The Real Triumph Rocker Ratio

[oldtuckunder]

Well this was an interesting little project, especially as I can find no trace on the web of anyone else doing it, will I go down in history as the only guy anal enough to do this, or the guy that exposed the myth about rocker ratios.

Firstly if you get bored quick the highlight is that the standard Triumph (no pun intended) is not 1.5:1 Its lots of things at different and there is one magical calculation you can do that does arrive at a 1.51:1 figure, but its meaningless as at no point does the actual assembly use other than as a brief transient point on its way to doing something else. If you want to know the real ratio(s) read on!

Basically my intention was simple, set up two dial gauges, one directly off the follower and one directly off the valve tip (neither can use the rocker tip or adjuster as these both transcribe an arc that throws the readings out) set the tappet clearance (or lash to our colonial friends) to zero, and directly measure for every 0.010 of Cam Lift what the resulting Valve Lift was.

My simple exercise was somewhat prolonged because when I first did the readings what I ended up with (apart from full lift at 110, which was gratifying as it means my initial set up two years ago at 108 to allow for a bit of stretch in a new chain has paid off) made no sense either in terms of cam or valve lift I expected from the cam. So much so that I suspected how everything was set up, re did it, tried again, got the same results, and then went around in circles for a bit until by chance I discovered that Kent who ground the cam cant tell "Their Ass from their Elbow" or as we old timers call it "Their Inlet from their Exhaust"  I have a cam ground perfectly with Inlet Lift on the Exhaust lobes and Exhaust Lift on the Inlet lobes!   

Anyway enough red herrings, back to the task, nb the unexpected lift makes no difference, as one they are not wildly different, two it meant the lift I was measuring was fairly identical to the standard cam, and three all we are doing is measuring direct lift on one side of a lever and resultant depression on the other. 

So the results.  I actually took measurements at every 0.010 of cam lift, but have consolidated the results to every .030 as it just makes everything clearer. Apart from the first two steps I combined into a single 0.060 step at its a bit erratic in the initial lift, but has settled down by 0.050 cam lift.

As you can see as I suspected from a bit of research the effective ratio being applied alters through the lift cycle, Triumph having nicely engineered the maximum lift ratio (i.e. maximum valve acceleration point) around the 50-75% lift point where I understand maximum inflow is desired, and starts heading for the floor at higher levels of lift.

What you can deduce from the table is that it is impossible to take a quoted cam lift and turn it into a valve lift figure without knowing the profile of the rocker lift, just saying the rocker ratio is 1.5:1 (which it isn't) is meaningless other than as a means of comparison to another meaningless figure for another cam calculated in the same way. And as you will have gathered the higher the lift the lower the ratio is at the upper end of the lift so the law of diminishing returns starts cutting in.

I think what the results also highlighted to me is that buying different rockers with higher quoted single figure ratios like 1.55:1 or 1.6:1 is a bit like buying a pig in a poke, unless they can quote the lift profile you have no idea where (if anywhere) in the lift cycle that lift occurs, and you also have no idea if they have completely bollocks'd that maximum lift acceleration in the critical 50-75% phase.

My only other comment is that these results were taken using standard push rods, standard rocker assembly, and a head only slightly skimmed from standard, my expectation is that if you heavily skim a head, block or both, you effectively end up with longer push rods, altering the geometry a bit (i.e shorter adjuster lengths) this may well have some impact on where in the cycle specific ratios are in effect.

Anyway next time someone asks for the Triumph Rocker Ratio, you can answer "It Depends" "Somewhere between 1.3:1 and 2.0:1"

Oops having trouble adding picture, will post and attempt to re edit, before I loose this.

Alan 

Edited April 17, 2018 by oldtuckunder

[yorkshire_spam]

Cracking data! "To measure is to know" 

[JohnD]

"Facts are sacred, but comment is free"     I don't doubt for a moment your careful research, Alan, and the validity of your data.   But to discuss what you have found, and how you have interpreted it.

 

Here's another way of displaying your own data. Blue line, cumulative cam lift, brown, cumulative valve lift.    I've posted a large chart, so that the linear trend line of the latter can be seen as a dotted line.  As you say, the ratio over the whole range, is 1.51:1.      But this display shows how little it deviates from that.       You could chose other trends , which would deviate more, such as an exponential, but I see no evidence that this plot is anything but linear.

This means that we can extrapolate the lines, by adding more cam lift steps and using the overall ratio to calculte valve lift.  This simulates high lift cams, and  by adding another 0.06 of lift in two steps, you would see this:

 

No sign of dramatic drop off in extra valve lift, which I suggest you cannnot assume from your observations, but my assumption is that there will be no greater deviation than in the observed range.   As usual,  "More research is needed"!

