need help reading piston burn pattern

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bladefrost
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need help reading piston burn pattern

Postby bladefrost » Sun Sep 07, 2014 11:30 pm

Hi guys. Im from the philippines. Here very popular drag racing using mopeds. Please comment on my piston burn pattern. Bike is down on power.
Specs:
52mm bore
27mm intake valve
23mm exhaust valve
12.5:1 comp ratio
Cam 273deg. Intake open 33deg btdc and close at 60deg abdc. Lift is 9.2mm

Here is the piston https://www.dropbox.com/s/afx2dpdrhtd0opt/20140724_081753~01.jpg?dl=0

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Re: need help reading piston burn pattern

Postby Belgian1979 » Mon Sep 08, 2014 4:42 am

Is that a contact from the exhaust valve I see in that exhaust valve pocket ?

Also you have damage on edges of the piston.

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Re: need help reading piston burn pattern

Postby bladefrost » Mon Sep 08, 2014 4:53 am

Yes it is the exhaust valve contacted the piston. I fixed that already. Also the piston damage on the edge is cause by me dropping the piston on the floor. I didnt bother to deburr it.

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Re: need help reading piston burn pattern

Postby BrazilianZ28Camaro » Mon Sep 08, 2014 6:33 am

The A/F mixture isn't reaching the piston' clean areas , probably due a shrounded intake valve, bad intake port , incorrectly shaped chamber or piston design.

Those fins around the intake valve pocket are restricting overlap flow around the piston, I'd smooth them , making the piston dome favorable to propagation of the flame front, even if it costs a little CR.
'71 Z28 Pro-Street Camaro
Pump gas -T-ram - All motor SBC 427"
3308 lbs- leaf spring/Caltrac Car
28x10 Hoosiers
New best 1/8 mile ET
1.45 60' / 4.16 330' / 6.45@108.75mph

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Re: need help reading piston burn pattern

Postby mag2555 » Mon Sep 08, 2014 7:01 am

A classic example of trying to max out the compression and killing off the burn flame!
This example you posted of non use of that side of the chamber is also rocking the piston in the bore on the power stroke which in and of itself may unseat the rings and or ware the bore in relation to how the piston gets kicked around.
You will make more power my friend with less compression in all likelihood !

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Re: need help reading piston burn pattern

Postby bladefrost » Mon Sep 08, 2014 8:26 am

BrazilianZ28Camaro wrote:The A/F mixture isn't reaching the piston' clean areas , probably due a shrounded intake valve, bad intake port , incorrectly shaped chamber or piston design.

Those fins around the intake valve pocket are restricting overlap flow around the piston, I'd smooth them , making the piston dome favorable to propagation of the flame front, even if it costs a little CR.


Wow this is really nice sir. An eye opener. I thought my piston shape is already good. The clean areas is maybe due to piston very near the head at tdc? Im trying my best to explain in english language. I cant provide more pictures maybe the cylinder head also. My bikes response is really slow. Accleration is flat. Tried hard jetting and ignition timing adjustment but no luck

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Re: need help reading piston burn pattern

Postby bladefrost » Mon Sep 08, 2014 8:32 am

mag2555 wrote:A classic example of trying to max out the compression and killing off the burn flame!
This example you posted of non use of that side of the chamber is also rocking the piston in the bore on the power stroke which in and of itself may unseat the rings and or ware the bore in relation to how the piston gets kicked around.
You will make more power my friend with less compression in all likelihood !


Maybe thats why my accleration is flat and power at high rpm is really poor no matter how i try jetting and ignition timing adjustment. By the way my timings are
30 deg at 2500 rpms
35 deg at 3000 rpm
37 deg at 3500 rpm
37 deg at 9000rpm
36 deg at 9500 rpm
35 deg at 10000 rpm
34 deg at 10500
33 deg at 11500
33 deg 14000


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BrazilianZ28Camaro
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Re: need help reading piston burn pattern

Postby BrazilianZ28Camaro » Mon Sep 08, 2014 9:33 am

bladefrost wrote:
BrazilianZ28Camaro wrote:The A/F mixture isn't reaching the piston' clean areas , probably due a shrounded intake valve, bad intake port , incorrectly shaped chamber or piston design.

Those fins around the intake valve pocket are restricting overlap flow around the piston, I'd smooth them , making the piston dome favorable to propagation of the flame front, even if it costs a little CR.


Wow this is really nice sir. An eye opener. I thought my piston shape is already good. The clean areas is maybe due to piston very near the head at tdc? Im trying my best to explain in english language. I cant provide more pictures maybe the cylinder head also. My bikes response is really slow. Accleration is flat. Tried hard jetting and ignition timing adjustment but no luck



I'm not a expert on bike engines, but yes, the combustion need a path to travel and expand fast during the power stroke. If all the piston area is too close the chamber the combustion will be incomplete and very slow, you need a fast combustion to rev well.

I'd start blending all the sharp edges of that piston and smoothing the chamber around the valve job to increase low lift flow.

You might know that single cylinder engines are very sensitive to exaust design.This can make or break the engine performance.
'71 Z28 Pro-Street Camaro
Pump gas -T-ram - All motor SBC 427"
3308 lbs- leaf spring/Caltrac Car
28x10 Hoosiers
New best 1/8 mile ET
1.45 60' / 4.16 330' / 6.45@108.75mph

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Re: need help reading piston burn pattern

Postby Powertrip » Mon Sep 08, 2014 3:48 pm

I see a few problems. It looks like there is an angle quench machined into the combustion chamber, yet the piston has a flat quench? You mentioned you damaged the piston by dropping it on the floor, you may have damaged the top ring land and are pinching the top ring. Why does the timing retard so much at high RPM's? The floatbowl on your caruretor looks to be at an angle, if too severe it will make it impossible to jet correctly.

