Looking to boost my compression and did some calculations with kb399 pistons. These domed pistons would put me at 9.75:1 static compression. Would this cause me detonation problems on 91 octane with open chamber iron heads? Current cam has intake closing @0.050 38 ABDC and at 400 feet above sea level which according to the Wallace calc gives me 9.04:1 dynamic compression. Let me know what you think.
This is a test right;
this is what I get.
from your 38*@.050, I generate a compression duration of 142*
From that, I back calculate the intake duration at 218*. I assume an LSA of 110* And I assume a split of 6* so the exhaust comes to 224*.
This brings us to a common cam size of 218/224/110.
Adding the typical acceleration ramps of 46*, this gives us advertised specs of 264/270/110
installing that at 109* gives us an Ica of 61*. And so
Mr. Wallace says;
Static compression ratio of 9.75:1.
Ica of 61*/400ft elevation
Effective stroke is 2.64 inches.
Your dynamic compression ratio is 7.97:1 .
Your dynamic cranking pressure is 160.61 PSI.
V/P (Volume to Pressure Index) is 123
That will make a heckuva 318 running 91 under power, and 87 while cruising.
With a 904 automatic, I see a 2400TC with 3.73s down to 3.23s with a 2800.
With 47* of overlap, I see headers and hi-flo duals.
I see a dual plane and personally, I'd like a 650DP, but anything over 450 is actually adequate.
I installed it at 109 because with a VP of 123, and a 2800TC, she will have plenty of low-rpm grunt, and the 3.23s will make awesome fuel mileage at 65=2600@ zero-slip. Therefore, 109 versus 106 is an extra 100 rpm on the top. If yur lucky, the power will peak at 5000. With just a lil work on the heads, you may be able to pull a 5600rpm shift, which is what the 904 wants.
As already said; there is no good reason to try to run max timing at idle; that is just nuts. Even if you are 4* short, the power loss does not come until after ~3500, and it is minimal. IDK maybe 7hp at 5000.
But there is nothing stopping you from engineering a a two-stage timing curve, being conservative at idle (cuz of the 2800TC, building fast to something like 28*@2800, and then slowing to 34* at 3600. That is easily doable with the 2800TC.
I would build this combo in a heartbeat, because;
The Wallace doesn't know that at the advertised Ica of 61* , the intake is still open at rocker ratio times advertized spec; say 1.5x.008=.012! So compression doesn't actually start at 61*ABDC, but might not start until as much as 15 degrees later, on a particularly slow ramped cam !
Of course; garbage in/garbage out, and so if I made wrong assumptions then you get wrong answers,lol.