Tappet clearance

[StephenRae]

Another thread got me thinking about this...why not disregard manufacturers advice, get the engine stinking hot and set the clearance to say 1 or 2 thou. Wouldn't this be enough to ensure the valves are closing and probably give a little more valve lift?

[thebear]

Originally Posted by StephenRae

Another thread got me thinking about this...why not disregard manufacturers advice, get the engine stinking hot and set the clearance to say 1 or 2 thou. Wouldn't this be enough to ensure the valves are closing and probably give a little more valve lift?

That could possibly afect the valve timing.

[Eric Falce]

That depends on what engine it is and valve gear.One or two thou could easily be taken up under hard racing conditions but again,you may get away with it on overhead cam system but not on pushrod!

[graham bahr]

i've done it, some engines like it others hate it, the effect on valve lift is marginal to say the least, but it does effect the timing and particulary lift on overlap quite a bit, either way gains are not massive but losses could be (burnt valves etc), personally i wouldn't now bother unless i was racing in a formular the dictated i used a std camshaft, that said it can be worth trying a few tho either way of the std setting, one of my bmw cams gave 1.5 bhp and 3lbft more with a 2 thou increase over the recomended tappet clearance, not a lot granted but every little helps, but to experiment in such a away you need a very accurate dyno, or you cant be sure as to what if anything you have actually achived

[Al Weyman]

Why bother, the cam or original manufacturer will have done many hours research already to get the optimum settings and some are set hot anyhow. Also once that valve is held open from it's seat while firing, in a short time it will burn out and can also damage the seat so make sure you get it right as it could be expensive.

[thebear]

Originally Posted by graham bahr

i've done it, some engines like it others hate it, the effect on valve lift is marginal to say the least, but it does effect the timing and particulary lift on overlap quite a bit, either way gains are not massive but losses could be (burnt valves etc),

Burnt valves would be the result of insufficient clearance as they would not properly seat.

[Richy_Rich]

Wasn't there some car you had to set the tappets with the engine running!?! Vauxhall, I think it might have been...

I've done the above before in an emergency on a TR2, got us home - not sure if it's recommended practise but it's pretty hard to wreck what is ostensibly a tractor engine

[Dave Brand]

Originally Posted by Richy_Rich

Wasn't there some car you had to set the tappets with the engine running!?! Vauxhall, I think it might have been...

Yes - the pushrod Viva engine. It didn't have a rocker shaft; the rockers were cupped pressings which pivoted on a self-locking nut with a hemisperical lower surface, screwed onto a stud pressed into the head. As the adjuster nut was stationary it was easy to adjust the tappets with the engine running.

Chevy V8s, I believe, use(d) the same setup, so they are probably adjusted while running.

[Al Weyman]

Originally Posted by thebear

Burnt valves would be the result of insufficient clearance as they would not properly seat.

Well one thou clearance sounds pretty marginal to me, I agree with Grahame.

The Chevy as standard has rockers as you describe but as the standard engines that use this set up have a hydraulic cam you set the lash with the engine static. There is one hell of a lot of oil pumps up to those rockers so you would not want to be adjusting them with the engine running. I do remember the old Viva and you could adust them like that.

[graham bahr]

vauxhall victor 1600 FC engines 1963-67 were also supposed to be done running, they did have a normal rocker assembly

[graham bahr]

Originally Posted by Al Weyman

Why bother, the cam or original manufacturer will have done many hours research already to get the optimum settings.

those optimal setting will only be optimal for the tested set up, with extensive testing on my engine i found the inlet tract length had an effect on the optimum exhaust outlet size, likewise camtiming and tappet clearances

[zefarelly]

My Cosworth A6 cam runs 18 thou inlet clearance and 20 though exhaust, it seemed like a lot to me, but I've stuck to it with no problem thus far. In theory closing the clearance up will give a slightly longer duration and lift, you can also work it the other way, but I suspect its pretty marginal unless you have the kind of engine thats incredibly highly strung, the reality for most of us is probably that we can't afford that level of tune so it becomes irrelevant anyway, the 4 stroke tuning book I use suggests yo may adjust the clearances in certain circumstances (track dependant) but I've never bothered, I certainly wouldnt open thme up too much because of the risk to the pushrods

[graham bahr]

that seems a lot, but a big part of it is the shape of the opening and closing ramps on the cam profile, these ramps are designed to seat the valve at the right speed, too fast and it may bounce or at least make lots of noise, too slowly and the valve seats will deteriorate very quickly, which is another reason not to run with next to no valve clearance unless your planning very regular topend rebuilds., but generally its safe to experiment a few thou either way

[zefarelly]

I always ran 15 on my old A3 cam, If and when I have an hour of rolling road time to kill I might try dropping it slightly

[Red Dog]

Be really careful about closing valve clearances - you are very likely to reduce the performance and burn a valve. Closing the clearance ruins the performance because it changes the valve timing significantly. For 5 thou extra lift you may alter the opening and closing points by 10 or 15 degrees! Also if you check the base circle on most cams you will find it is rarely completely circular even when it is supposed to be. set minimal clearance at the wrong point and suddenly your valve does not close properly. Add the huge temperature seen by the exhaust valve in race conditions and resulting expansion and you are heading for trouble. With all due respect you cannot replicate the conditions of temperature whilst running, by getting the engine really hot in the garage. Even if you bring it straight off the track, by the time you measure the clearance it has cooled too much. See if you can get your head under the bonnet of a race car at 8000 rpm on a rolling road and look at the colour of the exhaust - if you can stand the noise!

