Basics of Boosting Engine Power

Discussion in 'Performance Mods' started by jaguar, Mar 12, 2011.

  1. jaguar

    jaguar Well-Known Member

    These Grubee engines are anemic. period. Anyone who denies that has never ridden a 50cc motocross bike which has some good ponies. If you are ready to really make yours a fun machine without destroying the engine then these are the steps I recommend:
    1. Increase the compression to no more than 150psi. (It's highly recommended you get a compression tester for around $20.) A 4.5% increase in horsepower results from increasing the compression ratio from 6:1 to 8:1 according to this paper. If you have a 66/80cc engine then do so by removing 1mm or more from the head. You can do it yourself with a piece of thick glass and #60 sandpaper. Press the head mating surface lightly down on the sandpaper on glass and move it about in a circular motion. (do the same to the top of the cylinder to make sure it is truly flat.) If you have a 48cc you can do the same or if you also want more torque and less top speed then take the cylinder down to a machine shop and have them lathe 1.5mm from the base of the cylinder (the surface that mates to the case via the base gasket). This will lower the cylinder ports for better low rpm power (which the 66cc already has). Some people advocate only removing the base gasket but that would only lower it around .5mm. If you don't want lower rpm power with the 48cc then you can lower the head and lower the intake port at the bottom edge by 1.5mm. That brings it into the "sweet zone" for both low and high rpm. The weak link of the engine which higher compression affects is the top connecting rod needle bearing. So order one next chance you get and replace the stock one.

    2. Replace the stock CDI/coil with a motorcycle CDI modified to work with these engines. That, in combination with a motorcycle ignition coil, will boost your engine power around 10%. Doesn't sound like much but it is a noticeable difference in power and also makes it run smoother (more consistent combustion) all thru the rev range. Click on my signature link to find out more.

    3. Experiment with carburetor jetting so that it runs right all thru the rev range. First play with different hi/low settings of the needle. You can easily change the position of the needle clip so that it rests further down (making jetting leaner) or further up (making it richer). Change its setting for better idling and acceleration from a slow speed. How it runs at top speed is affected by the main jet. With a candle and solder (not the leadless type) and micro drill bits You can experiment with main jet sizes. The main jet is the small brass piece that is surrounded by the plastic float. Just unscrew the main jet and use the shafts of the micro bits to manually determine its current size. Write it down. Then pull out the next size of drill bit to use. If the engine ran
    consistently but weak then you need a larger hole size. If it ran OK but combustion was inconsistent (occasionally missing) then you need a smaller hole size. Next heat up the jet with the flame from the candle and melt solder into the center hole. Then use the micro drill bit to drill a new hole size. Try it out and if it's improved then try the next size drill bit to see if it makes it even better. If you want a great replacement carb that only requires selecting one jet for your altitude then get the Dellorto SHA 14mm. It has an idle circuit (which your stock carb don't have) which will greatly improve low rpm power because it atomizes the gasoline properly. More info on it is at my site.

    4. Match the ports in your engine. You will need a Dremel type rotary tool with the little cutting discs.
    Better yet is some rotary bits such as the #114 high speed cutter. Make sure the transfer ports (left and right) are the same distance from the top of the cylinder. Use the rotary tool to remove metal from the top surface of the port which is lowest to match the other one. Widen the intake port 2mm on each side for better intake breathing. Then use a mini file or sandpaper to round off the sharp edges so that they don't catch the piston rings. A 30 degree angle from vertical is a good angle. Use an exacto knife to trim the base gasket so that none of it sticks out into the flow path of the transfer ports. The metal that extends down into the cases from the cylinder should not obstruct the flow of air/fuel into the bottom of the transfer ports. Cut them to match if they interfere. Same goes for the piston skirt. The intake and exhaust should not have any ridges sticking out into the flow path. Reduce them with the rotary tool. Use an exacto knife to trim the gaskets to match also.

    5. If you want more low rpm power (if you're overweight or have hills to climb) then you can create your own intake extension tube which takes advantage of intake resonance to cram 15% more air/fuel into the engine for more power. (more info on my site)

    6. If you want more high rpm power you can open up the exhaust port with your rotary tool. First just widen it a couple millimeters on each side. (Maximum allowable port width (straight line) for ring safety is 28mm.) That gives better high rpm power without ruining the low rpm power. The next two suggestions will give you more top power at the expense of low rpm power. Use your rotary to raise the exhaust port. Do so by only 1mm at a time. Don't forget to round off the edges for ring safety. Use your rotary to lower the bottom edge of the intake port .5mm or raise the piston intake skirt by .5mm. It's advantageous to modify the piston skirt instead of the cylinder because they are cheaper to replace in case you went too far removing metal. In the old days the piton port road racers used to have to be push started very fast because the intake port was so low it wouldn't hardly run at all at idle speeds. But they ran like a bat out of **** at high speeds!
    Last edited: Mar 12, 2011

  2. Motocruiser

    Motocruiser Member

    @Jaguar: Do you recommend sanding down the head on the Grubee 66cc? As you stated it already has low RPM torque. Any Benefits of increasing the compression? I'm definitely ordering your CDI/coil setup, porting the exhaust wider, modifying intake transfer port angle, and extending the intake manifold.

