SAH cam - valve clearances?

[88V8]

As you may have seen, I have had a knotty problem with the brakes (It Servos No useful Purpose) and have only this minute found the cause, which is that the manifold vacuum is only 6 in Hg at a 700rpm idle (should be 12.5 in Hg with a PI car). Hence virtually no servo.

 

In the engine, as from 1976, is a what the owner who had the work done describes as an 'SAH medium cam'. The profile, he recalled, was 'like the TR5'.

 

Now, the subsequent owner-muppet from whom I bought the car has lost all the data sheets, so I don't know the correct valve clearances. At the moment I'm running the standard TR6 clearance, ie 10 thou. If this is too tight, perhaps that partly accounts for the low vac?

 

The engine had a lot of other work done, including porting, polishing and higher comp - don't know how high - and idles very lumpy.

 

Compressions are in the region of 220psi (yes, two hundred & twenty) - hot to first fan cut-in, 12 seconds, full throttle.

 

So, question: does anyone have any idea what the valve clearances should be?

If not, what's the standard clearance for the TR5 please?

 

Ivor

 

PS I'll replace the inlet manifold gasket, just in case I have a leak

PPS I'll post the 'solution' in the Servos No Useful thread, just to close it off.........

[Terry Pruce]

If it's a hottish cam, then those clearances are too small. Try 22 thou inlet and 24 thou outlet. That is what I use on my TR6.

 

I once changed mine to 10/12 thou and found that the engine felt stifled, not lumpy. As if everything was too tight.

[JohnC]

I'm running a fairly hot cam (inlet valve opening at 43 and closing at 79; exhaust 75/40). With 12 thou clearances I saw 9" at idle, which increased to 11" when I opened up the clearances to 18 thou. More dramatic was advancing the cam timing by a tooth, which brought the idle vacuum up to 11-12" with 12 & 14 thou clearances (in/ex). It also gave me heaps more torque lower down, so all was good!

 

Based on advice I was given by many people, I would be cautious about clearances as wide as Terry is using. At 18 thou, my engine was a lot more tappety, and my cam supplier advised me not to go any further - he said the loading on the valve gear would increase rapidly. Something about the ramp profile, but we're talking second hand advice here!

 

On the other hand, I had no problems with braking when I 9" vacuum, so I do wonder if this is really the only source of your problem. Possibly a contributor of course. I'd try opening up the clearances a bit, and see what happens to vacuum and to brake effectiveness, and then see if you need to re-think.

 

Good luck, and keep us posted.

Cheers,

John

[88V8]

How do I do this....?

I thought I could measure the valve opening & closing points by putting a DTI on the pushrod (actually on the adjustment screw) to detect the start & end of the lift, and time it with a scale around the crank pulley. And this would give me the cam spec. Simple..

 

But the results make no sense, because the opening is pretty well 360.

So what am I doing wrong? Well, it seems that cams are specified on the basis of measuring from the point where the valve has lifted X amount off the seat. And X varies from manufacturer to manufacturer.

Could be 050" or 008" for example. And that's a big difference in angle of arc.

 

On my SAH cam, for instance, the first 10 thou of rocker lift (OK the valve lift is not the same, rocker ratio, but just to illustrate) takes 45 degrees of arc. So without knowing the value of X, I can't spec the cam.

 

I asked Moss, but the only guy who might know is away....

 

So, in the abstruse information department, does anyone know what X is for SAH cams?

 

Otherwise I think I'll just open up the valves to 14 inlet, 16 exhaust, and see what happens.

 

Ivor

[yoda motorsport]

without knowing if the car ran fine before you have two situations

you are very right to measure the lift of the camshaft in this way off the rear of the rocker directly above the end of the push rod you can determine lift and duration in this way using a timing disc which can be set on the tdc mark on the pulley as these are usually very accurate.

if the car was running fine it could be a number of things a sticking valve may have bant a push rod i have seen this before also the cam could have worn badly causing a gradual loss of inlet manifold vacume

if the car was not running and you have fitted new push rods make sure you have fitted the correct type there are two lengths

the best way to identify the cam and its condition is to pop it out place it on engineers v blocks on a thick metal sheet (to hold your dti magnetic base to ) and measure the lobe lift and duration and assess its condition

the camshaft can be removed without taking the head off using magnets to pull the follower bucketts up the pick up tools are ideal for this and are cheap enough to purchase 6 instead of a head set

also cutting a notch out of the front vallence (covered up when grill is in place ) will allow you to withdraw the cam without disturbing the engine mounts

[piman]

Hello Ivor,

 

I don't think it is possible. Cams may well be specified with X degrees of opening i.e. like my SAH cam type 307 is a 30-70 but cams are ground with quietening ramps so in other words my 30 -70 doesn't stay fully closed until 30 then open to full lift in 140 degrees (Duration being 280) but starts before that. You would need a full design drawing to identify it and where will you get that?

 

By the way did you notice my reply on the TR5 forum, my cam has (I still have the spec sheet) 10 and 11 thou inlet and exhaust clearances specified.

 

Alec

[88V8]

I don't think it is possible. Cams may well be specified with X degrees of opening i.e. like my SAH cam type 307 is a 30-70 but cams are ground with quietening ramps so in other words my 30 -70 doesn't stay fully closed until 30 then open to full lift in 140 degrees (Duration being 280) but starts before that. You would need a full design drawing to identify it and where will you get that?

By the way did you notice my reply on the TR5 forum, my cam has (I still have the spec sheet) 10 and 11 thou inlet and exhaust clearances specified.

Alec

 

Yes, thankyou Alec, I suspect you are right, I am not going to get anywhere trying to spec my own cam as I won't find the value of X, and even then it is to be honest jolly hard to measure, especially given that the different lobes have probably worn at varying rates so I'd need to measure and average all twelve of them, get a life I hear you say, yes, well.

 

And even popping it out as Yoda ingeniously suggests wouldn't help unless the cam has an id stamped on it.

 

I did notice your response and I thank you for it, my leaning towards a wider setting was in the main influenced by the data on the Moss site where the clearances for pretty well all the TriumphTune cams are in the 14 thou range & upwards.

At 10 thou it does sound suspiciously quiet.

 

I'll let you know, I'm sure you're dying to know, zzzzzzzz.

[Red 6]

It might be safer using larger clearances, ie 20 thou in and ex.

 

I would also suggest giving Kent cams a call. They are very helpful and should be able to turn what information you know into something meaningful.

[88V8]

Ok just to round this off..

 

Basically not able to measure the cam opening/closing without knowing the 'ramp measurement' as the value of X is called (at least I learned that) and Moss/Triumphtune/SAH as was, don't know the value, so taking heed of various forum suggestions I've picked a number any number, and opened up the valve clearances from 10 thou, to 14 thou inlet & 16 exhaust.

 

There is still no appreciable tappet noise which slightly worries me, but the alloy rocker box may have something to do with that.

 

On the plus side, less danger of frying the exhaust valves, and the manifold vac has increased from 6 inHg at idle to 9in which may have helped with the brakes.

One day I might remove the camshaft and find out what I've got, in the meantime, thankyou.

 

Ivor