How to Design a Two Stroke Expansion Chamber Performance Exhaust - Step 2

REPOST
Next we calculate the BMEP (Brake Mean Effective Pressure) being the average cylinder pressure.
The BMEP value will provide an indication of the average exhaust gas temperature we need to calculate tuned length of the epxnasion chamber exhaust, as well as providing a guide to the pipe mid section and outlet pipe diameters used in the exhaust design.
To calculate BMEP we need to first caclulate the Swept Cylinder volume of the engine and use a simple equation with Swept Volume as well as Target RPM and rated Power on the engine at that RPM.
Be it Horse Power of Kilowatts, in this case we use kW.
BMEP is given as Bar pressure.
Using Professor Gordon Blairs correlations of BMEP to Exhasut gas temperature we can arrive at a temperature to use in the tuned length equations.
All equations are provided in the video.
Freeware 2 Stroke Expansion Chamber Exhaust calculator program
Download at birdcagesoft.com
(Page link on sidebar at site)
#2Stroke #ExpansionChamber #BMEP #EGT

[@AuMechanic]

NOTE this video is a repost I munged the Swept Volume equation in the last one so redid the video.

[@matthewhopkins7042]

Mate I don't even own a 2 stroke and this is beautiful.

[@waterboy4124]

Hello Dave. Thanks so much for posting this. I built this pipe and had many happy miles on my 66cc piston port china girl. Pulled hard from 4500 to 9k rpm. You da man!

[@AuMechanic]

Glad you got good results.

[@realblakrawb]

Thank you for posting this. I was tired of the "black art" excuses from the big name mx pipe makers in town.

[@AuMechanic]

Yeah there are a few that try to paint expansion chambers as some dark arts as a sale point for their own designs and while they can be a bit complex and there is some variability to them the principles behind them are fairly well know, thanks mostly to Gordon Blair's SAE papers on 2 strokes.

[@yetticrap]

Thanks for these videos i have had to watch the over a few times to absorb all the knowledge. I saw your email down below so i sent you a lengthy question and some pictures of my build hopefully you can point me in the right direction. thanks Stephen

[@nathanchalecki4842]

Damn you are making some good videos. I really want to study this stuff and port my own engine + calculate exhaust design but just feel like my brain doesn't have the room lol

[@AuMechanic]

Thanks for the comment It can be a bit daunting but just take your time and don't rush any part of it, if you get to a point you are struggling with feel free to ask any questions. I'm doing a freeware pipe calculator which is nearly ready for download that will help to do that party of it, see the latest video.

[@nathanchalecki4842]

@@AuMechanic thanks for the reply. Guessing you are in Australia?

[@AuMechanic]

@@nathanchalecki4842 Yes in Australia, live in Melbourne.

[@250txc]

Sounds great but all too technical for me. Here is what I'd like to know. I'm going to clone another production pipe for my 250cc bike. I've had several other chambers, 1 type adds low-end grunt, another adds high-end power. So the pipe I have now is just a good pipe across the 0-8K RPM scale. If I wanted to increase the power across the RPM scale, what changes would I need to make to the cloned pipe? Would just increasing the diameter of the fat part of the pipe help or could something like that totally screw up my power-band? Thanks!

[@AuMechanic]

I would suggest you download the free pipe calc software and put the numbers in it and compare the results to your pipe. The software provides various calculation factors for the port to mid section ratio and the popup tool tip help explains the differences with them. In short increasing the size of the mid section will cause the cone taper to be a higher angle that in turn tunes the pipe for more peak power. What will risk upsetting the power band is changing the lengths of the pipes as that will tune the pipe out of (above or below) the power band of you engine.

[@emivesprini3345]

Entonces la potencia maxima y las revoluciones con la que debemos hacer la cuenta son las que nos da el fabricante de la moto?

[@AuMechanic]

In English "So the maximum power and the revolutions with which we must calculate are those given by the manufacturer of the motorcycle?" Yes the maximum RPM to use is the maximum engine RPM as in red line RPM not the peak power or torque RPM. Maximum power is to calculate BMEP and which used to estimate Exhaust temperature only it is not a direct calculation.

[@emivesprini3345]

The HP specified by the manufacturer of my motorcycle are 14HP at 6400rpm, but this power is what generates the crossing of the lights plus THE EXHAUST OF THE ENGINE FACTORY. However, can this value be used to calculate the effective mean pressure?

