968 Intake Valves - Factory Piston Reliefs OK?
05-01-2011, 08:11 PM
#16
Official Bay Area Patriot
Fuse 24 Assassin
Rennlist Member
Fuse 24 Assassin
Rennlist Member
Thread Starter
Thanks for the info Mark. On a side note I had a buddy that did his own engine balancing with a dremel too on his pistons and rods. He has never had his block let go on his 3.0L 911SC motor. And, that motor winds about a grand more than ours.
05-01-2011, 08:45 PM
#17
Rennlist Member
yep, i balanced scot's engine that way, (tip and tail on the rods ) the rest was done professionally. the amount of material coming out of the valve cuts is almost insignificant.
05-01-2011, 09:40 PM
#19
Rennlist Member
ill admit, they looked like crap, but they were all balanced within 1 gram of each other and tip and tail balanced as well. who ever takes this engine apart, will say. "WTF did someone do to these rods! " But in the end they were perfectly functional.
05-01-2011, 10:09 PM
#20
Former Sponsor
Sorry, Mark, I could never take a hand grinder to a piston....it just seems so wrong in so many ways....I know it would never probably hurt anything....but offends me in ways I can't even begin to describe. Much like the proverbial fingernails on the blackboard....maybe worse.
Now if you would use your axe....
Back to the original question....
Yes, the 968 intake valves will "touch" the pistons, if the reliefs are not enlarged. The airflow will also be terminally screwed up with the valve that close to the piston.
Yes, most people would be way better off with stock valves than the 968 intake valves. Most all of the heads (except the ones we do, because we saw this very early...long ago) we have had on the flow bench that have had the bigger valves installed flow less air than the stock heads. And....it is difficult to get the stock flow numbers "back". Blending the seat into the port, which is what one would think would help, lowers the flow signifcantly....from stock!
It is a very "tricky" port and most people would be way farther ahead to leave it alone, with the stock valves....enough said.
Now if you would use your axe....
Back to the original question....
Yes, the 968 intake valves will "touch" the pistons, if the reliefs are not enlarged. The airflow will also be terminally screwed up with the valve that close to the piston.
Yes, most people would be way better off with stock valves than the 968 intake valves. Most all of the heads (except the ones we do, because we saw this very early...long ago) we have had on the flow bench that have had the bigger valves installed flow less air than the stock heads. And....it is difficult to get the stock flow numbers "back". Blending the seat into the port, which is what one would think would help, lowers the flow signifcantly....from stock!
It is a very "tricky" port and most people would be way farther ahead to leave it alone, with the stock valves....enough said.
05-01-2011, 10:56 PM
#21
Rennlist Member
I totally understand, and feel your pain. But, in fuction only , racing-land, i seem to let a lot of things that would bother me, slip on by!
back to the 968 valves, since we are ONLY talking intake here, how will they touch? the piston goes down 2mm when the vavle is depresssed 2mm at 20degrees after TDC. at 26degrees, its more like 4mm, and the valves are only like 3mm. from that point on, the valve has no chance against the falling piston. by the way, thats with NO valve relief. however, rotating the cam, in a locked engine flywheel is a different story.
I hear you when you talk about the bigger valves and less flow. it happens a lot. proves they did a good job with the S4 heads. But, with a stoker, the good outweighs the bad, right?
Mk
back to the 968 valves, since we are ONLY talking intake here, how will they touch? the piston goes down 2mm when the vavle is depresssed 2mm at 20degrees after TDC. at 26degrees, its more like 4mm, and the valves are only like 3mm. from that point on, the valve has no chance against the falling piston. by the way, thats with NO valve relief. however, rotating the cam, in a locked engine flywheel is a different story.
I hear you when you talk about the bigger valves and less flow. it happens a lot. proves they did a good job with the S4 heads. But, with a stoker, the good outweighs the bad, right?
Mk
Sorry, Mark, I could never take a hand grinder to a piston....it just seems so wrong in so many ways....I know it would never probably hurt anything....but offends me in ways I can't even begin to describe. Much like the proverbial fingernails on the blackboard....maybe worse.
Now if you would use your axe....
Back to the original question....
Yes, the 968 intake valves will "touch" the pistons, if the reliefs are not enlarged. The airflow will also be terminally screwed up with the valve that close to the piston.
