My mods, and performance, how am I doing?

S

StevenMain

New Member
Hi, my name is Steven, I'm a highly experience auto mechanic, Have owned several race cars and crewed on the Main and Martinets Top Alcohol cars that my dad has owned or co owned.

NEW to two stroke.
Being as I'm using my GI bill to finish school I don't have money for projects any more so I built my bike since they are good cheap fun.

I have the 66 PK80 ebay motor kit on my Kent Del Rio and here are the mods
NGK #6 range plug
HD CDI kit
Stuffed Crank case
Arrow Reed Block
PZ20 Keihin Carburetor
Expansion chamber pipe (no name ebay but very Large high mount type)
Transfer ports widened, de burred, aimed at intake, roof angle elevated to converge just below deck.
Intake Widened, 3rd Transfer port added, port matched to reed block
piston windowed, added transfer ramp bevels to top
Exhaust port widened, port matched to ported exhaust flange.
Raised Compression (not measured but nearly removed the sealing ring around stock head, its about .008" left)

The mods have REALLY woke this little 66 up, it starts and idles fantastic, has amazing throttle response even with the large 20mm carb.

Running the stock gearing (10/44) I get 41mph @ 9,212 RPM
I was curious if this was respectable "race" performance or not. The best part is I got so lucky on balancing the thing is silk smooth with no vibration whatsoever. Im 185 lbs.

My friend has a stock PK80 with raised compression, a banana chamber with muffler, and delorto 14 Carb, and short billet intake on his bike. I can leave his ass like hes standing still. He tops out at 29 after a LONG time. Mine gets up to 41 in (aproximation) 8 seconds or so?

Wondering how to estimate HP on these things!
 
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My 66cc got up to 40mph, and that seems to be a barrier with 44 tooth rear sprocket. Further mods would not get it to rev higher or higher top speed, infact 35-41mph depending on what pipe or head or intake. What the further mods did was make it accelerate faster as you mentioned. My math seems to indicate a higher rpm than 8300 at that speed. How did you measure?

Steve
 
sbest thank you for pointing that out!
I confirmed speed via 2 separate gps apps on 2 separate phones.
The rpm was done with math, but my math was in error!
I thought the front sprocket was 11 tooth, it's not it's 10 tooth! So 27" tall tire doing 41 mph is 510.68 wheel rpm * 4.1 * 4.4 = 9,212 rpm holy cow that's screaming. I think the only other significant performance gain I could get can only be obtained through some type of aerodynamic fairings. I will edit my original post!
 
One thing I failed to try was I welded the totally stock muffler to the stinger of my chamber because the bike is so loud unmuffled, my 41mph run was done with the muffler in place. It blew an exhaust gasket doing the quick hard sprint maybe I'll remove the baffles and try again unsilenced for a better top speed, I also lowered the seat and welded a bracket to extend it 2 more inches rearward, added rear footpegs, and lowered the front handlebars, this will give me a better tuck. Unfortunately I'm running my speed tests in full motorcycle leathers which are very bulky and draggy.
 
Steven, actually check your wheel diameter, or more accurately, check how far it travels in one rotation with weight on it. You might find your revs are closer to 10,000 rpm.

Steve
 
Right on again with 80.25" rollout measured I must have been doing 9,732 rpm. Not accounting for possible tire growth.

Removing the muffler could help, beyond that is my motor probably maxed out as far as performance potential?
 
Yeah, I found mine would not rev any more even going down a very steep hill. I do not have an expansion chamber pipe yet but I am running a short pipe that is port matched and was picked out of all the ones Grubee sells for its ability to rev.

I ran the bike for months and hundreds of miles at WOT at speeds of 30-40mph up and down hills. I can feel some play in my lower rod bearing now, so it does come at a cost. Probably better to run it at 8000 rpm, but darn, that 44t sprocket is pretty much perfect for hills, start away, and enough speed. I don't want to go higher or lower without giving up too much on the other end.

I just finished building a jackshaft shifter with a 48cc motor. Not completely pleased with it either. The start away and low cruise rpm are nice, but more parts and complication, and starting the engine is more difficult. I like pedal assisting, but the shifter ratios (engine to pedal) make this difficult.

Oh, removing muffler is generally not much help with a 2 stroke tuned pipe because backpressure on the stinger pipe is useful, that is why the stinger pipe is so small.

I liked the single speed system as a tool for engine development because it focused totally on looking for broad power out of the engine. I'd say you found it Steven.
Maybe time for you to look into a jackshaft shifter?

Steve
 
I'm addicted to learning, I'm always wanting to try and build new things. Two stroke engines were something i've never been inside, which is funny because I have plenty of automotive experience just sold my race car, a Turbocharged 2002 Trans Am with 1,000 rwhp to a gentleman in Qatar. I love to build.

I feel like I must be still laying alot out on the table though compared to some of these 50cc two strokes which make twice the power. I don't know where I can begin to look though since my two stroke knowlege is litterally nothing. My porting is what really made the thing scream and ill be honest. I was probably very lucky.

Id like to experiment with a 20% nitro blend RC fuel I have, but to run it I would need to figure out how to get precisely 4 times the amount of fuel in the engine for the same amount of air. I don't think my 20mm carb can be rejetted that big lol.
 
I'm with you on lifelong learning.
These engines are the best tool you could ever get to learn about 2 strokes.

I took a "one at a time" approach to mods.
Taking advantage of easy top end swap time, I stuck with stock components with slight mods to gauge the effects of each. Like you, I found opening up the port windows really helped. My goal was broad power and this little engine can do it from 500rpm to 10,000.

As for making BIG power, these engines need more window area to do it. The whole cylinder circumference under the the transfer ports needs to be open.

I'm playing with torroidal combustion chamber shape. It limits rpm somewhat but gives large low rpm torque. The hemi shape is still king for rpm.

Steve
 
Im going to need you to stop being right about ****. I removed the muffler baffles, the bike ran slower. 38 mph it would pull like ****, then, almost as if it hit a rev limiter, stopped. I wonder if the Huffy Davidson has some sort of funky ignition curve to prevent the engine from going over x rpm. Which wouldn't make sense since I did already have it at 41 mph. Not sure a higher rpm is possible from this engine it just STOPS revving, stops from a decently strong pull no less.
 
Same experience here. Ported cylinder pulled strong and then quit like a rev limiter.
No more revs even on a downhill. It might be an ignition thing?

I did a bunch of exhaust experimenting this afternoon with stock cylinder 48cc and 66cc Grubees.
48cc (stock cylinder, converted back to single speed 56T) and 66cc (44t stock cylinder), both with modded heads.

I have a recently abandoned runway in my backyard.
Level but strong winds, saw speeds of 40-58 kph against the wind and then with it.
Interesting stuff, upwind/downwind, 48cc/66cc.
48cc is in a lightweight 10spd frame (with shifter frame still in place), 66cc in a heavy MTB and big tires.
stock pipe= 44/52, 44/57,
portmatched pipe= 46/52, 46/58
Straight pipe= 40/42, 42/44
Stock portmatched pipe, baffle removed= 42/48, 42/50

Not a lot of difference in power between the two stock engines as tested.

Steve
 
I think the stock pipe is better tuned than we have been giving it credit.
Gordon Jennings talks a bit about the value of the stinger in tuning for top end.
The restriction of the stinger keeps the end gasses in the cylinder and not escaping out the pipe and tend to return higher speeds. To properly size your stinger, you keep making it smaller and smaller and watch the effect on top speed. So it is with the stock pipe on these China Girls. Not so much restriction as a tuned effect.

Port matching is simple on these pipes. Bolt the gasket to the pipe and plow a 3/4" drill into the flange at the same angle as the pipe. BANG! Portmatched!

Have been looking at the port measurements today. Looks like these cylinders are designed for a 42mm stroke. We can lop off some of the intake skirt for that side, but the exhaust port and transfers lower edge on most of these cylinders is 41-43mm down from the headgasket surface. I have bumped my basegasket up 1mm but can foresee raising it 3mm and shaving some off the top of the cylinder or making a recessed cylinder head. Might work on this today.

Steve
 
For my components I bolted them to the cylinder, shot spray paint through the port, and unboltef and went to town with a dremel. The exhaust pipe isn't large enough diameter for a true port match on mine so I ran it out as close as possible. My pipe has a rectangular flange opening now.
 
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