Race Engine Built Question
11-20-2007, 07:56 PM
#76
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You misunderstood the point here. yes, any interference engine has valve pockets. why at 45 degrees can you rotate the cams? because the valves are not pushing down far enough to hit the final depth of the valve reliefs. this, in other words, is what im saying. in some cars, as in non-interference types you can rotate the cams while at TDC and not hit either, but if the pistons were flat top, with no valve reliefs, this wouldnt be the case. (Even though when properly timed, the valves would never get near the pistons and probably not need valve cuts .)
So, the point of the 45 degree clamping, is to allow for cam movement and valve movement as to not make contact with the piston or valve reliefs.
If properly timed, i would expect the pistons to get near a valve, so this would test with clay, might not tell you what you need to know. Again, from experience, a 1mm issue with the valve pockets, wouldnt be seen by claying a properly timed engine. (however it would be catostrophic if the belt moved a few teeth, or at the 45 degree setting of the crank if you were spinning the cams)
as we know, at 20degrees, the valves are only pushed down a scant, 2mm, following the elipical movement of the cam lobe where max valve movement of near 12mm would be at 90degree crank position. and 90 degree crank position is near half way down the bore for the piston, or near 40mm.
Again the point of this is that, why wouldnt you depress the valves in a clayed piston to see what the clearance is on the valve pockets? you would tell how deep you are, and you could calculate how many teeth you could slip before bad things happened.
care to site where " pistons will be half way up or down at 45 degrees ATDC" (paraphrasing)? However. I know what you are talking about. its a position where all the pistons are not at TDC and give ample room for the cams to be rotated and valves depressed to max lift, without contact. By the way, the 85 euro(with shallow piston cuts) should have this same spec as there is some contact when putting the belt on if the crank is at TDC.
Mk
So, the point of the 45 degree clamping, is to allow for cam movement and valve movement as to not make contact with the piston or valve reliefs.
If properly timed, i would expect the pistons to get near a valve, so this would test with clay, might not tell you what you need to know. Again, from experience, a 1mm issue with the valve pockets, wouldnt be seen by claying a properly timed engine. (however it would be catostrophic if the belt moved a few teeth, or at the 45 degree setting of the crank if you were spinning the cams)
as we know, at 20degrees, the valves are only pushed down a scant, 2mm, following the elipical movement of the cam lobe where max valve movement of near 12mm would be at 90degree crank position. and 90 degree crank position is near half way down the bore for the piston, or near 40mm.
Again the point of this is that, why wouldnt you depress the valves in a clayed piston to see what the clearance is on the valve pockets? you would tell how deep you are, and you could calculate how many teeth you could slip before bad things happened.
care to site where " pistons will be half way up or down at 45 degrees ATDC" (paraphrasing)? However. I know what you are talking about. its a position where all the pistons are not at TDC and give ample room for the cams to be rotated and valves depressed to max lift, without contact. By the way, the 85 euro(with shallow piston cuts) should have this same spec as there is some contact when putting the belt on if the crank is at TDC.
Mk
Care to cite a source for this..."But, the reason for the valve pockets is really for moving the cams when the belt is not on, and when a piston might be near TDC, or for a safety margin when a belt slips a few teeth like it did for me."..... ???? Porsche instructs you to lock the 32 valve engine at 45 degrees off TDC BEFORE removing the belt , pistons halfway up or down and not move it until the cam timing is correct.
11-21-2007, 07:10 AM
#77
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Mark,
I did not build my race motor to be a non-interference motor. So slipping of a cam belt or breaking will cause some damage. No need to "check" other valve clearances besides what I did when I clayed the motor.
Adrian,
Used the hyd. lifters. there was plenty of clearance even if the lifter was or wasn't pumped up.
I did not build my race motor to be a non-interference motor. So slipping of a cam belt or breaking will cause some damage. No need to "check" other valve clearances besides what I did when I clayed the motor.
Adrian,
Used the hyd. lifters. there was plenty of clearance even if the lifter was or wasn't pumped up.
11-21-2007, 01:15 PM
#78
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Mark WTF ..."You misunderstood the point here." ...No you failed to substantiate the statement. Then in the standard Kibort "debate" dance all around the topic. Just admit that you really have no idea how and why valve pockets exist.
11-21-2007, 01:17 PM
#79
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I understand you were accepting of the engine being interference. You went through all that work, wouldnt you want to check the valve pockets to see if they were correct? It is a variable that is not going to be checked when you clay the engine and build the entire top end and bolt it all down and time it out and rotate the crank. Not saying that it would have changed the outcome of the belt failure, but it would be interesting to see how far you are off, especially since you went through the effort to make those nice valve pockets.
you went larger on piston size and those valve reliefs are made from the S4 dimentions, widened for your new valves and then transfered to a larger piston, just like we do with the 5 liter 2 valve conversion. both of mine were off and could have caused a bent valve or two with a slipped belt, but not a broken one.