Yes, most people would be way better off with stock valves than the 968 intake valves. Most all of the heads (except the ones we do, because we saw this very early...long ago) we have had on the flow bench that have had the bigger valves installed flow less air than the stock heads. And....it is difficult to get the stock flow numbers "back". Blending the seat into the port, which is what one would think would help, lowers the flow signifcantly....from stock!
It is a very "tricky" port and most people would be way farther ahead to leave it alone, with the stock valves....enough said.
Now if you would use your axe....
Back to the original question....
Yes, the 968 intake valves will "touch" the pistons, if the reliefs are not enlarged. The airflow will also be terminally screwed up with the valve that close to the piston.
Yes, most people would be way better off with stock valves than the 968 intake valves. Most all of the heads (except the ones we do, because we saw this very early...long ago) we have had on the flow bench that have had the bigger valves installed flow less air than the stock heads. And....it is difficult to get the stock flow numbers "back". Blending the seat into the port, which is what one would think would help, lowers the flow signifcantly....from stock!
It is a very "tricky" port and most people would be way farther ahead to leave it alone, with the stock valves....enough said.
05-02-2011, 08:30 AM
#22
oh brother. yeah, im a barbarian. what are you racing, an F1 engine spinning 18000rpm??
cutting 1mm of material off the valve relief isnt more than a couple drops of oil weight on one piston vs another or the crank. it is pobably less than a gram each and certainy if you do the same for all pistons, it wont be a huge issue.
plus, we are talking 6500rpm engines here. not a big deal. Hey, no problem being a little ****.
so yes, if you had the choice, when buildig the engine make the cuts and then balance the entire rotating assembly. but, if you are doing a valve mod, there is no real need to pull the block to cut the pistons for this amout of material removal. I dont think even GB would have a problem with widening the smilie faces on each intake valve relief by 1mm. Greg??
what are you racing, an F1 engine spinning 18000rpm?? cutting 1mm of material off the valve relief isnt more than a couple drops of oil weight on one piston vs another or the crank. it is pobably less than a gram each and certainy if you do the same for all pistons, it wont be a huge issue.
plus, we are talking 6500rpm engines here. not a big deal. Hey, no problem being a little ****.
so yes, if you had the choice, when buildig the engine make the cuts and then balance the entire rotating assembly. but, if you are doing a valve mod, there is no real need to pull the block to cut the pistons for this amout of material removal. I dont think even GB would have a problem with widening the smilie faces on each intake valve relief by 1mm.
Greg??I personally think the flow issues with these heads on a standard bore with standard valves revolves around the poor castings. What I mean by that is around the valve seat there is often part of the alloy casting missing and as such when you fit the larger valves to these heads it allows you to properly shape this area.
However there is some material that can be taken out of the throat of the valve seat. You normally wouldn't want to go past 90% of the valve diameter for a street motor but upto 91.5% is used for race engines that see revs all the time. Its a velocity thing.
The issue with the larger valves on a low revving 5.0 litre engine is can you use the extra air? Are you losing velocity to create this extra flow. The part of the port past the guide is the diffuser in a 4V engine. So the extra valve size will slow the air down further, depending on demand you may lose efficiency. You need to balance that off against fixing up the casting issues. If they can be fixed by throating out the seat, that would be my preferred method on a 5.0 litre engine.
Personally I would bump the compression by using 86.5 pistons modified or the Australian versions. Then spend money on ITBs and cams such as Colin's or Mike's. The manifold is a much bigger problem than valve size. A nice straight air path is the way to go.
Greg
05-02-2011, 09:45 AM
#23
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Again interesting discussion..
I have been also thinking of using 968 valves with new stage II cams and headers, etc. However, I haven't seen any evidence of how they work with 5L 32V engine.
So far, I have read and understood(partially), that the port bowl diameter should be around 85-87% of intake valve diameter to get any advantage of larger intake valves.
Also, larger valves will help engine to breath more which will help to get more top end power. This also means, that engine may loose some low-end torque.
Engine needs high intake velocity to provide the most of the power. Now if larger valves are causing intake velocity to drop too much, we may loose even the high rpm power. In this case, we assume, that only valves have been replaced and rest of the engine stuff remains the same. Engine power will increase until we reach around Mach 0,6, where VE starts to fall.
