I'm planning a naturally aspirtated stroker build and I've been considering the importance of matching the headers and intake manifold to the camshaft. So here's what I want to know.

1. Will Exhaust cutouts (when opened up) provide the same "quality" of scavenging wave as a true "striaght pipe" or open collector? Or does it matter?

2. Has anyone here had experience with expansion chambers on a 4-stroke? When and why would you use them, if ever?

3. I've read in several places that collecter size and length is very important. Is it possible to achieve a stronger scavenging wave from a collector's exit by tapering the collector diameter? (like a megaphone put on backwards) I ask because according to some math calculations, the optimal collector for my car would be 3 inches in diameter and 20 inches long. My thinking is that, by slowly tapering the collector diameter from 3 inches to 2.5 inches, I can achieve an optimal primary scavenging wave (from the primary tube into the collector) and a slightly-stronger-than-normal secondary scavenging wave from the collector's exit into the atmosphere. Is that correct/possible?

Mark

I moved this post from the other tech section because I got no replies, not trying to spam it up.

 

1. Will Exhaust cutouts (when opened up) provide the same "quality" of scavenging wave as a true "striaght pipe" or open collector? Or does it matter?
- it does not matter, but you will need to place them at the optimum distance from the collector. Unfortunately, dyno time is the only reliable way to get length nailed down.

2. Has anyone here had experience with expansion chambers on a 4-stroke? When and why would you use them, if ever?
Yes. It works, as Vizard wrote back 20 years ago, it needs to be placed at where you would terminate the collector extension. Can be difficult to package. Something like a Mac flow chamber works well, but optimally should be at least the same cubic inch as the engine.

3. I've read in several places that collecter size and length is very important. Is it possible to achieve a stronger scavenging wave from a collector's exit by tapering the collector diameter? (like a megaphone put on backwards) I ask because according to some math calculations, the optimal collector for my car would be 3 inches in diameter and 20 inches long. My thinking is that, by slowly tapering the collector diameter from 3 inches to 2.5 inches, I can achieve an optimal primary scavenging wave (from the primary tube into the collector) and a slightly-stronger-than-normal secondary scavenging wave from the collector's exit into the atmosphere. Is that correct/possible?
Sure, that does work, but the back pressure you will introduce changes your math and will alter your camshaft requirements, and ultimately, I feel, lose more power than gained form the possible scavenging increase.



I moved this post from the other tech section because I got no replies, not trying to spam it up.
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Gotcha.

Okay, cool. I just read Vizard's article, so I see where you're coming from.

Hmmm. You really think it'll create that much backpressure? I did more math and found that the tapered collector (in this instance) contains 84% of the original volume of the standard 3" X 20" collector extension. Is there any way to translate that into terms of backpressure?

 

Just my opinion:

You do not know what the proper collector length is unless you try different collector lengths on the track with a consistant et type vehicle or by using a dyno.

Years ago people would use a painted mark on the side of the collector and at the point where the paint was not burned off, they would cut off the collectors. This gave them a pretty accurate tuning starting point. You can find this tip in some of the old Hot Rod magazines.

Exhaust Cut-outs get rid of exhaust back pressure but do little for tuning unless the cut-outs are at the end of the collector. This means you use a 3.5" Marmon Ring system to keep the exhaust in the closed exhaust position until you are ready to race and then you quickly remove the marmon ring clamp, the rear part of the exhaust swings out of the way (on a pivot hanger), and the engine now has open headers/ collectors at the correct tuned length.

The "Y" type cut-outs make noise and that is about it. There is no tuning as you have two depression points for the exhaust wave.

Tom Vaught

 

The more I think about it, the more I realize that there's no relationship between collector volume and backpressure. So that last post was kinda useless. What I was trying to communicate was my theory that the pressure differential between the exhaust gasses in the collector and the outside atmosphere would be enough that it would "recompress" the exhaust gasses for the engine (i.e. no backpressure) and thus the smaller exit would produce a stronger scavenging wave with seemingly no effort on behalf of the engine. Does anyone know of a way to predict this with math, or is trial and error the only way?

If it helps anyone, I found this. I don't know how to interpret it tho

http://www.pipeflowcalculations.com/

 

On a side note, can anyone explain how and why these work? What's the deal with the "reverse cone" on the end of it?

http://www.burnsstainless.com/Megaphones/megaphones.html

 

Can a boosted motor even get better scavenging? Isn't the point of scavenging to help exit the exhaust and pull in new air? Doesn't forced induction do that anyway? Especially with a good bit of overlap?

I've been debating selling my exhaust and getting rid of the x-pipe, which I don't like, and building a tuned length, completely divorced dual system.

Other words for expansion chamber are termination box and resonator. A chambered muffler like the mac or flowmaster will bounce exhaust pulses off, a completely open box will change the volume of air rapidly, also bouncing back the pulses. Both act like the end of a tailpipe.

There is a program called pipemax that you input exhaust valve timing and other engine variables, and it will tell you exhaust specs. I don't know how accurate it is.

 

Pipemax seems to be very good NA but not so sure on Boosted apps

 

TBH, most people do not bother with tuning the exhaust on a turbo motor. This being said, I would be very curious to see what the gains are by doing such not in peak HP but rather in average HP and most importantly the spooling and off boost characteristics.

 

I was thinking with a supercharger. I would think its impractical with a turbo, unless you have the room. I would think that a turbo would reflect the pulses just the same though. From what I have read, having a collector length of less than 14" would probably put the power increase too late in the rpms to be usable.

One chevy magazine gained 53 ft/lbs at around 3500 rpms from having the correct collector length vs open header IIRC. This was a 550hp 383. I'm just not sure how much of an effect it would have with FI. I don't even know who I could call that would know.

 

Supercharged engine (Like a Vortech or F2 ATI) will pick up with the right stepped header: right collector design/ right tuned length, right step lengths.
That is a fact.

Tom Vaught

 

Well, I guess I'll watch for cooked paint and see what I get on the dyno. A pair of 18" header extensions are only 30 bucks.

 

So can anyone tell me how a megaphone works?

 

They're like boomies. Nascar uses the same idea, don't I know why I've never seen on a street car though.

 

What are boomies? Does nascar use megaphones?

 

we used megaphones on outboard engines when I raced boats.
it helped scavenging on the old deflector top engines, plus it sounds cool 8)

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lighter forged flat top pistons came along then exhaust tuning on the two strokes became an art.

 

Boomies were those megaphone looking deals on motorcycles. I haven't seen one in a long time. Nascar uses a type of a megaphone I guess. The tip is greatly stepped up. I can get some pictures of a guy that aquired some and some more of a homemade one.

 

Is there any science behind this? If you painted it again would no paint burn off 2nd time?

 

I believe you are suppose to use a non heat resistant paint. I used 500 degree paint (for coating, not for testing) and it burned for about a foot after the collector flange.

I'd like to know the reasoning behind it too.

 

not necessicarily. Look at a well tuned two cycle exhaust system. the end of the exhaust its self is a reducing cone. Look at a header, there HAS to be an area reduction in the collector compared to the overall area of the tubes. This change in area causes the change in velocity that leads to scavenging.