Camshaft /Tappet settings

[marcusaurailius]

Can any one tell me why the gaps on a upgraded camshaft are bigger than on an OE one

eg, standard TR 6 , 35 65 35 65, 250 cam lift, 10 thou clearance, after market cam, same duration, but a little more lift, 25-40 more, but any thing from 16 to 24 thou clearance,

this is even if the cam is ground on a new ..blank..

also , why does one have to set the tappet settings ..wider.. with 1.65 ratio rockers, as opposed to OE 1.5 ratio ones, or so i have been told, but not why,.

i have been told conflicting things, but seeing as we have some..camshaft type... Guru,s on here, maybe I ,and others will end up being a bit wiser!!!

regards Marcus

 

[facty]

The cam profile is specifically designed such that hot lash will be taken-up at a sensible velocity with both acceleration and jerk at minimum values - this ensures minimal impact forces at this juncture.

The valve-lift profile is therefore designed with specific lash ramp characteristics that may change from one designer/engineer to another.

 

[Teacups]

Ivymike - I think your last comment makes a lot of sense and I strongly suspect that this is the basic reason that racing cams have higher ramps/clearances etc.

 

[patprimmer]

I strongly suspect this guy knows what he is talking about even if he is a bit of a braggart.

http://harveycrane.com/lashloop.htm

Regards
Pat
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[evelrod]

Well he may be a bit of a braggart as you seem to think, Pat.
However, all what he says id definitely old school and if my personal experiences are any indication, he is damn well right on.

In the beginning, say, with my '61 Vette race car in the early 60's...I was not knowledgeable enough to stray from the OEM specs (and, I did not trust my peers )...So, even though I used the Duntov cams, I stuck with the specs.
Now, for the last 30 or so years, when setting up a race engine, I always vary the timing by 4 or 5 degrees both ways and vary the lash by up to .004" both ways while doing the dyno run ups...Fiddle with ign. timing a bit, but that is usually less productive...

As to whether lash increases or decreases with temp change...There is no problem with setting pushrod lash cold, as long as you know the numbers...I set pushrods hot and OHC cold...Just a matter of convenience. Some gain, some loose. All by fairly insignificant amounts.

All this does not change my opinion that the clearance ramps and ultimate valve lash is often determined as much by the cam grinders experience as any technical reason. That is definitely old school as am I.

Rod

 

[marcusaurailius]

Pat, Off topic here, a little,

but have been looking for clues!!, and come across an article that says that Crane Cams have noo gone bust,

maybe you already knew this

regards Marcus

 

[patprimmer]

I knew that, and if I remember correctly, Harvey Crane despite being the founder had a bitter bust up with the company some years ago. He was probably pleased to see them go.

Regards
Pat
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[Teacups]

Braggarts I don't mind if they know what they are talking about. Braggarts who don't know anything are the annoying ones.
For more than 40 years I have accepted without thinking about it too much that a tappet's clearance reduces when it gets hot. But is this true? The rule in physics is that a gap expands as if it were solid material. Everything I have seen written about tappet clearance describes it as closing up as the temperature increases - but I now doubt if it does close up significantly.
Treating the whole engine as a solid mass says the gap must increase with temp. Does the valve itself get hot enough (and expand enough) to counteract the overall effect of increasing the gap?
Possibly the gap is there to allow for wear, base circle runout and reasons other than than tappet clearance closing up with increasing temperature.

 

[ivymike]

To quote my earlier post, It is common (but not universal) that valve lash will decrease as the engine heats up. It is almost universal (I am unaware of exceptions) that valve lash decreases as the valve/seat interface wears. The right amount of cold lash to have is (enough to accomodate gap reduction as the engine heats up)+(enough to accomodate valve/seat wear between adjustments)+(a little extra for safety).

Note that you have a short and long side of the rocker which must be considered seperately. If the valve grows by more than the surrounding bits of head, the gap shrinks. If the head grows by more than the pushrod, the gap grows. So on and so forth... If you think that all the parts are the same temperature, then how do you explain the choices about what touches coolant and what doesn't? It would certainly be easier to put all the coolant passages at one end of the block and be done with it (or skip the coolant altogether). Remember the discussion about what happens to valves if they can't transfer heat to the head? That doesn't apply if the valve and head are the same temperature (no heat transfer).

I can assure you that hot and cold lash are almost always different numbers, and that engine OEMs know pretty well (by measurement) how much lash changes from cold to hot. The engine manufacturer I currently work for requires a hot lash measurement test as a matter of course on every new engine program. For small changes it doesn't yield much interesting info - for larger changes... let's just say you skip it at your own (signficant) risk.

 

[patprimmer]

In my experience, engines with aluminium heads tend to open tappets when hot and cast iron heads tend to close them.

Engines with aluminium pushrods tend to close them. Notice I said tend rather than do.

Aluminium blocks will also have an effect vs iron as will relativity of oil to water temperature.

Inlet valves are more inclined to open and exhausts to close.



Regards
Pat
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[PEW]

I'm no expert but logic makes me think that on an overhead valve engine (side cam) like my little 850, if the (steel) pushrods expand less with heat than the cylinder head and block assembly (all alloy on my engine), the tappet clearances will increase with higher temperatures.

 

[ivymike]

alloy... aluminium? How much do the valves expand?

On large diesel engines (all steel/iron), the valve lash tends to decrease when they get hot. Common lash settings (cold) are in the neighborhood of 0.5mm exh and 0.25mm int. Common lash values (hot) are more like 0.2-0.3mm. These are typically cam-in-block engines with mechanical (screw) lash adjustment.