The valve diameter, duration and lift together with port shape, size, engine displacement and rpm will determine the velocity in the intake system. There are also some other dimensions, but this will explain how complex it is to make things work together for different applications.
I'm thinking of that two stock intake valves should be enough for me. Don't really want to start experimenting too much with this stuff
After skipping larger intake valves, I have read, that some engine builders are using groove machined intake valves to get better A/F atomization, does anyone have experience of this?
http://www.metricmechanic.com/pdfs/m...ne-booklet.pdf
Simo
S4 -87 AT
I have been also thinking of using 968 valves with new stage II cams and headers, etc. However, I haven't seen any evidence of how they work with 5L 32V engine.
So far, I have read and understood(partially), that the port bowl diameter should be around 85-87% of intake valve diameter to get any advantage of larger intake valves.
Also, larger valves will help engine to breath more which will help to get more top end power. This also means, that engine may loose some low-end torque.
Engine needs high intake velocity to provide the most of the power. Now if larger valves are causing intake velocity to drop too much, we may loose even the high rpm power. In this case, we assume, that only valves have been replaced and rest of the engine stuff remains the same. Engine power will increase until we reach around Mach 0,6, where VE starts to fall.
The valve diameter, duration and lift together with port shape, size, engine displacement and rpm will determine the velocity in the intake system. There are also some other dimensions, but this will explain how complex it is to make things work together for different applications.
I'm thinking of that two stock intake valves should be enough for me. Don't really want to start experimenting too much with this stuff
After skipping larger intake valves, I have read, that some engine builders are using groove machined intake valves to get better A/F atomization, does anyone have experience of this?
http://www.metricmechanic.com/pdfs/m...ne-booklet.pdf
Simo
S4 -87 AT
05-02-2011, 01:01 PM
#24
Rennlist Member
Greg, im not deepening the valve reliefs and we are using a very shallow cut with the machinist to begin with. ( you cant do '80 valve cuts on 85 4v pistons only .175" will work, like the euro 84-5 cuts)
the only dremmeling Im doing is to widen the cut by 1-2mm or so. no chance of punching through.
Mk
the only dremmeling Im doing is to widen the cut by 1-2mm or so. no chance of punching through.
Mk
Mark the issue would be that the piston gets very thin at the edges and with a hand tool and the average hand skill you could very well throw away a good piston for the sake of spending $200 on having the reliefs increased on a mill.
I personally think the flow issues with these heads on a standard bore with standard valves revolves around the poor castings. What I mean by that is around the valve seat there is often part of the alloy casting missing and as such when you fit the larger valves to these heads it allows you to properly shape this area.
However there is some material that can be taken out of the throat of the valve seat. You normally wouldn't want to go past 90% of the valve diameter for a street motor but upto 91.5% is used for race engines that see revs all the time. Its a velocity thing.
The issue with the larger valves on a low revving 5.0 litre engine is can you use the extra air? Are you losing velocity to create this extra flow. The part of the port past the guide is the diffuser in a 4V engine. So the extra valve size will slow the air down further, depending on demand you may lose efficiency. You need to balance that off against fixing up the casting issues. If they can be fixed by throating out the seat, that would be my preferred method on a 5.0 litre engine.
Personally I would bump the compression by using 86.5 pistons modified or the Australian versions. Then spend money on ITBs and cams such as Colin's or Mike's. The manifold is a much bigger problem than valve size. A nice straight air path is the way to go.
Greg
I personally think the flow issues with these heads on a standard bore with standard valves revolves around the poor castings. What I mean by that is around the valve seat there is often part of the alloy casting missing and as such when you fit the larger valves to these heads it allows you to properly shape this area.
However there is some material that can be taken out of the throat of the valve seat. You normally wouldn't want to go past 90% of the valve diameter for a street motor but upto 91.5% is used for race engines that see revs all the time. Its a velocity thing.
The issue with the larger valves on a low revving 5.0 litre engine is can you use the extra air? Are you losing velocity to create this extra flow. The part of the port past the guide is the diffuser in a 4V engine. So the extra valve size will slow the air down further, depending on demand you may lose efficiency. You need to balance that off against fixing up the casting issues. If they can be fixed by throating out the seat, that would be my preferred method on a 5.0 litre engine.
