Big valves vs. stock

[The moped learner]

Im getting a 100cc big bore kit with big valves not like 69mm or 64mm valve length do any of you think ill get better power band with bigger valves here is what im buying

http://m.ebay.com/itm/151047606403?nav=SEARCH

[blue]

Im getting a 100cc big bore kit with big valves not like 69mm or 64mm valve length do any of you think ill get better power band with bigger valves here is what im buying

http://m.ebay.com/itm/151047606403?nav=SEARCH

  carefull on the size I have seen a few that had long stems on the valves and blew the motor up.

[The moped learner]

This is stock 64mm length  valve but are bigger, do you think it will increase my power band?

[BettinANDlosing]

Bigger valves will increase torque but not top end power. It's a nice upgrade tho I've done it to the gy6 150cc

[The moped learner]

Thanks thats what i love to hear!

[BettinANDlosing]

Just be sure you a. Torque down the head nuts with a good accurate torque wrench b. Adjust the valves to .1mm and c. Spin the motor over by hand before trying to start it to make sure the valves and everything clear. Did the kit come with a cam too? Running that setup with a stock cam you will make %25 less power than with a good high lift cam. Best of luck with the build.

[The moped learner]

Yes it comes with a A9 cam

[BettinANDlosing]

Good deal. Make sure with all those new parts rotating you break it in nice n easy, let it idle for 5 minutes or so then shut it down let it cool of then warm it up and go drive it easily. Also even though the head is big valve you could spend an extra $50 at the machine shop and have them port it. Totally worth the extra money while you have it out.

[The moped learner]

How much do you think porting them would cost?

[BettinANDlosing]

Depends on what machine shop you go to. Just look around in your city and tell them you have a small motorcycle head to lightly port. It won't be more than $100

[The moped learner]

So what should I set the the intank and exhaust at

[BettinANDlosing]

Set the valves to factory .1mm too start, maybe tighter but only after the motor is run in. Make sure you set the cam timing correctly too

[Yager200i]

Valves and intake can be too large, and they can be too small.

If they're too small, you rob the engine of power by starving it for air.

If they're too big, you also rob the engine of power, because you no longer have high airflow speed and that momentum that packs more air into the cylinder just before the intake valve closes. And you open the intake to air reversion on valve overlap if your valve timing isn't right, or your exhaust isn't properly designed (most 4-stroke scooter exhausts aren't) to bounce negative pressure pulses back toward the exhaust valve just before it closes.

There's some very complex math to do if you want to determine the exact right valve size and intake volume... taking into account air temperature and density, engine displacement, valve timing, intake air speed, intake volume and length, and intake resonance.

Get it wrong, and your engine will still run, but it'll be a dog. Get it right, and you can increase engine power by 10-20% pretty easily.

http://mototuneusa.com/homework.htm

Now, you may be asking, "If I 'port / tune' the intake and valve size so it's most efficient at high engine RPM, won't it be a dog at lower RPMs because that ram air effect is gone?"

Not necessarily... you can cut what are essentially screw threads into the intake. At lower engine RPMs (and hence,  lower intake air flow) they create a laminar boundary that has the effect of squeezing down the size of the intake, keeping intake air flow speed high and thus still giving a ram air effect. At high engine RPMs, that laminar flow boundary gets squeezed further and further out toward the intake walls, creating the effect of a larger intake, while still maintaining high airflow speed.

The screw threads (Powre Lynz is what Mike Holler calls them) cut into the intake walls also help if you've got a carbureted engine in that they ensure any fuel dropping out of the airstream get broken up, bounced around and evaporated.

http://www.allpar.com/fix/holler/head-porting2.html

In short, try the following combinations:
stock valves, stock intake
stock valves, high velocity intake
big valves, stock intake
big valves, high velocity intake

I'm betting you'll find that the stock valves, high velocity intake combination will give you the most power and engine tractability.

[BettinANDlosing]

Nailed it^^^^ this it's why you'll want to polish the exhaust port and not the intake port, you don't want the intake to be perfectly smooth or else fuel will puddle. All and all these are Chinese gy6 engines and unless your building a race engine in Puerto Rico the bolt on big valve head will gain you a bit of torque. There's a great book, "Scientific Design of Exhaust and Intake Systems", that goes through really how much math and measurement goes into really maximizing the performance of the engine.

[The moped learner]

Well you guys well be hearing how it comes out in two days i cant polish the exhaust port myself i have the tools