2PG performance (with math! Yay!)

[clr.R4tch3t]

I am going to start documenting my findings in my engine build. I am using a stock crankcase, .50 mm over piston, stock jug bored out to spec for the new piston, new gaskets, new bearings, new seals, and a whole lot of luck to keep this thing from exploding.

I will attempt to answer my own questions as well as yours with mathematically backed responses and hopefully this will be fun and informative for anyone looking to get a little pep out of this Robin engine that so many people love. I am a mechanic by trade and have countless projects. Why not add one more? I have also ridden dirt bikes my whole life (2 stroke has always been favored) but I have never gotten into the theory behind the mechanics in a way that would allow me to make educated decisions on how to modify one. Everything has always been stock or a name brand bolt on performance modification. I believe it is time that I change that norm. HOT Carl is going to be my unwilling participant.

Let's break some stuff.

[clr.R4tch3t]

We can start with some basics. What gives you more power? That can most simply be answered by how efficient your motor can pump air and how fast it can do it. The biggest limiting power factor to a 2-stroke motor is piston speed (limited by the material that it is made out of). In broad terms, most motorcycle sized pistons are limited to a median speed of 4,000 ft/min. This is approximately the same as traveling from a dead stop to an excess of 120 miles an hour and then returning to a dead stop (all in 2.3622" in our case), and that is impressive. The little Robbin does not get that crazy, I assure you. Average piston speed is as follows:
Cm=0.166 x L x N
Cm is mean piston speed, in ft/min
L is stroke in inches (2.3622)
N is crankshaft speed in RPM

With this we find that the factory max suggested RPM of 4k puts us at 1,568ft/min, well within the green zone. 6k RPM = 2,352ft/min, and 8k RPM = 3,137ft/min. It is suggested to stay below 3,500ft/min for a reliable engine, anything above 4,000 really puts you into the twilight zone on the brink of failure. Our Robbin is considered a 'long stroke' engine and is therefore constricted to a lower RPM range (longer stroke= faster piston speeds at lower RPM range). Sadly, we won't be able to turn 11,000-14,000 rpm with this stroke... But we don't want to die either, do we? Of course not. But we must remember, there is no substitute for revs. We will get way more power from more RPM than we could ever hope to get from port tuning the jug, so we must push the envelope a little.

Excessive piston acceleration. We don't have to really worry about this, but here is some fun info. When a piston moves too fast it causes the piston rings to float (known as ring flutter) and that causes all kinds of bad things. There are a few ways to battle this, mostly ring design, but I won't get into it. My factory rings and my .50mm over rings are all 0.057" thick. Rings this thick will tend to float around 88,000ft/sec^2. The equation to find where we are in relation to this is as follows:
Gmax= N^2 x L ÷ 2189 (1+ 1÷2A)
Where Gmax is max piston acceleration in ft/sec^2
N is crankshaft speed in RPM
L is stroke in inches (2.3622)
A is ratio of con rod length (between centers) to stroke

I do not have the con rod length handy, so I can't run numbers on this one just yet. If you know what it is, please feel free to fill me in. Otherwise, I will just do some research and hopefully I can find it (I don't want to split my case again just to measure it).

Once these mechanical limits are known, you can then design and build your engine to maximize power for your target RPM.

[van the man]

When does class meet again?

[my1423]

The problem is gearing.
These engines provide a wide mellow powerband at low rpms..
Dirt bike 250cc regularly hit near the 50 hp range, but those engines have almost no power or torque at low rpm, witch is where these engines spend most of their life.
You need power to start right off idle.

You can lighten the clutch weights way up and get the rpms up into the power band, but then you will smoke belts left and right.

Side by sides and some sleds tackle this issue by making the belt not slip at any rpm. They use a separate centrifugal clutch to slip and get the rpms up prior to movement.
This pulls the heat and wear off the belts.


You want more power, most everything i have read says,
Shave the head. Most seem to recommend .025.
Or shave more and add a shim under the jug to change port timing. That is pretty involved on getting the shim and shave to match to get more power and keep the rpms low.
Port polishing.
Clean the pipe.
Good large air filter.
Rejeting to match mainly for altitude.

Saw a few posts where people milled out the stock reeds and welded up the case to take large soft power reeds. Thats a little outside of most peoples abilities but really cool.

Fancy pipes dont really work for the rpms we need.

[clr.R4tch3t]

I am going to attempt a good grunt bottom end torque while also spinning faster RPM. I am reading tons of papers to try to get the best over all grasp on building this motor. Heck, by the end I may agree and stick with something that maxes out at 5k to save my bottom end torque. I can definitely see a problem with smoking belts, and that I am going to take in consideration. I want this to be something that 'anyone' can feel comfortable tackling and know why they are doing what they are doing. I find it very hard to do something 'just because someone told me that this would work'. I want to know why and I want to decide for myself if I really need the modification that is being recommended. I think shimming a jug is easy enough. I think handing a jug to a machine shop and telling them to shave off X amount from the top should be easy. Shaving the head can technically be done at home, but I will probably use a mill (also can be done at a machine shop). Yes, some of these mods will be too much for the guy or gal that wants everything handed to them on a silver platter. The important thing is that if the average fuel burning junkie wants to get a little dirty and spend a few weeks in the garage over the winter they can have some fun the next spring.

I am not sure how often the class installations will come.. I am still balancing life on the homestead, 2 boys, 50 hour work weeks, and whatever the wife wants to throw at me. Haha. But I do have to get this cart up and running soon so I am ready for this drift season (it is our pit cart and a necessity) and that is coming up fast. Hopefully this all goes as well as I am envisioning!

