Combustion chamber shape, squish, compression

[StepVino]

Hi, happy to have found this forum.

I have an 02 Yamaha Vino, with hor. minarelli. Have
been tweeking it for about a year, and am currently trying to
find the correct cylinder head configuration. This is haw it's
set up right now:

Next R pipe, bored out stock carb, Tecnomoto vario with 4.5gr,
Jog 90 belt, ported exhaust and transfers for around 10k,
VForce reeds.

I sanded off the bottom lip of the stock cylinder head, and
ground the pear shaped, stock combustion chamber into a
small cone. Squish is huge, over 60%. Compression is only 100lbs,
but top rpm is 11.8k at 58+mph. Here is a video of this setup:
http://www.youtube.com/watch?v=R24p70vlvrs

Wanting more (of course).. trying for 12K and 60mph, I tried
an Airsal T6 head, shaved 2mm, and reshaped a 40% squish area.
Compression is up to 125lbs. Pickup is better, but top rpm is down
to 11.4k. I tried retarding the timing, but it made everything worse.

Question is, maybe you can save me some time, and might know
which direction I should go. Work on the large squish head, to
raise it's compression? Change the cone to a bowl shape, on either
head?

Many thanks for all input

 

[adas]

Aloha, If you want more RPM then you need to shorten your exhaust and raise your ports and not need to be messing with the head, but that does not mean you will get more MPH. You should try to find the rpm for the highest power and tune your transmission to stay at that rpm. Right now it seems you are starting to over-rev and loosing HP. To get more speed I would go with a gear up kit (Malossi or Metrakit Primary) or an overrange pulley setup. frank

 

[scootertrog]

Hi, happy to have found this forum.

I have an 02 Yamaha Vino, with hor. minarelli. Have
been tweeking it for about a year, and am currently trying to
find the correct cylinder head configuration. This is haw it's
set up right now:

Next R pipe, bored out stock carb, Tecnomoto vario with 4.5gr,
Jog 90 belt, ported exhaust and transfers for around 10k,
VForce reeds.

I sanded off the bottom lip of the stock cylinder head, and
ground the pear shaped, stock combustion chamber into a
small cone. Squish is huge, over 60%. Compression is only 100lbs,
but top rpm is 11.8k at 58+mph. Here is a video of this setup:
http://www.youtube.com/watch?v=R24p70vlvrs

Wanting more (of course).. trying for 12K and 60mph, I tried
an Airsal T6 head, shaved 2mm, and reshaped a 40% squish area.
Compression is up to 125lbs. Pickup is better, but top rpm is down
to 11.4k. I tried retarding the timing, but it made everything worse.

Question is, maybe you can save me some time, and might know
which direction I should go. Work on the large squish head, to
raise it's compression? Change the cone to a bowl shape, on either
head?

Many thanks for all input

StepVino, compression for the un-tuned Airsal T6 should be 145 - 150 psi, I've done several Vinos with the Airsal kits. Either your guage is not good, or you've got an air leak. Also 2mm off the head is an awful lot, are you sure about that? The Airsal T6 a/c makes enough compression as is, I would not try to modify the head. It may be possible the compression is too low due to you using the OEM carb, I'm not sure because I never checked that combo.
Ditch the OEM Teikei carb. That thing is the absolute worst carb I've ever laid my hands on, you have done well to get as far as you got with it, lol. It's a pain in the butt to modify the throttle cable for an aftermarket carb on a Vino, but once done, well worth it.
Have you changed your crank? The Vino has a very weak bottom end - trust me on this one. Steady operation over 9,000 rpm is too much for the crank bearings Yamaha uses in it. Your crank will not hold up long @ 11,000 rpms.

scootertrog

 

[StepVino]

I know... I was hoping that since it was so easy to loose 200rpm,
it might be just as easy to go the same distance the other way.
Maybe not though.

Any thoughts on the exhaust being hotter at lower compression?
Does higher compression lower exhaust temps, maybe decreasing
the pipe's max rpm?

 

[StepVino]

scooterdog, missed your post while answering previous one...

