Thread: Half Boost

Modified my engine - ported heads, XP1 Cam, springs, SS valves, headers, etc. Put on a 3.4 pully and took it to be dyno tuned. Engine would only develop half boost on its meter. Dropped the pully size to 2.9. Same thing. The dyno guy said it was because the ported lower intake manifold opening under the S/C was too large. He said I'd develop full boost if I put the stock manifold back on. We had a deep discussion for 30 minutes but we couldn't agree. I don't think porting the top side of the intake manifold does anything to improve performance and would have no effect on the pressure developed within the manifold. I put a ported manifold on the engine only because it was readily available and not because I thought it would make any difference. Does anyone know if an intake ported on the S/C side would lead to a loss of boost and why?

Full story - Initial dyno test had a boost gauge hooked up. Boost started out in the negative range and moved up to zero at full throttle. Dyno showed 242 hp at the wheels. Dyno guy thought issue was ported intake. Left shop and spent a week trying to find the issue. Found that the plug on the intake from the EGR didn't hold (EGR was intentionally bypassed). Couldn't see it at first but took out pipe that I'd put a plug into and replaced with a permanent solution. After that the lego blocks showed half boost when punched. On my own I put a 2.9" pulley on it, knowing that it wasn't a bright idea but willing to test it once to see if lego blocks would show any difference, which they didn't. And yes, I don't have a boost guage....yet. Took it back to the dyno guy and told him to tune it (with the 3.4 pulley on it). He wanted to know if I had changed the intake, which led to the 30 minute discussion. Since he's located across the street from me and is actually quite talented I decided not to press the issue and left without getting another tune. Does anyone have a explanation on how porting the intake would have no effect on the boost developed? The dyno guy argued that the opening below the super charger is like putting your thumb across the end of a hose, you end up with the same volume of water coming through as if not constricted but at a higher pressure. I argued that it's a fixed volume of space inside the intake manifold and covering half of the intake doesn't change the volume nor the rate at which air is allowed to escape into the cylinders. Don't abuse the dyno guy, he's pretty well known and does street rods to top fuel dragsters, but on this point I can't agree with him.

I agree with you. His hose theory doesn't seem right. Your manifold is more like putting a sprayer nozzle on the hose. Opening the sprayer is like the intake valves opening. The boost would be the pressure before the nozzle tip, not after it like he's suggesting. And how much pressure in the hose is dependent on how open the water valve is in relation to how much the nozzle is opening. At least that's how I understand it.