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 One size fits all ?

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Trevor Amos



Number of posts : 878
Registration date : 2010-08-13

PostSubject: One size fits all ?   Sun Aug 05, 2018 9:56 pm

A considerable amount of time seems to have passed since I last felt the inclination, or indeed enthusiasm, to contribute again to the Bantam website. It would seem that I am not alone in feeling the whole exercise to be a waste of time judging by the almost total lack of new posts. With the notable exception of Les and Jimmie, who between them have kept the site going, there has been nothing of consequence!

I discovered the following short article a few months ago on another 2T site and it struck a chord with me, so I have presented it here.
At first glance there would seem to be a series contradictions, who would graft an RS Honda pipe to a modest home tuned 125 engine with a non-race pedigree and expect the same pipe to produce the goods with more than a 50% increase in power and rpm?
Perhaps you guys might be intrigued and then inspired enough to play around with the numerous performance variables of your own engines, along your favourite exhaust pipe formulas and see what happens, you may be surprised.
                                                                 ------------------------
“The RS exhaust has proved to be a very effective pipe, very tolerant of my different engines.
My motors started at 16hp at 8,000 rpm and now 30 at 12,000 rpm.
It`s not too surprising the RS has worked so well on all of them. Because it has a good shape and a suitable length for lower rpm, power and exhaust temperatures. A length that is much the same as required for higher rpm, power and consequently higher exhaust temperatures. So basically the RS pipe has the right length for all my engines as they have developed increased power and rpm.

It works like this:-

A) Lower rpm = longer pipe.
B) Lower exhaust gas temp = shorter pipe.
C) Higher rpm = shorter pipe.
D) Higher exhaust gas temp = longer pipe.

A+B = C+D

I have tried to simulate better designs in EngMod but could not develop anything that was superior “
                                                          _________________________

It might be nice if someone could post a comment on this and maybe get a conversation going?

Trevor.


Last edited by Trevor Amos on Wed Aug 08, 2018 6:38 pm; edited 1 time in total
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nigel breeze

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Number of posts : 354
Registration date : 2007-12-23

PostSubject: Re: One size fits all ?   Tue Aug 07, 2018 7:02 am

Hello Trevor, hope your fit and well cheers
just looked in to see what’s going on. im assuming that the exhaust has been fitted to this fellas engines with no other changes. if i remember correctly the tuned length is dependant on the rpm at which the engine has been designed to make best power exhaust port timings and the exhaust gas temp which has a bearing on the speed at which the pulse moves within the exhaust. it would seem that he has stumbled upon a sort of rule of thumb configuration which works for a number of applications. forgetting any change of cone configuration it would seem the gas temp is the factor which has the main effect, allowing this pipe to be so successful for him.
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Trevor Amos



Number of posts : 878
Registration date : 2010-08-13

PostSubject: Re: One size fits all ?   Thu Aug 09, 2018 3:13 am

One of the most important of the almost infinite number of variables associated with a two-stroke race engine`s performance, is the relationship between exhaust wave speed and pipe length. That wave propagates along the pipe at the speed of sound which is proportional to the square root of the local temperature. The gas temperature is not constant throughout the engines rev range nor along the length of the pipe, being hottest adjacent to the engine, coolest at the largest diameter and spikes up again at the junction between rear cone and tail pipe. Other major influences on temps in the pipe are ignition timing, carburettor jetting to achieve the stoichiometric ideal, compression ratio and so it goes on. Every change to exhaust port timing demands a different pipe length along with rpm changes and so it all goes into the mix. Taking compression ratio as an example, the higher the ratio, the greater the burnt gasses expand in the cylinder before the exhaust event is actually underway. This means cooler gas flowing through the exhaust duct, and that may be great for keeping the cylinder cool on an air-cooled barrel. However the wave speed suffers from lower temps and consequently this is felt throughout the effective power range.
So it will be the combination of all of the changing variables acting on the engines in these two seemingly extreme engines that work in unison to enable the RS pipe to function well on both. I do suspect that the 30hp engine will derive a greater benefit than the lower output one does.
It`s a fascinating exercise trying to factor in all of the variables to come to a definitive conclusion, to answer what at face value is a contradictory proposition. But then it is just this that makes two-stroke race engines so satisfying a challenge!
Good to hear from you again Nigel, have you got another special enterprise coming together in you workshop?

Trevor
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nigel breeze

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Number of posts : 354
Registration date : 2007-12-23

PostSubject: Re: One size fits all ?   Thu Aug 09, 2018 6:20 am

Trevor, ive just had my 4 speed engine bottom end back. I had some gear change problems which hopefully have been fixed. As the engine was made up from bits it had some tolerance issues lol!

nothing really going on other than that. actually had my first full day off work in a month today ( and i feel guilty!) but as im knackerd ive been lying on the sofa all-day watching films. i think im falling into the trap of living to work again instead of working to live.

Seems that this site is very much on the decline as you say, but i look in from time to time to see if i have missed anything.

all the best , nigel.
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