Forget open chamber iron heads, you cannot make hi pressure with them.To run them with no squish, you are limited to about 160psi. And you cannot run much of a cam because the pressure drops rapidly with a later closing intake.
There are work-arounds for this, namely a hi-stall will allow the engine to spool up higher and get up on the cam quicker.But your engine will NOT spend most of it's life at WOT, nor at higher than 3000 rpm, so a hi-stall, for a streeter is, IMO, not the greatest idea. You will be waaaaaaay further ahead with pressure.
With closed chamber alloy heads and tight-Q, you can run up close to 200psi,still on pump gas. I have found 87E10 supports 185psi no problem.
Here's the deal; to make power requires rpm. There will never be a normally aspirated 318 that makes 300hp at 3000 rpm. Not a 4000, not at 4500. You are gonna have to spin it.
But don't lose sight of the big picture; 300hp with 3.55s, and 2.45 first gear, is a tire frier all thru it. That makes second gear your street favorite gear, and so 5800rpm in 1.45 second gear is ~75mph! Is that what you want?
Or do you want power at some lower roadspeed. To do that will require a lower rear gear? 4.10s will pull it down to 65mph. So now, you have spent money on a big TC, and the big rear gears,and the labor to install them, just to use that big cam in a low-compression 300hp engine, in a street-legal setting.
IMO, you will get better results with alloy heads, and hi-compression; and keep the current gears and stall.
To make pressure with a typical performance cam you might need ~11/1 Scr. With 318 cubes, your swept area is ~652cc, and that means your TOTAL chamber size will need to be DOWN around 65.2cc.
Allowing 8.6cc for a gasket, 5cc for eyebrows,and Zero-decks; that leaves just 51.6cc for the head chambers proper.
To see how this works, lets give the Wallace calculator a workout;
first, here is what a stock smog-era 318 might look like at 800ft elevation;
>Static compression ratio of 8:1.
Your dynamic compression ratio is 7.10:1 .
Your dynamic cranking pressure is 135.60 PSI.
Ica of 48*, VP of 113
read about VP here; V/P Index Calculation
notice the pressure and VP; this is typical of those smoggerteens. and you may know how that performance feels. Keep your eye on the VP.
>Next lets pump the pressure up with new pistons.
Static compression ratio of 9.1:1.
Your dynamic compression ratio is 8.06:1 .
Your dynamic cranking pressure is 160.88 PSI.
Ica still 48*, VP of 134
Check out the increase in VP; 134/113=plus 18.6%
I made no other changes, just cranked the pressure to the max for pump gas. This will feel similar to changing the rear gear by the same amount or from say 2.73s to 3.23s.. VP of 134 is pretty good. So now we are at the max for pumpgas and the stock cam with iron heads.
>Next; lets put some alloy heads on it and crank the pressure to the minimum for 87E10gas.
Static compression ratio of 10.2:1.
Your dynamic compression ratio is 9.02:1 .
Your dynamic cranking pressure is 186.78 PSI.
Ica still 48*, VP of 156 I made no other changes, just cranked the pressure to the max for pump gas and alloys.
156/113=plus 38%, so now like 3.77 rear gears. This is about the strongest your 318 can ever be,with the stock cam, and still burn 87 gas. Do you need 156vp? No. That is a total tire annihilator. A good target is over 140, anything under the stock 113 is kindof embarrassing.
These VP numbers are directly comparable between any size engines. So if you had a 1969 440Magnum with a VP of 156, then your stock-cammed 318 would feel similar to it, gear for gear, stall for stall, and pound for pound; from take-off to about 3000 rpm or a bit higher.
VP of 156 is a funtastic place to be.
>Next; lets change the cam to something a lil more sporty, with a new Ica of about 58*, or 3 sizes bigger than the stocker. This will get rid of some of that excessive VP and pick up a bunch of hp.
Static compression ratio of 10.2:1.
Your dynamic compression ratio is 8.49:1 .
Your dynamic cranking pressure is 172.41 PSI.
Ica of 58; VP of 134
See what happened to the VP? 134/156 means a loss of 14% and so from 3.77s, now we will need 4.30s to make that back. But as compared to the 113 of the stocker, we are still 134/113=18.6% plus
>next; lets pump the pressure back up
Static compression ratio of 10.8:1.
Your dynamic compression ratio is 8.98:1 .
Your dynamic cranking pressure is 185.69 PSI.
Ica of 58*, VP of 144 Badaboom! 144 is a great number!
See what's going on here? VP popped back up again. Now we are at 144/113=plus 27.4% over stock.
> next; lets max the cam out another 2 sizes, while simultaneously pumping up the pressure to maintain the 185ish psi;
Static compression ratio of 11.3:1.
Your dynamic compression ratio is 8.99:1 .
Your dynamic cranking pressure is 185.96 PSI.