 

Oh, and I can't tell how you derived that highlighted "1.51" but an average of averages is a mathematical fallacy.     You didn't use the Excel command "=AVERAGE(F8:F14)", as that gives the wrong answer of  1.42.    The overall ratio in range is 0.363/0.240, which is 1.5125, or 1.51, as we are not interested in the miniscule.

Small details are important, and as you say, if you want to change your cam, this is important.

My next engine will have a high lift cam shaft - the first rule of research is that it should be verifiable and repeated, so I will follow your lead and see where we go!

JOhn

Edited April 18, 2018 by JohnD

[oldtuckunder]

13 minutes ago, JohnD said:

Oh, and I can't tell how you derived that highlighted "1.51" but an average of averages is a mathematical fallacy.     You didn't use the Excel command "=AVERAGE(F8:F14)", as that gives the wrong answer of  1.42.    The overall ratio in range is 0.363/0.240, which is 1.5125, or 1.51, as we are not interested in the miniscule.

Further comments on your other points later John, however the above comment is interesting, several "experts" have written and in fact said to me on the phone that "1.42" is the figure to use for standard Triumph cams. Also interesting that we agree that the 1.51 number whilst it can be extrapolated is meaningless.

PS. I wasn't saying that higher cam lift won't give you higher valve lift, as the lift is all cumulative, what I was saying is that the effective ratio starts declining rapidly as the rockers get towards the top of the lift, i.e in the mid range for .030 of cam lift you get .060 of valve lift, whereas at the top end .030 of cam lift only delivers .040 of valve lift, so one cant extrapolate and say a cam with an extra .030 of lift, will deliver that times "quoted" rocker ratio extra lift to the valve, it will in fact be .030 times the effective ratio at that point in the rocker geometry which in the standard Triumph case at the top of the lift is down to 1.33:1 and falling.

What I had meant to add was a comment that not only is it important to know what the lift ratio geometry of your rocker assembly is, but also the lift geometry of your cam lobe. A cam that applies its lift in the 50-75% area of the induction stroke is likely to be much better than one that just adds it all in at the top where the induction flow has nearly stalled,  i.e. a total cam lift figure tells you very little on its own.

[aoie]

Thought I should share, I just recently sent my stock spitfire head into the machine shop and complied the following measurements before removing it from the engine :

[aoie]

A little off topic but could use some feedback on valve push rod geometry. The following picture is my cylinder head intake and exhaust push rods #4 Cylinder. The #4 exhaust rod has what looks to me a significant slant/angle compared to the rest. The head when removed did have significant damage/wear to the valve stem and guide on #4 Exhaust.

Can anyone tell me if this looks to be the norm, or do I have something wrong with the rocker or spacing that would cause such slant on the push rod compared to the rest.

Andy

[Nick Jones]

Flippin' heck......

Top work Alan.  Very thorough as always. Many interesting points brought out, tending to demonstrate that the original designers did in fact know what they were doing and leading me to wish to try the exercise on my own engine with rather higher lift cam (if only to determine whether the high lift is on the right lobes!).  Questions of head skim & pushrod length might also arise.

Aoie, I have noted pushrods running at odd angles in the past but don't recall seeing quite such an extreme angle before.  Difficult to see how you could influence it much though....

Nick

[RedRooster]

Does indeed look quite on the wonk, 

[oldtuckunder]

I can't do it where I am, but on a hunch check that rear rocker pedestal part number, the spit and the 2 & 2.5 engines use an almost identical looking pedestal, but differerent part numbers as the spacing of the sides of the pedestal are different, it would be easy to mix them up. Alan

 

[GT6Steve]

Shouldn't that be a left handed rocker rather than the right handed as the adjacent one.  They usually sit in opposing pairs.  Do yo perhaps have two sitting at extreme angles?

 

[GT6Steve]

Shouldn't that be a left handed rocker rather than the right handed as the adjacent one.  They usually sit in opposing pairs.  Do yo perhaps have two sitting at extreme angles?

