GT250A transfer ports versus piston height at BDC

[Vintageman]

OK I am on a role. Too many things, but here is something I found interesting or never noticed before on other engines as I ready my 76 GT250A

The piston when at bottom dead center, I found the edges of the crown still very much above the side transfer ports and above the extra ports added to the 76/77 design. I am remembering 1/8 inch maybe more for both. It was maybe only 0.025" above the exhaust port bottom at its the center only. The extra ports look like the pointed up so not so much loss maybe, but the side transfers, aim accross piston more and that seams to be significant transfer port time loss

Did Suz have a hipo version of this design where maybe these tranfers ports bottoms aligned better with piston crown?

It looks like there is potential here if one only shimmed under jug a bit and shaved head some for example.

It is about to go back together and thought I ask about in case a little can do a lot.

[tz375]

JA-Moo can comment on his Kawasaki experience and others may like to chime in, but the bottom of a port flow very little to no gas in the case of transfer and exhaust and it's better to have the port floor lower than piston edge to ensure complete scavenging.

It also means that you have more effective area than if the port were a perfect match to the crown with a radius in each corner. This way the corner radius is zero mm.

[ja-moo]

One of the big tuners through the years did some testing, and found the transfers flow better for some reason with the port bottom below the piston. And yes there is very little flow left at BDC.

[ja-moo]

One of the big tuners through the years did some testing, and found the transfers flow better for some reason with the port bottom below the piston. And yes there is very little flow left at BDC.

[Vintageman]

Make sense. Thanks

[Vintageman]

Looks I am not the only one considering this
see
http://www.macdizzy.com/1989bhopup.htm" onclick="window.open(this.href);return false;

He raises the cylinder to leverage the fact they are lower than piston.

I have since also reviewed some discussions and agree "experimentally” better if a transfer lower. But never seen supporting theory yet (which I enjoy )

Maybe if you are looking for Hop up, raising these is more of a gain versus what you loose from them not being lower than piston, maybe not either (I suppose this is a question).

[tz375]

Close but no cigar...

What that does is to raise the ROOF of the transfer ports to gain more time-area. We do the same on a GT750 with spacer plates and then lower the transfer port FLOOR again to about the same place as it was. In fact what MacDizzy says is that if he were going to modify the ports, he thought he could do it by simply raising the whole barrel to take advantage of that step. Had he done that, in all probability the motor would have made more power than stock, but would have made even more power if he then lowered the port floors back to where he started.

Exhausts and transfers are all about the Roof for timing and intakes are all about the floor.

We have to be very careful when we apply "Logical deductive reasoning" to 2 strokes. What we rationalize as an outcome is often not what is actually happening.

[Craig380]

Heh

Speaking of the 'black art' of tuning 2-strokes, I always liked this quote from Walter Kaaden (and he should know) about expansion chamber design: "You will know when you have the correct dimensions, because it will then be impossible to fit it to the machine without it dragging on the floor, burning the rider's leg, or forcing the relocation of at least one major component."

[tz375]

And that's a man that would have known. A modern version of that is " You will know when you have come up with the optimum port design for an old two stroke, because it will have broken through into fresh air, the water jacket or another port"

My previous comment is because so often we rationalize how we think something must work based on prior experience in some other field, and so often it just does not apply. In some cases it's simply because other factors we should have considered are far more important than the ones we included. In other cases, it's because port flow patterns are not as we might expect and can only find out the answers by exhaustive experimentation and research.

[Vintageman]

It is so much easier to lower the transfer for no casting there like top

To retain intake port timing you could simply shave bottom of piston skirt: same as lowering intake port floor.

But now the intake top is even higher than stock Raise jug height + shaved skirt amount

Any negative here?

What is interesting (for my ego ) MacDizzy, at the time of this writing, did not understand why transfers are lower and thought like I at the start of this post.

[tz375]

Well, that page has been up for a number of years now and to be sure, none of us knows everything. We are all learning.

So, raise the barrels say 1mm (just for an example) and the port floors and roofs all raise by 1mm. In addition, 1mm needs to be removed from the top of the barrel to maintain compression.

If necessary and desirable, it's relatively easy to lower the exhaust and transfer port floors so that they are still below the piston crown.

On the intake port, if the piston skirt is now 1mm below the roof of the port, it's OK to trim that off the skirt. In addition, it is now possible to determine if that particular engine will benefit from a raised intake floor and shorter duration. If so, port the floor to get the desired timing and duration. On other motors, the skirt is so far above the intake port roof that a 1 or 2mm spacer will not expose the skirt at TDC.

Of course not all motors need more exhaust or transfer duration, and others benefit from a 3mm spacer plus additional exhaust porting. It's horses for courses as they say.

