jim_in_dorris

This isn't a help me pick a cam thread, although what I hope to learn from it will influence my choice. What I am wondering about is LSA and valve overlap considerations when tuning a TBI motor. The 8-464-8 cam has an LSA of 113 and a valve overlap of 32 degrees. the 8-407-8 cam has an LSA of 110 and a valve overlap of 35 degrees. They both have a adv intake duration of 252 degrees. The SCR is 8.95. I would have thought that the 8-464-8 cam with a 32 degree overlap would have a higher DCR than the 8-407-8 cam, however I was wrong. the 464 cam has 7.48 DCR while the 407 cam has a 7.81 DCR. Why is that? I would think that less overlap means more DCR. Also does overlap affect vacuum? How? I have read countless threads indicating that you want a wider LSA for ease of tuning (to a point) yet comp cams CAMQUEST software gives both of these cams a "great fit" for computer controlled FI. Exactly how does LSA affect my tuning? VE? MAP? multiple things?

note: the 407 cam numbers were generated with 1.6 ratio rockers to keep the the same as the 464 cam.

alvanwie

Overlap has little to do with DCR. DCR has more to do with IVC relative to BDC.

More overlap less vacuum. During overlap both intake and exhaust valves are open, so effectively you have a vacuum leak out the exhaust.

If all else is equal, lower LSA results in more overlap and makes tuning more difficult. More overlap causes at least 2 problems with tuning. 1.) less vacuum decreases the range of map reading from idle to WOT, 2.) the more overlap the more unburnt A/F mixture that escapes directly to the exhaust and can case problems with the O2 sensor readings.

That being said, both of these cams are mild and should be relatively easy to tune.

Tuning parameters often effected by cam selection: timing, ve, AE, idle

jim_in_dorris

Thanks, I think I get it now. Yes they are mild cams, I want low end torque and am using stock heads and intake so they run out of air ~4900 rpm, so a big cam would be wasted.

thomas1976

Ceck out the 8-500-8 "line", much less lift for similar power.