Cylinder head chamber shape

Discussion in 'Performance Mods' started by Steve Best, May 7, 2016.

  1. Steve Best

    Steve Best Well-Known Member

    Spent a wonderful Saturday swapping heads:

    The front right is my modified head that I have been running for a while now.
    The rear right head is an original GrubeeGT5A head that was prone to detonation.
    Note the squish band is out to full cylinder diameter and in further on the modified head.
    The squish band is 3 times wider on the modified head.
    Compared to stock, no detonation, more low end torque and more power at all rpm.

    On the left is an aftermarket head I recently bought. Notice something missing? No bolt holes!
    They were drilled almost all the way through from the other side.
    What should I do? Call the seller angrily? No, I laughed and drilled them through.

    So I took some white grease and a flat plate, greased the head gasket surface:
    Then filled the chamber up with water to the bottom of the threads with a calibrated syringe.
    This syringe is really too big for this job, but I just wanted approximate numbers accurate to 0.5cc.
    A bit of dish soap in the water will help the syringe slide smoothly.
    Tip the head up so the water is square with the sparkplug threads and squeeze just enough water.
    Now read the amount of water off the syringe:
    Later I did the same thing but instead of a flat plate, I used a piston with a bit of grease on the chamber taper.
    Look carefully in this picture and you can see the water at the bottom of the plug threads:
    So what did I find for volumes?
    Stock head with flat plate = 6cc, with piston = 4cc
    Modified head with flat plate = 6.5cc, with piston = 3cc
    Aftermarket head with flat plate = 8cc, with piston = 3cc

    Whoa! See the problem here? Just telling you it is a 6cc or 8cc head does not tell the whole story.
    You have to measure the chamber volume with a piston, not a flat plate to get the true story.
    Seeing as everyone is talking about 5cc chambers, I don't think they are doing it.

    How did they work?
    Stock head not tested because previous use gave overheating, detonation and blown head gasket.
    Modified head works well, top speed 57 kph (today, it varies)
    Aftermarket head MUCH more low end torque, starts better, accelerates better, top speed 53 kph.
    WHAT??? I did the old double swap to be sure and true enough, only 53 kph on all runs.
    I actually got 58 kph on 2 of 5 runs with the modified head.

    Why? I believe chamber sweep, quench losses and burn speed.
    The deep chamber does not clear exhaust gasses well, the extra quench area invokes pumping losses and the faster burn speeds of the more turbulent mixture peak out too soon and probably need a delayed spark like from Jaguar's CDI unit. More testing will tell.

    Meantime, I'm running this head because I really like the low end torque. It will pull from low speeds and gets me up to 53 kph quick. It pulls strong on the hills too. I like it.

    Timbone likes this.

  2. HeadSmess

    HeadSmess Well-Known Member

    hi quality machining followed by lax didnt drill the holes all the way? who let that one go?

    i would have said something nasty to the supplier... i aint paying for something that wasnt finished!

    mmmm, low end torque.

    about all i noticed on my nsr when i rebuilt the top end and tightened the squish from 1.8mm down to 0.8 mm was the way it wont rev right out to redline when running higher octane fuels... screams on 91, feels like a slug on 98... but the low end improved somewhat, regardless of fuel. might pop the top off and have a peek inside soon, see how it looks.

    hard to say if the compression gain was from the reduced squish or the new piston/rings/barrel... anyways, way better than when i first got it, and definitely better than the last two weeks after having to dig the rings out of the piston after seizing it... have a feeling the issue had existed before i even got it. completely different engine sounds now. smoother.

    the issue with modifying your only gotta keep it running! just got myself a kawasaki as a main bike. nsr is for weekend thrashing now :)
  3. AutOtaku

    AutOtaku Member

    I dont mean to derail this a tad as it was a very interesting read, but doesnt the aftermarket head overheat after long rides? theres not much material there so i would believe it would.....
  4. Steve Best

    Steve Best Well-Known Member

    It actually weighs at least double what the stock head weighs.
    I've only rode it a few miles yet so no hard shakedown test. So far no sign of overheating, but weather is cool yet.

