# Transfer Port Theory

Discussion in 'Performance Mods' started by jaguar, Dec 17, 2012.

1. ### jaguarWell-Known Member

Factors affecting transfer flow: Transfer area, engine displacement, crankcase pressure.
Here's the formula for flow velocity:
Velocity (m/s) = Volume flow (m3/s) / area (m2)
Air/fuel velocity through transfer ports increases with greater air/fuel volume flow thru the same port area (which happens when you modify the engine with a larger bore), and decreases with greater port area to channel the same air/fuel volume (which happens when you enlarge the transfers or add boost ports).
Here's my theory on how air/fuel velocity change is related to powerband change. First off, increasing the crankcase compression ratio (CCR) increases the pressure that exists in the crankcase right before the transfers open. More pressure increases the volume flow and thus the velocity. So there are 3 things that can increase transfer velocity; enlarging the cylinder bore, reducing the transfer port area, and increasing CCR.
Why is velocity important? More transfer speed means the air/fuel charge will reach the spark plug faster. You want the flow speed to match the type powerband you want. The scenario most likely is that the end of the transfer charge arrives at the spark plug (when it fires) at the lower rpm of the powerband, and the beginning of the charge is at the spark plug at the upper rpm of the powerband. You can slow down that arrival time by changing the transfer ports roofs to be more level. That directs the flow so that there is more of a horizontal vector and less of a vertical. More of the flow energy is dissipated in the left and right flow collision with more horizontal expansion of the combined charge.
What is interesting is that small engines need less percentage of the cylinder bore circumference than larger engines. Here's an example to prove it: A 45mm bore and stroke engine has 1590mm bore area. The circumference is 141mm which is 9% of the 1590. Whereas a 90mm bore/stroke engine has 6361mm bore area. The circumference is 283 which is 4.4% of it. So if the two engines have the same percentage of circumference for transfer port horizontal opening the 45mm bore engine has twice the flow area. If they have the same percentage of the circumference (say 50%) then that means the small bore engine needs less CCR to achieve the same transfer flow velocity. Maintaining the same CCR allows the small engine to be more adept at high rpm running than larger engines.
I have always been perplexed at why the designers of my small cylinders (48cc, 55cc, 60cc) have only one transfer on each cylinder side and utilize a low CCR. Having narrow transfers strikes a balance with the low CCR. Both can be small just because it's a small engine which means its transfers are automatically more efficient.
Boost ports
Since the flow from boost ports is mostly vertical with no collisions then it arrives at the spark plug even faster than the transfer flow. I had experimented with boost port size and determined 30% of the transfer port is ideal. How some engines get away with huge transfers is beyond me. Maybe that is just a fad, and in reality the engines could run stronger if their boost ports were narrowed.

Last edited: Dec 19, 2012

2. ### FabianWell-Known Member

Jaguar, i like your theory and enjoy reading about it.

Please give us more!!!

3. ### jaguarWell-Known Member

If the engines are "square" then the 45mm one is 72cc and the 90mm one is 573cc. Let's double check my theory:
If the large engine has 50% of the circumference for the transfers and the boost ports then the horizontal opening of them totals 141mm. That divided by the bore area equals .022 (2.2%). Applying that to the small engine (.022 x 1590) you get 35mm total horizontal opening. 35 divided by 141 = .25 so that only 25% of the circumference is needed to have the same ratio as the large engine. Now if we figure in the port height in comparison to the engine displacement: small engine 8mm port height, large engine 16mm port height. 8 x 35 = 280 square mm which divided by 72cc gives a ratio of 3.9 to 1. For the large engine 16 x 141 = 2,256 square mm which divided by 573cc gives a ratio of 3.9 to 1.

4. ### jaguarWell-Known Member

Also there is less distance for the intake charge to travel to the spark plug with small engines, another factor that gives reason for less CCR and flatter transfer roofs.

5. ### Old BobMember

Read Jennings,Gordon,Bell and a few others.

You can not simply scale down ratios from larger engines.Also you forgot to consider the effects of rpm on velocity and the time duration the ports have to accomplish their job.

6. ### jaguarWell-Known Member

Bob, what do you think I did? get drunk and then make this **** up? I have studied my ass off for the past 1.5 years as well as done my own research because all info available is still very lacking. Why don't you fully understand what I've read and then give me a good comeback with details from your own emphasis comparing a small and a large engine? It's too easy to pretend that you are in-the-know but it's hard to prove it. Give me details, a technical example. You can't say "you can't do that" without showing why (with details).
If I am wrong I will admit it, no problem. I would like my fresh analysis to help improve the world of two stroking and if others can help me refine my ideas then that is fine. Sometimes I correct my own self. For instance, the increased velocity from increased crank pressure is most likely only significantly greater at the beginning of the port opening. It would take a direct measurement to reveal the whole truth. But the CCR doesn't just affect the transfer velocity, it also affects the intake velocity. More CCR has the effect on the intake of making its movement more "crisp" because the increased vacuum allows less lag. My next real life test will show me if it is able to affect low rpm power as well.
There are so many factors involved for an absolute complete understanding that it's mind-boggling. But I'd like us all to have a good understanding of the most basic factors and some generic rules to follow.