[@AuMechanic]

Yes it can Keep in mind BMEP value is used to estimate Exhaust temperature not a direct calculation.

[@sbarrett4826]

Hi, Dave,firstly thank you for the software, most helpful and I’ve used it to design myself an expansion chamber. My only question, I intend using my standard frontpipe which is 520mm in total length, 460mm pipe length and 60mm port length. My design shows a very short front pipe before the first cone. Can I fit this design to my std frontpipe and will it change any of the calculations? Thank you, S Barrett.

[@AuMechanic]

If you want a longer header pipe than the calculations give you would need to use a steeper angle on the cones to make them shorter and so end up with the same tuned length but then you might end up with a very peaky pipe.

[@heathlett275]

I have a 74 moped so no gears. Does a completely straight first section help with a broader power curve? If so how do I calculate that in? Not really an option on your program

[@AuMechanic]

A straight diffuser cone has a more narrow power range.

[@robcaven58]

A little more science to be investigated would be where you want to place the Power band in the throttle. This is determined by the length of tubing from exhaust port to the beginning of the chamber. I played with mine and have the bowerband start almost at the end of throttle, so as to get all the power I can out of engine. This can be adjusted depending on what kind of conditions exist. Weather straight track or motorcross.

[@AuMechanic]

The tuned length of the entire pipe will determine what RPM the centre point of the pipes tune will operate. Changing the header length simply changes to tuned length and will effect and RPM change, but changing it to the exclusion of the rest of the pipe can take the chamber away from the intended wave reflection timing. The header length dimension is a derivative of the Jennings pipe formula that was created in the 70's by crudely reverse engineering simply measuring factory race pipes, where the equation I use are form the late 90's and were created by comprehensive scientific method and peer reviewed papers published by SAE In short you might observe some effect by changing one part of the pipe by you may not realise some offset loss, as in 2 steps forward and 1 step back.

[@robcaven58]

I'm no scientist. I wasn't really working on a tuned system. Just got lucky I guess. Thank you for responding so quickly. I'm a welder, retired Union Ironworker. I worked at Disney World over 15 yrs fabricating all sorts of things. I had weld certs. for all materials. In the video your tig welds looked good for what I perceived was first attempt at such a complex project.

[@tamasbarta5691]

Absolutely great videos, keep going! I am a bit confused about BMEP. Assume I have an engine with a modified cylinder (bigger bore, modified inlet port timings). How can I calculate BMEP, if I don't know the kw what the engine can produce?

[@AuMechanic]

Calculating BMEP is not critical, it is only used as a guide to Exhaust temps in the case of pipe design save measuring actual temps. You can make some estimations based on how much power increase you have after such mods compared to stock. Also keep in mind that a compression test will give peak pressure where BMEP is average pressure so it'll be a bit lower than cranking pressure. Note BMEP is in Bar of pressure so you can convert that to PSI.

[@twintuned]

I was going to ask the same thing. For example, the yamaha AS2 125cc is 15hp, the Yamaha TA125 racer is 24hp. Same displacement but developed for speed. I see programs needing bmep, but never really understood, thank you.

[@tonywilkinson8041]

Excellent explanation. Big help. 🍻

[@AuMechanic]

Thank you for the comment.

[@tonywilkinson8041]

@@AuMechanic I am yet to wade through all the vids but its in the pipe line! I have recently purchased a yahmaha 125 2t that I plan to road race prep for historic racing. I am a panelbeater by trade , handy with sheet metal and oxy / accetlyne , mig etc so extremely keen to have a crack at putting an expansion chamber together. Thanks for sharing and making the time to do so! I'm guessing you cover rolling the sections at some stage? Thanks Tony Gold Coast AU

[@AuMechanic]

@@tonywilkinson8041 I have not done a video on the fabrication side of it, there are many videos around covering that. My days of making pipes are in the past when I road raced bikes myself.

[@heathlett275]

Material thickness? Your software seems to be port length short. How do I contact you for some help?

[@fririkjon7576]

Hi this is the video that I needed. but I was wondering if I could go a more simple way. I am changing a 2cylinder 2 stroke engine that has a single pipe to a 1 cylinder engine. Is there a way to calculate how my pipe would need to be for half the engine from the pipe that is for both?

[@MrFalcon195]

Hello... The pipe diagram is shown in a straight length fashion. Can those pipes be bend into any direction by keeping the length of the pipe or is there a new calculation required? Thank you.

