Cam choice finer points

[curtis73]

I'll try to keep this short but I wanted to discuss a cam choice I'm about to make. This is a vortec 350 for a boat. The two main concerns are:

- keeping an acceptably low overlap period since the exhaust is wet. Too much reversion will suck water back in
- low idle speed for fishing and docking maneuvers. The lower the better.

The other big concern is not overpowering my outdrive but that happens at about 325 hp and we're safe. So I've been dancing around with a few cams but need some help on the finer points.

Consider these three cams, all rollers measured at .050" lift:
-200/212 on a 108 LSA, -11* overlap
-206/212 on a 110 LSA, -10* overlap
-212/218 on a 112 LSA. -9* overlap

I can get them all to match curves in a dyno simulation if I install the biggest cam straight up, the middle cam retarded 3*, and the smallest cam retarded 4*. They play out like this:




Since the smallest cam provides the most average tq and best hp, I'm tempted to go with it, but a few questions plague me:

- will its idle speed be any lower than the others since I retarded the cam? I assume it will be lower since its lower duration and less overlap, but will I have to raise the idle speed back up by retarding the cam?
- what are the drawbacks to retarding a cam? I hear about people advancing it all the time; mostly to try and recover low end torque after choosing a cam that was too big, but what about retarding it?
- the smallest cam is a custom grind I came up with... can you see any problems using a duration spread that big? 12 degrees is a lot.

[GreyGoose006]

wouldnt retarding a cam shift the powerband up?
i guess that would be good if you want a low idle, and boats tend to run at higher rpms for most of the time.

[curtis73]

Yes, retarding the cam will shift it up, similarly to adding duration. I'm curious though if it will raise the idle like adding duration would. Usually shifting the powerband up raises the idle, but I don't know if thats related to IVC or strictly duration and overlap.

[GreyGoose006]

thats way beyond my limited knowlege...
lol

[534BC]

What do you mean by "raising and lowering the idle" ?

Althought the vac may be different , won't any cam idle at any speed above 300 rpm? and is mostly a function of the amount of air allowed in. especially with the very similar cams you are considering.