Personally I would bump the compression by using 86.5 pistons modified or the Australian versions. Then spend money on ITBs and cams such as Colin's or Mike's. The manifold is a much bigger problem than valve size. A nice straight air path is the way to go.
Greg
05-02-2011, 02:12 PM
#25
Former Sponsor
I totally understand, and feel your pain. But, in fuction only , racing-land, i seem to let a lot of things that would bother me, slip on by!
I've spend more than a few minutes in "racing land", during my lifetime....
I might consider hand machining a piston, if I was in the middle of the Sahara desert and I needed a piston to finish a race...but the reality of the thing is this:
The airflow out of the head is partially dependant upon the piston and what its shape is. In a race engine, we carefully try to equalize things, so that each cylinder is doing the same amount of work. Hand grinding valve reliefs, in pistons, is going to defeat any work we might do to make airflow, through the head, equal in each port.
back to the 968 valves, since we are ONLY talking intake here, how will they touch? the piston goes down 2mm when the vavle is depresssed 2mm at 20degrees after TDC. at 26degrees, its more like 4mm, and the valves are only like 3mm. from that point on, the valve has no chance against the falling piston. by the way, thats with NO valve relief. however, rotating the cam, in a locked engine flywheel is a different story.
You've got to realize that I've been doing this a few years and have seen a lot of engines....that's what is so funny when someone decides he can suddenly build "stroker engines", buys a bunch of parts from a previous stroker engine builder, who never had an engine run. Figure out how that is going to turn out!
They do touch. We saw this very early on one of Anderson's race engines...and those pistons had "enlarged' valve pockets, over stock. We ended up having to make the valve pockets even larger. I'm not going to tell you the specifics...for obvious reasons....that is proprietary information.
I hear you when you talk about the bigger valves and less flow. it happens a lot. proves they did a good job with the S4 heads. But, with a stoker, the good outweighs the bad, right?
No, far from that.
Loss of airflow is a loss of "potential" power. The naturally aspirated engine is an "airpump", for all intents and purposes. The amount of airflow through the head can be measured and the maximum power output of the engine can be directly calculated, from that number. The number is an "absolute"...and can only be changed by either increasing volumetric efficiency or adding another source of free oxygen (any "power adder"...nitrous or positive cylinder pressure, etc). While every "home" engine builder believes that volumetric efficiencies of over 100% are easy to achieve (thank you Dynojet and magazine articles, from the past 15 years)...anyone that has spent much time with a flowbench knows this is absolutely not true. Most engines are lucky to have volumetric effciencies of 85%.
Here's a sample....extremely simplified. There's more here than just this....but this example uses "perfect" cams, compression, exhaust airflow, etc.
Say that a 928 stock head flows 110cfm at 10 inches and .450" valve lift. At 100% volumetric efficiency (very hard to achieve), an "perfect" engine would make .43 horsepower, per cfm, per cylinder. Quick math tells you that this "potential" is 378 horsepower.
Put in 968 valves and reduce that flow by 10% (common number that we have seen.) Now you have 99 cfms, which will have a maximum potential of making 340hp at 100% volumetric efficiency!
Even reducing the airflow by 5% will result in a loss of 20 horsepower, in this example!
Note that I've already stated (previously) that it is common to loose airflow through these heads....when they are messed with. I've got a set of pretty nice looking "Threshie" "titanium valved, bigger intake valved" heads, here, which lost way over 10% airflow. They look very trick, but they absolutely suck...or in this case, fail to suck. Looking trick is not necessarily trick....try and imagine what this "loss" of potential horepower cost this customer, in dollars!
Understand the problem?
Mk
I've spend more than a few minutes in "racing land", during my lifetime....
I might consider hand machining a piston, if I was in the middle of the Sahara desert and I needed a piston to finish a race...but the reality of the thing is this:
The airflow out of the head is partially dependant upon the piston and what its shape is. In a race engine, we carefully try to equalize things, so that each cylinder is doing the same amount of work. Hand grinding valve reliefs, in pistons, is going to defeat any work we might do to make airflow, through the head, equal in each port.
back to the 968 valves, since we are ONLY talking intake here, how will they touch? the piston goes down 2mm when the vavle is depresssed 2mm at 20degrees after TDC. at 26degrees, its more like 4mm, and the valves are only like 3mm. from that point on, the valve has no chance against the falling piston. by the way, thats with NO valve relief. however, rotating the cam, in a locked engine flywheel is a different story.
