[Fot] Oil pressure and tolerance on valve clearance

[Michael Porter]

On 6/3/2014 7:03 AM, Duncan Charlton wrote:
> I've seen anywhere between 70 and 100 psi on our racing engine when 
> warmed up and on the track but never as low as 50.  I have read that 
> 70 to 80 is best but I never had any leaks of the engine or the oil 
> cooler with 100 psi.  By the way, where are you measuring the 
> pressure?  The oil line to my gauge was tapped into the oil gallery on 
> the left side of the engine rather than the stock location to get the 
> most accurate reading of what's going on in the oil passages.

50 psi does sound a bit low.  Still, the important thing to determine is 
if the low pressure is due to excessive clearances or leakage past the 
pressure relief or pump problems (or some combination of those).  The 
old rule of thumb is 10 psi per 1000 rpm, so at full chat, 50 psi isn't 
enough pressure.

>
> I don't think it's unusual to see a range of valve adjustment although 
> it most often seems to be stated thus: "0.010 to 0.012". I looked at a 
> list of specs for Briggs and Stratton small engines and most of their 
> engines are given an allowance range of 0.002" to 0.004": 
> http://tinyurl.com/m2vynmd
>
> I understand it can make a difference in power if you set valve lash 
> at the small end of the range since the valve opens earlier and closes 
> later but the amount of time the valve sits on it seat is shorter, so 
> there is a tradeoff in loss of time for heat transfer.

Larry Young could speak better to this, I'm sure, but comparing the lash 
of other engines is just an apples and oranges exercise that's more 
likely to mislead than help.  For instance, the stock cold clearance on 
the old VW aircooled engines was 0.006".  However, because of the 
engine's construction, its hot clearance was 0.015-0.016", and it's the 
hot running clearance that matters. There's a very small advantage in 
decreasing the clearance balanced against a much bigger chance of 
ruining the lobes or the lifters on the lobes' initial ramps. That's the 
primary reason why the clearances on racing cams (with steeper ramps) 
are generally quite a bit larger than stock, IIRC, a situation that gets 
worse with increasing spring pressure.


Cheers.

-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....