Auxiliary transfer port modification!

Yes! Auxiliary transfer ports! Now, I'm not going to start celebrating just yet, because I'm still waiting for some parts before the engine can run. I provided a bunch of pictures showing the details. This type of modification has been done before and proven to work quite well. If it works in this engine has yet to be seen. Of course, I'm happy with an enlargement of the transfer ports, but I'm especially in favor of this kind of modification because it might actually make the engine more reliable. The reason I say this is because the auxiliary port is fed through a hole on each side of the piston, as shown in one of the pictures. Cool air and fuel will be flowing deep into the underside of the piston, cooling it and also possibly providing the wrist pin bearing with better lubrication. If this proves to be a winner, I will make spares which others may purchase. The cool thing about this modification is that it is possible for someone with a hand drill and a die grinder/dremel tool to do this particular modification. Although, I must warn that this enlargement of the transfer ports gets close to where the end of the piston rings travel. If the machining is not exactly in the right place, it could be bad news for the piston ring if the end is not always supported by the cylinder. There are two styles of 66 cc piston. This modification does not lend itself well to the GT-5 engines which use a piston that has a wrist pin hole location close to the piston ring grooves. The GT-5 piston does not leave enough room for the "feed hole". I don't know what else to call it. So, it's interesting, I guess the early engines use the low wrist pin location, and the new GT-5A and GT-5SR engines also use the low wrist pin location, so that's pretty good news if this all works. Well, of course I'll keep you all posted on this!!

-Fred

Even though it seems like there is little interest in auxiliary ports here, I thought I should post the results for the modification in case someone else is thinking about a similar modification, as it might save them some time and money. The modification did not seem to help, in fact, I noticed a decrease in power, both in low speed and high engine speeds. It still could be possible, but this particular way did not work. Back to the drawing board!

-Fred

Hi Crmachineman, do u think maybe u didnt get increased transfer flow due 2 slightly lowering the already low crankcase compression ratio, as u have increased its volume by the amount of both feeder ports in the piston & the ramps below the transfer outlets. Just a thought. Anyway, cheers

I might be able to explain why it decreased the power... Think about what those holes in the piston are now doing; They're forcing air & fuel back DOWN the ports for that brief moment they are open to them instead of helping push the airflow up & into the cylinder.

I've had great success with carving a third intake port & transfer hole in the piston. (This will only work with a reed valve).

Ahh, you have done that on the intake port! very clever! Yes, I was thinking about something like that too. Works only with reed valve, eh? Are you sure it must have a reed valve to work? I very much like the idea of that modification you did.
Anyway, the first cylinder I tried this experiment with is going into the scrap bin. I'm trying another attempt with a new cylinder. $$$
Ivan, I've read some studies from a leading Yamaha engineer who says that worrying about crankcase compression ratio is not as important as many of us think.
In the first attempt with this aux. transfer port thing, I enlarged the transfer port a lot. I think that may have been a mistake because I believe it's important for the air/fuel mixture should come out of the transfer port at a high velocity, so it can properly fill the cylinder. When I enlarged it so much, I think I reduced the velocity, causing poor exhaust gas scavenging, and possibly a bit of short circuiting. I am currently experimenting with a different mod. on a new cylinder. It's working better that the first one, but I think I can do better. I am going to be very careful not to enlarge the transfer port too much. The engineers of this engine seem to have placed a small restriction at the exit of the transfer ports. I believe that was entirely intentional, to increase the velocity of the air/fuel mixture entering the cylinder. Instead, of opening up the transfer port, I want to try to feed it a little more, through the piston, so the wrist pin bearing will have better lubrication, and possibly giving the engine slightly better performance. The big performance increase will be from the modification that the guy from Venice motor bikes mentioned. I'll just have to buy a few more cylinders. Good thing these parts are so cheap. Just imagine doing these kind of modifications on an expensive motorcycle engine! I really appreciate all of the insight and theories you guys have brought to my attention because, let's face it, these 2 stroke engines have some really complicated dynamics. Understanding them can lead an engine builder to amazing performance!

-Fred

Yes, the boost port (u can c it surface cut into the cylinder above the port) needs a reeded intake. They can b cut behind the transfers without reeds. They go from the height of transfer "tops" all the way to the bottom when done behind transfers & u need 2 make sure they're in front of the ring ends. When done above intake port with reeds u keep the roof at no more than a 5 degree angle 2 direct mixture up at the rear of the cylinder head so it dont just go out the still open exhaust port. Cheers

When done behind the transfers they will cause a loss in low & mid performance, only coming into play at the top end. Cheers

I have recently added RSE reeds, unfortunately its rained ever since so I aint even rejetted yet. I'll c how it goes as is first, then take a few mm off the intake port floor & piston skirt adjacent 2, Port the piston so the motor intakes thru 180 degrees, boost port the barrell & add a feeder port 2 the piston, 2 c how much of a difference it makes. Cheers

Alright! That sounds great! I can't wait to hear how it goes! It seems that carving this port above the intake port has a distinct advantage of being far away from the exhaust. Also, if the fuel mixture must travel through the piston in order to get through the carved port, it will benefit the piston and wrist pin by improving cooling and lubrication.
Very cool!
-Fred