 

[oldtuckunder]

Ok back, just taken a look and that is a spitfire pedestal, looking at the two I have that is also the correct rocker with the adjuster set over to the right and the tip over to the left. Not having a complete engine I don't know if that push rod angle is normal or not, it just doesn't look right, is the tip central on the valve stem? If it is, it says the rocker is in the right place,

Alan

[oldtuckunder]

On 18/04/2018 at 10:23 AM, JohnD said:

Here's another way of displaying your own data. Blue line, cumulative cam lift, brown, cumulative valve lift.    I've posted a large chart, so that the linear trend line of the latter can be seen as a dotted line.  As you say, the ratio over the whole range, is 1.51:1.      But this display shows how little it deviates from that.       You could chose other trends , which would deviate more, such as an exponential, but I see no evidence that this plot is anything but linear.

This means that we can extrapolate the lines, by adding more cam lift steps and using the overall ratio to calculte valve lift.  This simulates high lift cams, and  by adding another 0.06 of lift in two steps, you would see this:

No sign of dramatic drop off in extra valve lift, which I suggest you cannnot assume from your observations, but my assumption is that there will be no greater deviation than in the observed range.   As usual,  "More research is needed"!

 

Hi John

This I'm not trying to be argumentative (well OK maybe a bit) but cumulative linear graphs are very deceptive, they have an awful habit of predicting the future based on the past, not the conditions ahead.  To use a motoring euphemism its like driving watching what has happened in the rear view mirror and using that data to predict where you should be turning ahead.

Having spent the 30+ years of my life writing forecasting software for manufacturing and distribution companies its a flaw we find in even very sophisticated software packages, and have spent a great deal of time getting them to behave better.

The simple example is, take a product that has had a demand of 100 a  month for the last year, a simple forecast would be 12 x100 /12 =100, you need to make 100 next month.   So lets add in some change, a 100 a month for 10 months, then 90 then 80, so we calculate 10x100 + 90 + 80 / 12 = 97.5, you need to make 97/98 next month. So as we can already see the forecast for next month is already dubious as we haven't really taken into account what happened in the last two months, (we have a tiny bit, but not enough) Now if those were cheap washers it really doesn't matter if we buy/make 97 instead of ? (and calculating ? is where I have earnt my living) but if they are jet engines the difference between 97 and 70 (as some might forecast) and ? (as I might forecast) could be significant.

So the problem is that a cumulative model of cam lift to valve lift has the same problem, it predicts that growth in valve lift is based on a sort of average of historic valve lift to cam lift, it doesn't factor in significantly enough what the recent/current ratio is, likewise just taking the current ratio is also likely to be incorrect, as in the above example you have to be very careful in forecasting the next step, is 97 a good number? 70? or is it likely to be a more complex calculation that works out the percentage drop on average 2 months ago, and then the percentage drop 1 month ago with a bit of clever weighting that applies a bit more import to the recent trend (but not totally) to arrive at ?

Which is exactly my argument about valve lift, whilst the cumulative trend is interesting and adds weight to the direction of travel of lift, as does the average linear ratio that has occurred, the most important numbers are what the ratio currently is if you are going to try and predict what the next n of cam lift is going to deliver in valve lift.

The fact that the ratio generated by the rocker assembly isn't a static figure, but is moving both up and down, means that you can't use a generic average ratio meaningfully , likewise even though I now know precise ratio figures for my rocker geometry and would have a level of confidence in predicting generally what would happen if I added an extra 0.030 of cam lift as a next step,  I can't be a 100% certain as I don't know exactly what ratio the rocker geometry will deliver unless I actually measure it. What I do know is that its far more likely to be in the 1.3:1 range (because that's what the current ratio is) than 1.5:1 which the linear average would predict.

Alan

 

 

[JohnD]

Thank you, Alan!  If I had realised the depth of your expertise in forecasting, I don't think I would have ventured a comment! 

And as you say, the variation on ratio across the range is a fascinating phenomenenon.   Given the action of rocker on valve stem, while the rocker rotates about a fixed shaft, I expected a change but one that varied as a sine wave, as the absolute distance from contact point, either push rod or valve stem, to rocker shaft varies with angle.  But yopu have proved otherwise!

John

[oldtuckunder]

Comment is always valuable, making someone question a theory or even an interpretation of data is what scientific process is all about, you may regard what I posted as target to be shot at, I certainly do, and given the time I would experiment even more!

Alan

[motov8id]

The rocker ratio is the ratio of the drive end to the driven end. the effective length of the driven end doesn't change as it slides across the valve stem but the drive lenght gets shorter as it moves through its arch and gets closer to the center the rocker shaft

[oldtuckunder]

Only because I would like to know, but are we sure the driven end length doesn't change? although it is roughly curved, does that curve actually compensate, or over compensate for the tip moving in towards the pedestals as the rocker rotates, anyone care to measure?