I don't have an Airsal cylinder, just the head. I bought it thinking
it would save a lot of work reshaping the stock one, but actually gave
less compression. So definetely.. ported stock cylinder, with 2mm off
50cc T6 head, is giving 125lbs. Had to change to shorter reach plug.

I've been trying to get to 60mph before it blows up. Definetely could
use crank.. carb.. better pipe. It's been a fun challenge to see what
moded stock parts would do though.

 

[scootertrog]

OK, my misunderstanding, sorry. You have done well, acheiving 60 would be outstanding with your current components, especially with 49cc displacement. I am surprised the Next-R pipe would even let you rev that high, but your video proves it can.
The problem with the Vino, it is so light and can be made to go fast, stopping it safely is another matter, as I'm sure you know, lol. Be forwarned, the oil pump is a rather poor non-adjustable type, and you are playing Russian-roulette with your crank.
Fortunately, there are a ton of Minarelli-based parts you can throw at your engine, it's only a matter of how much you want to spend. I believe Adas is on the right track - to get any more top speed with your current engine, you need to swap out your gearing and lower your overall rpm's, although your off-the-line torque might be quite poor.

scootertrog

 

[Big B NRG]

Maybe you should try a lower compression Ratio. It's often that a lower CR makes the scooter run more "easier".

It revs up easier. Also you could try to work with the pre-ignition. That might work alittle.

 

[StepVino]

Thanks B, I advanced the timing about 3 degrees last night,
but it's been raining. Grr.. If it doesn't help top speed, I'll
work on the Airsal head (125lbs comp), and change the cone
to a bowl shape, lowering compression. Might also round
off the edge of the squish band a bit.

Scooterdog, I have been adding 2% premix to the stock oil.. just in
case. Interesting thing is with low copression plug looks pretty nasty,
although runs good. With higher compression the plug looks absolutely
super.. but doesn't run as fast (or overev as much).

I re-read frank's post (thanks frank), and it makes more sense now.
The transmission is tuned to around 9.5-10k. That's the rpm range it
holds while the belt is shifting. I tried 4grams, down from current 4.5,
but it was awkward driving out of the neighborhood. ~10k rpm at 20mph .
4.5 is a compromise..7k at 20mph...

I've hesitated raising the exhaust port more, not wanting to leave the
pipe behind. Might go 1/2mm higher.

My plan was to get an upgear after reaching 60. I was thinking +10%,
since the 100/80 tires give an extra +5 (calculated some time
ago, could be wrong). Also was thinking of trying sportier 100/90,
though it might be a tight fit. The Sava has a very wide center tread..
could be loosing some speed there? I pump them to 36lbs for test runs.

The brakes... I bought a set of Eton Bemer forks on Ebay. Waiting
for the rest of the parts to show up there.

 

[StepVino]

After some fiddling, the increased timing just overheated the engine.
Set it back to 1 degree or so advance... better, but would still
overheat, and loose power around 11.4k. This is with 130lbs compression,
for some reason it's gone up! Probably misread it last time.

Anyways, I had purchased a new cooling fan off Ebay, from a Jog 90.
This has the fins curving opposite the stock fan, in the direction of
rotation. Installed it, measured the air flow at the inlet, and it's sucking
20-30% more air. Test drove it, and was able to get to 11.6k, with
no problems. Didn't keep that up for long, just long enough to know
it was making a positive difference.. no cutting out.

Back home, I have looked into exhaust tuning lengths vs port duration,
and engine tuning vs port duration. It appears that the exhaust might
need to be longer! That was a surprise. Since port duration effects pipe
tuned length, the current port duration of 188, puts the pipe at ~10750,
but the engine at around 10000. If I increase port duration.. pipe
tuned rpm goes up again, so pipe rpm always stays ahead of engine tune.
Unless, pipe is made longer.

Looks like you should be able to make a graph, with two lines. One
for pipe rpm vs duration, the other for engine rpm vs duration. Where
the two lines cross would be where pipe and engine are matched.
Then, fiddle with exhaust duration and pipe length, untill the lines cross
at the correct rpm... But I don't have time to go through all that right
now! .

