Is there a special piston for 4 valve heads

[anthonyvs]

On the piston i got with my 61mm big bore kit there is only 2 small clearance holes for the valves. how will 4 valves be able to clear the piston?



I know the mod sin did had the same piston but how can it work. especially with a high lift cam.

[earlwb]

They have a larger combustion chamber so the valves have more clearance, but you get a little less compression. If you use a high lift cam you might have problems, I don't know. You would have to put in some clay and turn the engine over, then remove the head and check the clearances (how thick the clay is where it touches. The crown of the piston is fairly thick, so one could dremel or machine off a little material for valve clearances too. it sorta sucks that it is on the piston edges though.

[anthonyvs]

Dam i wish somebody would of talked me out of buying this head. I'm using the cam that came with the head. They advertise it as a racing cam. i emailed the place i bought it from. i'll wait and see what they say.

[earlwb]

Isn't that the same cam that Syn used on his conversion. it must be OK as he never complained about it or had problems.

[gy6rocket]

Yep the 4 valve head is taller than the 2 valve for clearances, thats why you need to use a new chain. This does however make the combustion chamber larger, hence the compression lower. Perhaps this is why the 4 valve does not show a lot of improvement, it has more flow but lower compression.

If you could get the right piston for the 4 valve head i'm sure it would show huge improvements. Since the head is slightly higher i'm willing to bet a 59, 61, 63 etc - dome top piston would yield great results. \

I am not aware of any manufacturer of these yet but you could always have a custom one made

Here is what I am talking about, I found this pic, not for a GY6 but it gives you an idea of what the piston should look like in a high performance 4 valve head.



Either way, don't regret the purchase, it will still work with that piston, just don't expect big gains.

[anthonyvs]

yeah its the same cam he used. I should of done more home work on this. ok lets say that i do get a hold of a domed piston. The only way i could get more performance out this head is by geting more compression out of it. so would that mean shaving it down?

[gy6rocket]

No you do not need to shave the head to get more compression, The domed piston will give you more compression by the nature of the design.

Permit me to elaborate with a quick sketch, it shows why this works:



As you can see in the pic the domed piston will create a smaller chamber, squeezing the air/fuel mix more thus creating more power. The higher you compress the mix the more power an engine will have. Also the higher you go the higher octane you need to run to avoid detonation.

[tp555]

Shaving the head changes your cam timing.The cams open later by whatever amount you cut over the radius of the cam gear in degrees.Trying to keep it simple.Custom pistons big $$.Try to find the
piston made for the head.

[earlwb]

TP555 is correct, you cannot shave the head as that might change the cam timing. Resurfacing the head will retard cam timing. A serious gearhead can get a new cam sprocket made with the gears recut to whatever they need to advance or retard the cam timing too. There has been a couple of guys who fabricated (machined) an adjustable cam timing setup with is pretty neat. But obviously they haven't done that with a four valve camshaft and head as far as i know. But the scooter racing people probably have done it.
If you want to increase compression, you need a piston with a higher crown, like a domed piston for example. Idealy you want a domed piston with recesses for all four valves.

But if I remember right, that 4 valve head was, very likely, intended for markets where they use low grade gasolines (think low octane fuels like regular gasolines or worse). So it'll work fine with the regular two valve piston. But eventually someone will start making high compression pistons for it. They may already be doing so, but no one is importing them yet.
Not everyone has easy access to premium grade gasolines.

[anthonyvs]

excellent explanation. thank you.

[gy6rocket]

I'm keeping my eye's open for the 4 valve domed piston, its the only thing holding me back from the 4 valve head

[anthonyvs]

I dont feel its the wind resisatnce anymore and here is why. I took it out for long ride yesterday and even going down hills i never got over 58 mph. its like the engine just couldn't get enough air into it. It felt restricted. with my new exhaust and K&n filter i should be getting plenty of volume thru it. And a clue to me is i have read post where a person would have really good performance top end wise only to find it was becuase they had no valve clearance and it was allowing the valves to open up more then usual.
As a experiment i should try and install the cam that came with the new head and see if i get more speed out of it. does anyone think the cam for a 4 valve head would not work for my stock head? they look exactly the same

[gy6rocket]

It should work, the rockers on the 4 valve are designed so 2 valves share 1 cam lobe. The cams are in the same spot so you are good to go.

[motomech]

Maybe Wisco, don't they list dimensions? I can't download their cataloge here. He could call.
The combustion chamber seems to be recessed a bit in the four valve head, so a dedicated design piston crown would seem appropriate. Want to keep combustion chamber vol. sm., while at the same time not obstructing flame front propagation with overly large piston dome. Tight squish band and the resultant deck height being critical to running higher compression.
Unknown cam dimensions could be determined with degree wheel and dial gauge.

[anthonyvs]

nobody has had any luck finding a domed piston for the 4 valve head. I looked as well. I'm not going to have 1 special made.
As far as geting a degree wheel , i wouldnt have a clue at this point. This is my first attempt at modifying engines. so I think i have a long way to go before i'm thinking like an engineer when it comes to modying engines. For now i listen to the scooter gods. Thats you guys in this forum

[earlwb]

Well , heck, just go ahead and use the head. See how it works and all that stuff.
Syn liked it a lot when he put one on. You ought to be able to like it as well.
Besides I think you can run regular grade gasoline too.

[geri]

Hi everybody,

Iam from Europe, Austria. Maybe you know Wiener Schnitzel, Mozart, Falco or Schwarzenegger. Thats were I come from

Beside this, I would like to tell you that I use to ride a GY6 with a 4 Valve 125ccm Motor
That machine is called "Dealim Otello" and is very popular here in Europe.

of corse I also would be interessted to get this domed piston. Cause we have very high premium fuel (100 Oktane ROZ) detonation would be no problem. I wounder if its possible to get this piston here in EU, never heard about before, but I will keep my eyes open!

Anyway, once I tried my machine with 91 ROZ and 95 ROZ, but with 100 ROZ it runs much better.

[earlwb]

geri, Thanks and let us know if you are successful.
I think that it is far more likely that someone in Europe or even Austrailia might have a four valve high compression piston available. Here in the USA scooters aren't all that popular yet.

[anthonyvs]

hey i found out today that 4 valve head cams are not compatible with 2 valve head cams. the 2 valve head cams are longer. i can tell just by looking at the new cam that it has more lift. the lobes are way longer with more duration because there not as pointy as the 2 valve cam. here i'll take a pic. I have access to calipers at work . i could measure both to compare them.
<a href="http://s253.photobucket.com/albums/hh68/anthonygy6/?action=view¤t=Picture095.jpg" target="_blank"><img src="http://i253.photobucket.com/albums/hh68/anthonygy6/Picture095.jpg" border="0" alt="Photobucket"></a>

the cams are not even pointing to the same location when i line them up exactly. also at first i thought there was a big difference in lobe height. but its deceiving because the rear bearing is smaller making the lobes look bigger. guess i'll have to measure them.but there for sure longer duration.