You've got to realize that I've been doing this a few years and have seen a lot of engines....that's what is so funny when someone decides he can suddenly build "stroker engines", buys a bunch of parts from a previous stroker engine builder, who never had an engine run. Figure out how that is going to turn out!
They do touch. We saw this very early on one of Anderson's race engines...and those pistons had "enlarged' valve pockets, over stock. We ended up having to make the valve pockets even larger. I'm not going to tell you the specifics...for obvious reasons....that is proprietary information.
I hear you when you talk about the bigger valves and less flow. it happens a lot. proves they did a good job with the S4 heads. But, with a stoker, the good outweighs the bad, right?
No, far from that.
Loss of airflow is a loss of "potential" power. The naturally aspirated engine is an "airpump", for all intents and purposes. The amount of airflow through the head can be measured and the maximum power output of the engine can be directly calculated, from that number. The number is an "absolute"...and can only be changed by either increasing volumetric efficiency or adding another source of free oxygen (any "power adder"...nitrous or positive cylinder pressure, etc). While every "home" engine builder believes that volumetric efficiencies of over 100% are easy to achieve (thank you Dynojet and magazine articles, from the past 15 years)...anyone that has spent much time with a flowbench knows this is absolutely not true. Most engines are lucky to have volumetric effciencies of 85%.
Here's a sample....extremely simplified. There's more here than just this....but this example uses "perfect" cams, compression, exhaust airflow, etc.
Say that a 928 stock head flows 110cfm at 10 inches and .450" valve lift. At 100% volumetric efficiency (very hard to achieve), an "perfect" engine would make .43 horsepower, per cfm, per cylinder. Quick math tells you that this "potential" is 378 horsepower.
Put in 968 valves and reduce that flow by 10% (common number that we have seen.) Now you have 99 cfms, which will have a maximum potential of making 340hp at 100% volumetric efficiency!
Even reducing the airflow by 5% will result in a loss of 20 horsepower, in this example!
Note that I've already stated (previously) that it is common to loose airflow through these heads....when they are messed with. I've got a set of pretty nice looking "Threshie" "titanium valved, bigger intake valved" heads, here, which lost way over 10% airflow. They look very trick, but they absolutely suck...or in this case, fail to suck. Looking trick is not necessarily trick....try and imagine what this "loss" of potential horepower cost this customer, in dollars!
Understand the problem?
Mk
Here's food for thought.
05-02-2011, 02:31 PM
#26
Rennlist Member
I get it. dont get me wrong. I dont have any experience here, other than crude measurements Ive made on engine builds. everything I have done , has just been assembly work, asside from the widening of valve reliefs.
how is it that the intake valve can touch the pistons. did they do it running, or was it when you clayed it all up? Im asking, because im only going on what i have physically seen from the piston doing down in the hole on a stock 928 crank. how do they touch if the piston goes down 2mm at 20degrees, and the valve goes doen 2mm at 20 degrees? (and then 4mm at 26degrees, etc). I would think the stroker, the piston is even going further for a given rotation, due to its longer stroke and thus piston travel ? theoretically, you wouldnt even need valve reliefs. but those, are very deep on an S4 piston, or 968 piston.
I think you are talking the wider valves, hitting the corners. So if that is the case, I wonder how they can touch given the distance of the movement i measured.
I see the physical logic, but if they hit, it would be interesting to know why, and Im sure you have found this out, or studied this.
how is it that the intake valve can touch the pistons. did they do it running, or was it when you clayed it all up? Im asking, because im only going on what i have physically seen from the piston doing down in the hole on a stock 928 crank. how do they touch if the piston goes down 2mm at 20degrees, and the valve goes doen 2mm at 20 degrees? (and then 4mm at 26degrees, etc). I would think the stroker, the piston is even going further for a given rotation, due to its longer stroke and thus piston travel ? theoretically, you wouldnt even need valve reliefs.
but those, are very deep on an S4 piston, or 968 piston.I think you are talking the wider valves, hitting the corners. So if that is the case, I wonder how they can touch given the distance of the movement i measured.