 

[Teacups]

The laws of physics say that a gap expands when it gets hot. (Just why this is I personally find hard to visualise - logic could just as easily tend to make one think that it got smaller). So a gap decreasing on an engine with increasing temperature must be generally considered to be something of an anomoly. I think the valve is the main cause of this odd behaviour. The valve is constrained to expand only in one direction (like the mercury in a thermometer). Presumably the valve's amount of expansion is double what would normally be expected. Does this make sense?

 

[patprimmer]

No

Regards
Pat
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[patprimmer]

I will waste a little time trying to explain again.

1) If you have a block of one substance and drill a hole in it, THE DIAMETER OF THE HOLE DOES GET BIGGER AS YOU HEAT THE BLOCK UP. That happens. It is fact, not opinion whether or not you can personally envisage it. This is the most elementary of high school physics.

2) There are a number of different metals used in the components that influence tappet clearance. These different metals do have a different co-efficient of expansion. Between aluminium and iron, there is a large difference. Between various grades of steel and iron there small but real differences. This means that for the same temperature increase there is a different amount of expansion. This is advanced high school or first year uni physics.

3) Different components in an engine are at different temperatures while the engine is running. Even different spots in the same component are at different temperatures and these temperatures are constantly changing as the engine runs and goes through its compression, fire, exhaust and induction cycles. This is more complex but can be understood by anyone with basic uni physics and some knowledge of an engine.

We are not here to train those who do not understand basic engineering principles. Please read site policies.

Regards
Pat
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[SomptingGuy]

A three word explanation:

Uneven temperature distribution.

- Steve

 

[patprimmer]

Steve. Isaac already tried that explanation but it went straight through to the keeper, hence my resorting to high school student level explanation.

Regards
Pat
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[Teacups]

I didn't intend my meaning to imply that a gap didn't get bigger as it got hotter - of course it does. My meaning was that it is hard to visualise why it does.
I suspect my knowledge of physics is better than most people on this forum.

 

[patprimmer]

Whatever

Regards
Pat
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[marcusaurailius]

Ok ,ladds , been doing a bit of detective work here, and also got some good feed back, from some ..cam manufacturers..

but one thing is clear, the gaps that are given with a cam, all vary, as to what the cam grinder thinks/wants

some say that the gap is needed to ..take..up the gap on a ramp

some say its for thermal expansion

some are saying its both

some are saying that it is because the cams have a lot more lift

but some cams have more lift, and less gaps required,.

all are right , and every one has there own opinion of it

there are so may variables.

EG, My engine, an old castiron lump, the rocker gaps ..doo.. get bigger with a hot engine,

also , when setting the rockers, as stated before, the gaps keep altering, as you turn the engine round, for final checking

this i thought was due to the cam base circle not being a ..circle.., so getting the back of the cam on a high,or low spot, thus altering the setting

this was eliminated, by setting the rockers at the same spot , but still got conflicting readings

but what aboot all the oil on the different parts!!!

cam bearings, cam base/tappet base/push rod ends/inside tappet [full of oil ]/rocker tip/valve tip,.

all are covered in the stuff,

noo when i was setting the rockers, i was getting different readings, so I pushed doon hard on the screw adjuster, and the gaps opened up 1.5 -- 2 thou more

so some thing was getting compressed!!, oil,me tinks,!!!

also when engine had been ..turned over , to re check the rockers, then the gaps would also increase,

this is I think, due to the oil getting ..thinned oot.. by turning over, and no oil pressure to re supply the bearings,

so cam was sitting at the bottom of its bearing, and not supported by as much oil film

and as for pushing doon on the rocker, then i was squashing all the oil oot frae the contact surfaces of the components

Plus , it all depends on how thick your oil is too

so all these things can and doo alter the rocker gap

I actuall ended up setting the gaps to 6 thou, on the original rockers, as 1.5-2 thou bigger when hot, and 2 thou bigger when compressed == the 10 thou OE setting, when not compressed,and cold

on the cam i got noo, and this is where the post was from,

the gaps are to be set at 17--19 thou,

the engine ran noisy with these settings, about twice as noisy the originals set at 6 thou, as stated above

but when i put on the set of 1.65 roller rockers i got [v,v cheap,and new too ] then the engine was a lot quieter

but when , as i found oot, that the gaps had to be increased ,by the amount of extra ratio,

then the gaps went to 19--21 not 17--19 as for original setting for new cam

the extra noise that has come with the 2 thou gaps is a lot,

how can 2 thou diff, make so much more rackett!!

so I have gone and set them to my old formula , as mentioned further back, and took 2 thou off the original setting of 17-19, to end up with 15--17, this should i think still give me enough ..left over gap, to compensate for the extra ratio change

The engine noo, is very quiet, and seems to go as good with a small gap as a big gap

Also as people have siad, the valve timeing has also increased a little

with the big gaps on the roller cam, the ..actuall valve.. was opening at 23 degs btdc, now with my closed gaps its 26 degs btdc, which is not far off, the 28 of what the cam is supposed to do,

So as you can see/read, there are so many variables in a cams gaps specs, that it is best to try and find oot, what works best for you,.

and thanks for all the feed back, well done

regards Marcus

 

[BrianPetersen]

Marcus, sometimes you just have to tinker a little to find out what the engine wants.

I've got this very same job to do on this rainy afternoon ... tightening up the exhaust valve clearance just a smidge on a Honda single with an aftermarket cam, which has been making a bit more mechanical noise than I'd like.