Making a slip-on merge collector for a header

[RallyBob]

My good friend Roger Wilson has been hard at work since his retirement preparing his racing Opel GT. Time and money seldom converge simultaneously, so while he now has the time to work on his racecar, cash flow is not the same as it once was when he was working. In order to help him expedite things, I offered to build numerous components for his racecar which would have either taken him large amounts of time (which he can now spend doing other things), or large amounts of money buying custom specialized parts from racing shops.

In another thread I posted some pics of a new watt's link I built, and that item is slated for Roger's racecar as soon as I get the differential cover back from the powdercoater.

A while back, I built a custom merge collector for the racing header that Roger is building for his car. He was all excited about building the header, and knew that he wanted a nice merge collector for it, but when he found out they cost $300 and up he just about freaked out!

So I discreetly asked him what size header he was going to run and a few other things about the engine's specs, and using nothing more than some scraps of 1.625", 1.75", and 2.5" tubing I had in my recycling bin, I put together this slip-on merge collector as a donation to Roger's racing fund, and sent it to him without him knowing about it. Hopefully this gets him one step closer to being on the race track in a timely manner.



I started out with 4 short lengths of 1.75" tubing, which I then welded together on the abutting edges. This tubing has an ID that allows the 1.625" header tubes to slip inside. There will undoubtedly be some slight leakage initially, but once the engine and exhaust come up to temp the expansion takes care of that. Then I cut some scraps of tubing into triangular wedges, arranging them to form a cone-shape in the center of the four merged tubes. They were tack welded into place, massaged a bit into the final configuration, then fully welded. After welding, I ground them smooth with a carbide burr and a 2" disc sander, then blended the cone into the four merged tubes with some 80 grit cartridge rolls.

The basic outer collector body was made in my typical fashion, again from scraps of tubing (1.625" OD). Since I knew that I had four 1.625" tubes that I wanted to merge into a single 2.125" outlet, I did some basic math to create a cardboard pattern which I used to lay out each of the 4 sections of the collector body. The 1.625" tubes have a circumference of 5.105", however you can see by looking at the intersection of the four tubes than only 3/4" of that circumference is actually utilized at the entry of the collector. So that leaves me with 3.8288" of circumference at each of the entry tubes. And with the outlet being 2.125" OD/6.6758", with all four tubes merging to create one single outlet, then we know that only 25% of each 1.625" tube's circumference is used to form the new OD. So that's 1.6689" worth of circumference being used from each tube. I made the collector body 7" long, so I ended up with a tapered cardboard pattern that has a 3.8288" width at one end, is 7" long, and has a 1.6689" width at the other end. Once the pattern was made, I taped it to a full piece of 1.625" tubing, and transfered the pattern with a Sharpie marker. I did this on 4 tubes, then cut out the tubes to size. Finally, all 4 tubes were final-trimmed and hammered into shape, then welded together to form the outer collector body. While this sounds like a lot of work, in truth it takes about two hours to calculate it, make a template, cut everything to size, and weld it together. Still a lot cheaper than $300!

[RallyBob]

Once the outer collector body was welded, I finished the interior by grinding it with some carbide burrs and smoothing it again with some 80 grit cartidge rolls. I then fitted the four entry tubes with the inner cone to the outer collector, welding 360 degrees to make it leak free, and going back inside to smooth the new weld joints with some 80 grit sanding rolls.

For the venturi cone at the exit of the collector, I took a piece of 2.5" exhaust tubing, and made a slit down the length of it. I made a pie-shaped wedge cut, so the tube would taper from 2.125" back out to 2.5". Then this was welded to the end of the collector body, and again metal-finished at the weld joint inside.

[RallyBob]

Lastly, as this is a slip on collector, I welded two bent-wire hooks to the collector entry. These are placed 180° across from each other, and are there to allow stainless springs to be hooked up between the collector and the header tubes themselves. So the collector can move with the expansion and contraction of the exhaust system, but won't fall off the car!

[Gary]

Great work Bob. Once Roger sees that, he won't need any Viagra for a week!

[RallyBob]

Originally Posted by Gary Great work Bob. Once Roger sees that, he won't need any Viagra for a week!

