air and exhaust flow

S

speedwaydesign

New member
In my last post on exhaust reversion. It was suggested that I run a larger pipe size to prevent reversion. That was good advice, but I have been told that in comparing pipe diameters that the smaller pipe will empty more efficiently. Could you who work with engines explain to me,
1. How is the exhaust diameter and length determined?
2. What are the effects of a 3 stage pipe? Will they help with reversion?
Thanks Tom
 
There are some formulas for exhaust sizing.
That and experience and testing.
,3 stage pipe should help.
There are pressure waves in a 4 stroke pipe.
 
Get yourself 4 stroke performance tuning by A. Graham Bell, that has a good chapter with some easy to use formulas to get you in the ballpark.
 
Oman said:
Get yourself 4 stroke performance tuning by A. Graham Bell, that has a good chapter with some easy to use formulas to get you in the ballpark.
Click to expand...

Thanks for you who replied. It is apparent that few have really studied the subject - or - it is a builder secret. lol- Since Al has lots of formulas and reasoning I was sure that he would reply. I Don't just want to know what works, but how air flows in the exhaust and intake. I am positive that some builders on here know and have done testing on the subject. As a former race car engine builder I am familiar with multi cylinders air and intake, but as ya"ll probably know the single cylinder is a different animal. Help me if you can or point to some information that can help.
 
Your questions are vague, your statement that few have studied the subject is insulting, and if you are an engine builder you would realize there are no secrets. This engine although it is a single cylinder,is of the most basic OHV design, it has been studied, designed over designed and utilized way beyond it's intended purpose.

If you asked a specific question and provided adequate information, you would most likely get a reasonable response.
Oman referred you to an excellent resource, no one here can condense exhaust design, and fluid fllow into a twitter 150 character statement.

There is no optimal length, or diameter, to attain correct pulse tuning for all single cylinder engines. Generally a pipe larger than the port works best, but there are situations where the dia changes according to the length of the pipe.

Your camshaft and the valve overlap period is the most determining factor for pipe design.

When you speak of reversion, this is usually occurring into the intake tract and it has little to do with the exhaust system. It is caused because the pressure in the cylinder is higher than the intake tract at the point that the intake valve begins to open. This is quite normal, it is not caused by exhaust flowing backward in the pipe, it is a pressure difference. Perhaps the valve timing is not optimized to your operating RPM.
 
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speedwaydesign said:
In my last post on exhaust reversion. It was suggested that I run a larger pipe size to prevent reversion. That was good advice, but I have been told that in comparing pipe diameters that the smaller pipe will empty more efficiently. Could you who work with engines explain to me,
1. How is the exhaust diameter and length determined?
2. What are the effects of a 3 stage pipe? Will they help with reversion?
Thanks Tom
Click to expand...

Find yourself a good repeatable dyno. Go buy or make all the headers you can and start testing them. After about a month of this you will know more about header design than 95% of the people including most engine builders.
Word of advice......also test the header that the big dogs are using, its probably pretty good!

Don't waste your time on a flow bench for header design!

Steve
 
Many years ago, George Owenby (Roadkill) had sent me a Briggs Modified to run.
I asked him what kind of pipe to put on it, after a short silence, George said "Well, I'd put a good one on it". :)

Kind of goes along with Steve's comment about the headers the fast guys are using
 
There is no simple answer when it comes to exhaust flow, gas dynamics are a tricky thing, especially in non steady flow devices like our engines. Length is mainly determined by camshaft events but the speed of sound is dependant on temperature which varies within each cycle, so not an easy thing to model. Likewise for the steps in the header. At the end of the day most exhaust design are more empirical than theoretical. Suck it and see!
 
It's called exhaust theory.

