Improving Intake and Exhaust

[Dennis Detweiler]

Exhaust:
What was the basic scientific thought process behind the old megaphone style exhaust headers compared to the more modern expansion chamber? Does one style effect a certain rpm range more than the other?
Intake:
The single carb intake manifold reed cage is positioned below the intake ports of the block. Why isn't it positioned directly in front of the ports for more direct flow? Is there an advantage to altering the stock intake manifold by curving (grinding) and polishing the the manifold for more upward flow?
I know this can take an extensive answer. To simplify it, maybe some "yes", "no", and basic reasons? :~)
Thanks

[REAR]

Dennis,

The thought behind the early open exhaust was three fold.

First, it was open so it would be loud as hell and it would scare the guy in front, on the side , and behind you so he would steer clear of you.

Second, it was open so it would blow fuel all over the goggles or face sheild of the guy behind you so he couldn't see and this would prevent him from attempting a pass.

Third, the design allowed it to lubricate your chain, chassis, and pants all at the same time.

As for the modern exhaust, sorry can't help you.

R.E.A.R.

[Rob Voska]

Think about it.............. Why would the intake be positioned across from the intake ports? What would the advantage be? Remember to think.....

[Dennis Detweiler]

REAR: Makes sense now Another disadvantage of the old open header, it makes it easier for the cops to track you (noise and oil trail) after you outrun them in the streets of a residential area.
Rob: Duh me! I'm looking at the 3 intake ports above the manifold opening without regard to intake into the block. I have seen some manifolds that are ground and polished behind the reeds to make a bell shaped expansion into the block. Is this a plus or a waste of time?

[steveohara]

Dennis,

A thorough explantion of the expansion chamber theory would take up several pages so here it the short version.... a two cycle engine with an open exhaust is very inefficient as it loses a portion of the fresh charge out the exhaust on every cycle. The expansion chamber was designed and developed to assist in scavenging the burnt gases from the cylcinder and trapping as much of the fresh charge in the cycliner as possible. The design evolved to the point where it is believed a well tuned exhaust system actually causes some of the fresh charge to be drawn into the exhaust and then pushed back into the cyclinder after the intakes close but before the exhaust closes producing a supercharging effect. In general, a good expansion chamber will increase horsepower by around 20% over a simple open header regardless of the shape of the open header.
As for the intake manifold... there is nothing to be gained from any shaping, polishing, blending etc and the position where it is mounted makes very little or no difference in the performance. The size and design of the carb(s) and reed cage can make significant differences in power output and good carburation is the single most important tuning element on a two cycle motor. A very basic motor with a very well sorted carb will beat the radical modified motor with a crappy carb every time.
Steve O'Hara

[Dennis Detweiler]

Thanks Steve, that covered it well in a brief explanation.
Is there a "best" exhaust system recommendation for the 91 models?
New manufactured or hunt for old?
Thanks

[Ron Carbaugh]

Steve...what is a cycliner? Just kidding and Happy New Year. The expansion chamber does in fact let the cylinder bring back unburned fuel after the piston ported engine has gone through it cycle. That is scavanging. If you ever get a chance and care about some of the early 2 cycle develpment in the USA read the book The Ledgend of Mercury by Jeff Rodengen. The early chamber that is talk about in this book is in the hands of my old race boat sponsor. It has so many welds that it is a piece of art.

Rob..do we really have to think or can we just depend on natural selection?

Just want to thank every one for a great site and I hope to make it to TBO this year.

[steveohara]

Ron,

You have witnessed my true typing skills under pressure from the wife to get out the door with no time to proof read and fix the mistakes You might be interested to know that I built dozens of expansion chambers for Mac engines during the years I was racing and developing those motors and had the pleasure of working indirectly with the well known Irish 2 cycle research scientist Gordon Blair through my good friend and mentor George Bosco. George and Blair were working independently but sharing information and ideas on computer modeling of two cycle engines during the 70s and several of my pipes were based on mathematical models developed by the two of them for the computer programs. None of the pipes designed using the math models worked worth a damn but they still contributed to the learning curve. In the end, the best pipe designs were the result of trial and error and the results often contradicted the formulas. During the 70s and 80s I read extensively on the subject covering just about everything written and published by the SAE and the one thing that became clear was that the theory and the test results proved inconclusive or contradictory more often than not. At the time I stopped following the research, they had yet to conclusively prove that any expansion chamber was actually capable of generating a pressure gain above atmosphere in the cylinder that was trapped at the closing of the exhaust. It was a good theory and supported by all types of good data but the scientists still conceded that they were just educated guesses. I stopped following of the research after the publication of Progress in Technology Series No 26 as published by the SAE in 1982. Someday I would like to find the time to get my hands on subsequent SAE papers dealing with expansion chamber research and other two cycle related subjects to see what came later. Anybody out there have any of the later volumes published by the SAE covering two cycle theory?
For those who have an interest and enough engineering background to understand research papers the Series No 26 publication contains some very interesting articles on transfer port layout, scavenging, combustion chamber shape, reed valve behavior etc. I found it very interesting to learn five years after the fact that I was doing "on track" research that paralleled one of Blair's projects dealing with reed valve performance and behavior and our results and conclusions were very similar. The most interesting aspect of that research was the discovery that properly tuned reed valves never close all the way when a two cycle engine is running in the upper half of the power band... they just open more or less through the cycle! If reeds were fitted with enough tension or strength to close completely they resulted in significant loss of power. Of course, Blair and I were both dealing with 4 petal reed valve assemblies and it is very likely that configurations such as the GEM V12 or the V16s had so much reed area that they could meet the engine demands with much less lift and duration so that the reeds might actually be able to close completely for a portion of the cycle. That is a good example of why the study of two cycle engines can be so interesting and frustrating at the same time.... every thing you change effects everything else and there are no absolute facts or truths when it comes to two cycle design and tuning.
Regards,
Steve O'Hara

[Ron Carbaugh]

Steve,
Thanks for accepting the humour...since I have may years of being involved with the motorcycle industry I almost always spell cylinder...cyclinder.

Your comment:
That is a good example of why the study of two cycle engines can be so interesting and frustrating at the same time.... every thing you change effects everything else and there are no absolute facts or truths when it comes to two cycle design and tuning.

I really have to reluctantly agree with you since I worked with R&D at Honda and built and/or tested a lot of different configurations of carburetor, cylinder and exhaust systems for both Honda and a R&D shop that probably did more testing with their patented dyno that most big corporations. The difference in people like you and me testing is that we can go from the build straight to the track and see if it works. So many times math just doesn't work when trying to develop 2 strokes. Most of my early work was done (where I lived) at sea level so I had a great bench mark on tuning and only had to try and re-tune the carbs for different altitude. You and I could go on and on, but I always appreciate reading your posts...thanks for always taking the time to explain your position.

In closing just a short story....we were testing small race boats and I was always so confused as to why someone would say this is my best prop?? I would ask why and I would always either get a blank look ( I always like that over some obvious BS) or they would say it ran the best? Of course I ask how do you know it ran the best...because I would win with it. About that time 2 and 4 cylinder tachs that were acurate(and the needle didn't bounce) came on the market so I set up test with our boats with acurate speedometers and tachs. Man was there a difference on speed than from what people thought was their best props...conclusion. The driver does make a difference.

I hope I didn't mis-spell anything........Happy New Year and thanks to REAR for this great site.

[Rob Voska]

"None of the pipes designed using the math models worked worth a damn but they still contributed to the learning curve. "

All I can add to that is truer words were never spoken and AMEN !