If you have an overhead cam direct on the bucket you are seeing the true clearance - if you have ohv pushrod you cannot see the true clearance and the rocker ratio has to be considered i.e. Clearance at valve 10 thou, with 1:1.25 rockers at cam lobe it is 8 thou. If you then install a 1:5 rocker the clearance must increase to match - 10 thou x 1.2 = 12 thou, and thus the clearance at the cam stays at 8 thou. NOTE:These are not suggested settings they are just convenient for calcs! BTW most A series engines have settings around 16 to 18 thou at the valve with high lift rockers.

If you want to get more performance from standard cams concentrate on getting the timing optimised. Never trust the manufacturers marks to time the cam. Find and use the Lobe Centreline Angle. There are other methods but the LCA method is very good particularly when you have one cam and cannot vary it. Try advancing or retarding the cam from the manufacturers settings by 2 or 3 degrees and don't forget to reset your ignition timing. Optimise the valve seats (within the rules) and make sure you have good guides and valves as even slightly leaky ones don't help. Check your valve seat pressures are high enough.

[zefarelly]

that makes perfect sense Red Dog

when I came in at Combe I noticed my exhaust was red hot at the collector! ( I looked as soon as I opened the bonnet and theres a tiny gap in the exhaust wrap when you look at a certain angle)

[Al Weyman]

Zefarelly are you sure you are not running the ignition without enough advance as this as well as cam timing out can cause a glowing exhaust manifold.

[zefarelly]

good question, but I think it was because a bolt came loose on the manifold, blowing a gasket and I did halfe a lap with a misfire ands running a bit lean !

[Dennis.Doyle]

Hope this thread isn’t too old to re-open, but there’s still something bothering me about tappet clearances. I take the points above about the need to allow for expansion, production tolerances/wear on the base circle & faulty measurements. However, presumably it’s still the case that the smallest acceptable gap is still the best as it gives the smoothest the take-off & landing as the cam goes off & back onto the ramps on the cam’s base circle?

If this is the case, why do we see so much variation in tappet clearances suggested for cams with relatively similar characteristics & intended for the same application? The example I’m thinking of concerns xflow race cams:

Holbay K3A: Inlet 0.015” Exhaust 0.021” (figures from Coltec)
Piper XFA6: Inlet 0.016” Exhaust 0.018” (figures from Piper)
Cosworth A6: Inlet 0.018” Exhaust 0.020” (figures from Kent Cams)
Kent 254: Inlet 0.022” Exhaust 0.024” (figures from Kent Cams)

That’s 7 thou spread on inlet & 6 thou on exhaust. Even stranger, the minimum difference between inlet & exhaust is the Piper at 2 thou & the maximum difference is the Holbay at 6 thou.

Presumably they all make the same allowance for heat expansion & production tolerances as I guess the A6 sold by Kent means it’s an A6 profile manufactured by them. Once you have subtracted this allowance, then the relative differences between them are even greater.

Any suggestions as to why there are these variations?

One rather worrying thought is that there are one or more misprints in there.

BTW I’d need to see a pretty powerful argument to get me to go below those numbers as I’m working on the assumption that the cam designers know rather more about this than we do!

[trikesrule]

But do they practise what they preach. I guess so. Years ago I dirt track raced a Valiant slant six. It had mechanical lifters and adjustable tappets. When the track was hard and slick (fast) I used too open the clearances a little for more top end and vica versa for a wet soft track. Gave me an edge when Qualifying. When I'm done I look to see if I've reshaped the box I'm trying to think outside of -meaning was it worth it..........trikes

[johnny yuma]

Remember the valves are not cooled by the water flowing through the engine they are too isolated.The incoming charge of fuel/air cools the inlet valves but the exhaust valves only gets cooled when they actually touching the valve seat-if you minimise this or accidentally eliminate it by going for 1 or 2 thou clearances the valves will suffer and compression will be lost.

Anyone know what Sodium filled valves do?

[Dennis.Doyle]

Not sure, but I think it might be just to help heat transfer, e.g. up the stems to the guides.

My question is really about, why all these differences on both inlet & exhaust?

