exhaust valve to piston clearance
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exhaust valve to piston clearance

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    Default exhaust valve to piston clearance

    I am building an engine for gn racing. I had a problem with valve to piston clearance on the exhaust side. I advanced the crank sprocket 4 degrees and the clearances came into acceptable at .110 on exhaust side and .070 on intake. cam is an Isky 747d 288 292 @.050 on 107 lobe center. My question is: Is this an acceptable fix or is performance going to suffer. The cam already has 3 degrees of advance built in and I just added 2 more for total of 5 degrees advanced.

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    Quote Originally Posted by gn 724 View Post
    I am building an engine for gn racing. I had a problem with valve to piston clearance on the exhaust side. I advanced the crank sprocket 4 degrees and the clearances came into acceptable at .110 on exhaust side and .070 on intake. cam is an Isky 747d 288 292 @.050 on 107 lobe center. My question is: Is this an acceptable fix or is performance going to suffer. The cam already has 3 degrees of advance built in and I just added 2 more for total of 5 degrees advanced.
    Performance will suffer if it's not where the cam is designed to run.
    Are you checking with checking springs or actuall running springs? How are you checking, with clay or a dial indicator? What size pushrods and what kind of spring pressure?
    Have you actually degreed the cam or are you just arbitrarily moving it vor PV clearance?
    Last edited by scott foxwell; 06-16-2013 at 11:07 AM.
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    Quote Originally Posted by gn 724 View Post
    I am building an engine for gn racing. I had a problem with valve to piston clearance on the exhaust side. I advanced the crank sprocket 4 degrees and the clearances came into acceptable at .110 on exhaust side and .070 on intake. cam is an Isky 747d 288 292 @.050 on 107 lobe center. My question is: Is this an acceptable fix or is performance going to suffer. The cam already has 3 degrees of advance built in and I just added 2 more for total of 5 degrees advanced.
    That cam will never work in a GN

    Heres my 2 cents

    If this is not custom ground, then that cam is not ground on 107 LSA, and its not 3 degrees advanced.
    Isky cams are ground old school and they make no pretense as to where an engine they know nothing about wants the IC. So they are all gound straight up unless the customer orders it that way. Which I can't imagine why anyone would need to order a racing cam "ground advanced." So to somewhat correct Steel's comment "where it was designed" to run, Isky didn't design it to run in any particular place. Thats the builders job to determine. I wouldn't go so far as to say you "lose", as much as it will simply move the power band as opposed to straight up or retarded.
    That said, I have never seen a cam that was installed advanced from "ideal" for a given engine that lost as much power as installed retarded from "ideal", Where ever "ideal" is for your engine,

    I have run that profile. In fact I think it may in one of engines right now But its not ground on that LSA.

    You didn't post any actual degree numbers at .050 lifter rise so I don't know if it is or isn't advanced or how much.
    So I will throw a number out there, and an opinion, If the intake comes in around 32* at .050, I'd call it good to go. JMO



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    Quote Originally Posted by gn7 View Post
    That cam will never work in a GN

    Heres my 2 cents

    If this is not custom ground, then that cam is not ground on 107 LSA, and its not 3 degrees advanced.
    Isky cams are ground old school and they make no pretense as to where an engine they know nothing about wants the IC. So they are all gound straight up unless the customer orders it that way. Which I can't imagine why anyone would need to order a racing cam "ground advanced." So to somewhat correct Steel's comment "where it was designed" to run, Isky didn't design it to run in any particular place. Thats the builders job to determine. I wouldn't go so far as to say you "lose", as much as it will simply move the power band as opposed to straight up or retarded.
    That said, I have never seen a cam that was installed advanced from "ideal" for a given engine that lost as much power as installed retarded from "ideal", Where ever "ideal" is for your engine,

    I have run that profile. In fact I think it may in one of engines right now But its not ground on that LSA.

    You didn't post any actual degree numbers at .050 lifter rise so I don't know if it is or isn't advanced or how much.
    So I will throw a number out there, and an opinion, If the intake comes in around 32* at .050, I'd call it good to go. JMO
    Thank you for your reply. This is my normally aspirated engine. It is a custom ground Isky with 3 degrees built in. on a 107 lobe.
    I checked clearance with running springs just 1 intake and one exhaust. no head gasket and preload with dial indicator. I did not degree cam but I will say with all same parts installed straight up engine kicked ass. problem was not enough valve to piston clearance and engine failed. I did not build it. New pistons and valves head repair and back together. Same problem I advanced crank gear 4 degrees and proper valve to piston clearances happened. I am going to put it together and race it.

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    Quote Originally Posted by gn 724 View Post
    Thank you for your reply. This is my normally aspirated engine. It is a custom ground Isky with 3 degrees built in. on a 107 lobe.
    I checked clearance with running springs just 1 intake and one exhaust. no head gasket and preload with dial indicator. I did not degree cam but I will say with all same parts installed straight up engine kicked ass. problem was not enough valve to piston clearance and engine failed. I did not build it. New pistons and valves head repair and back together. Same problem I advanced crank gear 4 degrees and proper valve to piston clearances happened. I am going to put it together and race it.
    Whoa!! Checking without the head gasket changes the relationship between an open valve and the piston not JUST the distance!
    Not that I think it will change much in your case, but its not good practice.



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    Last edited by gn7; 06-17-2013 at 01:41 PM.

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    FWIW you don't need .070 on the intake and .110 is more than enough on the ex.
    Checking w/o head gasket is a waste of time. Not sure I understand that. I think you're fighting to get more p/v than you need.
    What do you mean by "preload"?
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    Quote Originally Posted by gn7 View Post
    Whoa!! Checking without the head gasket changes the relationship between an open valve and the piston not JUST the distance!
    Not that I think it will change much in your case, but its not good practice.
    x2
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