(Robert Legere and Rüdiger Cordes, 5. + 6.5.2000)    


I want to weld a exhaust manifold myself. What about the length and diameters of the tubes? Is there a common knowledge of the best solution? What about the cam shaft and the rpm of max power or torque, do they influence the manifold design?

Thanks for everything answered, Rüdiger.

There are numerous factors which influence the design of a "header" or tubular exhaust manifold.

I would recommend 1.5" (38mm) tubing for a 1.9 or 2.0 engine used for the street. The length of the tubes depends mostly on the rpm's and camshaft, but definitely no shorter than 32" (813mm) and as long as 38" for high torque (965mm). I usually choose 34" (864mm) for a fast street car or ITB racecar (Nearly stock race class here in the US).

Of course, for a larger engine such as a 2.2 or 2.4 litre, 1.625" (41.27mm) tubing would be a better choice. And very high rpm engines with modified heads and camshafts might need even larger tubes. I use 1.75" (44.45mm) tubes on engines which rev over 9000 rpm's.

If you have specific information about your engine I can maybe help out more.

Bob Legere

Thanks for your very fast answer!

My engine is a big bored 2.0 CIH. Now it has 2.4 litre with a Schrick 302° camshaft, 45mm/41mm valves, and distributor limited revs to 6,300 rpms, modified ports, I have no information about rods and pistons. So it has 174 hp. I bought 90 degree bends and straight tubes of 40 mm x 1.5 mm. That means 40 mm outer diameter with a thickness of 1.5 mm results in an inner diameter of 37 mm. Your value of 32 to 38" is the length of each single tube before they are connected? So I had to join them under or behind the engine but not where they are connected now with the stock sprint manifold? So I need much more space for 4 tubes running down the engine. I had dreamed of a service friendly solution. And then all four tubes should be connected at the same position? Not first cylinder 1 with 4 and 2 with 3?

I don't particularly like 4-2-1 type exhaust headers. I prefer the power band of a 4 into 1 design. They are more flexible in power output if designed well. This is one reason why there are very few proper manifolds designed this way. They are admittedly more difficult to package under the car. More space is needed, and installation is more difficult. The 4-2-1 designs are easier (cheaper) to build as a result, and this is one reason most manufacturers build them this way. I've tested various 4-2-1 designs, and didn't like the power band. They were a little better at mid-range, but never made the peak power or even the low-end power of a proper 4-1 header. And the sound...yuck. A 4-1 sounds much better, at least to my ears. All four tubes will be the same length. This can sometimes be difficult I admit, I always make #1 tube as straight as possible to make it go as far to the back of the car as I can. This way #4 tube is not as twisted under the hood. They will be all joined together at one point.

And with what tube shall I then go to the catalyst muffler combination?

I would use a 2.75" (about 70 mm ) collector that was 10" long (254 mm) and then out to a 2.5" (63.5 mm) exhaust for that sized engine.

I want to only use one muffler with catalyst in front of it to have a light and aerodynamic exhaust system. No tubing over the rear axle, just a one tube end to the passenger side in front of the rearwheel. I have bought a Trush CVX muffler and will weld a longer duplicate of it out of non rusting steel. Comments? I want to make it a bit longer to eliminate higher frequencies and reduce more noise. At now the sound is very good but a bit too loud and at speeds over 88 mph the exhaust is no longer hearable. Maybe the other noise sources are louder ore the muffler is then full working.

I have found that a street Opel needs two mufflers to be quiet, but maybe the catalyst will do a lot of that for you. A longer exhaust is better for torque as well. I would point the exhaust tip down towards the ground, as this also makes the car a few decibals quieter. I know in Europe sound is a great concern, as it is becoming over here lately. We have to pass 86 db I believe now.

What do you think of the advantage of such a header. Can you give an amount of hps or percents?

I think you will see about 7 or 8 hp over a cast iron manifold, but the main improvement is throughout the ranges with better overall torque. Two hp at 6000 rpm's means nothing compared to 4 or 5 hp at all other rpms. This will improve the acceleration. You will probably notice better fuel usage though, and the spark plugs should have better color compared to the 4-2-1 design.

Bob Legere