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  5. Steve Best

    Steve Best Well-Known Member

    Gave the head a good test today, top speed on my test track was 55kph today, same setup as my last post. I kept on going and put some miles on it, running full tilt for mile after mile after mile. No problems with overheating. Temps today are about 20c or 68f. My long steep test hill needed no pedaling and there was no detonation or overheating even as the speed and rpm dropped under maximum load.

    So time to try more "stuff". Exhausts! I bought a few to try:
    The bottom three are Grubee/ parts.
    The black pipe comes with the engine and has a catalyst. It has been tested before and is noisier and less top speed. Both worse with the core removed.
    The small chrome is a $20 replacement and is slightly shorter. Quieter and slightly higher speed when tested against the black pipe.
    the long straight pipe is the "Fancy Poo Poo Exhaust Pipe"
    and the banana pipe is off EBAY.

    NONE of these are a bolt on. All needed slight bending to fit. Without a vice and a torch you will need to be creative.

    First test was the snakey Banana Pipe:
    As you can see from the picture, it barely let the pedals spin and my feet hit it.
    I noticed slightly more take off power and it accelerated well, but didn't feel like the tuned pipe on my KTM.
    It ripped up the long steep hill with no problem at all and maintained a good speed (not recorded).
    I was kind of shocked when I looked at the GPS.
    Top speed was 60.2kph! 5kph faster than my regular pipe and 10kph faster than this head did earlier this week!
    So what will it do with the faster head? 62kph!

    Next up, the "Fancy Poo Poo Exhaust Pipe". Unlike sham-poo, this is the real deal!
    Oh-oh! I could not pedal as is, but wait! I didn't have to. Enough low end torque I could start away wit no pedaling.
    Not as perky as the Snakey Banana, I was suprised to see I was doing 58kph!
    Speed up the long steep hill was not as fast as the Snakey Banana but it was unstoppable.
    Great gobs of low end torque. Surprisingly, it wasn't all that quiet.

    So what did I stick with? My $20 shorty silver because I need to pedal.
    Otherwise it would be the Snakey Banana for sure.
    As crude as it was it offered more low end and top end.
    Very slightly more noise was a problem. Worse, it is in the way. I cannot live with it. I'll have to make my own.

  6. Frankfort MB's

    Frankfort MB's Well-Known Member

    You should also try MZpartsmiami's expansion chamber it's not that loud (when your riding it) ;)
    It seems to have great mid-high rpm power and sounds awesome on a bicycle

    To other people it sounds like a pissed off weed wacker lol
  7. Steve Best

    Steve Best Well-Known Member

    Actually my head was from MZpartsmiami
    The snakey banana pipe was from Boygoesfast.
  8. jaguar

    jaguar Well-Known Member

    Nice report. Your bike needs a Jaguar CDI if you are increasing the cranking pressure.
    To the volumes of head with piston touching the squish area you need to add in the area that would be there in real life. Use solder to measure the squish clearance and then use the volume formula. (=2 x 3.142 x bore/2 x squish clearance)
  9. 45u

    45u Active Member

    Nice wright up thanks! Yes a good pipe with a expansion chamber is the way to go. I have the MZ pipe but have not tired it yet.


    Attached Files:

  10. Fabian

    Fabian Well-Known Member

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  11. 45u

    45u Active Member

    Attached Files:

    Last edited: May 14, 2016
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  12. Steve Best

    Steve Best Well-Known Member

    Jeff, it looks like it has a more shallow chamber and more reasonable squish area, as well as plenty of fin area.
    To my eye, it looks like a winner.

    FurryOnTheInside likes this.
  13. jaguar

    jaguar Well-Known Member

    people don't heed my warning about having high compression and then wonder why the chrome plating is flaking off their cylinder,
    or wonder why their wrist pin bearing went south so quick.
    Fabian likes this.
  14. 45u

    45u Active Member

    This is why I will be running a bushing and not a wrist pin bearing. I am all so going to see if I can find a better wrist pin bearing as I know someone has to make good one. The 6cc head is not going to give that much higher compression. I have been told by many the 6cc head is the best and not too much. If need be I will send it off and have it nickle sealed. I all so am starting out with a genuine Gurbeen and not the clone. Only time will tell as I will find out just what works and what does not.
    Last edited: May 17, 2016
  15. Fabian

