Advanced Engineering Tech - lobe separation, and how it affects low end
04-14-2011, 01:22 AM
Ok, we have all heard "tighten up the lobe separation and it will help your low-end". But doing that also adds overlap. Overlap is supposed to help top end, isn't it? Is it perhaps the earlier intake valve closing that usually comes with the tighter lobe separation that helps the low end? For example, if you have a mild cam, say 220/224 on 115 +0 it will not have the low end that a 220/224 on 110 +0 will have. Is this due only to the fact that cam 2 has an earlier intake closing point? What if we ground the cam like this: "220/224 on 115 +5"? Would we gain back that low end torque?
04-15-2011, 02:54 AM
Lobe separation is just a number, a byproduct of the correct valve events. Overlap can help a lazy port down low, or it can hurt an over valved engine everywhere. It's very engine specific. So no, the generalization "tighten up the lobe separation and it will help your low-end" doesn't always fit.
04-15-2011, 01:50 PM
yeah, more or less what i was referring to is the earlier IVC that usually comes with the tighter LSA. I was wondering if you could sometimes (but not in every case) get similar results by advancing a cam with the same lobes but less overlap.
04-15-2011, 01:54 PM
But I wasn't aware of the notion of overlap sometimes helping down low.
04-18-2011, 04:17 PM
My last little LT1 made 400 rwtq at 2800 rpm, with 31 degrees of overlap @.050 and some pretty quick lobes. This was a 383ci motor with a 3" intake runner. That's pretty solid in LT1 land, cam was only .610 lift.
Tighter LSA doesn't necessarily mean earlier IVC... You could keep the ICL the same and bring the ECL in. That's why it's good to talk intake and exhaust centerlines instead of LSA.
If advancing the cam helps you got the cam wrong ;) The engine or combination likes the earlier IVC because you gave it the wrong one to start. Here a tighter LSA could have been the answer to keep your exhaust where you had it, or maybe if it didn't lose anything up top, you had a pumping loss issue and advancing the cam was the answer :) If you're looking straight at LSA and advancing the intake, realize there's an equal and opposite reaction on the exhaust lobe.
04-18-2011, 10:30 PM
Thanks, since I have started this thread I have dug into an old thread originally started by Pat G. This was explained in it. It seems I have been neglecting the exhaust lobe valve events in my thinking. BTW, the torque you are describing from your Lt1 is SICK.
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