Exhaust Size Differences
 

Looking into engine intake and exhaust acoustics it's pretty interesting to see how different sizes can change the engine's behavior.

My question is about the sudden increase in size by the introduction to an upsized catback system on a stock cat converter.

My car has a 421 header into a magnaflow cat. The header and cat are the stock pipe diameter at 2". My cat back is at 2.25". Not a huge difference but I'm wondering if creating a smoother transition from the 2 to 2.25" diameter would make a difference.

I'm thinking in reverse, a velocity stack is a significant improvement in air flow rather than a straight edge. Does that apply with exhaust systems too?


My idea is to put a reducer from the 2" cat to the 2.25" intermediate pipe. Or better yet, cut the header front pipe at 2.25" and do the same with the cat converter if I can get it welded right.

Any ideas?

Thanks

Any time you can use a smooth transition and keep from using a straight edge would usually be of a benefit. That being said where you system is flowing from a smaller diameter to a larger diameter and the stepped edge is only 3mm (0.125"). I don't know that it will really be worth the time and expense.

Some automotive engineers have used reversion steps in the exhaust system to stop the wave pulses from flowing back all the way into the cylinder head. However, these reversion steps are usually closer to the head and a bit larger.

I have seen those. I believe they also extend outward too. Kinda like this:

_______________
. . . . . . . . . . . ____l___________
. . . . . . . <---- _______________ <----air flow
_______________l


I'm thinking about mimicking that in my intake since its pretty easy. I saw a write up on it somewhere.

So I suppose it may not be worth the effort. My exhaust really needs welding cuz I chopped it all up to get what I like and its just held together by clamps so maybe a few extra tweaks would be worth it while thats being done. But its always confused me a bit how a catback exhaust could make a difference even though the aftermarket cat converter and header are always stock diameter and not larger

So my next question is on resonators. I had my welding guy make one a while ago and it made a huge difference in exhaust sound. I love it but I want something smaller.

I've been reading up on helmholtz resonators since the 1/4 wave side branch resonator is slightly different. The helmholtz resonator covers a larger range and is smaller but I'm having trouble finding the correct size through my calculations, somethings going wrong. Plus I can't figure out if I'm supposed to match the frequency I want removed which would absorb the pressure differences or to 1/4 it in order to cancel it just like the side branch resonator.

They make a difference in exhaust flow too. Mine is tuned for 3k rpm. At that point the engine is smooth and accelerates beautifully.

I know of the exhaust gas reversion style you are referring to. I have seen them on a few headers. I was talking more of the reversion step used at the header flange going into the exhaust port. that would be more inline to the step you would see in your configuration.

These 1/4 wave side branches that you speak of, aren't they more for reducing certain frequencies of sound vs gaining exhaust gas velocity or stopping wave pulse reversion? I guess that I have never really looked into them as a source of maintaining exhaust gas velocity.

They actually do very well. Its basically an intake air resonator only for the exhaust. Helps the gas escape just as the intake helps air come in.

They would actually help with gas reversion too since there is a reduction in back pressure and the gases are being 'helped', so to speak. I'm wondering if an anti-reversion design at the cat converter would be a useful technique to take advantage of that size increase.

I've done a lot of research since I posted that last comment. The helmholtz resonator is a different design that is more effective but I'm having trouble finding whether or not the frequency is calculated to match the resonators resonance with that of the exhaust or if its 1/4 like the side branch resonator.

I'm hoping someone out there may know. The resonators can be seen on the s2000 exhaust but there isn't a lot of info on that but more on the diy 1/4 wave resonators which I already made.

Ill be doing a full write up on all this once I get it all done but thats just the last bit I'm missing.

Yeah I'm not going to be of much help there. All of the resonance tuning I have been doing has to do with wave pulses and tuned lengths. Nothing to do with the resonators. All of the documentation I have found about the intake and exhaust resonators refers to noise cancellation.

I would like to know more about the effects of pressure using one of these branches though! I look forward to your findings and write up!

Sure thing. Ill post the link or PM you once I get everything done. I cant get dyno tests but I have seen info on how IARs actually improve performance by increasing volumetric efficiency. Since they are resonators that are controlled through a diaphragm or valve and (in the case of the f22's) vacuum controlled as well, they do a great job of resonating throughout the RPM range. I would hope to find a way to mimic that in the exhaust, possibly through a spring loaded piston and a solenoid or something hooked up to the tachometer to control the cavity's volume.

So I'm comparing that to exhaust resonance. I definitely notice a smoother and stronger acceleration starting at about 3000 rpm/ 75 mph on my car as the exhaust gets quieter from the resonator. But while I'm trying not to be biased, I'm thinking there is definitely something to do with this since that was not a strong point of my car before.

Ill try to find those thinks and include them.

But maybe you could help if you wanna. The PDF link below discusses the sound dampening of a helmholtz resonator. But I'm having trouble understanding weather or not it is a solid resonator tuned to that frequency or a mesh one or a dampened one. I'm not too good when reading technical papers like this.

http://www.acad.ro/sectii2002/procee...2/05-Lupea.pdf