GT250A transfer ports versus piston height at BDC

[tz375]

Richard, just for information issues, does piston skirt overrun the intake port roof at TDC in a GT750? This is common issue in T500 engines, e. g. intake port roof is 79mm from barrel top, piston skirt height is 75mm.

There's a GT750 motor on the bench with a 3mm spacer and the skirt is just showing by say 1-2mm so on a stock motor, the skirt would be just above the roof. That's a pretty small sample size on which to extrapolate to all motors of all years and this one has been ported, so the roof is probably higher than a stock set of barrels.

I would speculate that a stock motor has the piston 2-3mm above the port roof at TDC

[tz375]

Tz375,
The port does stay open longer when elongated either way you do it, but your point about no real pressure at top means very little change in Blow back which is my concern.
Thanks

Let me try that again as my last explanation may not have been clear. The port opens when the piston uncovers the floor of the port and then the piston rises to TDC and closes as the skirt again cover the port floor. It make no difference if the port is 10mm tall or 40mm tall. Raising teh roof does not change port duration.

Raising the barrels will make for a shorter duration because it raises the roof and delays the time at which the port starts to open.

Cropping the skirt or lowering the port floor, both extend duration by making the port open earlier and close later.

Lifting the barrels AND cropping the skirt OR lowering the floor back to its original position can be used to increase the duration back to stock. That may be what you were referring to.

Tell me fully about the T350 Robert Knippleburg (sp?) aka Dr.Q mod. I have this bike too.

My bad. Two different stories.
The Dr Q change was to a GT750.
The T350/305/315 happened a long long time ago in a galaxy a long way away. That bike belonged to a racer friend and he raced as a stock production bike. It looked like any other old Suzuki apart from the fact that it looked brand new. But it had a thick spacer under each barrel. I was not involved in the modifications. I just transported it and took the opportunity to take a blast up the road and back.

If you have a port map of the GT250 I can enter some ballpark data and simulate taller intake and /or exhaust ports and see what a spacer plate does to the curve. The answers will not be 110% accurate but will indicate which way to go.

[Vintageman]

Tz375,

You take your time to explain your answers, Thanks you. I may not be clear or mature my responses as I am learning


"Lifting the barrels AND cropping the skirt OR lowering the floor back to its original position can be used to increase the duration back to stock. That may be what you were referring to." I just can't figure how the quote work

Yes,

I believe I understand port timing is referenced to the bottom edge of intake and symmetrical for piston ported engine. My concern is raising top still creates some blow back, but as you noted little due no charge built up yet.

My understanding why the bottom rpm power drops away on piston ported is “blow back”. If engine was a rotary valve or reed vale for example no concern.

Figure 2 of 1966 Yamaha article emphasizes that point for rotary intake versus piston port

Since GT250A already has that feeling (bottom poor), I was concerned of messing it up. The GT250A is blast to drive, I can’t drive it slow. It does seem like it should have some more top end. IMO Suzuki did OK making it more fun (criticized for killing bottom end) while still allowing a 12k mile warranty by limiting stresses caused by higher RPMs.


I would like to take your offer on a T350.

I sure would like to see the port map for GT750 to compare with T500.

[tz375]

...............Figure 2 of 1966 Yamaha article emphasizes that point for rotary intake versus piston port
.....

Do you have a link to that article? I couldn't find it. Thanks

[Vintageman]

Tz

In my "More Porting Stuff" post a couple below this one

Here it is too

http://edj.net/2stroke/jennings/" onclick="window.open(this.href);return false;

It is the "some Development" pdf.

It's old, Just barley thinking of reeds, They also thought 1966 scavenging is good enough.

But those three figs gives real measured data about piston port intake and exhaust timing affect and versus rotary.

Thanks

[tz375]

That one. Thanks. really old data but the principle is the same.

If you look at the way that those YDS3 curves tilt, GT250 does basically the same thing but peak power is higher and tilt point is around 6500 RPM. I took a basic motor with some assumptions and kept all other variables constant, which of course isn't strictly accurate. For example the port time are increases with duration, but I kept that constant and just changed the time of the event. To be more accurate the area changes should also be included.

Nonetheless, the curves look just the same. The difference at peak was 6hp and the difference at 4,000 was 5 crank HP in the other direction.

Being a small old 250 it doesn't have a lot of bottom end or top end to give up, so any tuning has to be well thought through to avoid a huge hole in either powerband or pistons .

