O.K., so I think the exhaust mods I have recently completed made an improvement to engine performance. The more I look at the air flow design for my '98 Suburban, the more I think there might be an opportunity for improvement.
There have been many posts regarding the older trucks and folks have suggested two main fixes: going to the round filter and opening the LD intake extension casting as per JK's instructions. Since my '98 doesn't fit into either category, I'm looking to do something else to improve air flow to the engine. Currently, there is a rather small air inlet directly behind the passenger side headlight assembly. This plastic intake assembly takes a 90 degree bend, runs up and behind the grill, takes another 90 degree bend and then dumps into a cavity/space behind the fender well. Any air making it to this cavity then travels thru a 4" oblong/oval opening in the wall that separates the engine compartment from the cavity/space behind the fender well. A very short tube connects the 4" oblong/oval opening to a 3" opening in the K&N filter box, which contains the round filter. So, we have a passive air supply system to the turbo that appears to be restricted at several points along the way to the turbo. It just doesn't seem to be a good design, but I don't know. The system is clearly not a ram-air style. In fact, the inlet is obstructed by the headlight assembly. Whatever air is getting to the turbo seems to be getting there solely by the effort of the turbo itself. If air supply to the 6.5TD is in fact inadequate, as many people suggest, it seems that you would need to determine what CFM the engine "requires" and determine if that flow is being supplied by the design of the Suburban's '98 system.
I found earlier information that attempted to answer the first question:
A 6.5L = 397 c.i.
397 c.i. X 2500 rpm = 992,500 c.i. per minute (No boost and no loss)
992,500/1,728 = 574 cfm
Let's say we're reading 7 psi of boost (1/2 atmosphere?)
574 cfm + 287 (1/2 atmosphere) = 861 cfm at 2500 rpm
Assuming this calculation is correct, is there an instrument that can be used to measure air flow so that I can determine what flow is actually reaching the turbo?
If the 6.5TD isn't getting all of the air it could possibly receive, I'm thinking of a more radical modification. I could cut an opening in the fender opposite the 4" oblong opening that currently exists in the engine compartment wall and install something like the following:
http://www.westmarine.com/webapp/wcs...51&storeNum=24
I view this set-up as something of a ram air hybrid. More air would be directed to the air box because it would have a shorter distance to travel (less loss) than stock and it might even be delivered to the air box with some small increase in pressure. Water infiltration would be an issue but this could be corrected thru the use of a baffle in combination with the drilling of holes in the bottom of the air box as members have suggested in prior posts.
Anybody want to take a shot at this one?