I'm posting this on behalf of Bob Dowie, last years ITAC CRB liasion, and current GT Ad hoc liasion and CRB member. As the SIR technology is new to us, he thought this explanation and listing of sources might be helpful and useful.

I would like to point out that this is the first time that I am aware of that the CRB has posted such a response to the IT community, and I'd like to thank Bob and the CRB for their efforts and the open channel of communication.



Regarding the BMW E36, the CRB chose to institute use of the SIR restrictor instead of increased weight because the car was already listed and competing at the current weight. The committee target weight would have had the competitors locating and installing legally removed stock components or adding a great deal of ballast. Neither were felt to be are good options.



An SIR is an optimized (in other words, the shape is designed to pass as much air as theoretically possible) opening through which all intake air must pass. It is the entry point for all air entering the intake tract. Instead of being a flat plate, it is more like a funnel, and is designd to have no affect on the engine, or it's characteristics until it's design limits are reached. The goal is to trim the top of the HP curve, but not affect any other part of it. Typically SIRs are smaller than flat plate restrictors in equal applications, but are more transparent during operation.




The CRB and the GT committee have been working on the single inlet restrictor system for the last few years. Certainly not a new idea as its been in use around the world for years, but not within the SCCA. There are many benefits; the same type of restrictor is equally effective regardless of the intake system used. Determining what actual cars are capable of making in the real world is subject to rumour and innuendo, and sources are often conflicting. So, the biggest benefit by far is the fact we have some idea of the power output of an engine equipped with an SIR. Flat plates are based on a percentage of the stock throttle plate size, what we don’t know is if the stock throttle body is oversize to start with in the particular car that is being adjusted. It’s a nasty catch 22, and one that leads us to set a restriction and keep going down in size until the performance goal is met. Which is difficult in other categories, but even more so in IT where on track performance isn’t utilized in the same way it is in other categories for classification decisions. The result is often unhappy competitors who feel targeted, and must re-engineer their cars, and suffer the expenses with every “adjustment”.



Thanks to David Finch and his associates at Raetech engineering www.raetech.com we have a solid picture for what the output potential is for an engine equipped with a single inlet restrictor (SIR). Raetech donated hours of engineering expertise and computer modeling time to establish what we can expect from a given restrictor size. In the testing done so far the modeling appears to be accurate. Raetech followed it one step further by making an optimized restrictor available. But they are not a spec part and competitors are welcome to make their own or choose from the other machine shops making them.

The design spec of the 27mm size is to limit crank horspower to 218-220.



cdamachine SIR info on website

.racegearbox Call for info

raetech SIR info on site



We don’t see this as an experiment that’s being put on the BMW owners back. We had a GT3 Nissan running at this years runoffs equipped with one as well as a couple of GTL cars; with all the restricted GTL cars using them this season. It is the direction we are heading in restricting engines through out the categories; yes its new technology for us, but the SCCA has been observing the implementation by other sanctioning bodies and feels confident in the devices effect.. And like any thing new it will take time and require some effort as it’s implemented but we firmly believe in the system and its ability to restrict power to known level allowing us to bring in new cars without displacing the old. We certainly intend this to be the last change given the current class structure.



No committee or board in the SCCA likes to make competitors change their cars. But if changes are necessary they have a responsibility to make the hard decisions. This is one of these times.



Bob Dowie, CRB member, 2005 ITAC liasion, 2006 GT liasion.

Here's a computer model: