273 solid Crower cam opinions
The thought was more LSA would build dynamic compression and for it to not have a super choppy idle. I never really payed any attention to the actual overlap. It was $40 BNIB at a swap meet so i scooped it up with that thought.
It's not the LSA per se that builds compression, it is in fact the ICA ;Intake Closing Angle.
Let's see if I can diagram this;
Suppose that you had 2 cams of the same exact advertised durations, ie both cams had identical lobes. Now let's make one of them with a 114*LDA, and the other on a 108LDA. This is about the range you find on street cams.Let's install them both at 4* advanced, which is also typical.And finally let's make them both 260intake/268 exhaust. Let's not talk about lift yet.
This is how they will fall in;intake open/intake closed/ex open, ex closed/o-lap
next I will list the number of degrees of compression and the the number of degrees of power extraction.
the 114; 20*B/
60*A/72*B/36*A/36*o-lap.....120*compression/108*extraction
the 108; 26*B/
54*A/66*B/22*A/48* o-lap....126*compression/114*extraction
Notice the bold numbers.The sooner you close the intake the sooner you begin to build compression, and the longer you build it.Also notice it comes on the 108cam, the one with a tighter LDA.
Now check out the degrees of compression; notice that the 108 has 6 extra degrees of compression
Now check out the extraction. the 108 has 6 extra degrees of extraction.
And finally check out the overlap. this is the penalty of the 108, lots of overlap.Now overlap is of no concern at the racetrack,cuz the engine spends almost 100% of its time at rpms well past those where overlap is a concern.
--But for streeters that do spend a lot of time at idle and rpms in reversion, this is what produces most of the rumpity-rump. By itself,it serves almost no useful purpose. It also doesn't matter if some is there,in fact some has to be there. This is the period where both valves are open to some extent. This allows exhaust to flow into the intake, and also allows fresh charge to flow straight out into the header. Headers can make use of this event. Logs not so much. In fact this phenomena can be considered a 5th event subsequent to the 4 of a 4-stroke engine. When it's right it can help an engine get closer to 100% volumetric efficiency.
But let's get back to the compression and extraction events.The extra compression is felt by the driver as more giddy-up off the line, and the extra power extraction means you get to drive a little further on your tank.
This becomes bit of a balancing act, cuz the 108 is wasting gas at idle, while it's saving gas at rpms above reversion due to overlap.
Hope thats not clear as mud.
I think 273 hit it pretty good, but I'd like to add that if you get the fastest lobes possible/available, and the highest lift the grinder will give you and or get some longer arms, then you will get the most bang for your buck. Also careful co-ordination of the Scr/Dcr/quench will get the most out of the little engine.Lobes like this may have special spring needs.
