Two-stroke performance tuning?

[Chambers]

SS90, from a SAE paper you refered TZ350 too.

Last paragraph page 175: The drop in delivery ratio caused by increasing the crankcase volume can be fairly well compensated for by tuning the inlet and exhaust system.

http://www.edj.net/2stroke/jennings/...ase_volume.pdf

Second paragraph of the discusion section page 188: Small crankcase volume is best for high speed and large volume for low speed. Explanation: with small volume and low speed the crankcase discharges to fast into the cylinder and for the rest of the transfer-port-open period there is reverse flow.

Forth paragraph of the discusion section page 188: If the engine speed is selected to best suit the crankcase clearance volume the clearance volume does not effect the delivery ratio. Tuning either the inlet or exhaust pipe or both increases the delivery ratio.
.

So from reading the SAE paper, you have to have the right crank case volume for the intended engine rev's and it needs to be a smaller clearance volume ie., a higher primary compression ratio thats closer to 1.5:1 the higher the rev's.

I think your friend has it back to front and is wrong in what he has told you.

So, I emailed a friend of mine (who last year worked for KTM, and now works in Italy), and asked him his opinion of crankcase volumes (and in particular minimum values)

The following is an excerpt from his reply. "the primary compression is often misunderstood. for low rpm, a high primary compression is benefitial (because the crankcase then works as a pump). for high rpm, a low primary compression is benefitial (because the crankcase can then store enough gases for a good filling)."

"A low primary compression (larger clearance volume) is beneficial because the crankcase can then store enough gases for a good filling", sounds similar to what Speedpro was saying on the ESE thread too.

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[Yow Ling]

"I have decided to change the boost port angle, " change to what?

"increase the exhaust port duration" increase from where to what?

Its very interesting to be able to watch your progress. Thanks SS90.

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i think you are dreaming there bucketracer

[speedpro]

I also took the crank to a machine shop today to calculate the balance factor... I believe that balance shafts are for FAGS, but it would be nice to know!

I hope you took a bit more than the crank, it's a bit hard to weigh things if they aren't there.
I agree about the balance shafts for what we're doing. Mine's in the bin and I dealt to the crank and it's good. It shakes at low rpm but is fine where I use it so it doesn't matter.

[SS90]

SS90, from a SAE paper you refered TZ350 too.

Last paragraph page 175: The drop in delivery ratio caused by increasing the crankcase volume can be fairly well compensated for by tuning the inlet and exhaust system.

http://www.edj.net/2stroke/jennings/...ase_volume.pdf

Second paragraph of the discusion section page 188: Small crankcase volume is best for high speed and large volume for low speed. Explanation: with small volume and low speed the crankcase discharges to fast into the cylinder and for the rest of the transfer-port-open period there is reverse flow.

Forth paragraph of the discusion section page 188: If the engine speed is selected to best suit the crankcase clearance volume the clearance volume does not effect the delivery ratio. Tuning either the inlet or exhaust pipe or both increases the delivery ratio.
.

So from reading the SAE paper, you have to have the right crank case volume for the intended engine rev's and it needs to be a smaller clearance volume ie., a higher primary compression ratio thats closer to 1.5:1 the higher the rev's.

I think your friend has it back to front and is wrong in what he has told you.



"A low primary compression (larger clearance volume) is beneficial because the crankcase can then store enough gases for a good filling", sounds similar to what Speedpro was saying on the ESE thread too.

.

Hmmm,


I think someone who designs two stroke engines for a living is "on the money" (personally)

I'll take his advise any day.

You are of course free to make you own mind on that.

Experience tells me that (as was mentioned) with modern high revving engines (Aprilia 125GP bike was used as an example) run high crankcase volumes.

What was mentioned was what is high RPM and what is low RPM.

It's a little subjective really.

So far my reasearch is pointing for an engine designed to operate in a rev range of around 10,000 RPM a ratio of 1:42 (roughly) would seem best.

Of course using modern exhausts and ignitions.

The lower the revrange, the higher (closer to 1:5) the better.

We will see.

Like I have said before, you need to put a value on every change you make "just lower it let the pipe do the work" is not very good thinking (in my opinion)

Have you ever read anything (anywhere) giving an indication to what is the correct primary compression ratio for a particular set up?

