Build your own intake manifold
Last year at Watkins Glen my car was fast but it really just fell on its face in 4th and 5th. The car was using a Cyclone Intake manifold with a waste gate actuator fabbed up to operate the secondaries. As much as I love it I think its holding me back. I normally take the car up to 8500 rpms and at those rpms the intake plenum is just too small. With a limited budget and the price of intakes going up I decided to build my own.
For the design I am following some basic manifold building rules. First the runners should be 9" from the plenum to the intake guide seat. The distance on a 4g63 head from the gasket surface to the valve seat is just over 4". We all know that the longer the runner the better a manifold performs at lower rpms. But there is another rule to follow. That is the plenum should be at least 2.5 times the volume of 4 runners (gasket to base of plenum). The tricky part is that you can move the whole power curve and peak horsepower numbers either up or down depending on plenum volume. Bigger the plenum the higher the peak horsepower.
I want to make good power all the way up to 8500 rpms but I want some grunt down low too. My best design so far uses a 6" runner with a plenum volume just under 3 liters. I am planning on building the manifold by machining a 1/2" plate to act as the plenum floor that would feature velocity stacks (radiused entries) built in. That means they would be milled into the stock rather than rolling the end of a tube. I found ready made velocity stacks but they are $40 each and that's just too much money for me. I am planning on a plenum length of 16-17" inches with it tapering from 4" to 2.5".
I got most of my design ideas from Google including this site.
The plan is a single plenum just under 3L in volume with 6" runners. That would give me about 10.1" from the beginning of the runner to the edge of the valve seat. From what I've seen I thought the 6" runners would be longer than what others have done and the .125 thick runners yield a 2" ID which is also smaller than what others have done. I think the slightly longer and smaller runners would help low end power when combined with a slightly lower than 2.5 times plenum volume. I think making more power up top is easy and I really don't think I can mess that up. Making power down low is another thing all together.
What I think is that intake charge velocity controls cylinder filling at lower rpms and that like the 2G and EVO head design, smaller is better. At what point (rpm) does this smaller runner become a bottle neck I don't know. I am betting the farm that combining 1G port size dimensions with a big plenum would give me the best of both worlds. Much in the same way it would if I were to merely cut the plenum off a stock manifold and just weld on a bigger plenum. From my research that has been done with success.
Table saw with carbide metal cutting blade
Wood router with ½ round-over bit and dado bits
Drill press with bits
This is where I sourced the materials for the build. I had some ½ aluminum fixture plate that I got from the scrape bin at work but that can be purchased online as well.
For the plenum I am using 4x4" OD 6063 T52 aluminum square tube with a .125" wall thickness. The runners are going to be made out of 2.25" OD 6061 T6 aluminum tube also with a .125" wall thickness. That would give me a 2" ID on the runners and 3.5" on the plenum. I chose the thicker .125" wall thickness to ensure maximum strength while giving the welder lots of meat to weld with. The cost between thinner stock was minimal.
Building the manifold-
Lube the blade with metal cutting fluid (Home Depot) and have at it.
This next photo shows how nice the of an edge the blade left.
Then I milled a lip on both sides of the 4" strip I just cut out. This will receive the box top after its cut.
Then I cut the bottom of the plenum box off so it would fit on the milled base plate.
Later I'll cut an angle so the plenum will have a taper to it. For now its just dry fitted to see how it fits.
Then I cut the 2 1/2" hole for the TB in another piece of 1/2" plate I had. I just randomly cut the hole so I could practice milling the velocity stacks.