Bruce
01-17-2004, 10:25 AM
While there are lots of theories and design principals, I'm going to talk about what I've found, and what I've noticed that works.
For openers the best Normally Aspirated intake manifold is an Independent Runner design (obviously when well executed). There is no cross stealing of Intake charge, from cylinder to cylinder, and with no waves or pulsations to effect things.
In normally aspirated applications, with clever tuning of runner lenghts and cross sections, and with using a plenum, you can generate some pressure Waves that approach 1-1.5 PSI above ambient pressures in the manifold and get some excellent peak results. Peak meaning these designs only operate over a very finite rpm range. But, we immediately get back to ying, and yang. For the good you accomplice in small areas, it's at the expense of other areas.
So now we'll take a little look at our Buicks.
We don't have to worry about trying to develope any pressure waves, for addition cylinder filling since we already have boost to ensure cylinder filling. As a matter of fact, we just need to min., the pressure waves in the plenum, since all they can do for us (if we have enough plenum volume) is make the motor peaky.
If you've played much with N/A engines then you've probably noticed how runner lenght and volume can effect how fast an engine can change rpm when under load. High numerical axle gears, close ratio trannie, and you can run some fat short runners and have a set up that grabs revs in a hurry.
I've taken a stock manifold and welded up the sides (welding these manifolds is a huge PITB), and then cut back the runner floors. Swapped manifolds with no other changes, and had to instantly add 6% more fuel to maintain the same WOT AFR.
Experiment II was welding a section of square tubing to the upper plenum, that extended back into where the coil packs used to be. While there was no serious change in WOT AFR, thou this change needed some more AE (accleration enrichment), and the engine just "seemed" to spool faster. I know some folks will immediately look at the upper plenum, and go owww, that's ugly, and they're right. Trouble is engine's often like ugly stuff. Case in point, I used to work at a shop where we did porting, and dyno work. We tried various finishes on the innerds of the runners, to pick up HP, and in every case, a 100 grit roll left the best surface. And when someone would bring in a head or manifold we'd always show them the difference between the roll finish, and polished, and tell them the polishing lost a few HP, and without exception the street guys would still want the polished finish, thinking that there just HAD TO be some advantage.
In the pics you can see my upper plenum, then the sides of the modified stocker, and a M+A Manifold. Atta glance you can see what I would consider problems with the M+A. While not ready to install yet, you can bet the guts will be very different then they designed.
For openers the best Normally Aspirated intake manifold is an Independent Runner design (obviously when well executed). There is no cross stealing of Intake charge, from cylinder to cylinder, and with no waves or pulsations to effect things.
In normally aspirated applications, with clever tuning of runner lenghts and cross sections, and with using a plenum, you can generate some pressure Waves that approach 1-1.5 PSI above ambient pressures in the manifold and get some excellent peak results. Peak meaning these designs only operate over a very finite rpm range. But, we immediately get back to ying, and yang. For the good you accomplice in small areas, it's at the expense of other areas.
So now we'll take a little look at our Buicks.
We don't have to worry about trying to develope any pressure waves, for addition cylinder filling since we already have boost to ensure cylinder filling. As a matter of fact, we just need to min., the pressure waves in the plenum, since all they can do for us (if we have enough plenum volume) is make the motor peaky.
If you've played much with N/A engines then you've probably noticed how runner lenght and volume can effect how fast an engine can change rpm when under load. High numerical axle gears, close ratio trannie, and you can run some fat short runners and have a set up that grabs revs in a hurry.
I've taken a stock manifold and welded up the sides (welding these manifolds is a huge PITB), and then cut back the runner floors. Swapped manifolds with no other changes, and had to instantly add 6% more fuel to maintain the same WOT AFR.
Experiment II was welding a section of square tubing to the upper plenum, that extended back into where the coil packs used to be. While there was no serious change in WOT AFR, thou this change needed some more AE (accleration enrichment), and the engine just "seemed" to spool faster. I know some folks will immediately look at the upper plenum, and go owww, that's ugly, and they're right. Trouble is engine's often like ugly stuff. Case in point, I used to work at a shop where we did porting, and dyno work. We tried various finishes on the innerds of the runners, to pick up HP, and in every case, a 100 grit roll left the best surface. And when someone would bring in a head or manifold we'd always show them the difference between the roll finish, and polished, and tell them the polishing lost a few HP, and without exception the street guys would still want the polished finish, thinking that there just HAD TO be some advantage.
In the pics you can see my upper plenum, then the sides of the modified stocker, and a M+A Manifold. Atta glance you can see what I would consider problems with the M+A. While not ready to install yet, you can bet the guts will be very different then they designed.