I’m trying to get a grasp on cam logic. One question that I can’t figure out is will a dual pattern cam have more of a lope to it than a single pattern? Or is the lobe separation what has more effect on it? Also is 112 degrees of separation a lot or a fairly common amount?

Thanks for any help!

 

I think you will find that the "lope" is determined by the lobe seperation angle "LSA". The 110 LSA cams will have the hot rod drive-in lope at the expense of having too much overlap and killing low end torque.
112 is a step in the right direction, 114 is better as far as your low end goes, it will idle better than a 112 LSA cam. I suppose 112 is a compromise, a good dual exhaust (not necessarily headers) is a must with aftermarket cams. Your compression ratio will be a huge determining factor of what cam you should choose.

 

The lope is a function of when the intake valve closes. Late closing causes reversion in the intake system and make the idle ratty/lopey. The later the intake closes the nastier the idle and the harder it is to make the motor idle at all. Late intake closing needs lots of initial advance and rich idle circuits to achieve an idle.

 

Scott's answer is basically correct,however,LSA is only a very rough indication of overlap, and comparing LSAs on cams with very different durations is meaningless. For example the base 300 HP cam has a LSA of112. The L-79 cam is 114, but it has more effective overlap because the duration at .050" lifter rise is 222/222 versus 194/202.

The greater the effective overlap, the more "lope" the idle will have due to combustion instability from exhaust gas dilution, and it has nothing to due with whether the cam is single or dual pattern. It's purely a matter of EFFECTIVE overlap, which is measured in square-inch-degrees. The Engine Analyzer simulation program computes this number and reports it in the output data.

Delaying the inlet valve closing shifts the torque curve up the rev scale, but at constant overlap and within what is considered the "normal" range for exhaust valve closing, it has very little affect on idle quality or idle vacuum.

Case in point, the LS7 cam has .050" durations of 211/230 with points of max lift at 124/117, which is an LSA of 120.5, so it has very modest effective overlap and a very late phased inlet event, and for a typical aftermarket vintage SB cam to have the same exhaust closing point at 110 inlet POML, it would have duration of 239.

Modern LS engines have low overlap and late phased inlet closings, and this philosophy will also work quite well for vintage engines with exhaust manifolds by broadening torque bandwidth with stable idle characteristics.

The modest overlap keeps the low end torque from getting killed due to exhaust gas dilution, and the late closing inlet keeps the torque curve from falling off too fast at the top end for excellent top end power.

Duke

 

Guess those engineers were pretty smart back when they made the 30-30 cam with a 116 LSA. Of course, that very late intake closing angle required high compression, but high octane gasoline was readily available and cheap.

Using 120.5 on a flat tappet cam would require either very high compression or very short duration, right? And with the higher compression, you'd be flirting with detonation (with today's gasoline) above the TQ peak, correct?

On the other hand, the area under the curve for an LS7 roller cam means it doesn't need as much duration. So, it can use a 120.5 LSA and still close the intake valve at an angle that doesn't require too much compression for today's gasoline.

Just trying to see if I understand this.

 

I believe the LS7 has a relatively high compression ratio and they run it on the ragged edge of detonation which is possible with all the sensors and electronics.

Actually with the 30-30 and LT-1 cams, that late closing inlet creates a lower dynamic compression ratio. Examaple being with the LT-1 cam, you might have a static of 10:25:1 but your dynamic compression may be around 8:1 which is fine for todays premium and even some regular gasolines.

 

Yes, which is why a higher compression ratio is required. Of course, I meant "static" compression.