Cylinder head decision for 318
Looking to get better flow and bigger ports. If compression rises a tad would I still be able to run on pump gas?
Here is my 2cents
Following is a comparison of the cylinder pressure at 800ft elevation, first with a stock 318 at a generous 8/1 Scr,with the stock 240/248/112 cam, then with the Hughes Whiplash.
Stock 318LA
Static compression ratio of
8:1.
Effective stroke is 2.86 inches.
Your dynamic compression ratio is 7.04:1 .
Your dynamic cranking pressure is
134.04 PSI.
V/P (Volume to Pressure Index) is
110
Next; 318LA with Whiplash318
Static compression ratio of
8:1.
Effective stroke is 2.78 inches.
Your dynamic compression ratio is 6.87:1 .
Your dynamic cranking pressure is
129.64 PSI.
V/P (Volume to Pressure Index) is
104
Notice the pressure has dropped~4.4 psi, and the VP also dropped to 94.5%
To get the pressure back requires .2 more compression;
As in;
Static compression ratio of
8.2:1.
Effective stroke is 2.78 inches.
Your dynamic compression ratio is 7.04:1 .
Your dynamic cranking pressure is
134.04 PSI.
V/P (Volume to Pressure Index) is
107
But notice the VP only climbed to 97% of stock
But hang on with cylinder pressure of just 134, this is a pretty doggy engine. To be fun, the pressure needs to be up over 150psi.
Like this;
Static compression ratio of
9.2:1.
Effective stroke is 2.78 inches.
Your dynamic compression ratio is 7.88:1 .
Your dynamic cranking pressure is
156.09 PSI.
V/P (Volume to Pressure Index) is
125
There see; the Scr needs to be up around 9.2 to get that. Also notice that the VP has risen from a really crappy 104, to a magnum 5.2 level of 125.
But hang on, I estimated the .050 to advertised ramps as being 47*.. I think that's a fair number, but the valves are still ~.012 open (.008tappet rise x1.5rocker-ratio) by the math! And your engine cannot start to build pressure until the intakes are closed and not leaking. Depending on the cam, that could be 5,10,15 or more degrees on down the road.....
What if the Hughes isn't closed until 60* Ica?
At 60*, I get
Static compression ratio of 9.2:1.
Effective stroke is 2.66 inches.
Your dynamic compression ratio is 7.58:1 .
Your dynamic cranking pressure is
148.16 PSI.
V/P (Volume to Pressure Index) is
114
Crap, lookit that, we lost 8psi right there, we're gonna need more compression.
So;
Static compression ratio of
9.6:1.
Effective stroke is 2.66 inches.
Your dynamic compression ratio is 7.90:1 .
Your dynamic cranking pressure is
156.62 PSI.
V/P (Volume to Pressure Index) is
120
Ok the pressure is back with 9.6 Scr, but the VP took a dump again. Well this is as safe as it gets.
Here it is pushing the cylinder-pressure envelope for pumpgas
Static compression ratio of 9.8:1.
Effective stroke is 2.66 inches.
Your dynamic compression ratio is 8.06:1 .
Your dynamic cranking pressure is 160.88 PSI.
V/P (Volume to Pressure Index) is 123
And the VP is back to 5.2M level. Now you can use that 2000stall TC and 3.23s again.
Read about VP here; V/P Index Calculation
The points are;
1) YES!,that Hughes Whiplash will run in your 318 at 8.1 Scr,
but under ~3000rpm it will be super super lazy, you will not like how it performs with the stock TC.. With 3.23s, 3000 rpm in first gear is ~22mph at WOT. That's a long time to wait for the onset of power.And
2) accidentally having too much compression is not likely gonna be your problem; because;
To get to 9.6 for instance will require a total chamber volume of 76.6cc no more. With the pistons down at .057, that's already 11.2cc. If they are dished, that's gonna cost you more. You gotta use the .028 gasket so that's ~6.2cc which leaves ~59.2 for the heads plus the dish if any.
Plus
all calculations are for 800ft elevation. If you are at higher,my numbers are wrong, and you will loose performance.
