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

 

[ivymike]

The primary reason that I'm aware of for having a gap at all in the cold/newly adjusted condition is so that you don't end up holding the valves open when hot/worn. Holding the valves open even a tiny bit reduces their ability to transfer heat to the seat, which quickly results in failure.
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).
If you produce more power with the same hardware, you'll likely see more reduction in lash in the hot state, so you need more lash in the cold state.
When you increase the rocker ratio, you increase the amount of motion on the valve side w/respect to the cam side. If your lash is adjusted on the cam side, then it would take less lash with a bigger ratio to accomodate the same valve expansion. That's apparently the opposite of what you've been told, so I can't give you a good explanation on that one.

 

[patprimmer]

Tappet clearance is necessary to prevent the valve ever being held off the seat by the base circle of the cam. The amount of clearance required depends on the relative degree of expansion of the valves, head, block tappets and push rods and the accuracy to which the base circle is ground.

For the same clearance at the cam to tappet interface, you need more clearance at the valve for higher ratio rockers. Whether or not the tappet clearance or the rocker to valve clearance is the more important to maintain is arguable.

In reality tappet clearance can often be reduced considerably from recommended settings thus giving an increase in duration and lift and reduced noise and wear. To tight runs the risk of a total loss of power if a valve is held off it's seat.

Regards
Pat
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[Lou Scannon]

"To tight runs the risk of a total loss of power if a valve is held off it's seat"
... and burned exhaust valves, making the loss of power irreversible other than by overhauling the head.

 

[patprimmer]

and burned exhaust valves, making the loss of power irreversible other than by overhauling the head.

Only if they are a complete idiot and continue to drive with something obviously wrong, Oh OK I see your point.


Regards
Pat
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[marcusaurailius]

Thanks for the intrest, but I dont think Ive got yhe answers im after!!!!

Ivymike, youve come close, by saying that the bigger gaps in the tappets/ [rocker arms ] as we call them over here, are for an engine with more power,

i can see where ye coming from with this, but read below.

Pat, i may have picked you up wrong, no offense meant, but I am well aware that engines need running clearances,, but ta for the reply any how, ,Maybe you can come to my help below!!!

But even on an 18-58-58 18 deg seat to seat duration, and 220 thou lift at cam lobe,

a very mild cam, peaking at 4500 rpm,and 100 bhp

the settings are still the same for a OE cam of 35-65 65-35, with 250 at the cam lobe, and 5500rpm peak,and 142- 160ish bhp, depending on exhaust and air box,s

same engine/engines just different cams, on above

what Im trying to find oot, is why after market cams , all have bigger running clearences, even when made on a ..new blank..., and even when they are ..very.. similar to OE cams, the running clearances are still nearly ..Twice .. as big,!!!

it just does not seem to be with my older type of engine either, but all across the board,

[ is it the same with the big V 8,s some of you Ladds run!!]

Why is this??????

regards Marcus

 

[patprimmer]

For the theory.

I thought this was implied earlier, but on the same engine, the only reasons one cam requires more tappet clearance than another are:-

1) The base circle has more run out.

2) Expansion of some parts closes the gap more. This can be because of higher temperatures of some components giving a higher temperature differential to other components. If the only change was the cam, I would not really expect any real changes. Maybe (it's a long shot) the exhaust valve might get a bit hotter, but I really doubt it.

3) The engine is used in a different manner, but even on a stock cam, the clearance has to cover all manner of operation.

For the practice.

Tappet settings are a catch all recommendation with safety factors built in so they are NEVER to tight. Maybe aftermarket incorporates a bigger safety margin and maybe the aftermarket thinks that clattery tappets sound like a race car, so they are desirable to the market sector.

I tune cars with up to 290 deg at 0.050" and over .700" lift making over 1000HP. At times, I close the tappets down to the minimum I can without holding a valve off the seat.

If I think I need all the cam I can get, I close the inlets to 0.010" and exhausts to 0.012" hot, ASAP after a full power run. If I think I need less cam, I will open them up to 0.030". I don't really consider the cam grinders recommendation, however if the grinder recommends tight, I have more confidence in him and his quality.

This is on a blown injected methanol SBC with 23 deg aluminium heads, roller followers, SS valves, CrMo push rods and 1.65:1 rockers.

I have never had any valve train damage attributable to any settings within that range.







Regards
Pat
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[ivymike]

could it be that the aftermarket cam manufacturer has not gone to the effort to determine the "best" setting, and is just reciting a particular value which has worked in the past?

 

[evelrod]

Let me add my two cents here...

In my experience with several racing engines, the valve lash has been different with almost every camshaft, even for the same engine. The often quoted reason being as Pat and Isaac have stated...However, I have found that further research into the "engineer" that came up with the specs is a much better indicator. Seems these designers have their own personal tastes (based on experience?). Perhaps that can account for some of the variation we see in aftermarket cams?

As to MY current engines...The Austin A engines have most of the cams (Kent SP310 and SP296) lash called out for 0.016"+ range by design..In fact, I use .018 and .020 hot (determined on the dyno) and when cold they vary only by a couple thou. Now the Lotus twincam settings are wildly different, anything from Cosworth's typically tight .002" to McCoys .008 and CTE's at .014"...In practice the cold setting of .002 work just as well as the .018" I have tried on the CTE G-3's. This variation has been virtually the same in every engine I have ever built. What I mean to say is, referring to my initial statement, the lash called out in an aftermarket cam can and often is a product of the designer/engineer's personal preference. I am sure that blanket statement has a bunch of holes in it, but that's the way it looks to me.