    Fabian Well-Known Member

    There is no issue with the standard wrist pin bearing. In fact it is a much more durable method of operation from my experience doing heavy haul, where the engine experiences high temperatures, and high mechanical load.
    This theory has been put to the test, because i reused a wrist pin bearing consecutively in 4 engines, just to see how long it would last. That wrist pin bearing is in the engine currently installed in my bike, and it's operated perfectly well for over 20,000 kilometers or 12,400 miles, and is still giving dependable reliability.
    By account, the engines that i have had with a bushed bearing system did not last anywhere near that distance. In fact the bushed bearing was the greatest source of wear in the engine.

    What does destroy the big end connecting rod bearing and wrist pin bearing is over revving the engine.
    The 2-stroke Chinese bicycle engine should not be revved more than 4,200 rpm between gear shifts, and should not be revved any more than a maximum speed of 4,800 rpm. Once you rev the engine over 4,800 rpm for any length of time, you are significantly reducing the life of the big end connecting rod bearing. If you go to 5,500 rpm, start counting the days before the big end connecting rod bearing fails.

    My experience has consistently shown that normal gear change points should be made no higher than 3,800 rpm for reliable engine operation and a life span of 6,000 kilometers or 3,700 miles, though i have had one bottom end give 10,000 kilometers, or 6,000 miles or operation, but cylinders generally last 4,000 kilometers or 2,500 miles using air cooled specification 2-stroke engine oil at 25:1, together with the jetting optimised for best combustion.
  16. Steve Best

    Steve Best Well-Known Member

    Good post Fabian. I am guessing that a lot of the wrist pin issues are poor materials, heat and detonation. Typical Japanese motocross engine run 12,000 rpm and 5 times the hp output on pin roller wrist pins with 50:1 oil. Industry moved away from bronze bushings 70 years ago because they wouldn't handle speeds greater than 5000 rpm without pressure lube.

    More rpm means more load, as you note, especially if clearances and balance are sloppy. To withstand the load you need better quality control on dimensions and material quality. Detonation, heat and cheap oil will cook a wrist pin bearing in short order. Keep all these in order and you can do the rpm.

    My KTM 125 ran 7 years of highspeed miles on the same piston and wristpin with good jetting and Ipone 50:1 oil.
    It was finally damaged by an unfortunate complete submersion incident.
    I am betting on it with my MB Grubee GT5A, running WOT near 8000 rpm as I pass 500 kms now, on Motul synthetic 32:1 and detonation resistant head.

  17. 45u

    45u Active Member

    I do not use or want gears at this point since in some states gears are not legal. Hell if I am only to rev 4 grand might as well get a 4 cycle as they rev 6,800 or better. If there where more hop ups for the 4 cycle I would be running one of them and will be for my 3rd build. Have not read near as much about the big end bearing as I have the wrist pin bearing and being my second build is a hot rod why would I limit it to 38K? It is not a problem for me to install a rod kit if need be done a many a motorcycle. I will all so be balancing mine as this has a lot to do with life expectancy.

    I have read many here claim they are turning 7 to 11k RPM not saying it last long but seen many turning 7K with out a problem and that is about what I am looking at many be just a little more. I do not expect a HOT ROD motor to last as long as stock as I have my stock motor bike as well. Once again I will be looking for better wrist pin bearings but till I find them I will be running a bushing.
  18. jaguar

    jaguar Well-Known Member

    45u, 175psi is typical with these aftermarket heads which is way too much.
    All anyone needs is to take off 1mm of the standard head mating surface by using sandpaper on glass.
    That will bring it close to 135psi, the ideal.
  19. 45u

    45u Active Member

    Only time will tell. Jaguar have you ever run this head? Has much better cooling then a stock head.
    Last edited: May 18, 2016
  20. butre

    butre Well-Known Member

    you've got to remember, you have a very weird usage scenario. heavy load, low speed all the time is not what the rest of us do.

    I choose bushings not because they last longer, but because their failure mode isn't totally catastrophic. and I've never seen or heard of a big end failure from anyone other than you.

    I haven't even hit my power band yet at 4800 RPM
    45u likes this.