That's why we measure everything and analyze the existing status, develop targets and explore what is needed to achieve those targets. What revs do we want the red line? How much power do we want? How wide do we want the powerband to be? How wide and tall do the ports needs to be to achieve those targets and is there enough metal in all the appropriate places. And so the quest for more power continues.

[Vintageman]

If one does raise barrel and then decided to retain opening timing two choices:

Port intake port in barrel or Shave skirt.

But there is a difference is what I am wondering. The two choices give the same opening time, but as piston comes backs down from TDC the port is effectively open longer when the skirt is shaved.

I am thinking on piston ported engines you get more blow back in shaved case. And, the fact you raised the barrel in the first place to achieve the higher exhaust and transfer (versus porting them) you raised intake port top so already some more blow back.

This is a trade off, but If I am learning it all depends which ports were week or out of balance to start. You could gain, loose, or break even with this method: for me this will by trial and error: try in steps (or leave well enough alone).

Regardless, I am going to loose bottom end with raised barrel approach due to blow back, even if it proves to gain top

Maybe some more research ->

I know GT250A has the extra transfers, but I am going to compare it with the earlier GT250 to see how the intake and exhaust port compare.

[tz375]

Intake timing is solely a function of when the bottom of the skirt passes the floor of the port. How tall the port is makes zero difference to the timing and that's the key. Let's say we were able to raise the barrels say 6mm (that's not sensible - it's so large that it's easier to imagine the consequences)and we left the skirt the same and lowered the floor by a matching 6mm. The inlet timing would not have changed.

What changed was the roof is now 6mm higher and if the piston skirt is NOT obstructing the port at TDC, we have a 6mm taller port for more area, but with the same timing. So any potential blow by as the piston comes back down towards the bottom of the stroke will remain the same. There is little real pressure under the piston until it comes down a fair way. QED no change to the reversion pulse. We do have slightly more port area which might help a little at the top end but will not hurt the bottom end. Think of it as like regrinding an exhaust cam but leaving a stock inlet cam in a 4 stroke.

But what if the piston does hang down into the port at TDC? Well then we have a choice to either trim the skirt until it's clear and then work out how much, if any to adjust the floor.

Raising a barrel will not change the "blow back" as you called it. It will of course change transfer and exhaust port timing and you need to determine if the engine needs more timing there. On the T350 (315cc IIRC) the trick was to slip in a 1/4" spacer and mill the tops to suit and the one I rode like that was smooth and fast and completely easy to ride on the street. I have not looked at an early GT250.

Robert Knippleburg (sp?) aka Dr.Q got that right with his spacer and porting ideas years ago. Unless the motor needs more intake time, do not lower the floor more that stock despite all the old seventies articles suggesting that it's the way to go. Longer intake timing does help at the top end, if your motor needs that, and it decreases power below about 5500. The power curve basically tilts or rotates. If the motor is inlet constrained, the top improves a lot, if it's not, there's little gain and big loss at the bottom.

For any motor you have choices:
You can do what they used to do, or you can analyze your motor and use modern technology to save a few sets of barrels and end up with a better result.

On your GT250, as a rule, a set of barrels with more transfer port area has the potential to make more power than an engine with less transfer port area.

[argo1974]

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.

[Vintageman]

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

I don’t want to change intake timing if I raise the GT250A: just put it back to stock

The 1966 (yes old) Yamaha Article from my other post figure 4 and figure 8 are still applicable IMO.
Figure 4 intake timing changes for the YD3 250 “rotates” the power band as you say. In that case pivot is at ~3250 RPM, but that bikes peek is only 6000- 6500 RPM: low to start and varied little though out changes. This change snuffs out bottom and improves mid to top end without really increasing RPM

The GT250A to me feels tuned like that, but, pivot is at ~4000 RPM. Gets real dead as you drop below that too. So I think intake is not the week port for my objective. But, it does not rev too much higher than earlier GT250 model, so I am thinking the exhaust would prove beneficial for my goal.

figure 8 showed me you can better extend peak power with exhaust timing It reduced both lower and mid some, but not a pivot effect, I am going to compare the GT250A, with a DS7 and GT250L.

Tz375
Tell me fully about the T350 Robert Knippleburg (sp?) aka Dr.Q mod. I have this bike too.

For example Did you lower the exhaust port and transfer port floor as well to retain as it was stock? Any more specifics on this please.

I don’t have software and I am not looking for a lot of gain… why I am being real careful and asking a lot of questions.
Also a proven example would be perfect (i.e. T350 Dr. Q)

I have not been able to find the GT750 port map yet. If you remember which it was let me know. I want to compare it to my T500