Bimotion has a good tool to calculate target port time areas from the target BMEP and revs and it calculates the actual or projected time area and that enables you to see the bottlenecks and to determine a plan of action. I suspect that it's transfer area on a GT250

[Vintageman]

Yes, I liked the concept of intake and exhaust of that old article and the graphs give you a pictorial explanation I had not seen before. I wish there was more data/graphs for the 70’s bikes like this too.

Your 6 HP peak more for GT250A vs. YDS3 makes sense.
GT250 4 HP less at 4000 than YDS3: probably true too.

What did you show for Peak RPM for your estimated GT250A port model?

The YDS3 turn was just over half way of its peek.
6500 RPM seams high or the top end is week

My goal with GT250A is to raise peak RPM by 10%... if straight forward.
I’ll be tearing down GT250A soon
I paper trace it and repost what I find. Save your simulation please and thanks for doing that

I'll have to see how much that software is... Interesting stuff.

[tz375]

Yes, I liked the concept of intake and exhaust of that old article and the graphs give you a pictorial explanation I had not seen before. I wish there was more data/graphs for the 70’s bikes like this too.

Your 6 HP peak more for GT250A vs. YDS3 makes sense.
GT250 4 HP less at 4000 than YDS3: probably true too.
..............

Once again my explanation has caused confusion. I was trying to explain that on a GT250 when I ran simulations of a range of different intake timings, the range from worst to best was 6hp at the top end and 4 hp at the bottom end on that same theoretical GT250. I created a rough pass at a GT250 and then I ran the simulation software for each different intake timing. Long duration made more top end and lost more at the bottom end. The shortest duration made most bottom end and least top end power.

I have no data for a DS3 to compare them. The other thing to keep in mind with simulation software is that it is far from perfect and only tracks to real world when all the parameters are really close to being right. What the software does well is to show up changes that should be expected from changes. It's a great tool to see the effect of changes but not to use as a predictor of real world absolute performance.

[Vintageman]

OK you got me looking at software

How does one find rod length for

GT250/T350/GT380 (must all be the same)

T500 and GT750 (must be the same)


If anyone has port map 750 please post

[ja-moo]

Back a bit to the top of the intake port being covered at TDC by the piston skirt, it is basically the same as the transfers. By TDC most all of the flow has taken place, so the importance is very low. Especially on a large port as the velocity is lower.

Piston ports are just tougher to tune for performance, as was said, more duration is necessary for crankcase filling at high rpms, but the later port closing increases blowback, messing up the carburetion. And why reed and rotary valves work better in spreading the powerband.

[Vintageman]

Piston port induction sound is cool

I like intake sound stock H1

http://www.youtube.com/watch?v=buga91RLVHg" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

or R5
http://www.youtube.com/watch?v=DofJuSg6wxc" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Gt250A
http://www.youtube.com/watch?v=90TLzYEkecA" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

YDS3
http://www.youtube.com/watch?v=wnRLCb9Fwds" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Even a piston ported SAAB
http://www.youtube.com/watch?v=s9nYbtB-rKA" onclick="window.open(this.href);return false;

If you remove air box baffles, cover, or add pods, the sound can make your ears hurt particularly when wearing helmet and the sound sneaks under . Man that smarts when it resonates just right.

I am thinking a minor tweak to either intake or exhaust port(s), or raise jug on these street tuned engines if not greedy can only help even if not perfect balance or ultimate tune.

I found rod length for Gt250 to be 110MM (T305 should be the same )
No luck finding T500 rod length yet?

I want to start just by finding what stock port timing is and compare to other piston port 250cc twins to see how close they compare.
Then enter port dimension info into

[ja-moo]

I kinda like the sound of a reed motor myself........

http://www.youtube.com/watch?v=l_XEdxi89Wo" onclick="window.open(this.href);return false;

[tz375]

That K machine sounded smooth, but lacks the character of a finely tuned piston port motor

http://www.youtube.com/watch?feature=pl ... JNrVqKLYRA" onclick="window.open(this.href);return false;

[ja-moo]

Sounds nice, but has that "UUUUGGGG" sound of a piston port waiting for the revs to catch up.

It's funny how we differ on "sounds" we like. I never liked the BBBWWWAAAAA intake when a piston port was gassed out of the powerband. But you guys do. And I can't stand the sound of open stingers or loud chambers, but other can't get enough of it........

[tz375]

I only posted that video because it's one of my bikes I hate that empty waaahhh sound as the intake opens its mouth and nothing much happens for a while. Mine is much more responsive than it used to be, but I prefer my TZ twins with reed top ends. They sound sharp and responsive.