Van has been running well, but it is about to go under the knife yet again. At WOT I have always seen the intake manifold pressure dropping as the engine revs up. I turned the MAF sensor off in the PCM calibration and started pulling apart the air intake. First the air cleaner. Then the MAF. Then finally the dumb intake hat that GM used to literally cram our engines into these vans. I temporarily installed the marine flame arrestor that is little more than a mesh screen on top of the throttle body. With the throttle body finally able to draw in air, the manifold pressure went from 88 KPA to 97 KPA at 5,500-6,000 rpm. The engine ran like it has never run before. I would say it probably gained 40-50 HP. That pressure difference observed at the manifold is equivalent to driving at 4,000 ft elevation. I found an intake manifold that I ordered with fuel rails. It will have a 102mm LS style throttle body on the front of it, connected to the 4" silicone inlet ducting and 100mm MAF. The manifold is only 90mm at the face but I already have a 102mm LS style throttle body and a matching 102mm to 90mm adapter laying around unused on a shelf at the shop that came on a LS intake I put the DBW throttle body back on. My 97 Express van has 11" of height between the china wall at the end of the engine block and the bottom of the engine tunnel under the wiper cowls. The upper plenum sits 8" off the china wall and the lip for the throttle body mounting is 9" off the china wall. It will clear both the engine tunnel on the body and the factory doghouse. It is similar to the GM Ramjet manifold. I found engine dyno data for both the Ramjet and the L31 Marine crossram manifold on the same long block. Both the Ramjet and 300 HP Marine engine use the same short block, same 9.1:1 compression ratio with their thicker head gaskets, as well as the 196/206@ 0.050 duration GM 395' camshaft. The Ramjet and Marine intake both make the same torque at 2,500 rpm. At 2,700 rpm the Ramjet intake with its 4" long runners and open plenum starts to pull ahead. By 4,800 rpm the Ramjet 350 is up 61 hp over the marine intake. This manifold has slightly longer runners at 6" long. It should make a little more torque than the Ramjet manifold. That being said a 9.1:1 350 with a Ramjet manifold is already making 390 ft/lbs of torque at 2,500 rpm. With 383 cubic inches of air hungry small block compressing that air to a 9:1 dynamic compression ratio, I do not see this manifold making any less torque than the ported truck manifold or marine manifold. If it does cost a little torque, it will be well under the 2,800 rpm stall speed of my torque converter and a mere touch of the throttle will make the torque loss vanish. I am a little worried about how twitchy the throttle will become with the 102mm throttle body. The 80mm that is currently on the van is already pretty touchy at lower speeds.
The terrible flowing intake hat. I think this part alone is solely responsible for the 10 hp loss both the 305 and 350 in the Express vans have compared to the same engine in a truck.
Casting appears to be very clean in the pictures. Will know more about the quality of the manifold when it is in my hands.