I have spent countless hours on the dyno trying to make that big improvement by juggling the valve lash and timing...Generally a waste of time and money. There simply is not much to be gained (much to loose) going to far afield the nominal specs.

Rod

 

[malbeare]

"A slappy tappet is a happy tappet" A common saying amongst the older mechanics I know here in OZ.
malbeare

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[marcusaurailius]

Thanks Ladds, I seem to get the jist of it,

I was running the cam at 17 inlet, and 19 ex,

but on recomendation, from some one, who said that cos im on , 1.65 rockers, instead of 1.50 ones, then the gaps should be opened up to 19 and 21

as higher ratio rockers need more clearance,

this I did, and the engine now sounds very ..clattery..

does no seem to go any better , just a lot more noisy


A slappy tappet, is a happy tappet, never heared that one before, maybe have to tell the neighbours this, when they complain me cars too ..clattery.. for them , hee hee hee

thanks for the info though all of you

regards Marcus

 

[MikeHalloran]

I read somewhere that cams for solid lifters include a 'clearance ramp', to gently take up the lash before intentionally opening the valve. You should be able to see this with a dial indicator, zeroed when the lifter is definitely on the base circle, e.g. diametrically opposite the lobe. As the cam rotates toward the area where the valve should start opening, the lifter will slowly take up the lash, and then begin moving the valve. For this to work, the lash has to be set _exactly_ as the cam designer intended. Too little lash opens the valve early; not far open, but enough to burn a valve. Too much lash, and the cam whacks the lifter up noisily instead of gently engaging it.



Mike Halloran
Pembroke Pines, FL, USA

 

[franzh]

Back in the 1960's, Chevrolet released a 375 bhp 327 cid engine for their Corvette that found its way into other cars too. It had a cam that quickly found its way into other engines. It was named the "30-30" cam, for its 0.030" valve lash.

It was noisy. One commonly used rationale was the wide lash allowed a little valve bounce without hitting anything.

Another cam had .018 int and .024 exh, called the Duntov cam, of the early 283 fuel injected engine.

Franz

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[dicer]

I think it has more to do with the ramps and velocity of the lobe pattern in that area. As well as expansion of parts.

 

[marcusaurailius]

Mike,im with you on that, as i know that cams have take up

ramps,the one i have has these on, it says so in a

diagram, that was in the box,

the cam runs solid tappets/lifters

too tight and it puts undue pressure on the cam lobe.



Franze, this is sound thinking, but dont rev that high,


Dicer, , this is what im tending to think tooo, as I said

before, one a mild engine, one 50% more power,[cam and

injection only ] but still the same rocker arm gaps, so

expansion is not really an issue, at least with the OE cams,


Have we a ..cam manufacturer.. on here, that can say why

their cams run wider gaps,!!!

thanks , Marcus

 

[Teacups]

The amount of tappet clearnce must match the amount of lift on the opening and closing ramps of the cam lobe. The ramps can clearly be seen on a lift/vel/accel graph of a lobe. They show up a horizontal constant velocity trace if the cam lobe is measured without any tappet clearance. There is no point in making the tappet clearance greater or smaller than the ramp height. Just why higher performance cams have higher ramps I am nor sure - maybe a high performance (or racing) engine might be expected to run at extreme temperatures?

 

[patprimmer]

I agree different cams have different ramps in both design and length. So long as the tappet clearance keeps the follower hitting on the ramp and never holding the valve off the seat it will work, at least in my experience.

I have seen anecdotal evidence that the 30 30 cam ran better with tight tappets, but I only ever ran the the 18 25 or so called Green Grind Duntov. I have seen the 30 30 refered to as a Pink Grind Duntov.

Regards
Pat
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[ivymike]

Just why higher performance cams have higher ramps I am nor sure - maybe a high performance (or racing) engine might be expected to run at extreme temperatures?

high temperatures are a worse application for tall ramps, since you'll be signficantly increasing the duration in the hot condition vs. the cold condition. having a tall ramp helps you seat the valve on the ramp instead of simply dropping it onto the seat when you're running fast and the valvetrain is vibrating, so you'll put up with the change in duration to keep the valves from breaking apart.

If you plot dynamic valve position v time overlaid upon kinematic valve position v time, you'll see that at high load/speed the vibratory aspects of the motion are of significant magnitude relative to the ramp height. Since you don't want to slam the valve shut, you need to have a tall enough ramp to catch it when the whole train is wiggling.

 

[marcusaurailius]

One thing i forgot to mention, dont no if it affects other engines, but the ..rocker gaps.. get wider by about 1.5-2 thou, when the engine is hot

depending as to how hot it is, then the gap is bigger

its a cast iron head, block

I would have thought that the gaps would ..close.. up, as metal expands, with heat,!!!

am i alone in this phenomonom !!

thanks for all the intrest so far

regards Marcus

 

[patprimmer]

If the block and head expand more than the push rod, the gap opens. How does your block temp compare to your oil temp when you measure. All other things being the same, exhaust valves normally expand and close the gap and inlets normally cool and open the gap.

Regards
Pat
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