Ica of 64*, VP of 138
VP dropping to 138/113= plus 22% over stock. This is meh, acceptable, and you can stall it up a lil if you have to, but l but I wouldn't go any lower.
>Ok one last time; lets put the Scr back to the nominal 8.0, with the big cam
Static compression ratio of 8:1.
Your dynamic compression ratio is 6.43:1 .
Your dynamic cranking pressure is 118.36 PSI.
Ica of 64*, VP of 88
This will feel like a 225 slanty up to about 3000rpm
Throughout it all, keep your eye on the Dcr. It likes to be up around 9.0 with the alloy heads.
Ica of 64* is a pretty big hydraulic cam for a 318. You will find that on a 340 cam; 268/276/114, which I absolutely do not recommend for your 318.
Ica of 58* might be on a cam two sizes smaller.
If you switch to a solid lifter profile, you might be able to make that same pressure with a lil more power.
>OK, just to be fair; howabout uncut X heads on a 318 short block with the flat-tops down .012
I get a total chamber volume of 2.4 +5 +6.8 ( .028 gasket) +72=86.2, and so the Scr will be 8.56, and
Static compression ratio of 8.56:1.
Your dynamic compression ratio is 6.86:1 .
Your dynamic cranking pressure is 129.38 PSI.
Ica of 64*, VP of 96/ quite weak
96/113=85%; so a loss of 15% compared to a stocker; which will require the same 15% more rear gear just to feel as strong as the stocker below 3000 rpm. So 1.15 x2.73=3.14 rounds to 3.23s and 3000 rpm will be around 25mph. There ya go, waiting for par again, VP of 96 absolutely sucks.
ok one more to be really fair, lets pump up the pressure for iron heads, to get 160 psi for best pumpgas.
Static compression ratio of 10.07:1.
Your dynamic compression ratio is 8.02:1 .
Your dynamic cranking pressure is 160.08 PSI.
Ica of 64*, VP of 119
So feeling like 119/113= plus 4.4% over the stock 318, or like 2.85 gears over the 2.76s
So there you have the open chamber iron heads optimized to 160 psi, with a VP of 119,(Ica of 64*) to burn 91 gas;
compared to the closed chamber alloys optimized for 87 gas and making a VP of 138.
138/118= plus 17%.. From 3.55s, the alloys will feel like 4.15s. Or you could say, to feel like 3.55s with the iron heads, you could run 3.55/1.17=3.03s with the the alloys; about same performance below 3000rpm.
However after 3000 the alloys will begin to kill the iron heads, cuz 185 psi over 160psi is a heckuva boost.
Ok so VP is a yardstick we can use to compare low-rpm performance, between any two engines. and it applies to 3000 rpm and down, sometimes creeping a lil higher depending on the two particular engines being compared. So if you are running a 3000rpm TC, this part of your powerband will never come into play.
However, if you have never driven an engine with 185 psi, and then you get a chance to? You will never go back to 140 or 150 or even 160.
Ok so now, you need to decide at what roadspeed you want the power to be, with what tires and rear gears. This will compute your rpm. From that you can chose the basic cam. From that you can generate the required Ica, then what Scr is needed to make 185 psi, with that Ica.
Then see how close you can get to it as cheaply as possible while maintaining a tight-Q. That means a minimum of machining. Which means pistons as close to the decks as possible right out of the box. If you can get to .012 below deck or higher,with flat-tops, then yur in business, The goal is to get the Q less than .040 but higher than about .028; Q being the piston to squish-pad clearance.
Once your Scr is thus established, you go back to the basic cam , and fine tune the selection, to work best with your final Scr, keeping in mind that with an automatic, you don't need perfection.
As for me; I like to run outta rpm right around 60mph, at the top of second gear with a 4-speed. So with 27 inch tires that requires a gear of 4.10s for 60=5900 Badaboom!
so I don't mind being a lil off cam, so I need a power peak right around 5500.
If you go this way, you would need, with;
3.55s, for 60=4900, and a power peak near 4600, or
3.73s, for 60=5200, and a power peak near 4900, or
3.91s, for 60=5400, and a power peak near 5100.
An increase of one basic cam size, translates to about a 200 rpm increase in the powerpeak. The stock 318 cam, with alloy heads might peak around 4400rpm
So now I take you back to
alloyheads;
Static compression ratio of 10.8:1.
Your dynamic compression ratio is 8.98:1 .
Your dynamic cranking pressure is 185.69 PSI.
Ica of 58*, VP of 144
Badaboom! 144 is a great number.
Ica of 58 could be on a
262/270/110+3/122comp/112power
@.050 this could be
262 less 46=216 on a lazy cam, to
262 less 42=220 on a faster cam, to
262 less 38=224 on a solid flat tappet.
There might be 7 or 8 hp increase in each cam.IDK
bedtime...