctiy in mm/cam degree and acceleration in mm/cam degree squared. The velocity and acceleration curves get somewhat more irregular as small lift versus degree errors get magnified when you start doing the derivative calculations in the spreadsheet but the main shapes can be seen..

The outcome is pretty good for a first attempt. When I get a chance I will repeat at smaller degree and lift intervals to get better resolution especially of the velocity and acceleration curves. I will also use my larger diameter degree wheel for better degree accuracy

The cam measured up to match it nominal Lotus specifications well
i.e
Lobe lift above the base circle of 9.14 mm / 0.360 inch right on specification
Duration above 0.25 mm / 0.01 inch, I measured 275 crank degrees versus Lotus spec of 272 crank degrees which is within the accuracy of what I measured.

The most interesting aspect is that the cam is assymetric with the opening acceleration being quicker than the closing accesleration. The closing accelration is limited by the valve spring load and is much less than the opening acceleration which is limited by the contact pressure between lobe and follower.

I have a Wade 137 cam which I believe is a Cosworth L1 copy and a couple of McCoy cams in my spares to measure and I will post details of these when they are done.

Just for amusement I have a photo of the box that my no name brand Chinese 20mm travel dial gauge came in. I normally use Mitutoyo instuments but decided I did not need ultimate accuracy for this application and the cost of a long travel Mitutoyo dial gauge was high so I bought the no name one off Ebay at 1/10 the cost. I appears to be Ok for accuracy but I need to do a more detail checks with my Mitutoyo gage blocks across its full range. The amusing thing was it came in a box that used the Mitutoyo graphics except for the Mitutoyo name. Clearly they also sell Mitutoyo knockoffs in the same box and just add the Mitutoyo name

cheers
Rohan

[nmauduit]

Thank you Rohan ! most interesting ... I must confess I've coined with the idea of an automated setup based on a digital caliper with serial/usb connection and a rotary encoder, which has remained a shelved project so far. This was meant to be a bench setup so the impact of varying base circles would have had to be taken in account separately... while your setup does not has this drawback.
As for acceleration, you may want to use a software to curve fit from the measured points so that the derivatives are smoother.

I'm looking forward the upcoming datas (please include base circle, esp. if you have reground cams in the lot)...

[rgh0]

I looked at setting up to measure the cam using my lathe or V blocks to hold the cam but in the end decided it was easier to do in a head. If you set up on a test stand of some sort you need to have a large diameter flat face follower set exactly on the cam centre line line and parallel with it to get accurate measurements and this would involve making a few accurate special jigs. if you just use the point of the dial gauge or digital calliper on the cam lobe you get a different lift profile to that created by the flat face follower as the cam contacts the follower off the centreline for most of its rotation. Using the head as the test stand
achieves all this and I am fortunate to have a collection of heads The one I used is an old one thats gone soft but is dimensionally good and i keep for reference and test assemblies.

I actually got a 30mm diameter flat face follower that fits on the dial gauge pointer end thread (which is the largest i could find but still a little smaller than the standard cam follower bucket diameter) but decided not to use it versus just measuring the actual follower movement in the head.

The base circle on the sprint cam I measured was the standard 1.15 in / 29.21 mm My other higher lift cams all have smaller base circles iof 1.00 inch or 1.05 in which is required to fit the taller lobe on the follower and I will quote those when i measure them up.

I agree the velocity and acceleration curves could be smoothed by doing some curve fitting to the lift measurements and then doing a mathermatical derivative of the fitted curve functions. I did the quick and dirty approach of doing a a simple slope average calculation at each point. i.e slope at x2 = (y3-y1) / (x3-x1) which gave an OK result and will be better I think when i measure the cam at smaller degree intervals

cheers
Rohan

[2cams70]

It might help if you could mount the head on a block and take degree measurements from the crankshaft instead of the cam itself. That way the gear reduction ratio between cam and crankshaft might help with the resolution of the data points

[rgh0]

Doing the measurements on an assembled block and measuring crank degrees may aid the degree measurements accuracy like you say if you keep the chain tight and have no movement of the chain on the sprockets.

I have all the spare bits ( block, crank, chain cover and back plate, chain, chain rensioner, sprockets, camshafts etc) . However my engine stand is currently occupied with the refesh of my spare competition engine and no room in my garage and small workshop for a second engine stand

Using my larger 200mm dia degree wheel versus the smaller 100mm diameter one I first used will double the accuracy of the degree measurement which will improve the results and i will give that a try. I can always buy an even bigger diameter degree wheel as you can get them up to about 600 mm diameter

In the end I am not trying to design cams or measure them to an accuracy that they could be replicated, i will leave that to the professionals. I am just trying to understand the lift, velocity and acceleration profiles of different cams I have used so i can understand directionally what worked and why.

cheers
Rohan

[Andy8421]

So I decide to build me own cam test rig. I cant afford a full automated rig with rotary and linear encoders and digital output to a PC, so I decide it build it based on manual measurement. I wanted the measurement to replicate the actual follower movement versus the cam lobe so I setup the measurement in a spare head I had laying around.

Rohan, please keep us updated with your analysis, I always find your posts very interesting.

FWIW, for the price of a few litres of that fancy Redline oil you use, you could pick up a stepper motor, driver card, arduino microcontroller and a linear sensor from eBay and build a fully automated rig. It would take a bit of messing around and a bit of coding, but with a toothed belt step down drive you should be able to get to 1/4 degree steps on the cam and a resolution of better than 0.01mm on the linear encoder.