Hi Crmachineman, yeah, with most of the mixture intaking & 1/4 transfering thru the piston its gotta help lube the small end. I probly use 2 side by side ports like old Yamaha's did. The restriction in transfer outlet u mentioned is towards the front right. Its 2 direct the mixture 2 the rear of the cylinder so it loops around & fills the chamber from rear 2 front. U can add fillets inside the transferrs 2 aid this & increase transfer velocity, which low revving motors seem 2 like. Cheers

Hmm, creating fillets, or in other words, rounding sharp turns, right? I will certainly try some different mods. to transfer ports in the near future, and will look at that possibility. Well, at least my bike is running really good, ever since I put a new exhaust system on. I'll maybe post some pics. of it soon. I made it from a Kawasaki dirt bike chamber that I cut up into several pieces, then welded up so it fits the bicycle. It was working so well, it broke the chain when I was flying at 40 MPH. I cannot EVER use one of those cheap chains on this bike. It's the second time a cheap chain has broken on me.
Talk to you later.

-Fred

U reshape the leading edge of the transfer runners from below the outlets up 2 the roofs so that the discharge is aimed 2 the rear of the cylinder, across the piston crown so that it loops up the rear of the cylinder wall to the head, then forward & sweeps any remaining burnt gas out the exhaust port. U angle them so the discharge from both transfers intersect each other no further forward than the centerpoint of the piston crown. Cheers

U'll notice that on these, the discharge from the transfers intersects forward of the center of the piston so u get mixture exiting the exhaust port, which an expansion chamber will cram back in there, but its not scavenging spent gasses as well as it can & an Xchamber may not b able to get all the lost charge back into the chamber. Every little bit helps. Cheers

Ha, yeah the stock chain isnt the best. I broke 1 when my jeans (ripped leg bottom) went into the chain at around 50k's goin round a roundabout onto the freeway 1 night. Locked everything up & broke the chain. The ragjoint leaves a lot 2 b desired on these 2, & the tensioner is a scource of vibration 2. I went 2 SBP shiftkit a while back & love it. They add a bit of resistance, using 6 sprockets, 3 bearings & 3 chains in total, but well worth it. Cheers

Hey Fred ! Are you out there ? It's been along time since you started this thread, Come Out! Come Out! Wherever you are ?
We ALL Really need some of your latest developments on reed valves and the set for the cylinders.

Come back to us!

Once i take some pics i will post my piston vent port mod. It does work quite well and helps cool the piston.

I did mine a little differant though ... i copied the 25 cc zenoah viper piston

I drilled 2 holes in the side of the piston above the wrist pin on the exhaust port side in line with the stock transfer port then carved a 1/4 in groove from that hole in the piston to the wrist pin hole then drilled out the wrist pin.

This allowes the underside of the piston to vent into the transfer port and also to vent through the wrist pin cooling the whole mass of it without changing the stock transfer port setup.

I will have to post pics its hard to explain

Send some pic's PLEASE! I am Going To Be building a Really Really High Performance Engine and I need all the Knowledge I Can Get.

And Have A Happy New Year!

truckd said:

Send some pic's PLEASE! I am Going To Be building a Really Really High Performance Engine and I need all the Knowledge I Can Get.

And Have A Happy New Year!

Click to expand...

If you're looking to make really Really Really good power, read up on some Gordon Jennings and Graham Bell's 2 stroke guides... Pay special attention to the port timings and pipe design... that's where a lot of power is to be had.
Roger is also very right about the piston mods, they work every bit as good and will enhance a good porting job whether it's a simple piston ramping job to get more port duration and some help redirecting the transfer flow or cutting windows in the side skirts to add more duration or to keep the piston cool enough to prevent self destruction... Read all you can about porting, get yourself a dial indicator and make yourself a way to mount it so it reads the top of the piston, then make yourself a degree wheel, either by googling an image of one and then printing it on card stock or printing on regular paper and gluing to a sheet of plastic, or even using a small plastic protractor with degree markings... you'll need something to show your crank degrees.

You'll need a good dremel and get a set of carbide burrs off ebay, and a set of diamond coated burrs for the fine finishing stuff, a set of needle files, a set of sanding rolls, and some emery cloth to do the cutting and smoothing.

Most of all, Read Read, Read, and ask questions if you're not sure before you start cutting. Get some spare cylinders and pistons in case you mess up because practice makes perfect.

I got a ton more torque, power, and rpm out of my engine after porting and setting the port timing.

An SBP pipe or a Snake pipe installed and tuned after the porting is done adds a LOT more power too, but the pipe needs to be tuned to the engine's powerband that was set by the porting to get the most out of it, tuning the pipe is the easy part tho and it does pay off quite well.

Just read all you can and find the books I mentioned online or find someone to PM and email them to you. Once you get the idea of how things work, don't be shy about asking... People like Roger, myself, and quite a few others will be more than happy to steer you in the right direction. There's a Lot of power hidden in these engines so read and ask... especially before you buy any "performance" parts or try to do any cutting on your own. The other part of making this power is making the power but keeping the engine safe from it's self while doing so. Read and Ask, we'll explain why when and if needed.