JerryTX

Ok, now that the NASA ruleset is in place for next year, my brother and I find ourselves in similar positions. We are both going down the path of slotting into the new ST2 AND ST3 rule set. This will allow us to detune or remove a restrictor to run in the class with bigger car counts. Both of us have LS3's going into our cars, both want to be able to run both classes.

My engine will be stock longblock with the baby Texas Speed cam (220/220/.581/.581), his will be a bonestock LS3. I need to limit the LS3 to 411rwhp for ST2 and 373rwhp SAE for ST3. My car will be using stock LS7 manifolds AND cats (because that is what I already have) and otherwise straight through exhaust. I'm guessing the car will want to make 420-440rwhp? How hard will it be on the engine to make it breath through a restricter and get it to 373rwhp? Anyone have experience with how much heat this will cause, impact on engine timing over a 25 minutes race etc? Is this too much (too little) to ask of the LS3? My brothers car, while it may make slightly less power, is going in a lot lighter car that does have LT headers, so he'll have similar issues trying to kill 70hp or so.

This thread is about how to limit the power of the LS3 and whether it can be done safely or not in a road racing environment. The ruleset and the pros and cons are being discussed elsewhere, lets keep this on topic please.

AU N EGL

TB restrictor plate. Not sure what size though

jaa1992

Ask Mr Farmer what size restricor plate did what when he was running one.

davidfarmer

I have a thin metal restrictor, the smallest physically possible that the butterfly would fit into, for our LS7. It was roughly 1/3 the area of the OEM opening, and it restricted the engine from about 480 to right at 400. You will need some trial and error if you go this route.

However, I think the tuners are starting to figure this out, and can do so more cleanly. With PCM tuning, you could theoretically keep all of your lower torque, but flatten the HP peak just where you want it. A restrictor plate will effect mostly high rpm, but will pull power/torque across the board to some extent.

J.R.

To limit to approx. 373 RWHP with your engine try an approx. 56mm restrictor.

That should put you somewhere close. Your tune can also influence this.

Tune without a restrictor and then install properly sized restrictor to obtain your specific goal. This will let you run different HP for varied classes and not have to retune every time.

Gordy M

You would be better off optimizing the tuning on PCM and using the rev limiter for controlling HP. A good tuner can flatten the torque curve to the gears you are running so you have great pulling power up to the rev limiter. Most tuning software have an autocal devise to plug in your OBD2 where you change a your tune from ST2. It would lower the rev limiter to meet ST3 HP requirements. For example, what is the difference from shifting at 6300 rpm (ST3 tune) vs 6900 rpm (ST2) if the car is pulling strong to that point.

Maynor29

I'm the brother with the bone stock LS3 going in. I'd rather do the PCM tune as well but I'm converting to a cable driven throttle for now so that's not an option until I swap out PCMs.

With the new ST2 rules, I'll be limited to 376 whp. Can't recall what bone stock (except headers and open exhaust) LS3s make whp wise but I'm hoping I won't need to run a restrictor plate.

Any insight on typical whp levels?

MarkDFW

My 2012 Grand Sport A6 had baseline dyno of 370 rwhp if that helps. You should be close to that.

geerookie

Don't use a restrictor plate.

Talk to Julio @ Cartek in Garwood, NJ He can tune it (without a restrictor plate) to whatever you need and still maximize the power under the curve and not have to set artificially low rev limits.
I'm not going to reveal the special code we came up with to do this but he can do it. Don't ask, I'm not telling

You can mention that I sent you there.

Maynor29

Thanks Mark. Sounds like I might have to ballast up slightly to play it safe but that's a good baseline to work with.

Greg, using Julio is my plan.

chuntington101

I would tune with the restrictor in place. At the point you reach max airflow of the restrictor (the air approaches supersonic) some strange things can happen. I have ben informed that engine are actually more dept. prone once they have breached the airflow levels of the restrictor (lower density fuel air mix will burn quicker i guess).

Could you not simply have the rev limiter cutting in lower down the rev range?

davidfarmer

in my case, the engine was stock, and I wanted to keep the PCM stock for warranty reasons. The restrictor game me exactly the max power I needed, and maintained good torque (higher than an comparably powered LS6) and it still pulled to redline.

A good programmer can tune for more overall torque than a mechanical restrictor, but not all tuners have experience doing that. There are lots of ways to do it, depending on the car, and it's just now something we are asking tuners to do.