[@AuMechanic]

So long as you maintain the centre line length of the pipe then it will not change the equations for tuned length.. And you need to ensure any bends maintain the area (diameter) along the lengths so bends need to be clean, for example a header using a bender would need to use a mandrel tp keep the pipe shape and the pie slice method will do to put bends in cones or middle sections as well ensuring there are not "steps" in joints from mismatching.

[@MrFalcon195]

@@AuMechanic Thank you so much. Awesome!

[@alfiolocatelli5093]

Hello, how can I split L2 so that I have a radius by combining lot of single elements(like 10 pieces)? Do you have any advice on exhaust port lenght/height according to a power valve that open gradually from 0-8000 rpm on a 125cc ? Thanks

[@hankburton2043]

When is the next vid coming there great love them thank you so much for them I get a lot from them thank you Dave

[@AuMechanic]

Thanks Hank glad you like the series. Next video will be up by later this evening.

[@hankburton2043]

Awesome

[@Anthal1010]

I have a standard exhaust system on my bike but i dont like the positioning of it, how would moving the expansion chamber in relation to pipe-length-to-exhaust-port affect it? It's an RD350 N2 if that makes any difference

[@AuMechanic]

Nice bike, I raced RD 350 LC-H model in the 1980's If you make the header pipe longer that will lower the effective RPM range the pipe operates at, conversely if you shorten it that will raise the RPM range it is optimal at so you need to be careful about doing that so you don't mismatch the timing of the pipe with the port timing it has.

[@emivesprini3345]

So the maximum power and the revolutions with which we must make the account are those that the manufacturer of the motorcycle gives us?

[@AuMechanic]

The RPM value used in the calculator is the Red line value off your tachometer, not the Peak power rating. Keep in mind that tuned length calculation is for port open to port closed where the effect of the pipe drops off since the port is now closed. You want this aligned with red line so the pipe effect shuts off at that point acting a bit like a rev limiter. You don't want that occurring right at peak power.

[@emivesprini3345]

@@AuMechanic So, should we put the maximum power of the motorcycle in Kw, and the maximum rpm that the engine reaches?

[@AuMechanic]

@@emivesprini3345 For estimating BMEP using the calculator use the Power in KW But for setting RPM value on the main page of the software for tuned length use the maximum engine speed.

[@edwinnel4704]

Hi Dave, I’m stuck on Part 3 of the expansion chamber video, I’m building a sand drag quad with a 1990 650 wildcat engine in it and I can’t find the engine kw @ rpm online to work out the BMEP. The answers I find are too different, I take it that the engine makes around the ball park of 100hp, I have no idea where to go from here, is there any other way I can go at this. Have you ever ran into issues like this? And how did you get around it?

[@AuMechanic]

An estimation on power to BMEP is ok, it doesn't directly translate it just uses a general scale to estimate exhaust gas temperature from a BMEP range.

[@edwinnel4704]

@@AuMechanic thanks mate 👍

[@ridefast591]

Thanks so much...bringing the gas bikes into your calculations. I love it will share on the zeda group over 3000 members and lots will love to see this. Everyone wants you to do it takes too long to watch your vids and learn haha much easier too. So I followed up until you came up with the temperature to use from the BMEP u went quick and lost me there. Is the temp you got close to the temp that graham bell uses as an average im not sure the temp but 1675fps in hot ex gas? Or do you find a better suited number for my caculations for my bikes tuned length.And the vid was great but when you say your calculations you make it much easier to follow when u show it on the calculator beside like in the vids for centerline of a cone... Thanks again.

[@AuMechanic]

Hi Ride Fast, thanks for sharing the video. The wave velocity figure given by Bell at 1675 fps is similar to Jennings at 1700 fps, its a fixed value assuming the same exhaust temp for all engines as well as being the velocity in air not exhaust gas. Keeping in mind gas temps changes that FPS value a lot. I use Gordon Blairs more modern method that calculates more precisely the wave velocity based on gas temp, in addition to using the 2 constants given for exhaust gases which is not the same as plain air like Bell and Jennings figures use. That will be in the next video where we calculate the tuned lengths from the figures we have so far. So knowing BMEP helps us get a close estimation of the exhaust gas temp to then use in the tuned length equation. Ill try to do the calcs using the calculator in the videos to make it simpler if it helps, just trying to keep the videos short.

