Aaron91RS

iTrader: (2)

Lets say you have two identical 305 engines. One with TPI and the other with TBI. Lets say both have 9.5:1 compression. Now you floor both of them from a stop. Off the line the sin waves will travel up and down the TPI's runners. Now when they are timed right to the valve opening doesn't this cause more air to be forced into the cylinder. Kind of like a supercharger on a much smaller scale? Of course the TBI does not have that supercharger effect.
Now once the air is inside the cylinders, both engines will compress the air to the 9.5:1 ratio. However the TPI has would have more total air and thus more density when compressed? Is this correct? In which case wouldn't you want to build a TBI engine with higher compression, say 10:1. Now with more compression on the TBI, the air in both engines would have the same density. So even though higher compression is usually thought to put more force/stress on the engine as long as the density was the same the force/stress would be the same?

Vader

Aaron,

In a nutshell, you've just outlined the entire theory of the TPI sysem. However, it isn't necessarily true that the TBI and carb systems don't enjoy a similar benefit from pulse-wave charging. For about forty years now, manifolds have been designed to take advantage of this phenomoenon. Chrysler had a dual Carter setup on old hemis with intak runners that actually rose up and over the rocker cover on the opposite side of the engine from the cylinder bank that was being fed. You could open the hood and peer in at what looked like an aluminum octopus sprawled out over the engine enjoying the warm air.

Chevrolet tried a similar manifold with slightly shorter runners. All of this was in an effort to use the charging effect to maximum advantage. Several of the aftermarket manifold designers (like Offenhauser) had already had success with this, and offered manifolds for this purpose. The more modern intakes still use the charging effect, but at higher RPMs.

Longer runners, resonant at lower RPMs, were selected for the TPI system since low-RPM torque enhancement was the goal at the GM powerplant engineering desk.

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Later,
Vader
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Vader

Aaron,

BTW - Static compression ratios are a relative indication of dynamic compression pressures. Dynamic pressure is what contributes to horsepower, and is what is enhanced by the charging effect. A similar change in dynamic compression pressure can be done just by changing the valve timing.

At lower RPM, compression ratio is more of an important factor. At higher RPM, compression pressure is the limiting factor in producing power. Many 11:1 engines starve for air and have very low actual compression pressures at RPMs above 7,500, so little additional power is produced above those speeds.

This is the same phenomenon that chokes a typical stock TPI at around 5,500 RPM. The longer, smaller runners just choke a little more effectively than a short-runner manifold would. With equal cam profiles, a TPI engine should theoretically have a lower peak RPM power output than a carbureted or TBI engine. Still, most of the driving we do is under 5,500 RPM, so the TPI does enjoy a bit of an advantage for most street drivers. If you're going all out for flow and compression at high RPMs, forget about the 350 and get a bore/stroke ratio that is more oversquare - but don't plan on driving it to get groceries.

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Later,
Vader
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"Make Me Bad"
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