Transfer Port Modification- Pictures and results here.

I did this mod to my engine. I noticed a increase in top end power and a decrease in low end power. I may experiment with a longer intake runner to see if I can get some of the bottom end back. My engine also has been port matched, lowered intake, raised exhaust ports. I also did away with the head gasket and lapped the jug and head. I don't know how this mod would effect a stock engine. I used a good quality sawzall blade clamped in a pair of vice grips to do the cutting. Then I cleaned it up with a dremmel. It would be very easy to ruin a jug by gouging the cylinder wall while doing this mod. Be careful.

seca40 said:

I did this mod to my engine. I noticed a increase in top end power and a decrease in low end power. I may experiment with a longer intake runner to see if I can get some of the bottom end back. My engine also has been port matched, lowered intake, raised exhaust ports. I also did away with the head gasket and lapped the jug and head. I don't know how this mod would effect a stock engine. I used a good quality sawzall blade clamped in a pair of vice grips to do the cutting. Then I cleaned it up with a dremmel. It would be very easy to ruin a jug by gouging the cylinder wall while doing this mod. Be careful.

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someone else was talking about this in another thread. i don't really see the advantage. the piston already stops before it hits the bottom of the transfer port, so the flow change would be virtually unmeasurable. i would think all this would do is reduce ring friction.

Hey guys,

The guy who owns the shop next door builds F1 race boat 2 cycle engines. I've got an engine apart, and will get him to mark up the cylinder, case and piston for any modifications he deems appropriate.

I'll do it based on his recommendations and then show him the parts before reassembly to make sure I got it right.

I know quite a bit about 2 cycle tuning, which is about 1% of what Brendan power knows. If we're going to modify these engines we may as well do it right the first time.

Here's a link:

Brendan Power

Jim

Sweet. It'd be nice to get a pro tuner's opinion on these little motors.

In addition, it might be beneficial to describe the common mods we do to these motors and see if he thinks any might be pointless, could be done differently, etc.

i mentioned before that i was talking to a 2 stroke authority who used to race GP bikes back in the 60's. and i brought the transfer ports up to him and he told me for now i should leave them alone. but depending on the resulting flow characteristics he would probably recommend that we bevel the Piston top to have a smoother flow from the transfer port.

the exhaust is on the left and the transfer is the center port.
and as you can see this picture is of the piston at BDC. so by opening up the entire port i dont understand what is gained? the transfer port still stays the same size where it opens.

That makes sense. But isn't the bottom portion of the transfer port severely restricted as is? I don't have a motor in front of me right now so I can't really check on which way the air flows.

jasonh said:

That makes sense. But isn't the bottom portion of the transfer port severely restricted as is? I don't have a motor in front of me right now so I can't really check on which way the air flows.

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i was told they were fine. but im sure they are another part that when changed will only change the power band, which makes sense. open= higher RPM power because of flow and when shut they create more Torque. but it seems like you could do enough porting to get them open without compromising the cylinder wall.

runslikeapenguin said:

i was told they were fine. but im sure they are another part that when changed will only change the power band, which makes sense. open= higher RPM power because of flow and when shut they create more Torque. but it seems like you could do enough porting to get them open without compromising the cylinder wall.

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This is exactly what I got.

i talked to the 2 stroke guru i have been emailing and he told me that opening the ports up like this is not necessary, and that it was only a means to an end. the same results can be obtained with a combination of match porting the cylinder to the crank case and opening it up a little bit as well as beveling the piston head. but he mentioned that besides the port work the biggest problem with these engines was the head. he couldn't believe how bad the head design is. he mentioned that its our biggest limiting factor, and that before people start opening up boost ports, balancing cranks and building expansion chambers we need to work on getting a new head made for these engines, or finding one from another 2 stroke that will work.

anyone know a good CNC machinist?

Bikeguy Joe said:

That is exactly why I made this thread....the original member who deemed this mod the "shizzy" (my words, not his) seemed a little dubious at best, and I didn't think it was a great way to go about the end result.

A hacksaw, indeed!

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Yea i believe my comment on his thread in addition to the others sent him over the edge... he even sent me a pm. I agree with others that this mod isnt worth the trouble Later -Justin

Thanks for the info i almost took it to a machine shop--saved lots of dough

runslikeapenguin said:

i talked to the 2 stroke guru i have been emailing and he told me that opening the ports up like this is not necessary, and that it was only a means to an end. the same results can be obtained with a combination of match porting the cylinder to the crank case and opening it up a little bit as well as beveling the piston head. but he mentioned that besides the port work the biggest problem with these engines was the head. he couldn't believe how bad the head design is. he mentioned that its our biggest limiting factor, and that before people start opening up boost ports, balancing cranks and building expansion chambers we need to work on getting a new head made for these engines, or finding one from another 2 stroke that will work.

anyone know a good CNC machinist?

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I've been busy the past few days and haven't had time to go next door for a Q&A session. I do know it's usually subtle things that make a world of difference.

What did he say to do to the head? I thought a hemi-spherical design was good? The compression ratio is low, but what else needs to be done?

Did he tell you how the chamber should be shaped?

I can make one if I know what it should be shaped like.

Jim

Creative Engineering said:

I've been busy the past few days and haven't had time to go next door for a Q&A session. I do know it's usually subtle things that make a world of difference.

What did he say to do to the head? I thought a hemi-spherical design was good? The compression ratio is low, but what else needs to be done?

Did he tell you how the chamber should be shaped?

I can make one if I know what it should be shaped like.

Jim

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well for one the compression is un-naturally low for a 2 stroke but when i brought up the issue he said that a bump in compression would be ideal but not to go nuts. normally built 2 strokes have a compression ration in the ball park of 15-17:1 and above depending on the use and where the power band needs to be. but because these engines have to run over a super wide powerband being only one speed if the compression is bumped up to what normal 2 strokes are the engine will have a lot of issues reaching high RPM's and running smoothly at high speeds. seeing as our compression ratio is around 9.2:1 on the 70cc he said to just take any casting inperfections off of the base and leave it at that until later, when your all finished with the bike and any other performance mods that your planning and work on tuning the compression as you ride to find where it needs to be. he also mentioned to see if you can get a handfull of head gaskets in different thicknesses so you can tune without having to machine the head each time.

but the problem with the head is the squish band. basically there's none. and thats not good. when you run an engine like up to high RPM's one of the bigges problems is making it efficient at those RPM's. your engine may be able to spin up to 14,000RPM's but can it make power that high? some people have been playing with balancing cranks and intake and exhaust mods but to really be able to make power at the newly found RPM's the head needs to be completley re designed. there are a ton of reasons in particular but its mainly the fact that there is no squish band at all. if a head was made with sufficient squish then that would open up a whole new world of tuning. the same power that boosting the compression would yield would be avaliable to you without the limiting factors that it has on the engine operating speeds and it would not only allow for better gas mileage and bottom end torque but it would make tuning easier and would help avoid fouling plugs.
yes a dome cylinder head is what you want but if you cane effectivly compress and chanel the intake charge into that area you just defeated your advantage there.

to give you a little bit of an idea about what im talking about above heres a little picture.
you can see here where the lip of the head is in the cylinder. that squish band is basically non existent.
the green mark is an estimation of where our squish band is and the blue is more where it needs to be maybe even farther. that and what little squish we have is completley flat and not really ideal for combustion chamber flow.

I read an excellent article on the best way to design a head for a two-stroke. If I have time I will search for it, I wish I would have saved it to my favorites.