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PostPosted: Sat May 18, 2019 3:24 pm 
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That's the easy part. :shock: :? :lol:

I'll punch those in later and see what the output files look like.

That file has a really fat pipe and super high compression, so I'll compare it to other GT380 data on file. He also assumes a 29mm carb and very mild ports. BMEP is rather low but with those timing figures that's not a big surprise.

The simple answer for more power is to replace teh bike with a larger engine, but let's assume that you want to keep the 380 for now.


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PostPosted: Sat May 18, 2019 5:05 pm 
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jabcb wrote:
Have no expertise in porting so this is more of a question than advice.

Doesn’t the RD250 have reed valves? And wouldn’t those reed valves impact optimal porting specs?
If the answers are yes, than would the RD250 porting be applicable to a non-reed valve GT380?


That's a great question. If the RD had been designed in say 2010 to work with big volume pipes, that would probably have been an issue, but in reality the RD 250/350 is basically a DS7/R5 with a reed box stuck on the back. Porting was pretty basic back in the day.


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PostPosted: Sat May 18, 2019 9:43 pm 
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That data looks familiar and appears to be created by "motoworks" for his "special". Oddest looking pipes but MOTA optimized them that way for some reason.

It's a pity you can't modify the transfers, but that's not necessary for a mild tune.

If possible, raise the exhausts by 2.5mm and widen by 3mm. Drop the intakes by 3.0mm and widen by 2mm and fit GT550 carbs and decent pipes. Raise the compression and back the ignition off to 22 degrees max. I don't have a set of barrels in front of me so cannot guarantee that you won't break through into fresh air, so take it slowly and keep checking your work.

Try that and see how it works for you


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PostPosted: Sun May 19, 2019 2:28 am 
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Thanks tz
Yes, you are correct that is the data from Motoworks creation. My curiosity was mainly how to interpret the information.
I don't need or want anything as radical as he states. When younger it was only pure power I was interested in now, other things take precedence.

That file has a really fat pipe and super high compression [/quote]
I don't think any larger than what I already have will fit - My pipes are 100/110mm diameter at the parallel section and compression has been raised by milling the head 1mm (this done 20 odd years ago). I don't think I will be going any more than this. I also has the 550 carbs which is the option of choice.

Does anyone know if the port parameters were changed by Suzuki? The dimensions I have from a port map of my 76 model differ to the 75 model.

Cheers
Carlos


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PostPosted: Sun May 19, 2019 3:30 am 
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I'm almost 100% certain that Suzi never changed the port dimensions and positions on the 380. They did on the 250 - the 250A model went from 2 transfer ports to 4, and also got small auxilliary exhaust ports, but the 380 motor was never developed in the same way.

_________________
1976 GT380 - wounded by me, and sold on
2006 SV650S - killed by a patch of diesel and a kerb in Feb 2019
2017 SV650 AL7 - naked and unashamed


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PostPosted: Sun May 19, 2019 8:28 am 
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If your ports are different to a stock 75, perhaps it was ported at some time in the past.

Could you post a copy of the port map?

Back to your question: what do all those numbers mean?

The port dimensions con rod length etc at are input by the user, along with carb and duct dimensions. That takes a while to collect and enter. In that example, MOTA X optimized the exhaust, so the system created those dimensions. Normally you would enter that data from your exhausts after you measure them. In this case, the user optimized for power at 9,000 revs and that's fine for a rider that likes to hear it sing all the time. For a bike that gets ridden more often, it might be more appropriate to optimize for torque at lower revs.

The system calculates everything else. First table is power and torque in metric and old units.

Next chart is ignition timing as entered by the user. That determines when peak pressure happens.

Next is Pressure. What I look for there is BMEP and where it occurs. That reveals how efficient the motor is and where it's most effective. It's a easy to compare that to other charts to see the effects of a change.

The next chart shows fuel efficiency which is not very good. We'd like to see around .65 pounds of fuel per HP or slightly higher. This one suggests a lot of fuel ends up going down the pipe.

That is confirmed by the next table which shows efficiency and the amount of fuel that goes down the pipe.

The chart after that is cylinder pressure and pipe temperature. The pressure shows us that the pipe works best at 9,000 revs but exhaust temps are all low - maybe because there is excess fuel in the exhaust gas.

And the last chart is the Specific Port Time area which decreases as revs rise. That dates back to Jennings et all in the sixties.

Most of the data is not used until the tuner starts to compare one set of parameters with another. So for example I compared that set of data with stock ports and with different options. I could see not only a HP chart change but I could look at the charts and see which change made the difference. To me, those pipe dimensions were too large for any normal street use, so I go back to a more normal pipe and see if I can get the same or more power from other combinations of port widths or opening times.

What you don't see in that document are the other MOTA outputs such as power and torque curves and it also creates wave simulations at any chosen RPM. That way you can see temperature spikes in the transfers and won into the crankcase at the end of blowdown or pressure pulses that are hurting cylinder filling from the exhaust waves and so on.

For our old bikes, it's a great tool and its value is in comparing one idea with another. It's a lot cheaper than making and testing dozens of exhausts or hundreds or port combination. Power is theoretical at the crank and in the real world we rarely get to exactly the same answer and that's probably because of inaccurate data.

Where it's really useful is to compare say early GT750 with later 750 ports and then change port timings from one set to the other - one port at a time to see the relative change. That's what revealed that the inlet port timing contributed to most of the drop in low end performance in later motors and showed clearly how it tipped the power curve.

In the end though, we run lots of numbers and then at some point we have to start cutting metal to test our ideas.

One thing to note is that production tolerances back in the day were pretty poor in the ports and just cleaning them up and matching them can often make a worthwhile difference.

We all have our own ways to approach a motor. One is to start with 90 degrees exhaust opening and 30 degrees of blowdown and that matches stock GT380 transfers I believe. Check out AG Bell's book for more ideas.


