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Post by chuckspeed on Aug 25, 2011 4:42:45 GMT -5
So I have the meanest 50cc 4 stroke in town - but it's still not fast enough. After having the exh break loose over the weekend and dumping the damned thing in the rain on the way home, I tore into it, thinking its 47 MPH top speed was a function of a head gasket leak. No dice. Opened up the combustion chamber anyway with a half hour of dremel rasp carving:  The new chamber is much bigger than the stocker as seen in the compraro pic. I figure the motor was above 12:1, so this should tone it down a bit; hard for the starter to roll over the motor! Anyway, the scoot would sound like it was breaking up (popping and cracking) into the intake above 45 MPH. I could squeak 47 out of it - but that was about it. This was odd, as it would do 45 given enough time as a 49cc scoot. I figured that in going to 88cc's, I'd pick up a few MPH top end... So after reshaping the chamber, I started lightening the rockers. The little battery Dremel pooped out after a couple hours of carving, tho - so I had to do something else. Bored, I dug out the cams to do a comparison between the stocker and the race cam I'd added. The lobe height/profile were identical between the two cams...odd. Started to look at overlap, as a race cam typically has more overlap, but I couldn't see it with my naked eye - and I'm attuned to such things. Then I started thinking about centerlines - how the cam is indexed relative to the crank. When setting up a race engine, it's not uncommon to degree the cam to make sure it's doing EXACTLY what it's advertised to do. This takes a degree wheel - but there's no way to stick one on these teeny motors. But - there's a keyway on the camshaft...I lined the gears up tooth to tooth and:  BAM! The "race" cam is on the left, stocker on the right. Turns out the two cams are IDENTICAL, except the "race" cam has been advanced almost ten crank degrees! This is huge! An advance of this amount will make gobs of low end power (which it did) at the expense of top end. The intake valve was opening WAAY too early, causing backfiring into the intake tract - something I'd incorrectly diagnosed as valve float. Soo...the stock cam is going back in. Beware "race" cams; they're simply stockers with the cam gear advanced! Wish me luck. I think burying the speedo is close at hand. |
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Post by hardd1 on Aug 25, 2011 5:55:57 GMT -5
excellent observation
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Post by dude on Aug 25, 2011 8:27:03 GMT -5
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Post by Scooter Elements on Aug 25, 2011 10:21:24 GMT -5
Here is an Ajustable camshaft i sell here at my store. Alot of people use it for racing only! Attachments:
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Post by chuckspeed on Aug 25, 2011 10:41:40 GMT -5
I didn't take a photo of the piston, but it's domed. I had cc'd the head before installation, and came up with the numbers you see - about 5.2. This motor's running a 52cc kit - that and the raised section of the piston give it quite a bump in the compression department - much more than it had stock. Even with round one of chamber carving (an oval chamber designed to help valve breathing), the starter would struggle to roll over the motor.
Thanks for the pic on the cam - that would be the hot ticket, but now that I'm in Guam, the really trick scooter parts are scarce. I'll put it together with the OEM cam and give it a whirl, but next on the list is to carve up the clutch bell.
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Post by Scooter Elements on Aug 25, 2011 10:49:31 GMT -5
I didn't take a photo of the piston, but it's domed. I had cc'd the head before installation, and came up with the numbers you see - about 5.2. This motor's running a 52cc kit - that and the raised section of the piston give it quite a bump in the compression department - much more than it had stock. Even with round one of chamber carving (an oval chamber designed to help valve breathing), the starter would struggle to roll over the motor. Thanks for the pic on the cam - that would be the hot ticket, but now that I'm in Guam, the really trick scooter parts are scarce. I'll put it together with the OEM cam and give it a whirl, but next on the list is to carve up the clutch bell. yea thats a high compression piston. I have one installed in my 150cc scooter. People here love them. Wow Guam is very far away. |
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Post by D-cat on Aug 25, 2011 15:05:01 GMT -5
Actually there are differences in the lobes. There was just a chart posted here ripped from a French scooter site that showed the differences clearly but if you have good observation you can see the differences with the naked eye. Good catch on the key shaft position though.
It appeared from the chart that all the race cams seemed to have a deeper thrust, anywhere from .5 to 1.5 mm over stock. The A9 cam has one of the deepest with a narrow duration and an apparent slight intake retardation giving it a strong top end boost. The A14 is slightly advanced over stock and has a longer duration, making it a low end torque cam (probably very similar to the one observed above).