I just think it would be interesting to see how far you were off with the valve pockets and how much farther the cuts would be needed to make a non interference engine.
mk
you went larger on piston size and those valve reliefs are made from the S4 dimentions, widened for your new valves and then transfered to a larger piston, just like we do with the 5 liter 2 valve conversion. both of mine were off and could have caused a bent valve or two with a slipped belt, but not a broken one.
I just think it would be interesting to see how far you were off with the valve pockets and how much farther the cuts would be needed to make a non interference engine.
mk
Mark,
I did not build my race motor to be a non-interference motor. So slipping of a cam belt or breaking will cause some damage. No need to "check" other valve clearances besides what I did when I clayed the motor.
Adrian,
Used the hyd. lifters. there was plenty of clearance even if the lifter was or wasn't pumped up.
I did not build my race motor to be a non-interference motor. So slipping of a cam belt or breaking will cause some damage. No need to "check" other valve clearances besides what I did when I clayed the motor.
Adrian,
Used the hyd. lifters. there was plenty of clearance even if the lifter was or wasn't pumped up.
11-21-2007, 06:10 PM
#80
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ouldnt you want to check the valve pockets to see if they were correct?
I just think it would be interesting to see how far you were off with the valve pockets and how much farther the cuts would be needed to make a non interference engine.
Do it on your new motor and we can see.
11-21-2007, 06:35 PM
#82
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Nor did Scots, nor would yours even if they were not correct. But, the valves hit the valve reliefs on scots motor when the valves were depressed. ( so we fixed them) Had we just timed the motor, and ran it, we never would have caught it. So, thats why i mentioned it. When he got hit and ran backward and ran the motor backward, the timing belt slipped. Had we not caught the misalignment of the valve pockets with this test, they may have hit and cost him the engine!
Yes, i will check this on the new motor. It will be interesting to see how far the valves go before contact. Im sure someone has a value for this, but noone has chimmed in, so maybe not.
So, when is the fixed motor going in?
mk
Yes, i will check this on the new motor. It will be interesting to see how far the valves go before contact. Im sure someone has a value for this, but noone has chimmed in, so maybe not.
So, when is the fixed motor going in?
mk
11-21-2007, 07:57 PM
#83
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Because I am too lazy to do a write up ...lifted from a Ford site "Overview
Piston to valve clearance deals with the space between the valve and the piston during a critical point in the cam timing knows as the "overlap period". At no other point in the four stroke cycle does the valve get closer to the piston. Thus it is imperative that enough clearance exist in order to avoid a collision between the piston and the valve.
The overlap period occurs near the end of the exhaust stroke and at the beginning of the intake stroke, when both valves are open for a short
During Overlap both valves are open as the piston rises.
period of time. Overlap is a critical period in the cam cycle, as the movement of exhaust gases out of the combustion chamber "draws" in a fresh charge through the intake valve.
Many people have even erroneously measured clearance at the point of maximum valve lift, thinking that is the most likely point at which interference would occur. However when a valve reaches its maximum lift, the piston is actually the furthest away since this occurs during the intake stroke, when the piston is moving down the cylinder bore.
Minimum clearances between the valve and piston should be 0.080" on the intake valve, and 0.100" on the exhaust valve. If you run aluminum rods, the clearances should be even greater. In this article we'll show you the two methods most commonly used to measure piston to valve clearance...... "
Piston to valve clearance deals with the space between the valve and the piston during a critical point in the cam timing knows as the "overlap period". At no other point in the four stroke cycle does the valve get closer to the piston. Thus it is imperative that enough clearance exist in order to avoid a collision between the piston and the valve.
The overlap period occurs near the end of the exhaust stroke and at the beginning of the intake stroke, when both valves are open for a short
During Overlap both valves are open as the piston rises.
period of time. Overlap is a critical period in the cam cycle, as the movement of exhaust gases out of the combustion chamber "draws" in a fresh charge through the intake valve.
Many people have even erroneously measured clearance at the point of maximum valve lift, thinking that is the most likely point at which interference would occur. However when a valve reaches its maximum lift, the piston is actually the furthest away since this occurs during the intake stroke, when the piston is moving down the cylinder bore.
Minimum clearances between the valve and piston should be 0.080" on the intake valve, and 0.100" on the exhaust valve. If you run aluminum rods, the clearances should be even greater. In this article we'll show you the two methods most commonly used to measure piston to valve clearance...... "
11-21-2007, 08:17 PM
#84
Rennlist Member
How funny, i was just looking at that site! (after a quick search )
How much overlap do we have with our S4 engines?
We do know that the intake valve doent open more than 2mm at 20degrees past TDC, so if the exhaust is closed, there wouldnt be too much overlap, right.