If not, what is your experience with different ratios?

I try not to take too much information off the web (reading the SAE papers written in the 60's in my opinion gives you an understanding of where people where in the 60's) but it does give you information on where others have been.

Like I indicated, this series of posts is trying to open discussion on what is a minimum primary compression ratio for a given engine set up, in particular operating RPM range.

You may remember in the ESE thread, I wrote about "what sort of power" you will get from a higher primary compression at higher RPM's.....much like a high exhaust port time area, you have a "stronger hit" of power (peakier power), and loose your ovverrev.

And from my experience, too lower a ratio will have almost the same effect (stops revving abruptly)

This is why it is prudent to put finite values on these these factors.

[SS90]

I hope you took a bit more than the crank, it's a bit hard to weigh things if they aren't there.
I agree about the balance shafts for what we're doing. Mine's in the bin and I dealt to the crank and it's good. It shakes at low rpm but is fine where I use it so it doesn't matter.

Logically I took "more than the Crank" (I was paraphrasing)

Like you, I have been doing this for a while

Sure, I don't expect it to be sewing machine, but limits in how I can increase the crankcase volume mean the crank is the logical step.

There is actually no option for a balance shaft in these engines, so it is really "what I get is what I get"

You seem familure with balance factors etc Speedpro, maybe you could contibute to this thread with some experienced information that we could all read?

[SS90]

"I have decided to change the boost port angle, " change to what?

"increase the exhaust port duration" increase from where to what?

Its very interesting to be able to watch your progress. Thanks SS90.

.

Just re-read a page or 2 back, Bucketracer....I posted what I would like to have the exhaust port duration at (as well as the original specs, with graduated changes achieved with different base gaskets)

As for the boost port angle, I posted above what other people have found regarding fuel consumption (in relation to the meagre power made)...what's your opinion on the boost port angle now you have the same information as me?

I'm interested in your feed back!

[bucketracer]

Hmmm,


I think someone who designs two stroke engines for a living is "on the money" (personally)

I'll take his advise any day.

You are of course free to make you own mind on that.

I try not to take too much information off the web (reading the SAE papers written in the 60's in my opinion gives you an understanding of where people where in the 60's) but it does give you information on where others have been.

Yes your right, you should do that, your pretty safe as there has never been an honest but misguided tuner or charlatan in the hot up business.

Although the SAE paper was a proper academic study, I expect the laws of physics have changed since then, after all the 60's were a while ago.

.

[bucketracer]

I'm interested in your feed back!

Specs recorded from some of the motors I have too-tooed with over the years.


.............................CLEARANCE........SWEP T........COMPRESSION
.............................VOLUME (cc)....VOLUME (cc).......RATIO

RM125A
.....CRANKCASE.................403.00.........124. 82...........1.31
.....CYLINDER.....................9.00...........1 24.82...........8.20

RS125
.....CRANKCASE.................380.00.........124. 82...........1.33
.....CYLINDER.....................8.50...........1 24.82...........8.67

TM125R
.....CRANKCASE.................400.00.........124. 82...........1.31
.....CYLINDER.....................8.20...........1 24.82...........9.21

TZ250
.....CRANKCASE.................313.00.........123. 67...........1.40
.....CYLINDER.....................9.00...........1 23.67...........7.61

R125 Kart Motor
.....CRANKCASE.................460.00.........124. 82...........1.27
.....CYLINDER.....................10.00.........12 4.82...........7.25

KT100J
.....CRANKCASE.................320.00.........98.1 7............1.31
.....CYLINDER......................9.40..........9 8.17............8.03

KT100S
.....CRANKCASE.................250.00.........97.6 9............1.39
.....CYLINDER.....................8.90...........9 7.69............7.37


SS90 it looks like you will have to cut some more off those flywheels.

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[Yow Ling]

Wonder what I was thinking ?

[SS90]

Specs recorded from some of the motors I have too-tooed with over the years.


