I really enjoy the discussions on induction systems. I would like to add that all the inductions systems being discussed as “Cross Ram” systems in this thread are actually quite different in their characteristics. In general, Cross Ram is a remark about the appearance horizontally positioned runners but in practice they are forms of Individual Runner and Common Plenum induction systems; and there are several variations in themes within these two categories.
<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com<img src=" /><o
></o
>
<o
></o
>
The Sheet metal intake that started the thread is actually identical in type to the original polymer LS intake though it has the plenum on top instead of under the runners compared to the original. I presume this intake was made to adjust runner length and or (optimum) cross section for a higher RPM and/or horsepower target than the original LS intake could support. As an aside, the original LS intake is a very good piece and produces excellent performance across a broad rpm range with peak power routinely North of 600 HP on 427 CI mills and does so with a very low profile that fits under the hood of street cars. FAST makes versions of this intake which are identical in embodiment but have different runner length, runner cross sectional areas, throttle body choices, and plenum volumes, all of which can provide subtle performance improvements for a given combination of parts. They are economical compared to that sheet metal lid. IMO, you would have to have some very special performance goals or perhaps aesthetics to justify the cost of that intake. As a further aside, the OEM LS style of intake is quite prominent in many modern EFI production engines now. It essentially does the same thing as the old 5.0 Ford Mustang EFI intakes but is packaged much better. It’s a good design.
<o
></o
>
Whether “downdraft”, “side draft”, “Cross Ram”, or whatever the nickname, the true IR intakes are a very different animals and behave quite differently than common plenum intakes. It’s a long discussion but like anything else, the rest of the engine components need to be carefully selected for optimum performance. In the past, IR intakes gained popularity in racing because the isolated runner provides a very strong signal to the booster and correspondingly strong engine response; lot’s of area under the torque curve in a usable rpm range. –Makes for a good road race engine. Engine performance always seems to be compared at peak power numbers but it’s only one reference point. I’ll take most area under the curve for a given rpm range. –Makes a better driver out of me. IR intakes get the wrap for falling on their face at a given rpm but this is only true on a very limited basis. This mostly occurs because they are being applied to larger displacement engines than they can support. These days we’re getting as much as 50% more displacement out SBFs and SBCs than when many of the IR carb systems were introduced. You’re never going to support a 600HP engine with Moon SBC intake and 45 DCOEs or even 50 DCOEs. IR systems require mucho more carburetor capacity than common plenum four barrel carb systems. Injected IR systems (such as Kinsler) that make big power have bores of 2 7/16” (~62mm!). There were never any IR carbs (other than custom built stuff) made to support this power and forget all about it for poked and stroked big blocks.
When EFI is introduced to the discussion, the advantage of booster signal is removed from the IR discussion. However, IR also can make tuning for the subsequent harmonics simpler than common plenum intakes and this can become more important in higher rpm engines. IR is well represented in high rpm mills. The (fuel) timing of EFI can also be of great benefit in managing an IR system. When used with EFI I would contend IR also has a place at the edge of the (high depending upon your view) performance envelope for street and club racing because it can help strike compromise in engine behavior. This comes at a price however in both dollars and tuning time. I often draw skepticism for my view but my retort is always so are the Kinlser and Hilborn people delusional? How about the 4 cylinder motorcycle industry? IR is more expensive so why use it? Are all the Japanese engine designers delusional? -Don’t think so.
A couple of other comments/observations about some of the induction systems mentioned in this thread:
The large common plenum SBF Shelby intake is in the family that many refer to as “Ram Box” intakes. They usually produce excellent high rpm performance albeit in a fairly narrow rpm range. They were good for super speedways in NASCAR but poor for closer road tracks and very poor street induction systems. They are sort of the polar opposite of an IR system; big plenum means very weak booster signal to the carb and low plenum velocity means poor low speed fuel distribution. However, they have an abundance of air/fuel “availability” and thus produce good peak results when properly tuned. Replace the carburetor with a throttle body and properly placed injectors and this intake can actually perform quite well over a much broader range.
The Chrysler “Cross Ram” is a very unique case. It’s really a common plenum carb intake. It obviously has very long runners. In fact, there was actually both a “short” and “long” runner version of that intake as the location of the divider in the runner was changed to affect the runner length and point in the rpm range where Helmholtz resonator affect occurred. Because of the cylinder firing order, this was not a 180 degree intake but it didn’t matter much. This was also not a racing induction system but one that was very purposefully designed to produce low rpm torque for moving a heavy street car.
<o
></o
>
Best,
Kelly