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Post by chuckspeed on Aug 25, 2011 15:53:13 GMT -5
That.s not a little advanced - that's a LOT in the pic above! Duration appeared to be the same. the most I've ever advanced a cam is 6 degrees at the crank, which is three at the cam. This has at least 5 degrees - which is why the scoot would fly up to about 42 MPH and then creep past 45, finally peaking at 47. stock cam's going back in - even thinking of lightening the sprocket. I'll look and see what cam I bought, but it was advertised as an A14, I believe.
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Post by ericrockstar on Aug 25, 2011 15:59:39 GMT -5
Cool
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Post by chuckspeed on Aug 25, 2011 16:08:13 GMT -5
I just checked the box - it's marked A9. I also checked clearances, as cam grind lift comes out of the lobe base. Clearances are the same for both cams. Lobe separation is the same, so unless it was a total mistake of packaging, I have an advanced stock cam.
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Post by D-cat on Aug 25, 2011 20:56:38 GMT -5
You got ripped, that's not even an A9 or A14 cam tooth. A9: A14:   From the similar angle photos, you can see the lobe differences pretty clearly. |
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Post by gregw on Aug 25, 2011 21:38:13 GMT -5
certainly the A9 has more lift. but might require stiffer springs...
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Post by darknight73 on Aug 25, 2011 22:28:43 GMT -5
a9 looks like possibly a slightly smaller base circle, making the lift look huge. you can only go so big b4 smakin a piston!!
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Post by D-cat on Aug 25, 2011 23:11:09 GMT -5
Don't forget that you set your valve gaps at TDC; a smaller base circle = larger lift. If I remember right it was the A9 that had the 1.5 mm taller lift than stock. And yes, you do want to be careful with your valve-piston clearance when using an A9, especially in conjunction with a big valve head.
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Post by chuckspeed on Aug 26, 2011 23:03:14 GMT -5
I bought it from scrappy. The scoot goes up hills a little better with the stock cam now, but I've lost 5 MPH on the top end...odd. And yes - I did have to relieve the piston.
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Post by chuckspeed on Aug 26, 2011 23:04:30 GMT -5
I'm going to rest the valve lash on the stock cam - then go back to the 'race' cam to see if there's a marked difference.
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Post by skuttadawg on Aug 26, 2011 23:38:10 GMT -5
Thanks to all who posted as I too was wondering about performance cams . I know in cars the lift ( dif in lobe from base circle ) is bigger so valve opens further along with duration increase which how long the valve is open . I was wondering if too much cam could cause the valves to hit the piston . I want a lil 4stroke rumble like with airfilter and exhaust upgrades .
I love the sound of a tuned V8 with hearbeat cam ( thats what I call em due to sound ) and a set of Flowmasters
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Post by chuckspeed on Aug 28, 2011 4:11:42 GMT -5
I've got the aftermarket cam in the the hole again; the advance seems to make a slight difference. I left the adjustment on the followers alone - so perhaps there is a little more lift to the second cam. Jetted down from a 110 to a 102 and picked up some top end.
The head carving made a big difference in docile behavior. The motor has too much compression before; there was a hot spot the size of a dime on top of the piston - the only thing that probably kept the piston intact was the rich mix.
I can charge up some pretty steep hills now. When I say hills - I went up Nimitz Hill today at 25 MPH - the speed limit - which is about 700' of elevation change in a mile. Been well north of 50 on more than one occasion; thinking about sticking the long case gears in the scoot now, as it spools up to it's top speed fairly quickly and smoothly. There are a couple of hiccups as it comes up to speed that I've not been able to dial out - I think I need either a 105 main or a different needle.
Almost time to sell it.
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Post by noday on Aug 28, 2011 6:07:47 GMT -5
I do not understand your thought process
speed = RPM X gear ratio
for a 50cc GY6 it takes 9,000+ RPMs for 50 MPH
you have upped the displacement & modified the engine to increase power but have done nothing to change either max RPM or gear ratio.
why would you expect more top end speed ?
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Post by chuckspeed on Aug 28, 2011 15:24:04 GMT -5
I do not understand your thought process speed = RPM X gear ratio for a 50cc GY6 it takes 9,000+ RPMs for 50 MPH you have upped the displacement & modified the engine to increase power but have done nothing to change either max RPM or gear ratio. why would you expect more top end speed ? Because I could turn nearly 11,000 RPM with my ported, polished and mildly modified '66 Superhawk, and honda tiddlers have a redline of 12,000 RPM. Big ports and valves mean better breathing; this translates to more top end. A hot little motor like this should turn well over 10,000 RPM, as it's not a big honking V-8. FWIW, I've turned some of my race V=8's over 8,000 RPM, so 12K should be an attainable goal. I may have a windage problem in the case - have installed a PCV valve and have a breather can but have not installed it yet. The idea is to increase case volume to minimize compressing air on the back side of the piston - cuts down on oil sloshing and loss. I'm not losing much oil out the PCV, tho. |
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Post by noday on Aug 28, 2011 20:03:03 GMT -5
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Post by chuckspeed on Aug 29, 2011 5:13:19 GMT -5
WOW! That's the best damned technical explanation anyone's ever given! I suppose you know man can't fly, either... |
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Post by stephensimmons on Aug 29, 2011 7:02:58 GMT -5
Why wont it?!? With the proper porting and a set of big valves and a CORRECTLY grinded cam anything is possible! Of course you have to have your compression right to match your setup and a quality carb properly tuned. Oh and your clutch have to be tuned also  Back in the day I use to race minibikes at the drag strip that was a 1/8 mile long and beleive it or not (because I really dont care if you do because I did it) I took a 13hp honda and a salisbury torqe converter system, added a billet rod and flat top piston to the engine, addes big valves and hellashous ports to the head and a custom header and a tillson 855 carb T-sloted for alky and never ran head up BUT if given 2 bulbs on the tree NO 600 0r 750 motorcycle could catch it EXCEPT the GSXR750 and that was righ at the light! So NEVER say what a engine CANT do.  The only pick I got of the beast! Sorry didnt mean to thread jack so lets return back to the topic! Dont sell your scooter you will get it right it just takes time. |
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Post by noday on Aug 29, 2011 8:40:57 GMT -5
OK..... there is a significant design limitation of the GY6 engine re: max RPMs
the head, cam, valve design does not lend itself to high RPMs
above about 8K you start to get significant valve float.
once the valves float you can not get air in and out of the cylinder, no matter how much you port and polish.
you can get to around 9K by tuliping the valves and adding stronger titanium springs
because the head is aluminum, the stronger springs cause the valve seats to sink.
even so you are stuck with the mechanical rocker arms.
once float happens, not only does power drop off quickly, mechanical failure looms. rocker arms are hitting the valve ends, not pushing them. valve piston contact happens
So.... unless you have a way around this design limitation, you will not get that 10-12K RPMs
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Post by Pony66 on Aug 29, 2011 10:04:01 GMT -5
OK..... there is a significant design limitation of the GY6 engine re: max RPMs So.... unless you have a way around this design limitation, you will not get that 10-12K RPMs This feels like logic, but do you have any actual facts? Im not calling you out or saying your wrong. I would like to read some dyno reports and see some engines blowing up. Is there any data showing the revs of a failing engine. The same holds for a performance 50cc, any dynos on a high reving engine? |
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Post by stephensimmons on Aug 29, 2011 10:58:14 GMT -5
OK..... there is a significant design limitation of the GY6 engine re: max RPMs the head, cam, valve design does not lend itself to high RPMs above about 8K you start to get significant valve float. once the valves float you can not get air in and out of the cylinder, no matter how much you port and polish. you can get to around 9K by tuliping the valves and adding stronger titanium springs because the head is aluminum, the stronger springs cause the valve seats to sink. even so you are stuck with the mechanical rocker arms. once float happens, not only does power drop off quickly, mechanical failure looms. rocker arms are hitting the valve ends, not pushing them. valve piston contact happens So.... unless you have a way around this design limitation, you will not get that 10-12K RPMs In your 1st post you said "it will not get 10-12k rpms". NOW your saying the engine is limited  . All engines are limited thats why we build them ;D And there is always a way around limitations of an engine is your willing to do yor research and some trial and error. With the cheap parts they make for these things I didnt say it would last long BUT it could be done. |
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Post by noday on Sept 2, 2011 8:14:17 GMT -5
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Back in the day I use to race minibikes at the drag strip that was a 1/8 mile long and beleive it or not (because I really dont care if you do because I did it) I took a 13hp honda and a salisbury torqe converter system, added a billet rod and flat top piston to the engine, addes big valves and hellashous ports to the head and a custom header and a tillson 855 carb T-sloted for alky and never ran head up BUT if given 2 bulbs on the tree NO 600 0r 750 motorcycle could catch it EXCEPT the GSXR750 and that was righ at the light! So NEVER say what a engine CANT do.
. All engines are limited thats why we build them ;D And there is always a way around limitations of an engine is your willing to do yor research and some trial and error. With the cheap parts they make for these things I didnt say it would last long BUT it could be done.