Exhaust gas pressure with a turbo
Here is a good article on int/exh pressure ratios. Under the sub heading "Turbo cam basics"
http://www.hotrod.com/how-to/engine/ctrp-1106-turbo-camshaft-guide/
All good information and an interesting "read," I think. One thing wasn't addressed, though, because the article, probably, was written for a V-8 audience that has access to aftermarket heads that have increased flow capabilities "to the moon." The slant six head, (the only one available,) can be ported, and can use bigger valves to give somewhat increased flow, but the limiting factor here is not something that can be changed; small bores, which limit valve sizes to some pretty restrictive "breathing" numbers. It is unusual to see where anyone has achieved flow numbers on the intake side, exceeding 220 cfm which seems downright pitiful, when compared with the 302 small-block Chevy that can utilize aftermarket heads that can be worked to flow over 300 cfm, on the same size cylinder as a 225 motor (39 cubic inches.)
So, we do the best we can, which usually involves tapping the slant six's "ace in the hole:" the ability to withstand really large amounts of boost with no damage, if the engine was built with the right parts. That includes forged pistons, forged rods, O-rings, ARP 220.000psi cylinder-head studs, and a coppper head gasket. I am sure there are other factors and parts that could be used to increase the longivity of a high-boost, high-output 225 slant six, but this technology regarding the slant six in in its infancy with regard to using forced induction, even though the engine design dates back to 1960.
The utilization of supercharging or turbocharging on slant sixes is a relatively new phenomenon, and has a scarcity of information available because this is just not something people DID with these motors for decades... unaware of how much power was available through the magic of blowing the mixture in... They, instead, tried using conventional methods of getting more power, with results that were always less-than-spectacular, because of that strangulated head design. It was built for the 170 motor, and they increased the displacement to 225 and ignored the head... What could they do; the bores are
so small there's no room for decent-size valves (the 302 Chevy has 2.02" intakes, stock, on the same size cylinder)
Turboing one of these motors is not particularly cheap, nor is it easy... (tuning the carb seems to be the hard part,) but the rewards are great for the folks who have the wherewithall and patience to endure the frustration and steep learning-curve long enough to make it work. Well-over 500 horsepower at only 5,500 rpm is available for those who have the tenacity and persistence to stick it out to the end, and that amount of power will move an A body into the tens... a result that will put a smile on almost anybody's face.:burnout:
