Speed vs RPM for Grubee GT5A on 26" tires

[Deleted member 12676]

about anti-lemon: compare your engine to all the other barely modified engines and you'll see that yours is in the top 5%

only racers should use castor oil because it requires a complete top end cleaning (even cleaning out the deposits from inside the piston rings) after every few hours of hard running (like a race or hard practice)
anyway its not available down here in Paraguay

 

[sbest]

I have a spare Grubee cylinder and will measure the exhaust height for you before the end of this post.

I port matched an intake, tapered the outlet of the manifold to match the cylinder port.
The result was very disappointing. I bought some extra parts when I bought my engine and an intake was one of them, so I put the stock intake back on. Power came back. The probable reason is due to reverse pulses being blocked by the mis-match.

The head mods were only done to solve detonation induced warpage and to get rid of the warpage.
It did result in a slight increase in power. This is the goal:

Create a donut shaped tornado in the chamber to keep fuel in suspension and rapidly propagate the flame front.

Likely the charge would be completely engulfed by the time the piston dropped this far.
The right squish and chamber significantly reduces timing advance needed.

Grubee stock exhaust port height on my spare cylinder is 1.111" or 28.22mm.

Steve

 

[sbest]

about anti-lemon: compare your engine to all the other barely modified engines and you'll see that yours is in the top 5%

only racers should use castor oil because it requires a complete top end cleaning (even cleaning out the deposits from inside the piston rings) after every few hours of hard running (like a race or hard practice)
anyway its not available down here in Paraguay

Ha! If I wasn't in the top 5% I would do something about it! - - - Wait a minute, I did...

You are mostly right, but we did use it on small street driven 2 strokes and motorcross and enduro engines which are typically driven hard, almost to racing levels. That same carboning-up sealed leaky rings and loose pistons, making engines silky smooth and tight. That is why Gordon Jennings found the more you use of it, the better. I can assure you we ran many, many hours before decarboning, often years. It was a mess when you did it however. Castor carbon sticks hard.

A good test for your oil is holdback compression on a hill. I noticed this quite by accident. Used a cheaper (but still good quality) oil because I was out of my good stuff. We used to park on a slight hill to watch others hillclimb. I noticed my bike was creeping downhill, not holding on compression like it used to. When I switched back to my beloved Motul or Ipone I got hill holding static compression back.

 

[Bufu racing#1]

I have no tach so I have had no idea what RPMs I was turning.
Cold weather and snow ruined my bike riding so I did math.
My tires are 26"x2.3" and measure an actual 26.5" high.
I have the "as issued" 44t rear sprocket, and 10t front.
The engine gear ratio is 20:82.

Much of you who have built "80cc" kits have the same specs.
My tires are a bit oversize but we are only talking 200 rpm at max speed.

So the math is:
tire-dia x pi x 25.4mm/inch / 1000 = distance of wheel rotation in meters
speed in kph / 60min/hr x 1000m/km / distance of wheel rotation = wheel RPM
(wheel sprocket / shaft sprocket) = 44t/10t = 4.4
(clutchgear teeth / drivegear teeth) = 82t/20t = 4.1
Wheel RPM x (wheel sprocket / shaft sprocket) x (clutchgear teeth / drivegear teeth) = Engine RPM

So, as first mounted and new, I got 40kph top speed or 5700rpm
As it wore in, speeds picked up to 50kph or 7150rpm
With some tuning and head work I can often see 60kph or about 8600rpm
my strongest torque is at about 35kph or 5000rpm
torque drops off fast under 30kph or 4300rpm
9000rpm is 63kph and I touch it occasionally but the vibes are terrible.

Aiming for 50mph? 80kph is about 11500rpm
60mph? 100kph will buzz this combo to 14000rpm!
Even with a 36t sprocket if I can still spin 8600rpm I will be lucky to see


Steve ..try a 30t and inbox me for more info on how to get the motor to come alive.. simple stuff ...inbox me