8 replies to this topic

[Old Bob]

The Big Yellow Book (AKA David Vizzard's guide) explains why tappet clearances should be increased when using high lift rockers.  What clearances are people using with 1.5 : 1 rockers and does engine capacity affect this in any way?

 

Thanks all.

 

Bob

[jaydee]

I dont remember what vizard said, anyway its the exhaust that runs hotter, inlets are cooled by air fuel mixture which is quite cold

And if you do some calculation of the extra heat and the consequent valves expansion you'll discover theres not a big difference between 1.21 1.27 and 1.5 rollers

Nominal clearance is 16 thou with hi lift rockers, that said, its not unusual to set them 12 thou HOT

I remember some on here are using exotic gaps like 14 thou inlets and 17 thou exhaust

The shorter the gap, the lumpier your engine will run, youre gaining a few extra degs on the cam.

[Cooperman]

On my 1.5:1 rockers on the 'S' I run 0.012" inlet and 0.015" exhaust set cold.

[Old Bob]

Thanks both.  Peter, quite a difference between the nominal 16 thou and your 12 thou inlet gap - is this to suit a particular cam or just down to experience?

 

Bob

[Cooperman]

Hi Bob,

 

I've never quite understood the engineering logic in increasing the clearance for higher lift cams or offset rocker ratios by 33% (i.e. from 0.012 to 0.016).

The clearance is there because the valves heat up more than the surrounding metal. The actual increase in length, especially with the exhaust valve, is determined by the temp difference between cold & hot. With a higher performance engine, the ex. valve will get a bit hotter, but if you calculate the increased length you will find that an additional 0.003" from the original 0.012" as recommended by BMC on the original 1275 car, the Cooper 'S', will more than cover this expansion. The inlet is cooled by the inlet charge and a more powerful engine will have more inlet charge which equals more cooling of the valve stem & head.

That is what I set and with a 286 cam and 1.5:1 rockers it works fine for me.

[Old Bob]

Thanks Peter - Can't argue with experience so that's what I will go for.

 

Bob

[jaydee]

Well its a bit OT from high lift rockers anyway:

Inlets wont heat up, the only reason i can see behind that, its due to the machining of a cam.

If you take a kent, and shorten the gap from 16 thou then the few extra degs will be at a much slower lift, at a guess it could be bad at low rpm but give a bit of extra at high revs.

If you take a swiftune grind, there wont be much difference in lift raise with the few extra degs.

[Old Bob]

Well its a bit OT from high lift rockers anyway:

Inlets wont heat up, the only reason i can see behind that, its due to the machining of a cam.

If you take a kent, and shorten the gap from 16 thou then the few extra degs will be at a much slower lift, at a guess it could be bad at low rpm but give a bit of extra at high revs.

If you take a swiftune grind, there wont be much difference in lift raise with the few extra degs.

According to Vizzard the A series likes a fast lift which could be one reason for not going too far from standard.

 

Bob

[Ethel]

The reason you need any clearance at all is to create a running clearance between the cam & follower when it's not on the lobe, about 3/4 of the time (the valve is for 1 of 4 engine strokes, "suck, squeeze, bang, blow". The gap at the valve, divided by the rocker ratio gives you the gap on the follower's side.

 

The argument I've heard for Rover increasing the specified exhaust valve clearance is that it left the valve on its seat a bit longer to transfer heat and cool itself, thereby reducing the risk of pinking & running on.

 

The fast lift thing is because flow is far worse when the valve is near its seat. The "curtain" area only (if ever) becomes equal to the valve area when the lift is 1/2 the valve radius (you could subtract a bit for the stem). At low lifts you'd also lose more to friction and drag - why multi angle valve seats work.