308/328/Mondial performance exhaust development

Something that has always been a big question for me regarding exhaust headers, is if anyone considers the radius of the bends when calculating length. In any other medium, a bend in a pipe is considered to be equal to a different length of straight pipe. For example, in an direct vent HVAC system that uses PVC pipe to exhaust outside, one 90 degree elbow is the equivelent of something like 7 feet of pipe. In a system that only allows you to use 60 feet of pipe to reach outside, for example, two or three elbows can really shorten up your length. To explain this in more detail, the system can only produce enough air flow to push out through 60 feet of straight pipe. Any longer and you could have problems. An elbow creates a disturbing restriction to that flow, and conversly is the equivalent of a much longer section of pipe. In other words, when you look at an exhaust header, some primary tumes have barely any turns, yet others are all twisted and turned. If they simply were to start with equal length sections of straight pipe, and not consider the bends, in reality they would no longer be equal length once assembled. I suppose to be truly accurate the header would need to be flowed on a flow bench, and the individual tubes tested to achieve equal pressures?

 

Depending on how serious you want to get with this project....

http://www.spectrum5racing.com/

http://www.spectrum5racing.com/Technical/s5headerRevA.pdf

 

There is a big difference between HVAC ducting and exhaust headers. One of the major differences is that HVAC flow is considered to be incompressible flow and header flow is compressible flow. The tuned length of the header is determined based on the pressure wave dynamics, not for mass flow considerations. The length is based upon the time necessary for the pressure wave generated when the exhaust valve opens to travel to the end of the primary tube, reflect back as a negative "suction" wave and return to the exhaust valve before it closes during the overlap period. This "suction" wave is what aids in filling the cylinder with intake charge. The time is based upon the length of the primary tube, regardless of bends since it is pressure wave, not mass flow that is traveling down the tube.

The number of bends and the tightness of the bends will affect mass flow, and should be considered when designing a header. However, the pressure differential in a header is much greater than in an HVAC system, so the losses due to the bends are less critical than in an HVAC duct system. In a perfect world, we'd like to have equal length straight tubes, but this is highly unlikely in an automotive application with a collected exhaust. Some header design rules of thumb are: 1. Use as large a bend radius as possible, bend radiuses no tighter than about 2 x tube OD, 2. Do not cheat bends when joining tubes, 3. Avoid "S" bends as much as possible.

 

i like the idea above the best. Stright out to exhaust and no bending pipes. If you get something like that going i'd be willing to buy a set.

 

In general you are right. Most of the tuning occurs in the header. There is secondary tuning happening after the collector, but in a well designed header with the expansion as you describe, it will add little as long as it flows properly.

The issue I see with your last proposal is that you may not get enough sound attenuation using the short mufflers. Mufflers of large volumes more efficiently reduce sound w/o sacrificing performance. Your system should perform well (assuming they flow well), but may not be quiet - but only you (and the local constable) can judge that.

 

I am also considering changing my exhaust on my 328 to a Euro type, but I'm concerned about what to do with the O2 sensor?
Will the O2 reading be different if I probe only one exhaust header as opposed to the two it currently probes as both headers go into the single cat?

 

I think the latest is your best if ti will fit, but I wouldn't add bends trying to make them equal length....and I don't see where the turbo fits?

 

Obviously not equal lengths, but on a transverse engine I don't know how you could get equal lengths?? You could probably get equal volume by using a larger diameter on the short run.

Just my home brew and I've been happy with this past summer.
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On a clear night with no wind you might be able to...=)

Cavallo Nero (I think) post #29 sent me an audio clip of his car when I was looking into doing mine. Basically the same canisters but in different orrientation. He might still have his.

BTW, how would you post an audio clip??

 

Hello Russ,

I have a Euro 77 308GTB with two straight-through oval stainlees steel Magnaflow mufflers in your type A configuration but without resonator tips and I think it sounds really good. It has two single outlets with Magnaflow non resonator wide mouth stainless tips. All the connecting tubing ( 2" inlet and 2-1/4" outlet) is also stainless.

I had my stock headers and right intermediary pipe JetHot coated after the pipe was modified with a flange to mate with one of the new mufflers.

Wil

 

Will do.

You should call JetHot and HPC to ask them that question.

 

Agreed but if you're unsure, have the injection system cleaned with a MotorVac machine to make sure all the injectors are doing the same thing. The MotorVac (Snap-on's brand name, there should be others) pumps fuel, mixed with powerfull cleaners, through your injection system from it's own reservoir and cleans gum and carbon out of the injectors and other fuel system parts. It also cleans valves and combustion chambers. There is usually a noticeable difference in running quality after the proceedure which should cost no more than a few hundred dollars. We usually see a increase in engine vacuum and RPM.

 

I had this done to a 91 Mazda 626 6cyl at 80,000 miles at the recommendation of my (ex) mechanic. I didn't notice any performance increase or decrease, but I went from a car that burned no oil in 6,000 miles to one that burned a quart every 500 miles. Clearly I wasn't happy.

 

Clyde,

Sorry you had a problem after your MotorVac service. We've been using our machine for a long time with good results. Maybe it unmasked another mechanical proiblem and maybe it was just a coincidence.

Not that you are doing the following but every mechanic has a story or two to tell of a customer saying something like this: "Ever since you balanced my wheels my windshield wipers don't work." Maybe your ex machanic screwed up you engine in some other way and maybe he's innocent.

Anyway, I don't own stock in MotorVac or Snap-on (although I should own part of the company by now with the money I've spent there) I was just just advising that if you're worried about the exhaust gasses being equal side to side with only one 02 sensor that cleaning the injectors is a good idea.

We also have a hand pump for cleaning CIS injectors one at a time,out of the car, and you can actually observe the spray pattern improving as you pump the solvent through but in that case only the injectors are being cleaned.

A container of injection cleaner in the tank can't hurt either. It just doesn't do as thorough of a job.

I look forward to seeing you at the Fall Run tomorrow. I think ,at least, that I saw you were planning to attend.

Alright, no more F-chat for a while. Got to get my car ready!

Sorry to pull this thread off topic.

Wil

 

Russ,

Here's what I've been running on my '78. A bit loud, but not too bad. Kind of like that cats and tips photo of yours but no crossover pipe.

Henry
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Russ,

I think you have the right idea in your post above. It's still not clear to me what constitutes a resonator, optimized for sending a return, low pressure wave up the collector, et al. True, a resonator is a local increase in flow volume, but how much volume? Is a flow area increase of say 2:1 or 3:1 sufficient? How long does the resonator cavity have to be?

The 2-stage muffler plus resonator tips you sketch out should provide adaquate opportunity to craft an acceptable exhaust sound while the header, plus down stream collector resonator, should handle the performance issue.

Have you looked at the actual collector lengths after fitment of the wave reflection resonators? How does the packaging work out? The obvious placement of these devices is in the volume previously occupied by the OEM cats. If the resonators are sited there, what do the collector lengths look like? Are these lengths consistant with Vizard's recommendations?

Thanks for you continued interest in persuing this matter.

Bill