As already mentioned, that piston top looks to be a flame front nightmare, some serious contouring and space around the spark plug needs to take place.

What does your exhaust look like?
The price of progress is trouble.

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Re: need help reading piston burn pattern

Postby bladefrost » Mon Sep 08, 2014 4:34 pm

I'm not a expert on bike engines, but yes, the combustion need a path to travel and expand fast during the power stroke. If all the piston area is too close the chamber the combustion will be incomplete and very slow, you need a fast combustion to rev well.

I'd start blending all the sharp edges of that piston and smoothing the chamber around the valve job to increase low lift flow.

You might know that single cylinder engines are very sensitive to exaust design.This can make or break the engine performance.


I can smooth out those edges but im afraid im going lower the compression ratio too much.
I did have a different exhaust installed once and the difference is really big.
Also i have very close quench clearance in this engine. Might also explain why i have clean areas on the piston?
The bike has small bore only 52mm and 54mm stroke totalling to 115cc.

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Re: need help reading piston burn pattern

Postby bladefrost » Mon Sep 08, 2014 4:42 pm

Powertrip wrote:I see a few problems. It looks like there is an angle quench machined into the combustion chamber, yet the piston has a flat quench? You mentioned you damaged the piston by dropping it on the floor, you may have damaged the top ring land and are pinching the top ring. Why does the timing retard so much at high RPM's? The floatbowl on your caruretor looks to be at an angle, if too severe it will make it impossible to jet correctly.

As already mentioned, that piston top looks to be a flame front nightmare, some serious contouring and space around the spark plug needs to take place.

What does your exhaust look like?


Piston also has angled quench just not clearly seen on the picture. Ive checked the top ring and it moves smoothly around its grooved.
The bike likes 34 to 33 deg timing at the top to rev more. If i put 36 deg it will have difficulty reving high especially when hot. Bike also like 37 to 40 deg timing at low rpm. This bike is really down on power and response.

Exhaust pipe
https://www.dropbox.com/s/nnsu1dzxoi1jai2/20140813_081800.jpg?dl=0

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Re: need help reading piston burn pattern

Postby BrazilianZ28Camaro » Mon Sep 08, 2014 4:51 pm

bladefrost wrote:[

I can smooth out those edges but im afraid im going lower the compression ratio too much.
I did have a different exhaust installed once and the difference is really big.
Also i have very close quench clearance in this engine. Might also explain why i have clean areas on the piston?
The bike has small bore only 52mm and 54mm stroke totalling to 115cc.



Everything on a performance engine is a compromise. If you choose max CR, the combustion will suffer due the piston design. IMO, if you blend the piston and lose 1 to 1.5 points in compression the engine might be still much faster.

Some old BBC pistons dome designs are 150HP WORST than a good shaped piston even with 1 to 1.5 point more CR.

Ideally the quench area directs the A/F mixture against the spark plug to accelerate the burn and make power with less ignition timing, but at the quench area theres very little combustion happening. On SBC FT piston engines the combustion won't reach the edge of the piston where the quench area is.

Additionally the adequate quench clearance will cool the piston top, supressing detonation; In you specific case, you must study carefully the piston top design you need.

Take a look on L4 multi valve engines and see the piston profile and chamber profile they have. Obviously your engine need a much bigger dome to get high CR, but you might notice how they treat combustion with each design.
'71 Z28 Pro-Street Camaro
Pump gas -T-ram - All motor SBC 427"
3308 lbs- leaf spring/Caltrac Car
28x10 Hoosiers
New best 1/8 mile ET
1.45 60' / 4.16 330' / 6.45@108.75mph

bladefrost
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Re: need help reading piston burn pattern

Postby bladefrost » Mon Sep 08, 2014 5:42 pm

Everything on a performance engine is a compromise. If you choose max CR, the combustion will suffer due the piston design. IMO, if you blend the piston and lose 1 to 1.5 points in compression the engine might be still much faster.

Some old BBC pistons dome designs are 150HP WORST than a good shaped piston even with 1 to 1.5 point more CR.

Ideally the quench area directs the A/F mixture against the spark plug to accelerate the burn and make power with less ignition timing, but at the quench area theres very little combustion happening. On SBC FT piston engines the combustion won't reach the edge of the piston where the quench area is.

Additionally the adequate quench clearance will cool the piston top, supressing detonation; In you specific case, you must study carefully the piston top design you need.

Take a look on L4 multi valve engines and see the piston profile and chamber profile they have. Obviously your engine need a much bigger dome to get high CR, but you might notice how they treat combustion with each design.


I will try to contour the piston making it smoother. Also you mention the wing on the valve pocket i will remove that also to unshroud the intake valve during overlap.
Is it good to make quench clearance at max so that the piston should barely touch the head at high rpm for max power? With regards to studying piston top design, i dont know where to start. Im just a backyard mechanic with no formal education on racing engines. I just read alot of articles online and try to apply it. I guess not all of it is giving good results on these small high reving engines. Or just maybe i miss something.
Thank you for the help.

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Re: need help reading piston burn pattern

Postby Dan Timberlake » Mon Sep 08, 2014 6:33 pm

Hi Bladefrost

"down on power" compared to when, and what engine condition and parts?
Did it run well when you first put that piston in? Any compression or leak down tests?
When you put in the cam did you "degree" it?

I'd dyno test the fuel system. 1/2 gallon of gasoline per hour per HP must come out the fuel line, thru the petcocks and filters.


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