.............................CLEARANCE........SWEP T........COMPRESSION
.............................VOLUME (cc)....VOLUME (cc).......RATIO

RM125A
.....CRANKCASE.................403.00.........124. 82...........1.31
.....CYLINDER.....................9.00...........1 24.82...........8.20

RS125
.....CRANKCASE.................380.00.........124. 82...........1.33
.....CYLINDER.....................8.50...........1 24.82...........8.67

TM125R
.....CRANKCASE.................400.00.........124. 82...........1.31
.....CYLINDER.....................8.20...........1 24.82...........9.21

TZ250
.....CRANKCASE.................313.00.........123. 67...........1.40
.....CYLINDER.....................9.00...........1 23.67...........7.61

R125 Kart Motor
.....CRANKCASE.................460.00.........124. 82...........1.27
.....CYLINDER.....................10.00.........12 4.82...........7.25

KT100J
.....CRANKCASE.................320.00.........98.1 7............1.31
.....CYLINDER......................9.40..........9 8.17............8.03

KT100S
.....CRANKCASE.................250.00.........97.6 9............1.39
.....CYLINDER.....................8.90...........9 7.69............7.37


SS90 it looks like you will have to cut some more off those flywheels.

.

Ohh, it seems like you keep all the data from engines that you have "too tooed with"....do you have any pictures of said engines?

I was under the impression that you where suggesting that I was going in the wrong direction with lower primary compression for a higher revving engine....yet these engines you claim to have "too tooed " with all have primary compression ratios lower than 1.5:1.....

Can you explain your reasoning?

[bucketracer]

Ohh,
Can you explain your reasoning?

Could but won't where would the fun be in that, I am enjoying watching you work through it for yourself and making your own mistakes, but if you read the SAE paper its obvious.

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[SS90]

Ermmm,

Hang on a minute..... why can't you explain why you "too tooed" with these engines (and I am guessing lowered the primary compressions).....you didn't clarify what "too tooed" meant, but I am going to assume "tuned"

So, you are saying that the SAE paper suggests that I should INCREASE the primary compression..........

Yet, you, yourself chose to lower all the engines you have listed to more closely to what I am currently advocating.............

So, what where the results from these "too tooed" engines......?

Did you end up stripping them all down and increasing the primary compression closer to 1.5:1?

[SS90]

Specs recorded from some of the motors I have too-tooed with over the years.


.............................CLEARANCE........SWEP T........COMPRESSION
.............................VOLUME (cc)....VOLUME (cc).......RATIO


TZ250
.....CRANKCASE.................313.00.........123. 67...........1.40
.....CYLINDER.....................9.00...........1 23.67...........7.61


.

also, while the final figure of 1.40:1 is correct for a vtwin TZ250, I can't work out how you arrived at these primary compression ratio figures (any of them, just using this as an example)

Primary compression is calculated by dividing the case volume at TDC by the case volume at BDC.......

From the figures you posted, I can't calculate that (for any engine you list)

Can you explain?

[bucketracer]

No. no changes these are the standard figures as is.

The way I read it, the SAE paper says there is an optimum clearance-volume, rev's, port-time-area relationship. Where there is an optimum clearance-volume, any more or less and the transfer delivery ratio goes down.

The key being port-time-area and how quickly the crankcase discharges through the transfer ports at the chosen rev's ie 10,000rpm. Below that the pipe makes up for the worse transfer delivery ratio.

I suspect in the 60's the transfer ports were smaller than they are now and required a higher primary compression ratio than being used by more modern engines.

But the paper did say as the target revs go up the primary compression ratio also needs to go up. Sounds logical.

.

[SS90]

No. no changes these are the standard figures as is.


But the paper did say as the target revs go up the primary compression ratio also needs to go up. Sounds logical.

.

Well, that's what i am trying to establish some figures for...... like the figures you posted.......a "modern" TZ250 is 1.40:1......an RS125 1.33:1

From memory (I have owned and raced both these models) A 1991 TZ250B revs till 11,000 (primary compression ratio 1.40:1) a 1995 RS125R revs to 13,000 (standard, A kits more), and have a primary compression ratio of 1.33:1

Hmmmm,

I see a pattern forming here!

Re-read Gordon Jennings chapter on Crankcase volume (he mentions the same SAE paper there)

He also gives some insight and opinion on the reasons people increased primary compressions in the first place.

Have you seen any modern literature that gives minimum figures for primary compression ratios (for different engine set ups (operating RPM,capacity exhaust design etc?)