Well it seems common around here that people are using gutted cats, test pipes, and big exhausts. I was reading up about backpresure and how it relates to fuel/air ratios, and fuel consumption. I came across this on Flowmasters website. I felt a few people here could use the information

Backpressure

This is a condition found in virtually any engine and is largely a function of exhaust system efficiency. In a racing engine, it is an issue of particular importance in as much as it relates to valve timing, exhaust port flow, contamination of fresh air/fuel charges, and other factors pertaining to efficient removal of exhaust residue from an engine's cylinders. Because of the effects of backpressure on an engine's ability to make power (torque), it is important to understand that as backpressure changes, other areas of an engine are affected. For example, as backpressure increases (regardless of the cause), other engine components are typically optimized to compensate (or provide a partial solution) for this loss in exhaust system efficiency.
Perhaps one of the first areas an engine builder then addresses is the camshaft. By increasing the amount of effective valve overlap (and to some extent duration), some additional time is provided to evacuate the cylinder. However, this tends to reduce lower rpm torque, which then becomes compromised by higher operational rpm levels. But the trade-off here is that backpressure continues to increase with rpm and a point of diminishing returns is established. And typically, spark timing and fuel enrichment are adjusted to this less-than-ideal set of circumstances. Given these conditions, lets examine the changes we can expect to take place as backpressure is decreased. By reducing the backpressure, some amounts of raw fuel (and fresh air) will be "scavenged" into the exhaust system. So by reducing the amount of air and fuel in the cylinder at the time of combustion. [ul] A degree of cylinder pressure will be lost (a decrease in available torque) Effective air/fuel mixtures will be leaner (possibly leading to parts damage). Ignition spark requirements will change. The exhaust system's temperature will rise (owing to the burning of air/fuel mixtures in the exhaust manifolding and pipes). Exhaust gas temperatures will increase (particularly notable during engine dynamometer tests). There will be a tendency of back-fire (or popping) during deceleration of the engine.
[/ul] And while other conditions may arise (depending upon how a given engine is configured), the ones listed here are probably the most common. In Flowmaster's experience, as an engine's backpressure is decreased, valve over-lap should be shortened. Again, for example, if an engine were running a camshaft of 106 degrees lobe separation before a backpressure reduction, selection of a cam of 112 degrees would tend to: [ul] Retain more fresh air and fuel (thereby increasing cylinder pressure or torque) Reduce or largely eliminate combustion in the exhaust system Require less high rpm operation for optimum power Require less ignition timing Require a decrease in jet size Decrease inlet charge contamination by a reduction of exhaust gas in the intake track [/ul] The purpose of all this information is not to focus on specific engine combinations. Rather, it is to address the issue of backpressure and how certain conditions will arise when it is reduced; in particular as with the use of Flowmaster's backpressure reducing components. In fact, the use of Flowmaster systems (collectors, Y-pipes, etc.) can provide backpressure levels below that of open exhaust system. In that context, the selection of other engine components (in particular camshafts) should be viewed in this light. And while additional tuning is generally required, such steps can initially be approached as if the exhaust system was unrestricted. We refer you to the attached illustrations to further explain the stages of air flow into and out of an engine's cylinders, including the order and periods in a complete inlet/exhaust cycle when backpressure (including contamination) are sequenced. Figure 1 Here's an illustration showing an engine in the final stages of its exhaust cycle. Note that as backpressure builds, P3 becomes increasingly larger than P2, depending upon engine rpm, cylinder volume and the amount of restriction being created in the exhaust system. Note also that any exhaust byproducts not removed from the cylinder will contaminate incoming fresh air/fuel mixtures.
Figure 2 Near the end of an exhaust stroke (when the piston is pumping residual gas into the exhaust system) and the exhaust valve is about to seat (intake just opening), there is opportunity for some amount of exhaust gas to pass back into the intake system. The duration of this overlap period and the amount of backpressure in the exhaust system will affect the volume of exhaust gas back-flowing into the inlet system.
Figure 3 This illustration shows the relationship between inlet and cylinder pressures during a typical intake stroke. Here P1 is greater than P2 since atmospheric pressure (P1) is forcing air/fuel mixtures into a lower pressure zone (P2).

Figure 4 Once the piston begins to descend on the intake stroke (and the exhaust valve is not yet seated), some amount of fresh air/fuel mixture will pass into the low pressure area left by the exiting exhaust. In cases where backpressure is sufficiently low, this condition of over scavenging will provide residual burning in the exhaust system, over heating and a reduction in burnable mixture in the cylinder following closing of the exhaust valve. It is for this reason that shortening the overlap period is required when using the highly efficient Flowmaster systems.
Figure 5 This is another representation of fresh air/fuel mixture passing through the cylinder and into the lower pressure region of the exhaust system (intake opening, exhaust closing) during the overlap period. During this period, P1 is greater than P2 which is greater than P3. Stated another way, P1 (atmospheric pressure) is forcing mixture into and out of the cylinder into the lowest pressure region (P3) represented by the exhaust system.

Figure 6 In instances of high backpressure, residual exhaust gas not eliminated from the cylinder will occupy some amount of space in the cylinder, thereby displacing fresh air/fuel mixtures not able to enter the cylinder. For this reason, engine builders address increases in backpressure by longer overlap periods (sometimes more valve duration). This line of thinking can lead to the inefficient combustion conditions described elsewhere in this document; reduced power (torque), additional ignition spark timing and fuel delivery, and a general requirement for higher operating rpm.