[@ridefast591]

AuMechanic that umber 1675fps bell says is the average speed of sound in hot exhaust gas not air. Was curious if you arrived at a close figure to use. Now I know you are going one further by using BMEP to get a better average temp reading for a more accurate calculation, I still don't see how you arrive at that temp or speed of wave to use from the BMEP. Try to clear that up in the next video please and thank you.

[@AuMechanic]

Much of Bells equations are derived from Jennings, Jennings states he calculates a value of 1670 but uses 1700 as an average, Bell just used 1675 to be safe and neither provide more detail than that. But looking at Jennings temps given his 1675 fps equates to hot gas as in air but not hot exhaust gas. Blair gives far more scientific detail and provides not just temperature base but the individual molecules of exhaust gas at various temperatures and fuel mixtures. With BMEP, Blair provides a list of expected Temperatures for given BMEP which I cover briefly in this video at 7:50, so we take that temp and apply it to the above exhaust gas figures Blair provides to get a more accurate velocity range for various engines not just an average for all provided by Jennings and Bell. Jennings study is from 1973, Bell is mid 1980's, Blairs studies are 1990 and 1997 second edition and are scientific peer reviewed papers published by SAE, where Jennings and Bell are more trial and error studies of 2 strokes and very dated.

[@ridefast591]

AuMechanic So Bell is mistaken when he got his numbers from Jennings...because he does say 1675fps in hot exhaust gas. That is understandable. Still wonder what speed you came up with to use for that calculatd BMEP is it anywhere near the 1675fps i guess i can do the math with your calculated 600° calculated temp ans see for myself.Will follow along I'm sure I will learn more. Please get into more detail of using the BMEP to arrive at a good temp to get the proper speed of the wave...... I think I understand now you are taking your BMEP and comparing that to a chart I can find in Blair's work to get a temperature I can then use in my tuned length equation?

[@AuMechanic]

BMEP Blair gives is about 350C for BMEP of 8 bar pressure or less to 650C for BMEP of 11 bar pressure or more. We only use the Temp value itself in the tuned length equation, if you know the actual temp by measuring it you would use that instead of just estimating it based on BMEP. At an air fuel ratio of 13:1 - 650C velocity in exhaust calculates as 614 m/s which converts to about 2000 fps - 350C velocity in exhaust calculates as 504 m/s which converts to 1653 fps Lower BMEP engines like the Zeda could be closer to 300C and velocity in exhaust calculates as 483 m/s which converts to 1584 fps This will be covered in the next vid giving the equations used.

[@samjackson5446]

What changes at calculation when I have more cylinders and single pipe?

[@AuMechanic]

Good question, bit beyond the scope of these video's which cover single expansion chamber per cylinder. A few issues to deal with, one being is if there is differing lengths of a log manifold to the final pipe means "tuned length" may not be the same for each cylinder. Also differing issues if its a twin, triple or 4 cylinder. Some of this is covered in Gordon Blair's Two stroke design book and you might want to read up on that.

[@coco-te1op]

Good morning dave. . . How does one guess the temperature of 13.32 bmep? I'm thinking 650?

[@AuMechanic]

Yes anything over about 11 you would use 650C as an estimated value. You would need to use an EGT gauge for an accurate reading. Keep in mind that is the peak temp. Under the peak RPM or lower gears that value drops quite a bit but we work with peak temp in top gear. For racing purposes that provides for peak RPM at the end of straights in top with some room for a bit of over rev. Also be aware that the RPM value is the maximum engine redline RPM value not the peak power RPM.

[@coco-te1op]

@AuMechanic OK thank you dave. . . Will be putting degree wheel on today to find the exhaust port timing for the rd400f. . . My guess its around 190 to 200. Could be wrong. Thanks again

[@tuduu]

@AuMechanic Hey there. I've read several comments on that particular topic in just 2vids of this playlist. You wrote this in your comment above : "...the RPM value is the maximum engine redline RPM." So, in my case, the Redline goes from 10k to 12k on the tachometer of this '93 aprilia rs50. Should I use 12K, as I understand from your above comment, as my input value for RPM ? I've made all my sims until now with a value of 10K (thinking "redLINE"). I'll run some more estimations using 12000RPM to see the differences. I'm working on a mod for the stock exhaust pipe. And I do because of your work, both teachings and freeware. Thanks a lot for your contribution to the community, cheers !