Alan

[Nick Jones]

What becomes even more interesting is when you start plotting the valve lift against actual crankshaft degrees.  Plotting the results of different cams against each other would be really interesting........

Nick

[oldtuckunder]

13 hours ago, Nick Jones said:

What becomes even more interesting is when you start plotting the valve lift against actual crankshaft degrees.  Plotting the results of different cams against each other would be really interesting........

Nick

That is a point I alluded to above, there are several learned references that say that you want maximum valve opening acceleration (i.e. not just how far the valve has opened,  but how fast the rate of increase in the valve lift is occurring) at between 50% - 75% of the downward induction stroke as that is when the maximum velocity and volume of intake charge can be obtained, which I think is 90-135 deg ATDC range, given that normal cam timing is full lift at 110 deg ATDC (after which the valve is closing) which to me indicates that actually there is a critical 20 deg crank rotation period (90-110) where you would like the rocker geometry and the cam profile to be doing their best together to be opening the valve as fast as possible.

Looking at my measured figures above and taking full cam lift as being 110 ATDC it does imply that the standard rocker assembly does have its maximum lift ratio at about 50% and + of the cam lift, however it does now with hindsight make me wish I had also recorded crank deg at each of the measurement points, as whilst it wouldn't alter the actual lift ratio the rocker assembly was generating for every 0.030 of cam lift, what it doesn't reveal is what the cam profile was contributing to the lift acceleration as whilst we have a start point of crank rotation and an end point 110 ATDC, we don't know if the individual 0.030 cam lift steps occurred within 17 deg cam rotation (the linear average which as we can guess is probably a useless calculation) or maybe as low as 10 deg rotation or as high as 30 deg rotation.

What I do know is that their are some weird games that can be played with cam and rocker ratio profiles. With a box of TR8 bits that arrived from the states last week I also received a new cam, that was developed by a TR8 racer over there with ISKY for the Buick/Rover 3.5 V8 . This isn't a race cam, it does have slightly higher lift than an SD1 Rover cam, but its not a cam designed for a high revving engine, the Rover/Buick 3.5 V8, unless it has a lot of work done doesn't really like going above 5.5K and the standard induction system (which was something I needed to retain) also starts struggling above that.  But what a bit of research showed was this cam had a lot of real world feed back from very happy TR8 users of delivering huge amounts of torque in the mid range where maximum advantage could be taken of the high 3.08 final drive ratio.  This cam's profile, lobe centre angles, and total lift are way different to the average "fast road", "track"  cams for the Rover V8 being sold by the normal off the shelf cam companies over here. With a bit of luck and a following wind I may get to find out.

Alan   

[michaeljf]

Hello, I have just found and read this post and found it very interesting. It would be even more revealing with Nicks last comment added to the equation? Regards, Michael.

[Nick Jones]

Yeah..... I really want to try this. I do have most things needed to do the measuring but I’m not starting another project until I finish one!

[michaeljf]

Hello Nick, I like your response. I have similar thoughts on the project front. Will keep checking to see if this project moves along! Regards, Michael.

[ed_h]

Here is one thing that can make the rocker ratio vary a little over it's arc.  The contact point on the valve stem moves outward slightly as the rocker foot rolls on the downstroke.  The rocker geometry can be manipulated somewhat to make that contact patch narrower.

Ed

[Nick Jones]

On 4/20/2018 at 11:37 AM, oldtuckunder said:

What I do know is that their are some weird games that can be played with cam and rocker ratio profiles. With a box of TR8 bits that arrived from the states last week I also received a new cam, that was developed by a TR8 racer over there with ISKY for the Buick/Rover 3.5 V8 . This isn't a race cam, it does have slightly higher lift than an SD1 Rover cam, but its not a cam designed for a high revving engine, the Rover/Buick 3.5 V8, unless it has a lot of work done doesn't really like going above 5.5K and the standard induction system (which was something I needed to retain) also starts struggling above that.  But what a bit of research showed was this cam had a lot of real world feed back from very happy TR8 users of delivering huge amounts of torque in the mid range where maximum advantage could be taken of the high 3.08 final drive ratio.  This cam's profile, lobe centre angles, and total lift are way different to the average "fast road", "track"  cams for the Rover V8 being sold by the normal off the shelf cam companies over here. With a bit of luck and a following wind I may get to find out.

 

Well, on this, sadly he never did get to find out, but he did get as far as buying the cam, a chain kit and valve springs. These are now for sale so someone else can have a go.

[Hamish]

Keeps his memory alive tho 

I bet he is still racing and fettling tho.