PS I came up with 743mm measuring the Next R exhaust length.
This includes the approx length from exhaust flange to piston, with
other end at just past mid point of reflector cone.

 

[StepVino]

ok.. so I did some quick calculations, didn't need any graphs.
Just for thowing around numbers sake... Exhaust port duration for
engine tuned at 11k, is around 194.7 . Pipe would have to be
lengthened only 10mm. Well, that's probably around my error in
measuring it's length, so looks like these measurements have to be
taken rather precisely. You could say it's in the ballpark already, so
as suggested, just go ahead and raise the exhaust port? Forget
about the pipe?

If there's any math buffs, this is what I used for the calculations:

Based on the data in Two Stroke Performance Tuning by A Graham Bell,
page 34, table 3.1, Exhaust port duration

Without using anything fancy, like least squares or anything,
the data approx fits this equation

exhaust port duration = (10/1700) x rpm + 130 (1)

For pipe, on page 71,

pipe length = (exhaust duration x 42545)/rpm (2)

Solving (1) for the duration at 11k, and using this duration,
and rpm in (2) to find pipe length comes out to 753mm.
753mm minus stock lenght of ~ 743mm gives 10mm.

 

[Big B NRG]

What's the Blowdown on your cylinder?
195 exhaustport for a std cylinder is high. Maybe with a smaller BD and a bigger carb, it should be better.

Maybe a higher reving exhaust would be better. That formula you used, is old. There are guidelines for making an exhaust, but it;s better just to test on a dyno. If you don't have acces to a dyno, try to do some adjustments on your exhaustflange. Easy to adjust and you will notice quick.

 

[StepVino]

I have exhaust at 188 degrees, and boost opens first with 135
degrees. That should be a blow down of ~26.5 . Transfers have
128 degrees. May have overdone it a bit on the boost port.

Hadn't ocured to me to just work on the flange. Could just add
a spacer, if I need to go other way, cut a ring section and reweld.
I have a mig welder which would work weel on that part of the
exhaust.... no dyno.

 

[Big B NRG]

Oh btw, timings does not say anything on RPM.
I would have done it the otherway around, transfers higher then boost.

If you have the time to cut and weld, then you can try. Working with the flange is easiest. Just make a few spacers starting from 2mm thick to 10mm thick.

 

[StepVino]

Oh btw, timings does not say anything on RPM.

This I'm not sure what you mean. Is the port duration vs rpm an
outdated concept as well (like the pipe equation)?

I read from same book that at the beginning, tuners tried
opening boost port first. Now it is done the other way round.
Also on McDizzy somewhere, it mentions that same hight for
everything is best for higher rpm, and slight differences in height
can widen the power band. I thought there was some room
for experimentation there.. so I pictured the mixture out of the
boost port leading the charge.. sweeping the back wall clear,
then the auxiliaries (ported slightly higher towards boost port)
broadening the rear sweep , and finally the main transfers clearing
out the center. This is just a picture in my mind though, really don't
know if that is what's going on.

I can try raising the main transfers, and reduce the duration difference.
Does 26 degrees BD seem a bit short? Hadn't paid attention to that,
should it be around 30?

Will try the spacers. Thanks Big B!

 

[Big B NRG]

You should see it this way. (atleast that's my vision of it)

Pri. tranfers open --> fill the cylinder with fresh mixture
Sec. transfers open --> more cylinder filling
Boost tranfers open --> flushed the mixture away into exhaust.

Exhaust gives pulse and sends mixture back.

Then your piston should look something like this:

Exhaust info you are reading are written in the 70/80 years. The basics are still the same, but alot has changed. Also Bell wrote his artikel for "non-vario" bikes, that also is a difference.

Timing mainly depends on your exhaust icm with cylinder filling.
Exhaust timing of a 190 degree can do 14k RPM. (Speed2 cylinders etc).

BD you have for the carb you use is good. The lower the BD, the more time you have for filling your cylinder. BUt it also means your `powerband´ kicks in early at low RPM.

Making your BD higher might give you extra power on higher RPM.
Your average BD is 28.25 btw. (tranfers and boost together).

