Dave at Random Technology and Chris discussed several times on doing dyno testing on this particular question and have considered with the larger displacement engines that are coming out trying a set of 2" primary headers.
Personally Rich you going with some bigger primary headers wouldnt hurt. I think even running a complete 3" exhaust system with you running that much amount of air would help.
I believe the LG Pro tubes have 1 3/4 primaries, if I'm not mistaken. From the collector back to the end of the input pipes for the turbos the pipes are 3 inches. Which then gets necked down to the 2 1/2 inch pipes (I think) that are standard on cat back exhaust systems on a C5. At first thought, I felt that having 3 inch plumbing all the way through the turbos would be the way to go, but the more I thought about it, the less convinced I became that this was the way to go. Turbos work on the principle of the turbine being moved by the exhaust acting on the driving vanes, which in turn moves air in a proportional fashion to the throttle body via connecting vanes that then push fresh air along. Logic would say that the more exhaust getting to the turbine and the faster the turbine spins, the more air being moved. Much like a hose nozzle on a large hose works better than the same size nozzle being on a small diameter hose, the necking down of the exhaust gases possibly HELPS provide a boost in velocity of the exhaust reaching the turbines in the turbos. Or at least it seems like this should be the case.
But turbos are weird critters to figure out. I never hear them spooling up when in first or second gear, much less sitting still just revving the engine, but if I am in higher gears (especially sixth gear) on an incline, you hear them wailing like crazy. So the amount of exhaust getting to them seems more directly related to the load on the engine, and not entirely on the rpm of the engine. I know they have to be spinning with ANY exhaust going through them, but they REALLY sing out when under some kind of load. I really think the ratings on the dynos are all rather underrated as to the amount of power that is actually getting to the rear wheels in a real world situation with turbo charged vehicles unless the drum on the dyno can put sufficient loading on the rear wheels to simulate a car weighing 3,000 pounds working against inertia to accelerate rapidly. In other words, the turbos apparently NEED something to work against that loads the engine and provides the peak power potential from them.
Heck, this all seems counterintuitive at times, but what sort of clued me in to this was when I was trying to show someone the boost gauge showing boost while just sitting still and revving up the engine. It NEVER showed any boost at all with no load on the engine. Even at 5,000 RPM, which SHOULD have been moving a lot of exhaust to the turbos, the boost gauge didn't show that the turbos were even there. Just struck me as rather odd.... Matter of fact, I thought that something was wrong and perhaps there was a break in the pipes somewhere.... But later on, out on the road, in third gear, nope, there wasn't anything at all wrong with the turbos.... Just testing the turbos, of course... :hehehe:
So, I don't know about 2 inch primaries. Sometimes you get caught up in the fallacy that if a little is good, then a LOT is better, when there always seems to be a point of diminishing returns to watch out for. But heck, maybe providing that larger piping from the exhaust ports of the heads down to being necked down right at the turbos might be worthwhile.... :shrug01: Just might be a bit too expensive getting those two inch pipes just to experiment, though...
But heck, if you guys want a guinea pig, heck, maybe I'm game.... :thumbsup: I just would want to be there to take LOTS of video.... :yesnod:
