Ht Performance Porting Success

[210061741]

Ok if you have been following things here you know i recently tore down my 48cc Gubee GT-2B motor.

Before assembly i decided to try some modifing of the transfer port area in the cranKcase.

This was my theory ......

The area of the transfers in the case is fairly restricted due to the casting and the way it is production machined.

Alot of material restricting the flow from the crankcase through the transfer.

Removal of material and modification of shape could greatly reduce the restriction ( allow the intake cycle to breathe better and give it a better chance of sucking more of a charge in the amount of time produced by the duration of the intake timing ).
--------------------------------------------------------------------------
Plus side
Removal of this material will not change the port timing.
This makes it less succeptiable to error and less of a chance that i would destroy the engine with this experiement.

The transfers in the Cylinder will ultimatly dictate the maximum amout of flow through the engine. This means even if i went a little nuts with it the transfers will compensate cause the flow restriction would occour there in the cylinder.

Therefor the least restricted and easier transfer into the ports should proove to be a bonus.
--------------------------------------------------------------------------
Down side
Lots of careful time consuming grinding and shaping would be required.
This took me about 2 hrs per side.
Also to do what i pictured certinly would increase the crankcase volume by several CC's.
Really we don't want to do that but later we can add some filler somewhere in the crankcase to compensate if nessessary.

At this time i havent figured out what the BMEP of the CrankCase should be nor do i know it's exact volume.
If someone out there does please chime in.
--------------------------------------------------------------------------
So i had to decide if the potential gain was worth the risk of ruining the engine. As this whole idea was bassed of off my theory which seemed to follow along with all of the studying i have done about porting.

As i had allready wrote the engine off and convinced the wife i needed a new one which she agreed, I figured to give this nutty idea a try.

Now i didnt do much to the transfers in the jug, and nothing nothing at the top of the transfers in the jug.
I really reccommend unless you have a cnc to machine the transfers from top to bottom don't touch them.
You won't gain much anyway there unless the whole transfer is machined and if you change the angles at the top of the transfers you could blow your intake charge right out the exhaust.
-------------------------------------------------------------------------

So what i did was grind out the entire step in the crankcase and round and smoothe and blend it all as nicly as possable to get the transfer area under the transfer ports larger than the transfer ports and promote flow without restriction.

Then i attached the jug to the crankcase half that i was working on and blended the crancase and transfer ports so ther were nice and smooth.
No steps anywhere. Just nice even flow.

I took a huge amount of matterial off each side being careful not to thin out the wall too much at the jug / crankcase seal.

--------------------------------------------------------------------------

I had some trouble with my magneto but i got it running yesterday.
I cannot believe the difference in power.
Now my low end seems to suffer a little due to the increase in volume of the crankase.
Only the very low end. And only until the engine warms up.

Overall this mod rocks.
I gained alot of rpm's more torque everywhere except the very very low end and it runs soooo smoooothe.

I't is worth the time.

But remember i run an expansion chamber and a boost bottle.
Increasing flow in one area but restricting it somewhere else won't help.
Everything has to work together in harmony.

I imagine with a good carb that could be finly tuned you could really get the baby pumping.

Check out the pictures and if you feel up to it give it a go.
I don't think you will be dissappointed.

 

[craisin]

sweet
If one could imagine the total mass of what one ground off and then attach the equivilent mass in the form of a small plate inside of the crankcase,

 

[210061741]

The amount of mass removed isn't as important as what the optimum psi in the crankcase should be.

There is pressure created in the crankcase by the piston and to a large degree the pressure in the crankcase will dictate what the max ouput of the motor can be. this is called BMEP "Brake Mean Effective Pressure".

So the proper thing to do will be to accuratly measure the total volume of the engine determine what the BMEP is and then adjust the volume of the crankcase to give optimal PSI.

This i will not be able to do until all of my porting is completed.

Now if you have followed my 1500 mile teardown thread you will know that i put the old not so good cylinder and piston back in.
The jug i have not ported yet.

So i plan to order a new upper end kit this week.
I will take the new cylinder and do a port map.
I posted another thread where you can find out how to do a port map.

Once that is completed i will know the Timing and Duration of the Exhaust and Intake.

With all the data that will be gathered through that process i'll be able to calculate the BMEP and have some mathematically calculated adjustments to use as guidelines for porting to get an optimal RPM and Torque from the motor.

Since i will have new parts on hand i will start with the old ones.
Cut them slowly a little at a time.
The piston can be cut to some degree to change the duration and port timing.
I will keep going little by little and keep track of the results, keep pushing and pushing to the max until i cut it too far and get negative results.
At this time the old cylinder and piston will be no good. "oh well"

Now after anaylizing all of the results i'll know exactly the measurements needed to cut the new cylinder and get the very best performance possable.

At this time the engine will be as good as it can get.

Then i'll have to start with the 66er but hopefully by then i can get a Grubee GT4 High Performance 66cc.

I am very satisfied even with the porting that i did.
I was passing cars riding down the road today.
I run a single speed 36T.

Wish i had a jackshaft.

 

[210061741]

geeze i figured you guys would have alot to add to this.

Guess i'm just not that popular

 

[AussieJester]

If i was playing with one of these china motors i would be cutting the skirt on the piston also, we used to "short stroke" 3.5cc motors in the 1/8th scale on road race cars using this method when i was a teenager, they would absolutely screeeeeam touching 40 000 rpm... I think the biggest performance 'upgrade' would be fitting a reed valve setup to these motors have you thought of this 210061741?

KiM

 

[210061741]

Yes that is a major flaw in these engines?

At tdc the intake port is blocked partially by the piston.
Also at BDC the port is partially blocked.
This mease restricted flow.
Bad

To cut the bottom of the piston is a common practice.
It will change the timing too.

First i want to do a port map and see if it is even possable to get the proper timing and duration with these HT jugs.

Figure out the optimal and try to achieve it.

 

[Quenton Guenther]

Hi 210061741,

When I raced Yam, Kaw, Bultco, & Carabella [Mexico] we always stuffed the piston to reduce the crankcase volume with great results. On most motors we were able to only cut the rear of the piston skirt approx. 1 MM without major low end losses. I don't know if 1 MM would be way too much on a small motor, but we cut the pistons on motors from 100 CC to 360 CC with the same results.

Have fun,

 

[210061741]

So how could you stuff the piston without adding much weight?
Interesting concept though.

 

[Quenton Guenther]

Hi 210061741,

I think we used epoxy, and mostly used Wiesco piston which were often lighter than stock. Chances are the stuffing simply made the Wiesco piston closer to stock weight. I do remember the piston didn't need much to decrease the volume.

I always worried the stuffing would work loose, but it never did.

Have fun,

 

[210061741]

I was thinking epoxy in the rear of the crankcase
Or even better.
Remove the steel plate flywheel weights and replace them with larger and lighter aluminum plates.

It would be interesting to see if the flwwheel would still have enough momentum for the motor to idle good.