7. ### jaguarWell-Known Member

I did not forget "rpm, velocity, port duration".
The background assumption between the two engines is that they both have the same peak rpm and the same port durations. (How could an honest comparison be otherwise?) If the percentage ratios between the piston area and the horizontal port open area are the same then the flow velocity is the same because the large piston travels at twice the velocity of the small piston but has twice the distance to travel between port opening and BDC. So the port duration time remains the same for both engines although the transfers are twice as high in the large engine. (everything in the large engine is 2x the small engines.) In the end the velocity would be the same because the transfers would have the same proportional area to piston displacement in both cases and the same port duration.

Last edited: Dec 22, 2012
8. ### Old BobMember

You are wrong.
You are assuming too many things, in the real world frictional loses and boundary layer effects will change the flow characteristics.

Read up on flow in a duct.Read Jennings about time areas.

I'll pit my 30+ plus years of study in addition to my dyno and flow bench experienceand building both four stroke and two stroke engine from raw material to your 1.5 years.

Trust me read up on this stuff before drawing conclusions.

Last edited: Dec 22, 2012
9. ### jaguarWell-Known Member

"frictional losses and boundary layer effects" is all you have? c'mon man, get real. prove to me in some kind of real example how I am wrong about the relation of transfer port widths to cylinder bore. It's just basic physics. It's the port area being a percentage of the bore area. Are you going to tell me that that is not important? You going to tell me that if a small and large engine both have accumulated widths the same percentage of the bore circumference, and that leaves the large engine with transfer areas as smaller percentage of the bore area that it don't matter? WTF? think, man, think. get out of the box (the limitation of what has already been written). Progress means building on top of the past, not being limited to it.
I have pointed out something that no one else has ever done and recorded for future generations to see and you are upset that it can't be possible that someone with so little study time under his belt could do so. Maybe you haven't seen my web site. Click on my signature link. I also have figured out completely about how expansion chambers work and how to accurately, using calculated return waves with an Excel file that took me months to make, analyze and design pipes. It's totally breakthrough. You gonna throw a fit about that to? Some people are forward thinkers and some people want to only defend the past. It's obvious which one you are.
The time-area idea for ports is obvious, again being basic physics, but the formula Jennings gave us from the Yamaha engineers was only accurate for the Grand Prix engines they were working with. If in your 30 years experience if you had ever played around with it you would know that and wouldn't now be throwing it at me as if you have some kind of valid point.
You keep telling me to read, but it don't seem like you know how, otherwise you'd know from what I've already wrote that I have read all that's available and then some. As an example, I brought the idea to this forum of intake extension for piston port intake engines, to increase low rpm power, from an obscure research paper I stumbled across. Crap, I bet you haven't read Blairs stuff. I barely made it through because everything he wrote was for engineers familiar with many complex formulas. You remind me of 2door, the dumbazz moderator on the other forum that had never even heard of using JBWeld in ports to modify them although famous 2 stroke gurus had been doing it for decades. 2door had lots of experience under his belt and thought he was at the pinnacle of knowledge whereas in reality he just knew a few things really well and had a very closed mind. Like I said, give me an argument with details. I'm waiting...

Last edited: Dec 22, 2012
10. ### jaguarWell-Known Member

Bob, you just shot yourself in the foot. I just looked at the classic port time-area formulas and they have the port areas in direct relation to the engine displacement. That's exactly what I am doing, and as a side issue have proven how small displacement engines don't need ports as wide (as percentage of bore circumference) as large displacement engines do. In the 3rd post I gave details proving my point. So don't throw "time-area" at me. As I said (jeesh, how many times do I have to repeat myself?) the "time" for both engines is the same, and the area (as a percentage of engine displacement which is what Jennings advocated) when it has the same percentage relation to displacement for both engines results in the small engine needing ports narrower (as percentage of bore circumference) than the large engine.
I gave details! I proved it!

11. ### Old BobMember

I'm not giving you anything, you don't know what you are doing or talking about and misleading the noobs that read your misguided theory.

There is no link in your signature.

Take a look at a 20-30cc modern 4 transfer port two stroke used in various applications such as gopeds. Shoots your theory down doesn't it?

12. ### jaguarWell-Known Member

The link to my website is right there in plain sight.
All I'm saying is that ports are more efficient on smaller engines. The fact that certain small engines use 4 transfer ports is of little importance to this conversation because port area is only one factor. The advantage of doing so is to slow down the transfer velocity. The smaller the port, the higher the velocity for the same initial pressure. again, basic physics. Having a healthy crankcase compression ratio of close to 1.5:1 is advantageous for maximum drawing in of air/fuel charge. Smaller engines have less distance for the intake charge to travel to the spark plug and so need means to slow it down. Having larger transfer ports accomplishes that. But the designers of these chinese engines wanted minimum power and so utilized low CCR which allowed them to utilize the simplist design for transfer ports, that being just two. We can change the design for increased power by stuffing the crankcase for increased CCR and adding boost ports.

