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WC Balance of Performance


Seasoned Member
Nov 4, 2011
Mustang, OK
V-Series Cadillac(s)?
'11 V Wagon, ( '13 427 Vette & '21 Vette)
I've been looking at the World Challenge Appendix A stuff - this Appendix provides the basic parameters for each car and tracks the adjustments to balance performance over the year.

There have been 10 Appendix A changes since February. On Feb 28, WC put out 2 Appendix A's. The first GT race was March 29th.

Initially, the CTS V.R was listed at 3050 minimum weight and a 50.8mm restrictor. Then the second one dropped the minimum weight to 2920 but bumped the restrictor size up to 51 mm.
Audi (second in manufacture points now) is shown as 2843 and 44 mm restrictor.
There are 3/4 different Porsche's listed. The latest 997 GT3 car started at 2850 and without a restrictor size (?).

March race at St. Pete, a CTS V.R finished second. A Lambo finished first.

A new Appendix A came out on Apr 4th before the second race on April 12 but did not change the CTS V.R parameters. CTS V.Rs finished first and second.

April 21 - an new Appendix A is out and the CTS V.R gets a smaller restrctor - down to 49 mm. Then on April 23, another Appendix is put out.
There are 2 races at Barber Motorsports on Apr 26. A Porsche wins the first and an Audi wins the second with CTS V.Rs finishing a best of second and third.

Next race is May 31st for the GT cars at Detroit. New Appendix As come out May 1, May 2 and May 27.
The May 27th "A" changes the restrictor for the CTS V.R from 49 mm to "1.780 SIR" - I looked in the rule book for this and "SIR" stands for "Sonic Intake Restrictor." I still don't understand the different between an SIR and a restrictor plate.
The 911 GT3 (not the 997) got a restrictor bump from 56 to 65 mm during this time.
The Dodge Viper (not the SRT Viper) got a bump from 48.5 to 58.5.

Detroit is a street race and very suited to the Caddy's. CTS V.Rs finished first in both races this weekend.

Next race is June 20th and a new Appendix is out June 18th - surprise, surprise.
The CTS V.R gets its weight bumped up to 3050 - 130 pounds! - and that's the last of the consistent good finishes for Caddy racing.

At Road America, CTS V.Rs finish fourth and second.
At Toronto, only third and fifth.
At Mid-Ohio, Johnny made a great start and hung on to second in the rain in the first race. In the second race, the best was fifth.

A Porsche won 3 of the last 4 races with a Viper winning the other. Both of these cars got a little help in the Appendix A changes but the big chance was the extra weight the CTS V.R got stuck with.

Next GT race is Aug 23/24 at Sonoma. I expect another Appendix A before then. I don't care to speculate which cars will be help and which will get hurt.
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I'm not seeing anything in here about horsepower settings, although I suspect that is governed by the resrictor. Talking to the guys at the Mid-Ohio race, they are not happy about the treatment of the Cadillac team from WC. Between the added weight, and reduced horsepower, they seem to be handcuffing the Caddy boys. IMHO, they should have a set standard for weight and horsepower and leave it alone.
As expected, World Challenge issued not 1 but 2 Appendix A changes this past week.

#81 didn't change much
#82 increased the weight of the 8200 cc Viper while leaving the GT3 Viper (8400cc) as it was before. The 911 GT3 weight went up some and the rev limiter came down some but the increased the restrictor size.

We'll just have to see if these changes actually balance out the performance (as they are supposed to do) this weekend in Sonoma.
Appendix A changed August 29th, a week after the Somona race. I see 2 changes between the 15 Aug edition (before the Somona races) and the 29 Aug edition:

Cadillac CTS V.R gets a weight reduction to 2975 pounds - a 75# reduction.
The Porsche 911 GT3 gets a 25# reduction in weight.

The Audi's remain the same and so does the Viper GT3 and Bentley GT3 cars.
Clarification comes

Well, I've had an email conversation with a guy at SCCA Pro Racing - World Challenge about the "balance of performance" documents and what some of the data actually means.

Since the start of the 2014 season, World Challenge has changed their "Appendix A" information 15 times - started with Version 70 and they are now on Version 84. I've been paying attention to the GT BoP changes but GT information is not always changed with each new version.

With Version 70, I noticed that there were difference values for "restrictor / boost" that were inconsistent and I tried to find out what they were talking about. In some cases, the value listed for the restrictor would be a millimeter size for the restrictor. Others cars would shows a number with "SIR" after it. Still others had a millimeter value and a "bar" value.

I determined that SIR meant "Sonic Intake Restrictor" but didn't know exactly what that meant. I started asking around to find out what an SIR is.
I also assumed that the millimeter values referred to a flat plate restrictor with a hole the size indicated.
I assumed that the millimeter plus "bar" values referred to a flat plate restrictor and a boost restriction for turbo/super-charged cars.

Well it turns out that SCCA Pro and World Challenge were publishing restrictor information without being totally clear as to whether the restrictor is a flat plate or an SIR.

My most recent email with SCCA Pro/WC is that the restrictor for the CTS V has been an SIR for some time in spite of the fact that it was listed on Appendix A as 49 mm.

The SIR is a venturi based restrictor that smooths the flow compared to a flat plate restrictor. Here is they way the man described them:

"A Sonic inlet restrictor(SIR) is a venturi with a cone shape attached to the exit side of the venturi. For a given size restrictor a sonic restrictor will flow more air than a flat plate restrictor. The flat sharp edge of a flat plate restrictor creates turbulence around the edge of the hole which restricts the amount of air that the hole allows to flow through it. The sonic restrictor has an aerodynamic shaped venturi which reduces the turbulence around the hole and lets more air pass through the opening. "

Back in February, the CTS V restrictor was listed as 50.8 mm. Then it got a slight bump up to 51mm. In early April, the restriction was reduced to 49 mm and the man tells me that the restrictor required is an SIR version at 49 mm or 1.929 inches. Appendix A just listed "49 mm". Then in late May, the Appendix A value changed to "1.780 SIR". When I saw this, I wasn't sure whether that was good or bad because I had no way of comparing 49 mm to 1.780 SIR. Turns out that Cadillac Racing was farther restricted from 1.929 inches to 1.780 inches. At this time, the CTS V was still at a minimum weight of 2920 pounds and they still kept winning or leading races.

In June, WC added weight to the CTS V, raising it to 3050 pounds. That pretty much took Cadillac racing out of the picture for winning or leading much.

Now for the last WC racing weekend - a double header - the reduced the CTS V weight to 2975 pounds.

Engine size, weight and rev limiter setting in Appendix A are pretty easy to understand, but the restrictor requirements are not so easy. They have not included in the Appendix A whether the restrictor must be a flat plate or an SIR in most cases. In one example, the Corvette with an LS3 based engine can use either a 63.6 mm flat plate or a 1.780" SIR.

The SCCA Pro guy says an SIR that is 76% the size of a flat plate restrictor produces about the same HP. So, the 1.780" SIR is about equivalent to a 45 mm flat plate restrictor.

Is everybody clear on this now?

I'm still getting my head around it.


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