But the death knell on breathing for these engines was sounded when the Chrysler front office told the engineers to build a new engine for the Valiant that was as short (fan-to-firewall) as possible in an inline six configuration.
That dictated small bore spacing (for the required "short length") which in turn, mandated small bores which required small valves. The head we ended up with works great on a 170, which is the engine it was designed for.
But when you stroke a 170 to create a 225, there's no more room for bigger valves... and, that's the one item that makes these engines so hard to get any real power out of, naturally-aspirated.
A 302 Chevy V8 has 39 cubic inches per cylinder.
A 225 slant six also has 39 cubic inches per cylinder.
But, the Chevy has a 4" bore, whereas. the slant six has a 3.4" bore.
This gives the Chevy room for 2.02" intake valves and 1.6" exhaust valves, stock.
You can, without too much trouble, put 1.75" oversize intake valves and 1.5" exhaust valves in a slant six head.
Let's look at the valve area with those oversize valves:
The intake, at 1.75" diameter, has an area of 2.4 sq. in.
The 1.5 exhaust has an area of 1.76 sq. in.
Total valve area = 4.16 sq. in.
The Chevy, with exactly the same cylinder volume to fill, has intake valves that measure 3.20 sq. in., in area
The Chevy exhausts have an area of 2.01 square inches, for a total valve area of 5.21.sq. in.
Thats a difference of twenty-five percent.
Put another way, one fourth.
I used the 302 Chevy as a typical example of a high-performance V8, with good breathing.
So, although its possible to get bigger valves than what I used as an example, in a slant six head, it falls under the category of heroic measures and isnt all that easy to do, so most people just go with what I suggested, I think.
The uphill battle that racers face in trying to make significant power with a slant six head (Im talking normally-aspirated 225s here,) makes a strong case for forced induction on these motors. The stronger-than-usual infrastructure (short, stout, forged crank, thick decks and big bearings) adapt to boost readily, and if you can deal with all the many complications associated with turbocharging one of these motors, there are significant rewards to be had at the flywheel.
No, they still dont make power like a 4-valve Supra, but properly built and tuned, they can old their own against normally-aspirated V8s that are much larger.
Given the alternative (put a V8 in it,) it is possibly a cheaper way to go for the same amount of power as a built 360.
Not easy, nor cheap, but satisfying, in the end. J