[@abbasigopedsports7973]

Hi I'm your subscriber I watch your vedios regularly can I asked I have G2d 23cc engine I bought it from japan recently I would like to install it on goped bigfoot but I'm thinking its carburetor is locked can I do to install walbro carburetor to enhance its performance please I need your best advise.

[@AuMechanic]

Best you have a look around for videos or forums info on doing that, this vid is on expansion chambers.

[@frankg3278]

Is that max rpm of the engine or rpm at peak HP? People say the pipe determines the max rpm, I seem to be getting mixed messages :/

[@AuMechanic]

Max RPM refers to the engine redline RPM or maximum engine speed for calculating tuned length.

[@Ahmad.Kassem]

the calculation of EGT is not clear in the video, can you please explain more

[@AuMechanic]

The EGT value is not calculated with an equation, it is an estimation given by Professor Blair based on the BMEP Otherwise to get an accurate EGT you would need to use a gauge to test it.

[@Ahmad.Kassem]

@@AuMechanic so i will correlate between 3.5bar/350deg and 11bar/600deg, right?

[@AuMechanic]

@@Ahmad.Kassem Correct

[@acurarl9929]

Would a 400cc expansion pipe be shorter if rpm range is 3,500-5,500 with max rpm is 5,500 vrs 10,000rpm? Or longer?

[@AuMechanic]

The higher the RPM is the shorter the pipe is.

[@acurarl9929]

@@AuMechanic yes ofcoarse that makes sense. Thank you.

[@acurarl9929]

@@AuMechanic would a non expansion chambered 200hp 4cyl 2stroke benefit from a single expansion pipe at all? If so what are we talking bout for a gain. 10% or less or more? Would love to do 4 pipes but not enough space. Just wondering if I should scrap the pipe idea on my fresh rebuild on my jetboat. An thank you. Your videos are great. Just getting into them so pretty new

[@ni3walia315]

I have 2 strok bike name yamaha RX 135 cc. I want to built my self a exhaust chamber . But I don't have any drawing will u help or sujest me how can I make it

[@AuMechanic]

You can use the free software here to calculate the dimension of the pipe but there is a lot involved in fabricating the pipe itself after you have done the calculations using software. You need to make up a template using the calculated values and then section it in parts to fit the bike and then cut out the sheet metal off the template parts and then bend and roll section and weld it up. There are a fair few videos on YT about how to do that part of it.

[@ProjectDaily]

Where did you get the 10000 figure

[@AuMechanic]

That (and RPM / 60) are just a scaling factors Blair uses in the calculation since power is usually listed in kW, Swept Vol in Cubic Centimetres and engine speed Revolutions Per Minute.

[@precisionancillarywerkz1402]

WHAT SOFTWARE IS THAT

[@AuMechanic]

I wrote that software just to help demonstrate a few things in the series of 2 stroke videos I am doing.

[@BigRichardsGentlemensClub]

How is the cc's of an expansion chamber calculated? Is it the cubic area of the expansion chamber itself?

[@AuMechanic]

In short it is not, the capacity or volume of the expansion chamber is not a consideration of the design. If you watch more of the videos I have or try to pipe calculation software you will see how the dimensions of expansion chambers are determined.

[@cristhianboza9450]

hello how are you, can you give me an email I would like to talk about the escape of an article that is more advanced to the calculation of gordon blair

[@AuMechanic]

If you go to the about page of my channel the Email is there but if you can ask it here it may help others reading. Its up to you

[@cristhianboza9450]

@@AuMechanic could you give me your email and I send you the article

[@AuMechanic]

@@cristhianboza9450 ausmecham @ gmail.com Just remove the spaces

[@captainsledge7554]

Why do u talk so low..

[@AuMechanic]

@@captainsledge7554 Oh right, that was the mic I was using at the time that was not suitable with the PC mic input, I got a new USB input since that solved it.

[@christopherknee5756]

Because he is from DOWN under! Get it? :-)

[@fun_ghoul]

@@christopherknee5756 That's a real Knee slapper! Get it? 🤣🤣🤣

[@Not1Edit]

Can I reach You by email or elsewhere privately? I made two breakthroughs that have apparently escaped the triple 2 stroke community for 50 years, & I just wanted to run it by You to be certain I'm not overlooking anything. Thanks mate.

[@AuMechanic]

Yes Click on "About" on my YT channel to see Email