Speed vs RPM for Grubee GT5A on 26" tires

Discussion in '2-Stroke Engines' started by sbest, Jan 19, 2016.

  1. sbest

    sbest Active Member

    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 73kph.


  2. butre

    butre Well-Known Member

    full out on flat ground with no wind I can hit 9400 rpm (give or take a hundred or so depending on density altitude) with a 32 tooth before it before it stops accelerating. I'd like to see 10162 on my tach some day
  3. jaguar

    jaguar Well-Known Member

    sbest, what mods have been done to it?
  4. sbest

    sbest Active Member

    Darned little so far.
    Broke in on 20:1 oil because the book said so, I'd recommend 32:1.
    Kept the gear greased very lightly. Switched to 40:1.
    Port-matched, exhaust, intake worked better not.
    Leaned out the carb, Had detonation on hills, sanded the head flat.
    Most useful was to sand a squish band into the head and set to 1mm gap.
    Align chain and tensioner, watched the tire pressure and spokes.
    Noticed air filter not working, scratched cyl bore. Cleaned up transfer port flaws.
    Switched oil mix to 32:2, Last runs were 55-60kph before snow and ice set in.

    9400rpm with a 32t would be about 90kph
    10162 would be 97 kph
    These figures make sense?

  5. jaguar

    jaguar Well-Known Member

    I've never heard of a mostly stock engine getting up to those high revs.
    the engine comes with too little blowdown time to rev high.
    make sure your speedometer is calibrated correctly.
    also pull the plug and see how many revolutions the magneto makes for one rear tire turn. then do the math to see what rpm you are reaching.

    the best up-to-date articles on engine oil are recommending using as much as possible, not little as possible.
    I was getting excessive engine wear using semi-synthetic at 30:1 and so have gone to 25:1 (because I found out how little synthetic is actually in these blends)
  6. butre

    butre Well-Known Member

    yep. that 10162 RPM is 60 miles per hour, by the way
  7. sbest

    sbest Active Member

    My speedo is a Garmin 76 GPS and I have verified all the ratios involved by counting the teeth several times.
    The only difference between my and Butre's figures is the size of our tires and our sprocket, and matches my calculations.
    My engine was made 10 May 2015 and seems well balanced. My RPM calculation is probably accurate +-5%.

    Gordon Jennings was my guru when I got my 1974 CR250 and wanted to go faster. Following his book:
    I was able to make that 1974 CR250 much, much faster.
    So fast that I sold it to a guy in 1985 when his brand new water cooled power-valved CR250 could not even come close to my old air cooled "fossil".

    In 1978 Gordon published this article:
    It changed the mind of a lot of people about mix oil and ratios, and was followed blindly by crowds for many years.
    The article is accurate, as is anything Gordon did, but only for the 1978 conditions represented.
    1978 quality oils and a low compression motor.
    It came out that castor bean oil at 20:1 was pretty much the best stuff going I recall.

    Oils have changed a lot since then, and chrome cylinders react different than iron, so what is the truth now?
    I wanted to find out myself, so I followed Gordon's and Grubee recommendations and ran 20:1 mineral based oil in my GT5A engine for break in. It puked oil out the muffler every time I parked and eventually fouled the plug. Break-in done, I switched to synthetic 40:1 that I use on my motorcycles (KTM 125, 250, 300) and I distinctly had more power. Was it the ratio or the better oil? I don't know, that is what happens when you make more than one change at a time.

    Later I found dirt had scored (from insufficient air filter) the cylinder, I wondered if more oil would help reduce blowby. I upped the ratio from 40:1 to 32:1 and had more power. Later I tried 20:1 again and noticed no improvement and am back at 32:1. Is this the best ratio for these engines? If they are scored and leaking it appears to be but I'll have to retest when I get a fresh cylinder.

    Also take note, I am using Motul or Ipone synthetic, much better than most of the "semi-synthetic" crap on the market. Pay the money for good oil. Castor bean oil is excellent stuff but makes smoke and smells funny (some love it!) and really carbons up the engine. High quality synthetics actually clean and keep carbon deposits soft (to keep powervalves working) and can be used in small quantities. Mineral oils are very poor 2 stroke lubes, even now.

    My first motorized bike was maybe a 2012 model I bought used. It worked well and the engine was in good condition but only did 30kph. My new Grubee GT5A only did 30kph during the first tank (50kms?) of break-in, before I started changing things.

    The changes I've made are small fine tuning, but come from years of experience.
    I also adjusted the brakes and repacked and relubed any bearings on the bike and engine. I use minimal amounts of lube and the right lubes for the job. A full packing of thin soft waterproof grease for the pedal crank bearings and a very thin coat (think match-head) of tenacious grease for the reduction gear. The chains get cleaned and a wash of evaporating teflon based lube. The drive chain gets aligned. These things are important when you are dealing with minimal power. Even with a well scored cylinder, my Grubee GT5A does make good power right out of the box, better than the older motor.

  8. jaguar

    jaguar Well-Known Member

    well then you've got one of the few anti-lemon engines.

    semi-synthetic is all that's available here other than regular mineral oil. I like the idea of using both types at once.
  9. sbest

    sbest Active Member

    Anti-lemon? Why do you say that? Because vibration is manageable to 8700rpm?

    Can you get castor oil, even at an RC hobby shop?
    Castor might be even better than synthetic in these engine, for the same reasons it is used in glow engine. It builds a coat of varnish that protects metal surfaces and seals loose tolerances. I think you would see a power increase.

    I saw a link to an earlier thread:
    I don't think we are talking the same engine. Grubee has obviously changed some things with time.

    Last edited: Jan 21, 2016
  10. jaguar

    jaguar Well-Known Member

    ?can you more completely explain this statement:
    "Port-matched, exhaust, intake worked better not. "
    did you raise the exhaust port? (the only way in the world you are getting the revs you are unless you know how to change the laws of physics)

    if you didn't then the distributor did because these engines have low exhaust ports and terrible little blowdown time, both to limit rpm to around 5500.

    the head mods helps keep you going at the same speed without slowing down due to piston overheating. the squish band does that. but it doesn't give extra revs just because of the squish band. cutting down the head so there's a squish band (which I also did on 2 of my heads) increases compression ratio which increases power which does help some in achieving higher revs. (but not from 5500 to over 8600)
  11. jaguar

    jaguar Well-Known Member

    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
  12. sbest

    sbest Active Member

    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.

  13. sbest

    sbest Active Member

    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.
  14. Bufu racing#1

    Bufu racing#1 New Member