    The goal is to get to top speed quicker as all my friends ride Puch, Garelli etc...They have more top end but not as much bottom end.
    I weigh 160lbs. just to give you an idea of what my motor is pushing around.
  3. Dave C

    Dave C Member

    All of his suggestions are right on. More compression is a part of the entire HP making package. It's not about any one mod, it's the combined total that's important. Because mods mainly add percentages not specific HP numbers so you can't say, "it adds X.X HP" because that's not how it works.

    Too much compression can be a killer, too. Really not much more than a 9:1 ratio can be used due to the difficulty in getting a clutch to grab and overcome the compression resistance. I have seen 11:1 heads that have a compression release. You hold it open, get the piston moving then release the compression control so it will start. But I think the theory is the best is 9:1, you sure don't find MX cycles with much more than that and if it helped to have more compression you KNOW they'd have it at the max.:helmet:
  4. Neon

    Neon Member

    I agree with everything that was said except for a few things. When have these engines ever had good low end ? I've owned 2 of these engines and the only way to get anything that resembled take off was to rev the snot out of them. One way to help offset that was to go with a smaller diameter wheel, and a larger rear sprocket. I'd also tend to think these engines would be a little better if the tolerances were kept a little tighter. The pistons i had in these engines flopped around in the cylinder like a beached carp. They do need better bearings in them yes, also better seals are also a good idea. Is there any ignition units that anyone knows of that are a decent fit without the need to re-engineer the whole engine ?
  5. Dave C

    Dave C Member

    jaguar's set-up is the only one I know of for a MB. For $51 shipped here for a hand built CDI and $17 for a Honda replacement coil I thought it was a pretty good deal. I might put micro switches in where he has jumpers to change the rate of timeing change in two of the ranges, a simple on-off switch would effect the change to give you some external influence on timeing without having to open the box(this is my un-authorized modification :) ) Only other thing I'm doing is siliconeing the holes for the wires. This will also serve to help shock isolate the circuit board from the box.

    There does seem to be a lack of choice as far as ignition setups go. This was all I can find but I'm sure there's more out there but I'd guess it would cost more than what jag's got going.

    He bought himself a short vacation(1 week) but he'll be back, I talked to him and he said so. If you want to order one of his CDI units go to the link in his siggie. I'm sure he'll be happy to hear from anyone ordering ;)
  6. technomancer

    technomancer New Member

    What's the piece of thick glass for while sanding down the head?
    I am thinking about only taking a mm or so off.
    i only want to take a very little off, but you said 1mm or more.
    how much is the 'or more' that others are getting away with?
  7. GearNut

    GearNut Active Member

    The thick glass is perfectly flat. No ridges or wavy surface to bugger up your sanding job.
    The mm thing I have no answer for.
  8. Neon

    Neon Member

    1 mm or more = more compression or better bottom end. You may or may not be able to get that much taken off without the piston hitting the head or spark plug.
  9. Al.Fisherman

    Al.Fisherman Active Member

    The only reason that I mill my head is not to increase the compression ratio, but to make a good head to gasket surface. In doing so this increases the compression ratio , so much the better. I NEVER put a engine together on a bike out of the box. I haven't found ONE head that didn't need to be milled.
  10. jaguar

    jaguar Well-Known Member

    Put the glass on the floor. Put the sandpaper on the glass. Put the head on the sandpaper. Put your foot on one corner edge of the sandpaper. Hold the opposite corner down with your hand. With your free hand move the head in a circular motion while pressing down. Taking off 1mm is the minimum necessary. You really need a compression gauge so you can keep reducing the head until you get up to 135psi. It's not easy to do by hand, by the way. It takes perseverance. You should do the same to the cylinder top to make sure it is flat. That way it won't be hard to get a good seal by using high temp silicone sealant.
  11. technomancer

    technomancer New Member

    sounds good. and yeah, i know all about persevering over a chunk of metal that needs to be sanded or filed. :(
    thanks for the info.

    i need to get another comp gauge now though.
  12. Fletch

    Fletch Member

    On my newest build (HT 66cc) I sanded down the raised circular lip on the head so that the mating surface was completely flat. I compared a new head with a stock head gasket, and that raised lip seems to be roughly the same size. So the way I figure it is I remove a head gasket worth of material from the head, which equals not using a head gasket and not removing any material (make sense?). The only difference is that more heat is being transferred to the head because of the flush mating surface.

    I don't have a tool to measure compression (would like to get one), but I was overheating going up a 9 grade hill for about 1/4 mile (with shift kit). There are multiple factors at play such as plug heat range and oil ratio, but I ended up adding an extra head gasket to in effect bring the slant head back to stock size.

