Hear is what I have learned from making intake manifolds. Back when I was drag racing Pontiac's, I had more time then money. I always wanted a tunnel ram so I whittled out some flanges and set about bending heavy sheet metal. I borrowed a friends mig welder and hooked it all together. It wasn't pretty but functioned almost as good as what I had been running. Turned out the engine didn't need the extra length runners anyway. The second manifold was a little lower but made much better. It too almost worked, but fell off at high rpm due to runner size and the plenum shape. Lesson learned, keep with what I had.
After I got my '37 on the road, I cam across three NOS Rochester model H carburetors from 1961 Corvairs. They are not very friendly carbs to work on, no moving parts, so no metering changes with throttle opening. To change metering, you change the jet which effects the whole throttle opening range. The manifold was easy enough to make with rectangle steel tubing and some flanges cut out from 3/8" plate steel. I used a drill press, hack saw and lots of files, sanding disk and grinding wheels. I had the benefit of an extra 250 head and block to work from. The engine ran well when jetted up, but as you can imagine, the fuel mileage suffered. Jetting lean brought the mileage up, but then it would lay over at higher speeds. Cold weather was a problem since I had no chokes or heated manifold. The next manifold built was the same design using the three Model H Rochester only with a header instead of the cast manifold. It was more of a tapered pipe with six little stubs and a single 2 1/2" pipe down and out. Since I changed nothing else, I saw no improvement in performance. The local drag strip was near by so I went and made six runs back to back. The following week I went back, same general weather conditions, and made six more runs back to back only this time with a stock manifold, Monojet, and factory exhaust. All twelve runs were with-in .005 seconds of each other and the MPH stayed with in .5 of each other. So basically it was a draw. I could say I built a manifold that run as good as a stock setup, or you could say it only ran as good as a stock setup, either way it made no difference having three small carburetors or one sized properly. Had I made the header with really long head pipes with 1 1/5" pipe diameter, I probably could have improved the lower rpm torque, but I didn't want to try and snake them down through and around the steering / frame area.
Jump ahead a year or two, I came across two Monojet's the same part number, same metering, linkage the whole bit. When building this two carb manifold, I used the same size rectangle tubing and some more flanges I cutout. I added a heating chamber under the manifold which bolted to a stock exhaust manifold. I now had heat out to the ends and under each carburetor. I took the time to round all the corners instead of just squaring them off. It looked really good, but doubling the carburetors where only one was needed did nothing for how it ran. I went for a decent drive and could actually see the fuel gauge moving. At full throttle, it did run pretty good but you had to get the truck rolling first. It didn't take to many trips before this went in the basement for storage.
Next, I talked with Jon Hargrove about the W-1 carters and decided to give them a try. I dug out the dual carb manifold, made adapters to change bolt patterns for the W-1's and added a different linkage pivot. I have been running these for three years now and am really pleased with how it performs. The W-1 is super adjustable with multi-step metering. I don't believe the intake has much to do with how the engine runs if the carburetor choice is right.
I measured the stock manifold inside and choose tubing very close to that size, I also made the runners the same length. I spaced each carb 1/2 way between 1,2,and 3, and 4,5, and 6 cylinders. Each carb can feed all six cylinders. The real advantage is the end cylinders have better access to the fuel mixture so all six cylinders run even. With one central carb, the center cylinders always ran richer then the ends, the spark plugs showed this. Fuel mileage is now higher then it's ever been, and performance is better. Our local drag strip closed so I can't prove it, but seat of the pants tells me it's running much stronger. With the addition of an Air/Fuel ratio wideband gauge, I now have it dialed right in on performance and fuel economy.
Send me a PM with your e-mail and I can get some photos out to you. If you go down this road of making something, stick close to the OEM measurements and be sure to add some type of heat. Aluminum would be easy to work with if you can weld it, but steel isn't all that bad. When welding the flanges to the runners, be sure to have it bolted to heavy plate steel or a head to absorb the heat and keep warpage to a minimum. Trying flatten out steel flanges is not a easy thing to do, I have been there!
Dad has a couple of Pro-version engine analyzer programs, so I played around with them trying to determine what would help. I would go to extremes just to see what changes I could make. Unless I was willing to raise the rpm, nothing was making much difference. The really long (35" x 1.5" diameter ) head pipes did improve the torque, but would have been a nightmare to build and install. Cams didn't make much difference as long as I kept the rpm parameters the same. Sticking to 4500 rpm redline really limits the amount of improvements that can be made. Using the dragstrip time sheets and knowing the weight of the truck, I calculated out the HP needed to run the mph (78.5), The engine was making about 140 at the flywheel and 110 at the tires, pretty much stock spec's for a 1970 250. Figuring off the et instead of mph, it made 150 and 120 at the tires.
We also have a SuperFlow 110 cylinder head flow bench. I used my second cylinder head and ported the exhaust and intake runners. I gained 18 to 20% more intake flow and around 15% more exhaust using stock intake valves, no lump ports or cutting of the head bolt boss, and bigger exhaust valves. I added epoxy to the intake floor to raise and change the short turn radius, but kept the head bolt boss. On paper it looked good, and the analyzers both said it should pickup hp and torque. It didn't, it actually ran the same at full throttle, but ran terrible at mid throttle. Around town you couldn't tell any difference. It took about a year of endless test drives and swapping of parts to figure out why it wouldn't run. With the head back on the flow bench, we determined the exhaust flow to intake flow ratio was way off. At mid throttle, the exhaust was working so good, that when the valve openings overlapped, the intake mixture was being sucked out the exhaust causing an extreme lean condition. This head is now in the parts that didn't work bin. I finally have learned that if no other changes are to made, don't bother looking for HP or torque. I ran all my stuff with the same 2' exhaust, gear ratio, tire size, and stayed at the same rpm limit. Thats why nothing ever really changed. And when I say this head wouldn't run mid-throttle, it would shake the engine trans so hard, it broke the shifter mounting points off the transmission! It was a violent shake, the only way out was go full throttle or let off.
Joe