[van the man]

Weight might be a good place to gain some power. Ive noticed some components seem a little heavy, overbuilt (suspension, axle, tires/wheels). Even the body panels could lose a few lbs.?

[clr.R4tch3t]

I think there is enough room for growth in the engine that we won't have to worry about dropping body weight.

Now we don't have much choice on piston design or ring choice (for now, I haven't looked very hard. Let's make oem work for us). We will assume that we don't need to worry about ring flutter, because we won't be turning crazy RPMs. The Robbin uses an 'upper side keystone type' ring profile. This is an improvement over a standard rectangular ring, so that is a bonus. It keeps the ring weight down to help battle ring flutter. Compression gasses push on the top of the ring and push it into the piston groove making a gas tight seal. That pressure also pushes on the inside of the ring to keep the gas tight seal against the cylinder wall. Compression gas plus ~30psi outward pressure of the ring is enough to make magic happen in the combination chamber. I have included a drawing of a ring properly installed on a piston.

Compression Ratio. This is something we can fairly easily modify. The formula is as follows: CR=(v1+v2) ÷ v2
CR is compression ratio, v1 is cylinder volume at exhaust closing, v2 is combustion chamber volume. The factory manual says 6:1, but I am not sure if that is full stroke ratio or exhaust port closed ratio. They are fairly different, so I will measure what I have and report back.

With a good squish band cylinder head (what we have, how 'good' is yet to be determined) up to 9.5:1 is good for a motocross engine with a wide boost range and no substantial peaks. Generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of bore area) combined with a 9:1 compression ratio is ideally suited for low to mid range power. This is what we want. This head design will generate high turbulence in the combustion chamber. This turbulence is termed maximum squish velocity, MSV is rated in meters per second (m/s).

There are some dangers to raising the compression ratio. If we shave too much off the head it will become weak and it could warp. Higher compression will also create a lot more heat, and heat is bad. Our weak little head gaskets will also take a beating, hopefully the stress won't be too much.

If compression is bumped up, the squish band needs to be properly set up. A 5 or 6:1 ratio will function without a squish and not detonate if heat is kept in check. Our heads were built on this design, let's use it like it is supposed to be used! The band needs to be smaller than 1.5mm (0.060"). Shooting for 1mm (0.040) ish would be better. Ideally it should be as close as you can get it without piston to head contact at maximum RPM at running temp. This is going to take some fiddling with to see where our engine is and what can be modified. I will get back to you guys when I have solid numbers to work with.

Edit: added picture

[my1423]

I was going to post this on this thread about modded muffler but it better belongs here.
http://www.buggiesgonewild.com/gas-e...sucessful.html


The piston, rod, bearings, SHOULD be safe up to 7000 rpms.
That would be with oem parts. The ebay india replacement parts that most of us have used, may not be of the higher standard like oem.
A proper balancing should be done. I highly doubt the stock engines are fully balanced.
A full polish of the internals and the case would be helpful as well.

The big problem is the plastic fan, starter generator commutator and center armature, and clutch weights. If you have a failure the unbalancing will grenade things.

For ultra high rpms the fan could be removed and replaced with an electric fan.
The flywheel might be good after the fan is removed but it should be balanced and inspected to make sure it will survive and not vibrate after shedding that mass.

The clutch could be replaced with a better one that can be reved high and not shed weights. Also at the same time lighter weighs to push the engagement up higher and a much stronger spring on the driven clutch to keep the rpms up at lower speeds could be added.

The starter gen is tricky. Could add a tach attached to a relay to an ac clutch that would disengage the gen above 4000 rpms.

Stock ignition and coil probably not the best over 5000 rpm. Hei upgrade for high energy will help as well.

[my1423]

Saw this below on a cart modded with a huge sled twin engine.

With modding the engine for high hp i would mod the start stop system as well for safety and reliability.

Eliminate the pedal start.
Go to the ignition switch only start.
Come off the keyed side of the switch with an new extra wire.
run it to 2 new kill switches.
One on the steering column for easy access and one on the brake pedal as a momentary interrupt. Run this wire to the coil / ignition module.

This allows you to bump the carb butterfly stop up to allow an idle. This will let the engine warm up and prevent blowing out seals from backfiring.
Oil ratio will have to go up to 60 or 50 to one since the higher rpms and more heat from idling.

You have to turn the key off to kill the engine to change directions!!!!

Add a gm one wire alternator for charging.
Move the pedal micro switch from the current spot.
loosen the starter gen and mount it on a handle ran out to the floor.
Add a long return spring to hold the assembly off the belt.
The starter gen pushes on the gm belt when you press on the floor pedal and this also engages the micro switch turning on the starter gen.
They also put a smaller pulley on the generator for more torque.
This should not be needed with the 2pg.

This gives you a pedal starter that you have to press on hard to engage and allows for starting the engine in both directions.
The gm alternator will charge the battery and will take the higher rpms the engine puts out without blowing apart.

[clr.R4tch3t]

I have read a few papers about balancing engines and most seem to agree that any motor with less than 4 pistons don't really need to be balanced. The theory behind that (I believe) is the differences in weight between parts doesn't compound fast enough to justify the time and money in balancing for performance.

Now a real good question about RPM capability would be the factory ignition timing. The higher the RPM, the more spark advance you would need to stay in front of the piston. And with the engine going forward and reverse, it is hard to retrofit in something like a dirt bike ignition.

An electric fan would be a neat upgrade, as long as the generator could keep up with the demands. I haven't even thought about looking into that yet. Haha

My compression testing and squish measurements have been on pause to build Hot Carl a new home. Have been snowed out for this week, so I will hopefully get more work done in the next few days.