Can someone explain TDC on a GY6 and extra space for stroker By dmartin95 (Read 4,300 times)

[dmartin95]

This thread is about the 150cc GY6

I'm confused about something... As I understand (and please correct me where I'm wrong) it, Top Dead Center is when the piston reaches it highest point of the stroke. BDC is the bottom most point of the stroke....

In the picture I'm providing, it shows a gap at the TDC point between the piston top and head bottom --- What is this gap for?

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2nd part of this post is about that gap.... I have been told that you can use a stroker kit as high as +2,5mm with out having to add spacers. 

So, I'm guessing that addition 2.5mm stroke boost would fill the gap?

In short, can someone explain what that gap is for and the consequences/benefit of filling that gap with a stroker is?

[rapidjim]

Jan 3, 2015 3:21:50 GMT -6 dmartin95 said:

This thread is about the 150cc GY6

I'm confused about something... As I understand (and please correct me where I'm wrong) it, Top Dead Center is when the piston reaches it highest point of the stroke. BDC is the bottom most point of the stroke....

In the picture I'm providing, it shows a gap at the TDC point between the piston top and head bottom --- What is this gap for?

---------------------------

2nd part of this post is about that gap.... I have been told that you can use a stroker kit as high as +2,5mm with out having to add spacers. 

So, I'm guessing that addition 2.5mm stroke boost would fill the gap?

In short, can someone explain what that gap is for and the consequences/benefit of filling that gap with a stroker is?

I have seen this gap on some kits before and can only assume it is for either valve clearance or a provision for a stoker crank. I would think more the latter.

Installing a stroker crank will give you more displacement and power. lets do the math to explain this.

The standard stroke of a 50cc motor is 41.4mm and the bore is 39mm.

The formula for figuring displacement in mm is Bore(mm)*Bore(mm)*Stroke(mm)*0.0031416/4

So 39 * 39 * 41.4 * 0.0031416 / 4 = 49.4561 or 49.46cc

Lets change just the crank to a 44mm stroker crank

39 * 39 * 44 * 0.0031416 / 4 = 51.612 or 51.61cc

Remember, it is always best to check valve clearance when building a motor with a stroker crank. Safe is better than sorry

[cyborg]

the name of the space is the cumbustion chamber,,,and i agree with jim it is a provision for a stroked crank,,and he is also correct that clearances need to be extensively checked as cam profiles may generate enough speed in the valve that they move faster than the piston and hit,,,this is why you see "valve pockets" or eyebrows cut into the piston tops of most modern bikes it is to make room for the invading valve edge,,you need to SLOWLY rotate the engine by hand thru a cycle or two to make sure there is no interference ,,,another thing to think about when you start squeezing stuff very tight is fuel sensitivity,,,see what you started?,,,and as a answer to your other question about "filling" the space,,the stroked crank draws in more air just by the fact that the cylinder just got longer by the piston moving up and down farther,,,and the compression will be more "enough for fuel sensitivity to be an issue",,also the redline will be lowered a bit or should be because the stresses put on the crank journals rod and everything is amplified,,,,,we could go on about this for quite some time,,,current thought in modern engines is short stroke moving very fast is better than long slow where you have to worry about rod angulartity and a whole other host of inherant problems with a long stroke motor,,,but it's a scooter and it's a relatively cheap way to make more power without getting too exotic,,,

[dmartin95]

Jan 3, 2015 9:58:55 GMT -6 cyborg said:

the name of the space is the cumbustion chamber,,,,",,,

Thanks for clearing that up. I always thought the "combustion chamber" was the entire Volume of the cylinder at BDC.

Do all engines have this provision for a stroker? Do some engines come factory where TDC goes all the way to the top of the head leaving no gap/combustion chamber?

It has me curious as to why GY6 produces would leave the gap and not take advantage of the potential extra HP.... 

and he is also correct that clearances need to be extensively checked as cam profiles may generate enough speed in the valve that they move faster than the piston and hit,,

Can you elaborate/expand on that.... Are you saying the valves and piston could actually collide from rpm ( speed = RPM? ) excesses/deficiencies?

The cylinder I have is a "special" cylinder, it's not the same height as a stock GY6 (It's 1.5mm taller) and is made to accommodate the specific 3500 (+3.5mm stroker) ....  However, 3.5mm(stroke) - 1.5mm(extra cylinder height) = 2mm extra stroke,,... SO even with my extra tall cylinder, I will be filling that gap leaving only a .5mm gap. 

another thing to think about when you start squeezing stuff very tight is fuel sensitivity,,,see what you started?,,,and as a answer to your other question about "filling" the space,,the stroked crank draws in more air just by the fact that the cylinder just got longer by the piston moving up and down farther,,,and the compression will be more "enough for fuel sensitivity to be an issue

When it comes to "fuel sensitivity", I may have a cheat....  I will be using an Ecotrons EFI and it "self tunes".... If I fail the EFI, I may be screwed for a while because I'm not a carb guy and still have a lot to learn when it comes to upjetting and tuning a carb.... 

also the redline will be lowered a bit or should be because the stresses put on the crank journals rod and everything is amplified,,,,,we could go on about this for quite some time,,

At sometime point you may get frustrated with me and tell me to go take a class.... But let me tell ya brother, I appreciate your help and hope you are willing to go on for quite some time. 