 

[StepVino]

Big B, you have given me a lot to think about. I have been
using my spare time today doing calculations, and haven't
touched the scooter.

From what you say, my engine should be set up adequately
to reach 12k. I was thinking about the battles before I replaced
the stock exhaust.. whatever I did, it wouldn't go past 8k (except
downhill). This now seems like the same situation, so maybe I should
forget the engine, and just look at the pipe.

I have been figuring out the difference in length of the pipe, and
how that difference would have to be distributed between the
header pipe and center section. Will post the calculations and
hopefully, succesfull results. There is one number though, that needs
to be fairly accurate... the speed of sound in the pipe. Bell says
it averages at 1675ft/sec, or 510.5 m/s.. does that sound right? He
doesn't give an average temperature. I can do some more research
on that.

I like those pics. My piston looks like the bottom of an ashtray .
From the cuts at the top of the piston, looks like you have your
transfers open early too? What does the small ramp going into the
right cut do?

Have you come across this paper?:

http://naca.central.cranfield.ac.uk/reports/1940/naca-tn-756.pdf

The pattern on your piston looks similar, but a more developed form,
of the flat piston configuration, with all intake ports angled up at 45
degrees. Interesting!

Main page, with more stuff is here:
http://naca.central.cranfield.ac.uk/results.php?Simplequery=two-stroke&st=AND

I like reading the old literature.

Thanks again for your input!

PS What is exhaust icm?

 

[Big B NRG]

Exhaust "icm" mean "in combinatie met" it's a dutch shortword. It means in combination with, sorry for that.

The cuts on my piston was just some experiment, didn't work out well.
That factor speed of sound is oke. Because you are working with hot gasses and angles on the exhaust "power" that gasses.

Your scooter can do 12k rpm, but does it give power there? Like you have now, your scooter climbs in RPM the faster you go.

Also if you could have a exhaustgastemperature meter it would help a great deal on finding the right angles/lengths. It should be mounted around 10 cm / 4 inches from the end of the exhaustport /begin exhaustflange. For sprint/high rpm cylinders it should be around 600C. for street scooters I don't know, abit less I suppose.

Steep short conusses give peaky power at higher rpm. Long and "flat" conusses give more spreaden power at lower RPM. You should be somewhere in the middle.

Another thing you could do, is buy a few other exhausts and try them on your scoot, measure them up and see where the difference are.

For now, forget the cylinder and try some flanges and maybe other exhausts.

Btw, you can polish the top of the cylinder so it looks like new again. Then drive for around sometime (30 min or so) and pull your head off.

 

[StepVino]

Hi again, I was able to put in a bit of work on the scooter
today. After a few calculation, considering only the length of
the header.. seemed like taking off about 12mm should be safe.

I was planning on just a couple of hours, but I did not like
what I found in the pipe, specifically its construction.

I tried looking at the FAQ page, to see how to include pictures,
but it's all in Dutch .

 

[Big B NRG]

Hi again, I was able to put in a bit of work on the scooter
today. After a few calculation, considering only the length of
the header.. seemed like taking off about 12mm should be safe.

I was planning on just a couple of hours, but I did not like
what I found in the pipe, specifically its construction.

I tried looking at the FAQ page, to see how to include pictures,
but it's all in Dutch .

Give me the part what you need to have translated and I will translate for you

Or the FAQ you want to have translated.

 

[StepVino]

Thanks B, I was looking for the FAQ that explains how to
post pictures.

I cut the first weld after the flange. Instead of an even
taper, the Next R has smaller diameter section welded onto
a larger diameter. When I cut the weld, I found that the
smaller diameter pipe actually extends into the larger one,
creating a large step inside the pipe. What seems even worse,
is that the second section of the header has the curve, and
the first section (from flange) does not follow it for the short
~10mm section that it sticks into it. This seems very bad for
the reverse pressure wave into the cylinder.

To answer some of your suggestions, my next purchase was
going to be a cylinder head temp gauge. Have to try and keep
this low budget for now. However, I do realize that at this point
it's like trying to squeeze a dry lemmon, so I should really have
more gauges.

I will definetely try cleaning the piston, driving it, and posting
what it looks like after a few miles.