WillDC7

When you look at carbureted engines, high performance camshafts typically have a lobe separation angle of 110 or even 108 degrees. I have always heard it said that closing down lobe separation builds cylinder pressure and increases midrange power.

Looking at some comparative dyno numbers here between the same engines, using roughly the same camshafts but with different lobe separation angles, 108 or 110 seems to beat 112 to 114 degrees almost everywhere in the powerband. Moreover the difference seems to be greatest when duration is in approximately the street-car range.

Yet for computer controlled cars you almost never see this type of lobe separation. Instead, 112 degrees appears to be the bottom end, with 114 degrees being the norm. It seemed to me that there would be additional power to be gained by closing down the lobe separation angle further, particularly in a TPI or Supperram car the cam could seriously reinforce the tuning effect of the runners. I always assumed, however, that some quirk in the black art of fuel injection made running less than a 112 degrees of lobe separation a bad idea.

Thus far, however, these are the only reasons I've found to stay with more LSA:

1. The computer will have fits
2. The idle quality will be reduced commensurately
3. It will be impossible to pass emissions
4. The catalytic converter will be unhappy if unspent fuel ends up in the exhaust (as will the e-checkers)

Is there something i'm missing here? Suppose you're programming proms anyway, and you live in a state with no emissions testing and thus have no emissions equipment on your car. Is there any reason to not use a 108 or a 110 grind? I notice that TPIS sells an SCCA cheater cam ground on a 108 LSA, but this is the only such cam i've seen targeted specifically at a computer controlled car.

At any rate, it seemed odd that I wasn't seeing more of this, given the amount of weird-a$$ and inexplicable stuff people do to their cars. I assume that since literally no one is doing it, there's a good reason why, but I would be interested to hear an explanation of why fuel injection likes 112 LSA. Much obliged, help me avoid econ work.

Willie C.

jcb999

Most cams are more radical than that.

The biggest problem is that the O2 sensor will see unburned fuel bc the overlap is allowng the some of the intake charge out the exhaust valve. To stop that, it will lean the mixture.. probably to the point that it won't run.

So, if you never plan on going closed loop, it might work.


ps, technically its not the overlap (or lsa) that builds cyl pressure. Its the intake closing point (which is relative to lsa and duration)
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89rs w/a 400 109lsa, some aluminum things and a 4.10 gear

WillDC7

Okay, Thanks. That makes sense then. If that's the case, then the lobe separation restrictions would apply to you even if you were building a radical non-emissions engine, which jives perfectly with observed fact.

Willie C.