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Post by benficacm on Jul 23, 2018 20:22:25 GMT -5
I'm putting a 44mm BBK in my newly acquired Taotao thunder. I just want to level off some small hills on my way to work. I decided on the 44mm over the 47mm for longevity and because I've never done anything like this before. Baby steps. I'll be using the stock head. With my decision made, I find that there is not much posted about the 44mm BBK and I was wondering if I would see any benefit or degradation in performance if I add an A9 performance cam. I understand you lose a little compression and I'm hearing that's not a bad thing in larger BBK's. Any input on this would be appreciated. Additionally, I have a genuine Keihin pd19J that I will be jetting after removing stock air restrictions. Do you think upgrading the exhaust is a good idea? Most of my riding is north of Boston with temps in the 70's and 80's and anywhere between 100 and 200ft elevations. Just not sure the stock exhaust will cause any issues.
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Post by 90GTVert on Jul 24, 2018 9:06:52 GMT -5
Most use 47mm bores and larger, so you don't hear so much about the 44mm stuff. You should be bumping up the compression a little by using the 39mm head with the 44mm bore, depending how it's all designed. I'd say try the cam. If you notice that it doesn't have much pep, then you may need to see what can be done to raise compression for a better result (after you're sure it's not just out of tune). Compression testing before and after the kit and the cam could be useful info for you. I don't think the exhaust will do much for you. Won't hurt. Some like the sound. I just don't think you'll notice a lot of performance gain unless the stock pipe is restrictive.
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Post by benficacm on Jul 24, 2018 12:53:33 GMT -5
thanks for the reply. That’s what I figured. Wasn’t sure. I’m more curious about the A9 cam. I would think it could only help.
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Post by humanshield on Jul 24, 2018 15:44:34 GMT -5
thanks for the reply. That’s what I figured. Wasn’t sure. I’m more curious about the A9 cam. I would think it could only help. TBH, just a 44 or 47mm kit....an A9 cam "could" hurt.
From my own experience, the A9 cam is gonna drop your compression because of valve timing overlap. So if your compression is already around 180, it could drop to 150. You'll still get there, but you'll feel the drop in power and torque.
What happened to me is I installed a 50mm BBK and then a few weeks later, the A9 cam. Suddenly after installing the cam the started could crank the motor over again without trouble. BEFORE the cam the starter was having a really hard time. Checking the compression it went from around 200psi (IIRC) to about 170ish. BUT....the engine could breath MUCH better and so overall I could feel a significant increase in power and top end.
It's kind of an untold tip that if your starter has a hard time cranking the motor over (and you have a good battery and good grounding, it could be your compression is too high (unless you WANT it that way) Just remember there's a price to pay for high compression on the bearings and extra heat.
It's kind of a balance. High compression is great for power, but hell on the bearings and your motor will run hotter. The trick (I think) is to get your motor as efficient as possible with big valves, BBKs, cams, porting, carb changes, manifold work etc, then get your compression in the range of 175 - 190ish.
In the world of forced induction, remember that you DO NOT want to start with a high compression engine. You want boost to do the work (which results in higher compression dynamically instead of physically (smaller squish area etc)
Keep us posted and have fun!
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Post by GrumpyUnk on Jul 26, 2018 9:58:01 GMT -5
I cannot argue with experience, but would note that there are cams(I think) labeled 'performance' cams that have 'fatter' lobes. They do not change the lift, but open sooner and maybe close later(have not examined that closely to compare to stock). That would allow more time to fill, less time to expand(power stroke) and more time to exhaust. I do not know if there is overlap. Opening the exhaust sooner decreases the amount of time the expanding gas has to push on the piston, but closing later increases the time for spent gas to exhaust. The cam is described as having 'fatter' lobes, and the measurements are given. It was so cheap I could not resist, but have yet to install it, so cannot comment on its effects. tom
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Post by golfmech on Jul 26, 2018 13:58:42 GMT -5
Yes a true a9 has fatter lobes meaning longer duration = the valves stay open longer. I have measured and the lift is the same
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Post by golfmech on Jul 26, 2018 16:21:43 GMT -5
I just remembered part of the reason I chose to do an A9 cam was that the longer duration slightly decreases compression so for a 47mm bbk with a stock head it helps the issue of having higher compression. I still need at least 93 octane to avoid spark knock. The longer duration also helps on the high revs by allowing more air and fuel to enter the cylinder per stroke.
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ratdog
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Posts: 342
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Post by ratdog on Jul 26, 2018 18:38:43 GMT -5
A9 cams do not have more duration. They do not have more lift. They do have slightly more overlap because the intake opening is restarted a bit
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Post by GrumpyUnk on Jul 27, 2018 13:34:34 GMT -5
golfmech: Post by golfmech on 21 hours ago I just remembered part of the reason I chose to do an A9 cam was that the longer duration slightly decreases compression so for a 47mm bbk with a stock head it helps the issue of having higher compression.
I do not understand how increased duration will lower compression. There is more time for the mixture to fill the cylinder, so, logically, there would be more stuff there to compress... and thus higher pressure. But, I think that all depends on rpm, the frequency of resonance in the intake manifold, the phase of the moon, and casting of bones on a drumhead... IOW, there's a lot involved.
Ratdog, from what I understand, there is no increased lift, but fatter lobes. That should not affect collision factors between valve & piston. Then again, there are more things on the market that I for sure am not aware of. tom
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Moat
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Posts: 88
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Post by Moat on Jul 28, 2018 4:31:35 GMT -5
I do not understand how increased duration will lower compression. Normal intake valve closing is timed past bottom dead center (BDC), with the piston already well on it's way moving upwards. A performance cam with longer duration naturally closes the valve even later still - the additional time open allowing an increase in the amount of the cylinder's trapped charge to escape backwards via the intake port (reverses flow) at lower RPM's. That effectively decreases the dynamic compression ratio at cranking speeds, making it easier for the starter to spin the engine over. Bob
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ratdog
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Posts: 342
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Post by ratdog on Jul 28, 2018 7:04:08 GMT -5
Ok, as we learned how a 4 stroke works we were told that the valves are closed, the spark is fired, the piston moves down. When we get to the bottom, the ex valve opens, and the piston comes up letting the burnt gas out. When it gets to the top, the exhaust valve closes and the intake opens as the piston goes back Dow. At the bottom of the stroke the intake closes and the piston compresses the mixture until it gets to the top and fires again. If you had a single speed engine to run some farm tools, it might work that way, but not in the real world.
In our little scoots, when the engine fires and the piston goes down, the exhaust valve does not wait until the piston is down befor it opens. It takes time to get the valve opening and the mixture moving, so the exhaust valve starts to open well before the bottom of the stroke. Now the confusing part. The piston comes up and pushes the exhaust gas out. But the exhaust gas, now moving very fast, is moving and dosent want to stop, so it actually creates a suction behind it. The little engine takes advantage of that by opening the intake before the piston gets to top dead center. It also takes advantage of the gas “still moving” out the exhaust valve, by leaving the exhaust valve open past tech. That’s right foxes, both the intake valve is open and the exhaust valve is open and the piston is at 5e top of the stroke. Of course it takes time to open the intake valve, so it isn’t all the way open when the piston is at tic and the excuse valve is already starting to close so it too is not all the way open, so the guys that design all this make sure there is room for everyone at the same time.
This is all time dependent. At low RPM, you have the exhaust gas and intake gas mixing and opening the exhaust before tdc has you blowing burning gas out the exhaust pipe. As the reps go up, that time when everything is open is much shorter and things work much better. This I what a “hot” cam, valves open sooner and close later with more overlap, does not give much torque at low RPMs. And why a mild (stock) cam “runs out of breath” as the rpms go up
When we start talking about lift, we introduce another issue. If you are going to push the valve wider open then the valve has to accelerate faster.. and we have to be more careful that at tdc on the 2nd stroke there is still enough room for everyone in there. But the faster acceleration of the vale can lead to another problem. Remember that valve has to stop and come back down. Of course that is done by the valve springs. Thing is, with greater acceleration, it takes more to stop. While we all think the valve behaves it self and perfectly follows the lobe profile, the truth is at very high RPMs, the valve keeps opening beyond the top of the profile. Sort of a ski jump effect. Well if the stock springs are not strong enough the vale’s might be stopped by the spring binding ( all the coils pushed together, or hittin the valve guide, or by hitting the piston. This is why I personally always upgrade to beaver valve springs when I go to a hot cam.
So if anyone got this far, I hope that answers more questions then it creates
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Moat
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Posts: 88
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Post by Moat on Jul 28, 2018 15:30:35 GMT -5
Yep - the "ski jump effect", when momentum causes the valve to momentarily lose contact with the follower/cam lobe, is of course commonly referred to as "valve float".
One important fact to keep in mind with these things is that among all of the valve timing events (intake and exhaust opening/closing, duration, overlap, etc) - the single most important one which determines the overall powerband/torque peak - i.e.; whether a low-speed "torquer" or high-speed "screamer" - is the *timing of the intake valve closing*. It determines at what RPM is trapped the greatest amount of fuel/air charge, and it's effect with even minor changes can be quite profound.
This is why many variable valve timing (VVT) systems are only applied to the intake cam (as on Suzuki's latest GSX-R1000). Even the timing retardation that results from normal cam chain wear (lengthening) can be quite noticeable - I've replaced many worn out cam chains on motorcycles, and the improvement in low speed torque and response was very apparent.
Motorcycles often attach their cam sprockets with bolts, and an easy mod is to mill the sprocket's bolt holes into slots - allowing playing around with cam timing. I wish these GY6's were that way - a lot of tune dialing-in could be done without necessarily having to mess around with replacing the cam!
Bob
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Post by benficacm on Sept 9, 2018 18:42:09 GMT -5
Gentlemen. I did it! And wow. What a difference. Acceleration is miles better. The hills are much smoother and I definitely got a higher top speed. This from just the 44mm bbk and 5 gram dr pulley sliders. The beefier sound is pretty cool, too. It scares me to think what I would have got from the 47mm. Needless to say, I’m very satisfied and got exactly what I was hoping for. I did not get the a9 cam or the exhaust upgrade. I don’t really think it’s needed as far as I can tell. Special thanks to Brent for his carb tuning tutorial. Perfect.
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Post by scooterpimp on Sept 9, 2018 19:08:11 GMT -5
Glad to hear you got it setup to your liking!!! I remember the first time i did a bbk (47mm) with just up jetting & it made a big difference. Them 5g sliders help bigtime too. Exaust sounds nicer too.
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Post by benficacm on Sept 9, 2018 19:30:42 GMT -5
Like a kid with a new toy. I’m giddy. Ha. Next project is going to be to matte black the thing. About time I learn to paint and I always wanted to get an hvlp spray gun.
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