    I've had two bikes overheat and seize up on that hill when I removed the head gasket or material. In retrospect I was using the either the stock Chinese plug which is pretty hot (equal to a NGK B4HS), or the extended tip NGK BP6HS which is 2 heat ranges cooler but increases compression further because of the protruding tip.

    I guess what I'm trying to say is that in my experience so far (minus trying a cooler plug), removing a head gasket from a 66cc HT slant head will cause it to overheat with extreme load (huge hills) or long full throttle runs. I don't know much about motorcycles or high compression heads, but am I correct that they are meant for short distances of WOT, and NOT for climbing up very steep hills?

    I just ordered some NGK B7HS plugs which are the coolest I will have tried. Hopefully this should help some. Also because I have a short billet intake, the carb was heating up and the gas bubbling (boiling)- hard to believe I know, but it's happened to someone else (search). A regular intake manifold would keep the carb cooler, but the ported short intake has little area for heat displacement.

    Also, what octane gas are you supposed to use in an increased compression engine, and at what level (for example #91 if your compression is over X)? I like the idea of increased compression but I need this engine to last.

    One last thing... I have read that these engines aren't built to tolerate increased compression (bearings, imbalanced crank, etc.). Even if you get it running well heat-wise... won't it cut the life of the engine?
  13. Fletch

    Fletch Member

    Al.Fisherman- How much are you taking off? Are you leaving some of the raised portion, or milling the head completely flat? Are you using just one head gasket still?

    I was thinking that maybe I removed some of the already barely existent squish by making it flat like I did.
  14. technomancer

    technomancer New Member

    Balls. i just realized what compression testing on my bike motor entails for me.
    basically a few good pedals in gear should do it, right?

    stupid city streets...
  15. jaguar

    jaguar Well-Known Member

    Overheating going up hills is just due to the stock CDI which advances the spark timing too much. I really believe it was designed for a four stroke engine but was put on the Grubee to lessen its power so it can be allowed into all countries. If you don't correct that problem then just jet the carb richer.
    I've been running 135psi on my engine quite a while now with only the top connecting rod bearing (wrist pin bearing) changed to a slightly better one. I don't think 135psi is really considered "high" compression, just higher. There's guys running much higher than that.
  16. Fletch

    Fletch Member

    Thanks... I measured compression on my bike at 25psi yesterday. I'd been using the NGK extended tip which raises compression some unknown amount too. That's with 2 head gaskets because I sanded the head down. If I remove one of the head gaskets I don't think I'll be able to start it because it's a shift kit and you have to pedal with the clutch engaged.

    How do you correct the cdi... replace it? With my other bike I couldn't jet it richer without it messing up performance. I don't know if anyone else is climbing 2000 ft on their bike regularly, but it takes its toll. I'm running a cooler plug now.

    I was under the impression that the first bearing to go was the crank bearing? Is it the connecting rod? Where did you get a better connecting rod bearing? The bearing that came with this latest engine I'm installing looks different than the ones I've seen before. The bearings are the same length, but they look slightly thicker, and the the total size of the "barrel" is longer. I wish I had a pic. I don't know if this is a better bearing or not?
  17. jaguar

    jaguar Well-Known Member

    I hope you meant to write 125psi instead of 25psi.
    Yes, replace it with a modified motorcycle CDI which is not as advanced in timing and retards the timing after 3600rpm like normal 2 stroke CDI's do. I'll message you more info.
    No, the weakest bearing is the small wrist pin bearing, then the bigger rod bearing, then the crank bearings. Yeah it's better to have the wider upper rod bearing since the load is spread out more on the rollers. Any of the replacement bearings are better than the stock one.
  18. Fletch

    Fletch Member

    Thanks jaguar, I've actually bookmarked your page in the past but forgot about it. I have a 66cc though. Unfortunately $ is tight now so some of this stuff will have to wait. I just posted a thread asking for opinions on what to do while I have the bottom end opened to clean out some shavings. I'm running an 18mm Mikuni carb on a ported engine (don't know exact measurement off the top off head). I lowered the intake 2-3mm, and raised the exhaust to somewhere around 1.07 from top of cylinder, as well as widened it to like 26mm. I have a SBP expansion chamber that has some dents in it from falling off and being run over! I'm eager to see how it runs (new build). Looking at your site just now it looks like if I experience too much vibration, the easiest way is to add/lower weight to the wrist pin? (meant to reply to PM...too many tabs open!)
    Last edited: Apr 22, 2011
  19. 25psi seems very low to me. i havent done a true reading yet on my "65.9"cc sd stinger. but with just a couple slow peddles mine was at about 80 psi. i had my bike in the basement and i didnt have any room to really peddle the bike. im sure when outside and if i get going, it would be at about 100-115 psi. and thats just a stock engine with about 20 miles on it.
  20. jaguar

    jaguar Well-Known Member

    Yes, the piston/wrist-pin is heavier than those of a 48cc engine and the crank is balanced for the 48cc (somewhat). You can just drill a bigger hole in the wrist pin to see if that takes care of most of the vibration.