REDLINE is something I'm glad you brought up... With this build, it's a Taida 4 valve head.... Is it the case that in order to get the benefit of the 4 valves you need high RPMs? With this build I was planning for high RPM's.... It will have ceramic coated cylinder, a water cooler, and an oil cooler to deal with the excess heat produced from the higher RPMs'....

In where you see I'm making a mistake or using poor terminology, please correct me.

current thought in modern engines is short stroke moving very fast is better than long slow where you have to worry about rod angulartity and a whole other host of inherant problems with a long stroke motor,,,but it's a scooter and it's a relatively cheap way to make more power without getting too exotic,,,

Ok short stroke moving fast is good, long and slow problematic... But what about long and fast ( <----- Ok, this just sounds wrong for two grown men to be saying these things to each other....   ) 

[dmartin95]

Jan 3, 2015 8:19:20 GMT -6 rapidjim said:

Jan 3, 2015 3:21:50 GMT -6 dmartin95 said:

Remember, it is always best to check valve clearance when building a motor with a stroker crank. Safe is better than sorry

What is the difference between valve lash and valve clearance?  

[Alleyoop]

Valve Lash is the valve gap between the rocker and valve stem, Valve Clearance is when open will the piston come up and bang the heck out of it.

[cyborg]

Hi speed strokers do exist ,,, special parts are needed ,,,one Carrillo rod is $400 minimum,, a pankle rod $600 plus ,,, then you need a special crank then the cases aren't up to the task,, it's a vicious circle,,,,Alley knows this better than anyone he used to drag race,,, doing a mild stroker with decent parts and run it like a normal person it will last a good long time,,, if you try and build a radical engine it will not last,,,even one of our members was drag racing a scoot recently it ran better with the stock dimension crank,,, but for the street a stroker can be a real nice build,, good real world power ,, nice take off with a real nice meaty midrange,,, then the final drive mod that Alley did on his and you have a nice package

[dmartin95]

Going back to original post, can someone tell me how much space is available for a stroker kit in a stock GY6 150cc?

I'm pretty sure it's either 2, or 2.5mm but that extra .5mm is critical info I need to know.

[cyborg]

some stroker kits provide a spacer for the bottom of the cylinder,,,and DM i'm sure it varies engine to engine / manufacturer to manufacturer,,,and as we've seen on this forum and i've seen on others there is no "standard" when it comes to these scoots,,you must measure and verify,,,and also you need to calculate growth if you start squeezing it tight,,my last desmoquatro mill had a warning lite in the tach,,,when it was lit no going over 3-4000 rpm,,,or stuff would start running into each other,,,Alley had a stroked crank put in his scoot engine,,,build done by Jim at rapid repair,,,i'd give him a shout,,,click on his scrolling tile ad at the top of the page,,,i'm sure he has all the answers to your questions

[dmartin95]

Mar 28, 2015 11:22:23 GMT -6 cyborg said:

some stroker kits provide a spacer for the bottom of the cylinder,,,and DM i'm sure it varies engine to engine / manufacturer to manufacturer,,,and as we've seen on this forum and i've seen on others there is no "standard" when it comes to these scoots,,you must measure and verify,,,and also you need to calculate growth if you start squeezing it tight,,my last desmoquatro mill had a warning lite in the tach,,,when it was lit no going over 3-4000 rpm,,,or stuff would start running into each other,,,Alley had a stroked crank put in his scoot engine,,,build done by Jim at rapid repair,,,i'd give him a shout,,,click on his scrolling tile ad at the top of the page,,,i'm sure he has all the answers to your questions

I can't get a measurement for a couple of days and was hoping someone had the info on hand.

Let me clarify. The engine I'm talking about is a "stock GY6 150cc 157QMJ". While you may find different accessories like different stators, the engine will always be the same in the sense that it will have a a 57.4 x 57.8 (bore x stroke). They also have the same clearance volume.

I'm building a high compression motor with forged piston and ceramic coated cylinder. 

The cylinder I'm using is a water cooled Taida and it's taller than the stock 157QMJ cylinder by .8mm. If memory serves correctly the clearance volume in a stock 157QMJ is 2.5mm. That gives me a clearance volume of 3.3mm but the stroker I'm using is 3.5mm. 

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Here's the thing, Taida's techs designed this engine for me personally. They machined the piston so It wouldn't hit on the down-stroke, provided chain with extra links, extra long studs, etc etc... Taida knows their products and even though this one in particular is their first run like this, I just can't believe they would make a mistake like this.

But I'm still coming up .2mm short..... Smallest riser is 2mm. Perhaps I'm supposed to double or triple up base gaskets.  

[cyborg]

Are you sure they didn't machine some off the piston top? Do you have the kit already?,,, at any rate I would be giving them a call and voice your concerns,,,they probably have the answer or a part you may need