Something for those quiet semi-retired afternoons?

[rgh0]

Hi Andy
Building an automated setup would certainly be interesting to do and possible with cheap off the shelf items and a bit of programming like you say. When i get tired of oing it manually i will add it to my worklist

But you have me thinking about it already!!! Holding the linear sensor above the cam lobe is easy and mounting a stepper motor on the head to drive the cam also easy

cheers
Rohan

[englishmaninwales]

Very interesting information, Rohan. Any chance you would be able to repeat this on a fully assembled engine and give the in/exh lift at TDC for any commonly used cams (eg B, C, Sprint) that you might have installed?
(Apologies if that is a bit of an ask!)
I have found this an easy way to cam time the K series in my Caterham, but I’ve never been able to find the correct data for the TC.
Malcolm

[rgh0]

Hi Malcolm

You can work out the cam follower opening at TDC based on the cam curve and is one of the reasons i did the curve plot.

For the sprint cam

Inlet Timing is 26 degrees BTDC to 66 degrees ABDC for total duration of 272 crank degrees as quoted by the workshop manual. This duration is typically quoted from the end of the take up ramps which is typically at 0.010 in / 0.25 mm lift on the lobe. The curve I measured shows 137.5 cam degrees or 275 crank degrees from the 0.010 / 0.25mm lift on the lobe. This is about as close to the specification as i could expect given that at this point the lobe lift rapidly accelerates from the end of the take up ramp as you can see by the curve so a little tricky to get the degree measure absolutely accurate. From zero cam degrees and lift, the lobe lift reaches 0.25mm at 18.5 cam degrees

The point on the the cam lobe curve that corresponds to TDC in the engine is calculated as follows

18.5 degrees from zero on cam plus 13 cam degrees ( i.e. 1/2 of 26 engine degrees BTDC) = 31.5 cam degrees on my lobe plot at engine TDC. This point represents 2.0 mm lobe lift which when allowing for 6 to 8 thou inlet clearance equates to 1.8mm +/- about 0.1 mm follower lift when installed in an engine and engine at TDC by my calculations.

I have a complete Sprint engine sitting here and when i get a chance i will pull the cam cover and check the lift at TDC to verify, but i will not get a chance to do that for a few weeks as heading off for some skiing in Japan soon

cheers
Rohan

[rgh0]

Just looked up the Kent cams catalogue and their CPL2 cam which is a Sprint cam copy and has a similar recomendation of lift at TDC so i am in the ball park according to them i.e. They say inlet at 1.80mm lift at TDC

cheers
Rohan

[englishmaninwales]

That’s an excellent piece of work, Rohan!
Enjoy the skiing!

[rgh0]

Repeated the Sprint cam measurements with the larger degree wheel at 2 cam degree intervals.

I got the 0.010 in / 0.25mm lift duration at 272 degree per the specification now with the more accurate degree measurment. Curve below now uses 4 point moving average for the velocity and acceleration curves which has smooth them out a bit.

Now to do my 0.44 lift McCoy cam

cheers
Rohan

[nmauduit]

Now to do my 0.44 lift McCoy cam

if that's not a cliff hanger... can't wait

thank you Rohan

[rgh0]

Took me a little while to setup for the McCoy 440 cam as all my high lift cams have been modified for a long sprocket bolt to minimise the risk of breakage at the first bearing. This required me to make up a long stud to hold the timing wheel in place as the needed 5 inch long 7/16 inch UNF set screw I could not source. I also had to do a small amount of grinding on the follower sleeves as the sleeves had shifted in the soft head and were fouling on a couple of the higher lift cam lobes

McCoy 440 cam curve below. I have also include the Sprint cam lift curve in grey for reference. The McCoy cam has significantly greater acceleration on the initial lift and thus higher peak velocity to get the 0.440 inch lift into 280 degrees seat to seat (10 thou lobe lift) duration. This cam was ground on a 1.05 inch base circle which is needed to fit the cam lobes on the followers compared to the 1.15 inch base circle of the Sprint cam. For a high performance road cam or a high torque race cam that pulls to 8000rpm this is as good as it gets I believe with optimum combination of max possible lift in 280 degrees duration.

Below I have attached the full Sprint cam curve I published previously but this time plotted on the same Axis scale as the McCoy cam for easier comparision.

I have a Wade cam which is a Cosworth L1 copy and a McCoy 490 cam next to measure. i also have a McCoy 460 cam but thats in my current Elan engine so cant measure it at this time.

cheers
Rohan

[rgh0]

Measured up my Wade 142 Cam which is a copy of a Cosworth L1 with 300 engine degrees at 0.25mm lobe lift and 0.412 inch max lift. The cam was ground on a 1.06 inch base circle This cam puts out a bit less power at around 160+hp compared to the McCoy 440 cam which puts out 170+hp in an otherwise similar specification engine. Peak power is in the same range of 7000 to 7500 rpm but the McCoy cam has more midrange torque due to its higher lift and shorter duration.

I have plotted the lift of the 3 cams measured so far on the same graph so you can clearly see the differences in lift and duration due to their different acceleration and velocity curves

Around 100 degrees you see a glitch in the accelration and velocity and lift curves for all the cams. I thought this was due to measurement error initially but it remains after repeating the measurement and it is larger than the noise otherwise seen in the measurement curves. i am not sure why this is present but it appears real and may be due to how the curve over the nose of the cam is matched to the curve down the back cam flank.

cheers
Rohan