[Notso Swift]

Originally Posted by johnny yuma

Anyone know what Sodium filled valves do?

They help heat transfer away from the valve face and up the stem, so the have the benefit of transfering heat through guides/tappets /lifters/any thing else that is touched to the oil (primarily) Where valve seats put most of their heat into the water.
A solid valve does the same thing, but not as evenly or as fast

Why inlet exhaust different
Exhaust gas on Turbo car 900 to 1050 deg C (some special wheels will take 1200), Non Turbo 800 to 850 deg C
Inlet gas for Turbo maybe 70 deg C, often 30 sometimes 100, Non Turbo around 30 deg C
Exhaust has to work a lot harder to move the heat way as there is no air cooling

[Dennis.Doyle]

Sorry, I'm not making the question clear. It is the differences between different cams, with rather similar profiles, that I'm askng for an explanation, i.e.:

1. Why can Holbay K3A get away with Inlet 0.015” wheras Kent 254 needs: Inlet 0.022”?
2. Why can Piper XFA6 get away with Exhaust 0.018” wheras Kent 254 needs Exhaust 0.024”
3. Why do Piper, Cosworth & Kent reckon you only need an extra 2 thou on exhaust, whereas Holbay reckon you need 6 thou. I understand you need more clearance on an exhaust valve beacuse it will expand further and need to dissipate more heat. What I don't understand is why the manufacturers have such different views on this. They don't know what valves we are going to run or what temperatures they will run at & they are all for racing applications in the same type of engine.

[phoenix]

My guess is that although they don't know what valves you are running, they do know what acceleration and deceleration ramps they have ground onto their cams.

Two cams that have the same max lift and the same valve timing at, say, 1mm lift may have an identical 'main' profile, but these two cams might have very different characteristics on the acceleration and deceleration ramps, particularly from different cam grinders as they all seem to have their own in-house preferences. As these ramps may be of different 'lengths' in terms of cam degrees, different clearances might be specified so that the foot of the main opening ramp is met at the same cam angle for both cams.

My own humble thoughts are that reducing the clearance specified might use more of the cam profile, but areas of the profile which have no positive effects but may have damaging effects:

1) An inlet valve does not flow much below 1mm lift, so there is no point in taking longer than necessary in getting the valve open by a useful amount. The slow opening of the initial ramp would not, I imagine, make much if any improvement. Without any effective flow the 'timing' of the opening event will not be much affected either by the earlier opening.

An example I can give is an Abarth cam. The difference in timing between using 10 thou and 20 thou clearance is about 16 (crank) degrees so the valve would begin to open 16 degrees earlier but very slowly, as after the first 16 degrees the lift would be 10 thou. Flow would not be achieved until 27 crank degrees after valve lift began.

Using the correct 20 thou clearance, the same 10 thou lift is available after only 6 degrees and after another 6-7 degrees flow would begin as lift would 44 thou. By 16 degrees the lift is 87 thou - an improvement of 77 thou over the slow opening above.

Opening the valve 16 degrees earlier by reducing the valve clearance would seem to give only a 2 degree advance benefit (< 1% of the valve opening duration) and 10 thou peak lift benefit. Both are pretty insignificant in terms of flow improvement.

2) Unlike flow into the cylinder, when the inlet valve closes pressure inside the cylinder will be building to greater than atmospheric pressure and this will escape out of the cylinder through even the smallest valve opening. Late closing of the inlet valve by using smaller tappet clearances could lose cylinder pressure (compression) and therefore power.

3) Opening the exhaust valve as swiftly as possible increases the energy in the exhaust manifold, which is then used for scavenging to improve cylinder filling and thus power. Opening the exhaust valve too slowly could lead to the valve burning, so using small clearances could lead to premature valve failure.

4) An exhaust valve that isn't closed 'swiftly' but left hovering near the point of closure for longer would similarly be likely to burn out quicker than a valve operated correctly by following the cam profile intended.

These are my humble opinions, and I may be utterly wrong, but I do think most of the cam guys know their stuff - and I have used a number of them - so I am happy to use their clearance figures (even if I don't always use their suggested timing!)

Originally Posted by Dennis.Doyle

Sorry, I'm not making the question clear. It is the differences between different cams, with rather similar profiles, that I'm askng for an explanation, i.e.:

1. Why can Holbay K3A get away with Inlet 0.015” wheras Kent 254 needs: Inlet 0.022”?
2. Why can Piper XFA6 get away with Exhaust 0.018” wheras Kent 254 needs Exhaust 0.024”
3. Why do Piper, Cosworth & Kent reckon you only need an extra 2 thou on exhaust, whereas Holbay reckon you need 6 thou. I understand you need more clearance on an exhaust valve beacuse it will expand further and need to dissipate more heat. What I don't understand is why the manufacturers have such different views on this. They don't know what valves we are going to run or what temperatures they will run at & they are all for racing applications in the same type of engine.