Thanks Gary. Roger already has the collector in his hands, just hasn't seen the watt's link yet. And I still have yet to build some tubular upper and lower control arms for his GT when my time and money allow it. Those parts will definitely give him a 'boinger'...

He told me he walked around with the collector showing it to all his buddies at his rental space. Old people are funny....

[wrench459]

Sweet.. Bob
With all the care you put into the exhaust. I can't wait for your input on the other side of the motor.. sheet metal (hopefully steel) intake.

[RallyBob]

Originally Posted by wrench459 Sweet.. Bob With all the care you put into the exhaust. I can't wait for your input on the other side of the motor.. sheet metal (hopefully steel) intake.

Sadly no. Per the rules he must either run the stock Opel EFI intake or sidedraft carbs with 40mm throttle plates. He chose the EFI intake with Weber-Redline programmable fuel injection.

[Redskinsjbs]

First of all I'd like to say that you are the nicest guy who knows how to use a welder!

Secondly, you know when you are reading something and then all of the words start to blend and mush together, that happened right about here:

Originally Posted by RallyBob The basic outer collector body was made in my typical fashion, again from scraps of tubing (1.625" OD). Since I knew that I had four 1.625" tubes that I wanted to merge into a single 2.125" outlet, I did some basic math to create a cardboard pattern which I used to lay out each of the 4 sections of the collector body. The 1.625" tubes have a circumference of 5.105", however you can see by looking at the intersection of the four tubes than only 3/4" of that circumference is actually utilized at the entry of the collector. So that leaves me with 3.8288" of circumference at each of the entry tubes. And with the outlet being 2.125" OD/6.6758", with all four tubes merging to create one single outlet, then we know that only 25% of each 1.625" tube's circumference is used to form the new OD. So that's 1.6689" worth of circumference being used from each tube. I made the collector body 7" long, so I ended up with a tapered cardboard pattern that has a 3.8288" width at one end, is 7" long, and has a 1.6689" width at the other end. Once the pattern was made, I taped it to a full piece of 1.625" tubing, and transfered the pattern with a Sharpie marker. I did this on 4 tubes, then cut out the tubes to size. Finally, all 4 tubes were final-trimmed and hammered into shape, then welded together to form the outer collector body. While this sounds like a lot of work, in truth it takes about two hours to calculate it, make a template, cut everything to size, and weld it together. Still a lot cheaper than $300!

[RallyBob]

Originally Posted by Redskinsjbs Secondly, you know when you are reading something and then all of the words start to blend and mush together, that happened right about here:

Yea, well that's probably from the vikodan and the bottle of merlot I drank trying to get my back muscles to relax! Everything becomes a blurr after a while...

[opelwasp]

Originally Posted by RallyBob Thanks Gary. Roger already has the collector in his hands, just hasn't seen the watt's link yet. And I still have yet to build some tubular upper and lower control arms for his GT when my time and money allow it. Those parts will definitely give him a 'boinger'... He told me he walked around with the collector showing it to all his buddies at his rental space. Old people are funny....

Oh, Bob please lake lots of pics and give lots of dimensions. I have been wanting to do this for a coil over project/kit. I think I may have figured a way to make an effective unit but don't have the A arms done yet. I may even be able to make it bolt in.

[RallyBob]

Originally Posted by opelwasp Oh, Bob please lake lots of pics and give lots of dimensions. I have been wanting to do this for a coil over project/kit. I think I may have figured a way to make an effective unit but don't have the A arms done yet. I may even be able to make it bolt in.

It'll be bolt-on, but won't be compatible with most street cars.

First of all, the a-arms will be 1.75" longer than stock, so unless you are running fender flares they physically just won't fit unless you have very narrow front-drive offset wheels.

Secondly, everything will be heim-jointed. Besides the cost, they are noisier, ride rougher, and typically must be replaced annually. They don't tolerate wet and dirty conditions well either.

Third, I am correcting some of the geometry and packaging by designing the a-arms around a Manta spindle/steering arm rather than the weaker and taller GT stuff. The Manta steering arms have the tie rods fitted at the bottom, the GT steering arms have the tie rods at the top. So unless you are running a race-car ride height, bump steer will also be an issue with this setup.

However, I'll still take pics!