Anyway, absolutely, the valve reliefs are for the overlap area, but there would not need to be much of a cut to cover that in most engines. I contend that if you only had overlap clearance, you wouldnt be able to rotate the cams at the 45degree mark.
mk
How much overlap do we have with our S4 engines?
We do know that the intake valve doent open more than 2mm at 20degrees past TDC, so if the exhaust is closed, there wouldnt be too much overlap, right.
Anyway, absolutely, the valve reliefs are for the overlap area, but there would not need to be much of a cut to cover that in most engines. I contend that if you only had overlap clearance, you wouldnt be able to rotate the cams at the 45degree mark.
mk
Because I am too lazy to do a write up ...lifted from a Ford site "Overview
Piston to valve clearance deals with the space between the valve and the piston during a critical point in the cam timing knows as the "overlap period". At no other point in the four stroke cycle does the valve get closer to the piston. Thus it is imperative that enough clearance exist in order to avoid a collision between the piston and the valve.
The overlap period occurs near the end of the exhaust stroke and at the beginning of the intake stroke, when both valves are open for a short
During Overlap both valves are open as the piston rises.
period of time. Overlap is a critical period in the cam cycle, as the movement of exhaust gases out of the combustion chamber "draws" in a fresh charge through the intake valve.
Many people have even erroneously measured clearance at the point of maximum valve lift, thinking that is the most likely point at which interference would occur. However when a valve reaches its maximum lift, the piston is actually the furthest away since this occurs during the intake stroke, when the piston is moving down the cylinder bore.
Minimum clearances between the valve and piston should be 0.080" on the intake valve, and 0.100" on the exhaust valve. If you run aluminum rods, the clearances should be even greater. In this article we'll show you the two methods most commonly used to measure piston to valve clearance...... "
Piston to valve clearance deals with the space between the valve and the piston during a critical point in the cam timing knows as the "overlap period". At no other point in the four stroke cycle does the valve get closer to the piston. Thus it is imperative that enough clearance exist in order to avoid a collision between the piston and the valve.
The overlap period occurs near the end of the exhaust stroke and at the beginning of the intake stroke, when both valves are open for a short
During Overlap both valves are open as the piston rises.
period of time. Overlap is a critical period in the cam cycle, as the movement of exhaust gases out of the combustion chamber "draws" in a fresh charge through the intake valve.
Many people have even erroneously measured clearance at the point of maximum valve lift, thinking that is the most likely point at which interference would occur. However when a valve reaches its maximum lift, the piston is actually the furthest away since this occurs during the intake stroke, when the piston is moving down the cylinder bore.
Minimum clearances between the valve and piston should be 0.080" on the intake valve, and 0.100" on the exhaust valve. If you run aluminum rods, the clearances should be even greater. In this article we'll show you the two methods most commonly used to measure piston to valve clearance...... "
11-21-2007, 08:20 PM
#85
Rennlist Member
There must be more than meets the eye in this conversation. It seems reasonable to wonder if there might be a way to make a 32 valve engine non-interference while you're rebuilding it.
11-21-2007, 08:38 PM
#86
Rennlist Member
Jim, if you look at the spec book, there is NO valve overlap. (for 928s)
intake doesnt open until 11 degrees past TDC. (meaning the piston is already on the way down) and again, as we all know, this is why at 20degrees ATDC, the valve is only 2mm depressed
AND, with the exhaust cams, both on the 85s, 86s S4s and Club sports, the exhaust is closed before TDC, by about 5 degrees.
The only model that has a valve still open at TDC is the US 2 valve, and it is only 2degrees after TDC for a closed exhuast valve.
mk
intake doesnt open until 11 degrees past TDC. (meaning the piston is already on the way down) and again, as we all know, this is why at 20degrees ATDC, the valve is only 2mm depressed
AND, with the exhaust cams, both on the 85s, 86s S4s and Club sports, the exhaust is closed before TDC, by about 5 degrees.
The only model that has a valve still open at TDC is the US 2 valve, and it is only 2degrees after TDC for a closed exhuast valve.
mk
Last edited by mark kibort; 11-21-2007 at 09:22 PM.
11-21-2007, 08:39 PM
#87
Rennlist Member
Ill know for sure in a while, but im guessing, that the valve cuts like JV has done, might just do it for the S4s
on a side note, i wonder what JV's cam profile was. just more lift, and the same basic durations and timing, or something all together different? I think Anderson and Fans cams are something like stock, plus about 1mm more lift (like the GT cams have 1mm more lift than the S4cams)
mk
on a side note, i wonder what JV's cam profile was. just more lift, and the same basic durations and timing, or something all together different? I think Anderson and Fans cams are something like stock, plus about 1mm more lift (like the GT cams have 1mm more lift than the S4cams)
mk
Last edited by mark kibort; 11-21-2007 at 10:23 PM.