13. ### Old BobMember

Nope don't see any link.

I'll say this again, you CAN NOT SIMPLY SCALE DOWN A LARGER DESIGN.

I can see why you aren't welcome on the other forum.

14. ### jaguarWell-Known Member

yeah, you refer me to Jennings but his time-area formula shows that the port area is in relation to engine displacement, no matter what size engine.
please tell me, in detail, why I can't "scale down a larger design". The only reason I compared the two engines was to prove that ports on a small engine are more efficient.
Your attitude is very much in alignment with the Nazi controllers of the other forum. That's where you belong. This forum allows people to share new insights and viewpoints that have never been considered before. Any person of any intelligence knows that knowledge is an un-ending road that leads to many varied sights. To say that everything that can be known about 2 strokes has already been discovered is very narrow-minded and stifling.

Last edited: Dec 23, 2012
15. ### jaguarWell-Known Member

Although small engines can achieve a better port area to engine displacement ratio with the same percentage of bore circumference as a larger engine that doesn't necessarily mean small engines should be designed with small ports. There are other factors to consider, especially the CCR and the intended powerband.

16. ### lazylightning@mail.rActive Member

Hi Bob,

Nice to meet you. Alot of us here are trying to learn how to make some improvements to our engines performance and this fellow Jaguar here has done alot for us. I didn't see the link to your page with tuning recommendations.
Though some of us may have some learning to do, I doubt anybody on this forum would like the word noob being thrown around in any ole direction it might happen to fly. I suppose that means any of us here without 30 years of experience in motor tuning. Noob is one of those words that would make one look younger than 50+ though, so I suppose it's tempting to use it, even if having not learned any good manners when coming to tech chat with some strangers. Or maybe your really younger than you present yourself.
Actually here's the link to Jaguar's page: http://www.dragonfly75.com/motorbike/index.html There's alot of really useful information there and it's a darn shame you haven't taken into account all the hard work he's put into it to help us out here.
Presenting a theory after all, is just a theory. He didn't claim to be absolutely correct and has shown himself as reconsidering previous ideas when evidence shows otherwise. Now I think that he's being open minded and ready to accept scientific facts as he finds them.
It seems to me your statement deeming his theory incorrect was not built on an argument. You just stated it and accused him of misinforming us. Wow! How about some formulas?
So these little engines usually do only 5000 - 6000 rpm's at most. If they have had bearings changed, balance work done, carb, pipe and basically everything, then maybe you'll get 8000. It might blow at 9000 though. So, it seems that velocity isn't that much of an issue with these. If it were a Franco or Peugeot 103 or something, then maybe so.
Please be kind enough to build your theory or case in more detail. It is beneficial to us all.
And I can't understand your need for rudeness. This guy is proven as a good guy. Why just go around offending people. Please build your argument with intention to show him the error of his ways instead of coming off so high and mighty over all the noobs and anybody trying to explain their theories.
Please don't take offense, it's just very important to keep an atmosphere of good manners and friendliness here.
Again, it's very nice to meet you. Please feel free to contribute to our collective knowledge of maxing the low rpm HP out of these little teaser engines ;-)

Cheers,
Paul

17. ### FabianWell-Known Member

@ Old Bob

One thing is for sure. Jaguar has brought forth a needed product to the market place, being his 2-stroke CDI for Chinese 2-stroke bicycle engines, and that can't be taken away from him.

Old Bob, I would like to see you make a youtube video demonstrating and refuting Jaguars theories using your flow bench and the data that comes out of it, also setting up a few different cylinder and crankcase combinations and running them on the dyno to prove things one way or the other.

The old saying: the bull-s.h.i.t stops when the dyno rocks, but what we really need is a video to prove it.

18. ### FabianWell-Known Member

Oh yes, i have also been banned from the other forum, as have many others at the hand of that dipstick moderator 2-door

19. ### Old BobMember

If there were valid engineering principles being discussed my attitude would different.
Thats why hes banned for life from the other forum?

Can you say plagerism? anyone can cut and paste...

http://www.dragonfly75.com/motorbike/forums.html

This is the garbage that he posts about people who disagree with him.

20. ### FabianWell-Known Member

I am not only banned for life on the other forum, but for a further 3 reincarnations of my future lives. 2-door had a hissy fit when i advocated 25:1 oil/fuel ratio in relation to the low quality castings of the Chinese bicycle engine, when used at high duty cycle in a low airflow environment.
I cited extensive research from Suzuki Corporation back in the 1970's when they were experimenting with leaner oil/fuel ratios and the associated piston scuffing at anything over 35:1

2-door couldn't accept that research and i was banned for not just one life, but many future lives.
Ok, it didn't help my cause when he advocated opti-2 oil at 100:1, and i called him a d.i.c.k-head for giving out such advise.

So, does that mean i am also qualified to be a f*&kwit because i'm banned on the other forum?