Re: 13-1/2 to 1 compression ...on the street?

Stan Weiss wrote:
> 
> Steve wrote:
>> M.Burns wrote:
>>> Looking at a nice Pro Street car with 13.5 to 1 compression pistons. 
It
>>> apparently has been at some statewide cruises, rod runs, etc, so I
know it's
>>> been driven. My question is, what kind of fuel must the owner be
running for
>>> the engine to survive. Heard claim that by retarding the timing
somewhat,
>>> pump gas with octane booster works(?)  Race fuel?  Any ideas  (...from
guys
>>> familiar with similar cars or situations). Thanks
>>>
>>>
>> It depends on a lot of things. How radical is the cam? If its a HUGE
cam
>> with a ton of overlap, then the actual compression pressure might be
>> manageable enough to live on pump premium with a mild octane booster.
> 
> explain to me how overlap and not IVC will effect low rpm cylinder
> pressure?

You obviously understand the issue. IVC timing is the key event that 
controls low RPM cylinder pressure. But late IVC which lowers cylinder 
pressure usually goes hand-in-hand with a bunch of overlap, so the slang 
creeps in. Lots of overlap means that the IV has to stay open even 
longer after the EV closes for optimum cylinder filling at high RPM, so 
it leads to lower pressure at low RPM.