The length of the pipe needed for optimum performance depends on several different factors. As the exhaust pulse, (sound wave) a positive pressure wave, reaches the end of the pipe and expands into the atmosphere, it creates a negative pressure wave in the opposite direction. If that negative pressure wave arrives at the engine while the exhaust valve is still open, it will lower the pressure in the cylinder and allow more intake charge to enter the cylinder. The speed of that pressure wave (sound) is very dependent on the temperature in the exhaust pipe. It will increase, or decrease, at about 1.5 ft./s per degree. The fact that the exhaust valve and the intake valve are both open for such a short period of time limits the effects of the four stroke exhaust system as far as increases in performance are concerned. In a two cycle, the exhaust ports and transfer ports are both open, together, for a significant period of time. A well tuned exhaust system for a two stroke can increase the horsepower by 50% or more, versus a muffler.

There are a huge number of variables at work. If you know the exhaust temperature (EGT) you might be able to calculate, as a working hypothesis, the length of the pipe needed. Still, you must realize, the temperature of the exhaust gas drops very rapidly as the exhaust flows out the pipe. The temperature at the port, and the temperature at the exhaust pipe end, are very different. This effects the timing of the negative pressure waves arrival at the exhaust port.

As the length, and the diameter, change, it affects the speed of that pressure wave. I doubt there's any formula that can give you a specific wave speed without extensive testing and measuring. As others have already told you, it's mostly a cut and try procedure.

In any case, you're only going to get a small boost from the four cycle exhaust, and the timing of where you get that boost is going to happen only in a narrow rpm range.

I'm sorry if this is a very vague explanation, but it's a very complicated subject. As far as performance increases are concerned, the four cycle exhaust cannot be compared to a two cycle exhaust system. They are worlds apart.
 
Consider me scolded- I never meant to insult anyone I apologize. I will be more careful to stay specific to the topic and ask clearer questions. But thanks for your reply as it does help me to understand the subject better. Tom
 
I want to thank everyone who replied to my questions.
I will examine them carefully. It is good to get replies from people who have the knowledge.
Thanks - Tom Terry
 
speedwaydesign said:
Thanks for you who replied. It is apparent that few have really studied the subject - or - it is a builder secret. lol- Since Al has lots of formulas and reasoning I was sure that he would reply. I Don't just want to know what works, but how air flows in the exhaust and intake. I am positive that some builders on here know and have done testing on the subject. As a former race car engine builder I am familiar with multi cylinders air and intake, but as ya"ll probably know the single cylinder is a different animal. Help me if you can or point to some information that can help.
Click to expand...

This right here .
Thanks but obviously no one knows.
Lol.
I'm a engine builder.
Personally thought he had the answer in his other posts.
Kart43 nailed it.
I have researched it ,forgot most, never wrote a college thesis's though.
Reversion in the exausht is a pretty rare thing.
 
I know it is not easy to relay thoughts via Forums and Social Media. I also realize personalities are not projected well.

Some of the statements I see here that confuse me: "I am an engine builder, but don't know anything about these"
"What are the rules?"
"I want the fastest , and the best, but I think they cost too much"
"We are slow everyone else is cheating, what can I do?"
"We found a kart in a barn I bought an engine with a sticker on it, we have raced 3 times and we get lapped every race, who sells the best, cheapest engine?".
"Buy an engine destroy it, take it back for a refund"
" I know all about this stuff, I just want others to tell me what they have learned."
"I have done everything, I did before anyone else, they all just copy me"
 
flattop1 said:
I have researched it ,forgot most, never wrote a college thesis's though.
Reversion in the exausht is a pretty rare thing.
Click to expand...

This a true statement, rare means unlikely to happen, although not impossible. What is occurring, many aspects contribute to insufficient scavenging of the cylinder, the perfect situation would be, when the intake valve opens the area at the face of the valve is at a pressure lower than atmospheric. Al has always been correct when he states the high pressure moves toward the low pressure. If the pressure in the cylinder is higher than the pressure in the intake tract fluid flow will be out of the cylinder(reversion).

I am really surprised nobody spoke up about the thread recently regarding flowing fresh air back into the cylinder through the exhaust port. This goes against all concepts, theories, and knowledge known about flow through an internal combustion engine. There is a saying you can't suck and blow, tryi I know you will.
 
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