But for my dad and a 347 street the N-21 would do the trick.
Overlap vs cubes is what drives idle vacuum, so...
Overlap = (int dur + exh dur)/2 - 2(lsa)
N91 = (240+248)/2 - 2(110) = 24° overlap at .050" = nasty idle for a 347
The N21 is 219/229 @ .050 (again with a 106 icl and 110 lsa).
N21 = (219 + 229)/2 - 2(110) = 4° overlap at .050" = nice idle for a 347
The N21 is going to have great idle, but look at what happens to the IVC below.
IVC = (icl +(int dur/2)) - 180
IVC on an n91 = (106 + (240/2)) - 180 = 46° abdc
IVC on an n21 = (106 + (219/2)) - 180 = 35.5° abdc
The n21 is going to have extra 10.5° of compression time. That makes a huge difference in the cranking. You can lower the cranking by going to more duration or moving the icl farther out. Either makes for a later IVC.
My point is that I don't want him to get a cam that causes cranking to be too high and makes the engine ping on pump gas.
That's why my first question was how much static compression did the 347 have.
Here's my thought process when it comes to cams.
1. Static drives IVC.
2. desired idle drives overlap at .050"
3. IVC and desired idle drive int duration and ICL
4. exh duration is normally 8-15° more than intake duration
5. ECL is driven off exh duration and the desired overlap.
Not trying to argue or push my opinion. Just wanted to explain why I was concerned with 10.5 static compression and the n21.
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