supra5677
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posted January 30, 2007 10:00 AM
piston to valve clearance
Im getting an intake cam grinded to .380 lift with the same duration.. Im running a thin head gasket which is .018 thickness stock is .030. My question is can I run 105 lobe center with this intake cam? Muzzy runs 105/107 with .410 lift, and webcam runs 103/105 with .383 lift. My math tells me 107 but that can't be right.. Can somebody help. I believe 1 degree of retard buys you ..010 more clearance..
Supra
   
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entropy
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posted January 30, 2007 10:55 AM
Supra, here is some info from one of my experiments:
quench .037-.040"
int cam: duration =260 @ .040 lift = .383
Intake LC at 104,
valve clearance with clay:
radial clearance = .046-.048 at the tightest spot
axial = .063-.065 where pocket breaks over to dome
axial = .072-.075 deep in the pocket
According to some experts that is pretty tight, so i had planned to set it up at int LC at 107-108.
TAKE ALL THIS WITH A GRAIN OF SALT; I AM A STUDENT, NOT A TEACHER!!!!
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supra5677
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posted January 30, 2007 11:02 AM
I took my calculations from Y2Ks' zx12r engine math on zx-12r.org.
stock piston to valve clearance .041 at 104 lobe center. His piston to valve was .022 but his intake cam was advancd 1.5 degrees from stock at 102.5 lobe center.. Ide like to keep it at 105 so I can keep some of my midrange anybody else has an idea?
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dougmeyer
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posted January 30, 2007 02:16 PM
Supra,
First of all WHY less lift? But whatever.....
As usual, you're working with only part of the story. You really have no way of computing this unless you know the lift curve of the lobe shape you are having "custom ground". As I'm sure you know the closest point of inlet valve to pistin clearance occurs not at TDC but at a point prior to tdc. The inlet valve is already opening of course (during overlap) and the exhaust is closing. (this is why the clearance on the inlet is more critical - because the valve and piston are on a "collision course" whereas on the exhaust the piston is "chasing" the valve.
Anyway, your valve is open a certain amount of lift per dgree of crank rotation ( hence, distance upward or downward of piston movement). The RATE of valve movement changes with the shape of the lobe and until you know exactly how much lift you have at, say.... 5 crank deg BTDC, you can't know for a given stroke length how close the valve is to the piston. This changes with every cam lobe shape or "grind". Your cam grinder should be able to answer your question I would think, BUT he would have to know every dimension in play including the shape and depth of the valve relief in the piston.
My personal choice has always to run the highest cmompression I can and adjust the L/C to get the minimum clearance I can live with.
I love ya', man.
Doug
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GUNNER
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posted January 30, 2007 02:21 PM
It's widely accepted that .040 PTV on the intake side is living large. I try and never go below that myself.
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Y2KZX12R
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posted January 30, 2007 03:01 PM
 Edited By: Y2KZX12R on 30 Jan 2007 16:32
You can run the intake valve to piston pretty close on these little engines.
Like you posted my bike from the factory was .022" I was shocked!!! the cam was at 102.5
I ran .028" for a long time. No problems. Even bouncing it off the limiter almost every time out on it.
The intake valve is closest to the piston at about 8 degrees ATDC plus or minus a few degrees. but it doesnt really matter, you need to run it thru a cycle anyway to check it.
Like doug said, get the intake valve as close as you can to the piston. I've never seen a big cam engine that didnt like it as tight as possable.
You cant possably know what cam timing you can get away with untill you have all the components and do a mock assembly and check clearance. Theres just too many factors.
Set your squish first.... compression second... and intake cam timing as tight as possable.
Remember when setting cam timing to account for lash. (oops)
This time my engine is going to be dialed in much tighter in several areas. The main reason is I'm not going to try and start it on 12v this time sence I had to go with 24v starting anyway. So its getting 1-1/2 points of compression and much tighter squish and much tighter intake to piston clearance.
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dougmeyer
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posted January 30, 2007 03:19 PM
8 ATDC Roger that. I need to slow down and proof.....
I also run less than 1mm, BUT I'm not dragracing, I can shift carefully. I wouldn't ever tell someone else to do it.
Y2, Do't you always set your cam timing with zero lash or some preload?
D.
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Y2KZX12R
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posted January 30, 2007 04:30 PM
Edited By: Y2KZX12R on 30 Jan 2007 17:15
quote:
8 ATDC Roger that. I need to slow down and proof.....
I also run less than 1mm, BUT I'm not dragracing, I can shift carefully. I wouldn't ever tell someone else to do it.
Y2, Do't you always set your cam timing with zero lash or some preload?
D.
Shit with my spelling errors its a wonder anyone knows what i'm talking about. 
Yea I dont usually reccomend ultratight clearances either. Some carbon build up or bad fuel and its a bad day quick. But I'll take that risk on my own engines.
Yea, I was thinking lift correction.... checking duration not cam timing.... dah. stupid me.
My mind is on 9 things.
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dougmeyer
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posted January 30, 2007 06:32 PM
OK were even - I'll proof for you, you proof for me!
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entropy
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posted January 31, 2007 04:20 AM
DM:
the closest point of inlet valve to pistin clearance occurs not at TDC but at a point prior to tdc. The inlet valve is already opening of course (during overlap) and the exhaust is closing. (this is why the clearance on the inlet is more critical - because the valve and piston are on a "collision course" whereas on the exhaust the piston is "chasing" the valve.
Y2K:
intake valve is closest to the piston at about 8 degrees ATDC
DM:
8 ATDC Roger that. I need to slow down and proof.....
could you guys please explain this a bit more???
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Y2KZX12R
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posted January 31, 2007 05:06 AM
The intake valve starts to open well before the piston is at TDC starting the intake stroke.
So the intake valve is opening and the piston is still coming up in the cylinder. The piston is decelerating then hits TDC and starts to accelerate down on the intake stroke. The intake valve is accelerating open faster than the piston is accelerating down the bore. Eventually the piston is out accelerating the opening of the intake valve because its opening rate is decreasing, and the pistons acceleration down the bore in increasing. The crossover is usually at about 8 deg. ATDC give or take a few degrees.
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entropy
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posted January 31, 2007 05:35 AM
Edited By: entropy on 31 Jan 2007 05:36
Jim,
sorry to be so dense...
so is the primary danger zone for the intake at 8 degrees BTDC as the int valve starts opening and the piston is coming up (collision course?)
or
Is it 8 degrees ATDC as the int valve continues to open and the piston starts down?? (chasing?)
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Y2KZX12R
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posted January 31, 2007 07:03 AM
8 deg ATDC
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entropy
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posted January 31, 2007 07:18 AM
good!
and where is the danger zone for exhaust???
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Y2KZX12R
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posted January 31, 2007 08:05 AM
I'm not sure on the exhaust. But it would be somewhere about the same but BTDC.
Doug?
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dougmeyer
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posted January 31, 2007 04:53 PM
It can actually be on either side depending of the lobe center.
Go here and have a look at the diagram where the overlap occurs :
http://www.muzzys.com/articles/lobe_centers.html
Visualize the piston rising to tdc versus the valve motion as the intake is opening and the exhaust is closing.
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slug
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posted January 31, 2007 06:15 PM
how much of the clearance is used up when parts expand as engine gets hot? I know there is some expansion and all that.
Just curious, stuff like this is just inherently cool to know. 
 
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