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PostPosted: Mon May 20, 2019 2:33 am 
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Hi to all,

Herewith a copy of the port map for my GT380 76 (A model). Unfortunately it doesn't include the transfers.
Unless I have read the MOTA print out incorrectly my dims seem different to theirs.

Can someone please post the standard port map to allow me to have a safe base to start from should mine not be standard.

Thanks
Carlos


Attachments:
Porting Map GT380A.jpg
Porting Map GT380A.jpg [ 64.06 KiB | Viewed 705 times ]
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PostPosted: Mon May 20, 2019 6:34 am 
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Hi and thanks for all the feedback - greatly appreciated.

tz375 wrote:
If possible, raise the exhausts by 2.5mm and widen by 3mm. Drop the intakes by 3.0mm and widen by 2mm and fit GT550 carbs and decent pipes. Raise the compression and back the ignition off to 22 degrees max. I don't have a set of barrels in front of me so cannot guarantee that you won't break through into fresh air, so take it slowly and keep checking your work.

tz375
Do I understand this correctly - Where you state widen the ports is this dimension given as a total and not on each side and would this be measured on the curvature of the bore or as a straight dimension (chord length)?


sportston wrote:
ground the inlet tract fork until it was razor sharp

Sportston
In what way would this modification help?

Cheers
Carlos


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PostPosted: Mon May 20, 2019 7:35 am 
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You are correct that those port dimensions are different than a stock early model. The exhaust has already been lifted by 2mm and widened by 2mm and inlets are slightly wider than stock too.

What does the finish of the ports look like close to the liner? Stock are not very smooth and often have small gaps at the liner plus the finish is as cast. If ported, they will be smoother and will look more "finished".

I would just clean out any carbon and run them as they are if they have been ported, and if you want a little more top end raise the exhaust roof another 1mm.


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PostPosted: Mon May 20, 2019 8:45 am 
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I will need to check when I’m next home towards the end of the month.
Can you please let me have the standard port dimensions or a port map as I have a 75 (M) model motor at home and can compare those to the stock dimensions.
What are the chances of them both being touched?!

Cheers
Carlos


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PostPosted: Mon May 20, 2019 2:03 pm 
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A while back I measured the port dimensions on a standard M / 1975 cylinder. They were not done with a micrometer or anything, so assume the accuracy is plus or minus 0.5mm :wink:

All measurements are the distance measured from the top of the cylinder liner (i.e. where the cylinder head gasket sits).

Inlet: bottom is 88mm, top is 65mm (port height 23mm)

Transfers: bottom is 57mm, top is 43mm (port height 14mm). Transfer width is 26mm (measured straight across, not "around" the cylinder wall). Just one transfer port each side.

Exhaust: top is 33mm, width is 30mm edge to edge (measured straight across, not "around" the cylinder wall). The mouth of the exhaust port where the header pipe mounts is 36mm internal diameter.

As Richard / TZ said, someone has raised the exhaust port 2mm already on your cylinders.

_________________
1976 GT380 - wounded by me, and sold on
2006 SV650S - killed by a patch of diesel and a kerb in Feb 2019
2017 SV650 AL7 - naked and unashamed


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PostPosted: Tue May 21, 2019 3:17 am 
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I’m surprised that the PO would have altered the exhaust port and not the inlet. Possibly a common practice I’m not aware of.

Taking your stock dimensions into account there is a 2.5mm difference between your dimensions and mine to the base of the inlet.
Even taking casting and production tolerances into consideration I feel this to be excessive even back in the day.

I will remap and re-measure both sets of cylinders when back home and advise of my findings.

Thanks for all the feedback.

Cheers
Carlos


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PostPosted: Tue May 21, 2019 12:40 pm 
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CFS wrote:
Sportston
In what way would this modification help?

Cheers
Carlos

Not an expert in this field so I may be showing my ignorance, but I thought it might allow the gasses to flow a little faster if it was not hitting a fat obstruction on its journey to the piston. So I made it so it has a more gradual splitting of the gas-flow. When gas has to change direction it slows down, so why not make it easier for it to change direction by reducing the curve a little? e.g. Compare Concorde's nose to that of a freight plane. Simple aerodynamics, I thought.
Like I said though, I am no expert in 2-stroke port dynamics, just a general grease monkey and diagnostic technician. So I am happy to be proven wrong. I realise that port dynamics and gas flow is highly specialised and there are many factors that determine the efficiency and performance the engine other than flow speed. I was just tinkering at my own expense.


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PostPosted: Wed May 22, 2019 7:50 am 
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That's an interesting question, that we often fall for. Air actually flows better around a round object than a sharp pointed one. Look at wing shapes. The leasing edge is round and the trailing edge is more pointed.

When air hits a sharp edge it splits the flow like a V and consequently flows less air than a round leading edge.


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PostPosted: Tue Jun 04, 2019 5:58 am 
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tz375 wrote:
What does the finish of the ports look like close to the liner? Stock are not very smooth and often have small gaps at the liner plus the finish is as cast. If ported, they will be smoother and will look more "finished".

I checked the finish and it seems that the 76 barrels have been touched as the ports are pretty smooth. On the other hand the 75 barrels have definitely not been altered (rough casting). Transfers to both sets of barrels have not been touched.

Below are the dimensions of the port mapping I undertook to both sets of barrels and find it surprising that the 75 port dimensions differ to Craigs by quite a bit. Even taking fabrication and casting tolerances into account I find this excessive.

Please note that the widths are the arc length (measured along the bore cure) and not the chord length (straight dimension.


Cheers
Carlos


Attachments:
Port Map Dims.JPG
Port Map Dims.JPG [ 60 KiB | Viewed 501 times ]
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