Well, I had an idea for a performance exhaust part. I even
went as far as to have a prototype fabricated. I wanted to
make a pre-cat H pipe to balance the pressure across
the cats, since a large part of the back pressure comes from
the cats rather than the mufflers. I reasoned that you'd get
half as much gas, twice as often through each cat, and the
end result would look the same to the o2's.

I went to Dave at Northstar in Mountain View on advise
from the forum, and he was downright rude about it. I
don't have a problem with him not wanting to do it, but he
got upset when I wanted to talk about how it would
affect the emissions. I just wanted to understand it a
little better. What's the difference between my H pipe and
headers? They both are after market enhancements that
modify the exhaust before the cats. Why is one okay and
not the other? If I can understand why, maybe I can
better figure out how to do what I want.

 

The TIPS headers have an H pipe between the headers and the cat's. Good idea but, it's already been done, however, I haven't heard of it being done with stock headers. You may be on to something, find a reputable muffler shop that's looking for work. :yesnod: :cheers:

 

You mean something like this? :D



I did this when I installed my Borla XR-1 three years ago :cheers:

 

does that help with the resonance at all, how's it affect sound overall....did you see any gains.....why'd ya do it :D...thanks :cheers:

 

This could be the next A&A still Mod... Weld it on and go!!!

 

Actually I did it more like four years ago not long after I got the car in 1998. Obviously I didn't invent this as there was a lot of talk back in the alt.corvettes newsgroup (pre-Corvetteforum) about this mod. The talk back then was that it would quiet down the resonance which was a concern with the XR-1. It was also supposed to add a bit of HP. However, I did the mod at the same time as the XR-1, I have a vert which I believe will have less resonance than a coupe and I have never ridden (or driven) anyone elses car with an XR-1 so I am unable to say for sure if it reduces resonance.

I will say that it was a fair amount of work because I chose to split the exhaust down the middle (see the welds on the strap joining the cats) and used a drill press with a 2 1/2 hole saw to drill into the pipes. Note that the pipes are double walled and have some sort of a steel wool liner between the walls. The holes and the H pipe are carefully matched to the inside pipe although of course it is welded to the outside pipe. The H pipe itself was re-cycled from the back half of the stock exhaust which is not used on the XR-1. I built a small jig to realign the parts and took the assembly (out of the car) to a welding shop and had it professionally wire welded.

Back then, several folks had this done at muffler shops on the car :eek: . I chose to do it externally because I did not want bits of slag and junk going into the cats and clogging the small passages in there. As well as wanting to make sure that the welds were completely around the pipes which would be difficult on the car.

One option that I have considered is to add a bung for a wide O2 sensor in the H section so that it can see both banks. I have been satisfied with the mod since I did it and did see gains in HP with the XR-1 of course.

 

From what I understand the torque band is effected by how far the crossover pipe is from the heads or collectors. How you would determine the optimum location I don't know -- but I don't think you can just arbitrarily put it wherever you want and not possibly loose some torque. :nono:

 

This depends on a lot of things. Shorty headers, or long tubes. Heat tape wrapped or not. Length of the crossover tube,....

With long tube headers, the NASCAR guys build 3 tubes (out of 4 cylinders on a bank) to one collector and the other to the other collector. This leaves them with equal length headers for each exhaust, and equal exhaust pulses in each collector. This is not an H-pipe, not an X-pipe, but a pure solution. Unfortuanately it is very difficult and costly to make fit under our cars.

If you H-pipe was connected to shorty headers, ran down one side of the engine, under the bottom and back up to the shorty on the other side; my guess would be that the H-pipe would be too long to cause any benefit, AND loose a lot of thermal energy, causing the CAT to light up slower than without the mod.

Why is one OK and not the other--flow my boy--flow. Airflow, and aerodynamics is just not intuitive, and often works exactly opposite as to how intuition assumes it will work. But if you really want to understand, pay tohave one built, and pay to dyno before and after. Then you will understand.