I see the physical logic, but if they hit, it would be interesting to know why, and Im sure you have found this out, or studied this.
05-02-2011, 03:30 PM
#27
Former Sponsor
I get it. dont get me wrong. I dont have any experience here, other than crude measurements Ive made on engine builds. everything I have done , has just been assembly work, asside from the widening of valve reliefs.
how is it that the intake valve can touch the pistons. did they do it running, or was it when you clayed it all up? Im asking, because im only going on what i have physically seen from the piston doing down in the hole on a stock 928 crank. how do they touch if the piston goes down 2mm at 20degrees, and the valve goes doen 2mm at 20 degrees? (and then 4mm at 26degrees, etc). I would think the stroker, the piston is even going further for a given rotation, due to its longer stroke and thus piston travel ? theoretically, you wouldnt even need valve reliefs. but those, are very deep on an S4 piston, or 968 piston.
I think you are talking the wider valves, hitting the corners. So if that is the case, I wonder how they can touch given the distance of the movement i measured.
I see the physical logic, but if they hit, it would be interesting to know why, and Im sure you have found this out, or studied this.
how is it that the intake valve can touch the pistons. did they do it running, or was it when you clayed it all up? Im asking, because im only going on what i have physically seen from the piston doing down in the hole on a stock 928 crank. how do they touch if the piston goes down 2mm at 20degrees, and the valve goes doen 2mm at 20 degrees? (and then 4mm at 26degrees, etc). I would think the stroker, the piston is even going further for a given rotation, due to its longer stroke and thus piston travel ? theoretically, you wouldnt even need valve reliefs.
but those, are very deep on an S4 piston, or 968 piston.I think you are talking the wider valves, hitting the corners. So if that is the case, I wonder how they can touch given the distance of the movement i measured.
I see the physical logic, but if they hit, it would be interesting to know why, and Im sure you have found this out, or studied this.
Don't forget that on "more aggressive" engines, the intake valve is opening before TDC, so that the piston can immediately be "sucking" air into the cylinder on the downstroke. The closer the valve opens to TDC...the lower the volumetric efficiency.
05-03-2011, 12:47 AM
#28
Rennlist Member
yes, those valves hit the piston reliefs, but remember, i just pushed the valve down when the piston was at TDC. I dremeled out 2mm of width, no depth to make sure there was no possible contact, related to diameter of the valve.
now, even with the GT cams, they are timed 8 degrees before TDC, but 20 degress ATDC the valve depression is only 2mm. So, how can they touch. (26degrees, the piston is down 4mm and the valves are only down 3mm. ) so , my question is how can they touch, even if there wasnt a valve cut at all?
in that picture, the valve hits the pistons with .175" valve movement. with the .175" deep pocket, they touch when vavle is pushed down .3" or so. (about 8mm) however to get 8mm of valve depression, the piston is down in the hole over 20mms.
now, even with the GT cams, they are timed 8 degrees before TDC, but 20 degress ATDC the valve depression is only 2mm. So, how can they touch. (26degrees, the piston is down 4mm and the valves are only down 3mm. ) so , my question is how can they touch, even if there wasnt a valve cut at all?
in that picture, the valve hits the pistons with .175" valve movement. with the .175" deep pocket, they touch when vavle is pushed down .3" or so. (about 8mm) however to get 8mm of valve depression, the piston is down in the hole over 20mms.
If you look at your picture in post #3 (that has the clay on it) the pistons end up looking just like this....the "edge" of the valve touches the piston (just like the area where your clay is removed, on the "eyebrow"). The valves and pistons, in your picture are, obviously, going to touch...and they will actually touch pretty damn hard, when the engine is running. I'd guess you will end up bending these valves, unless you changed something, after this picture was taken....at the very least, you will be using the valve to "scrape" metal off of the piston (mobile machine shop inside the engine.)
Don't forget that on "more aggressive" engines, the intake valve is opening before TDC, so that the piston can immediately be "sucking" air into the cylinder on the downstroke. The closer the valve opens to TDC...the lower the volumetric efficiency.
Don't forget that on "more aggressive" engines, the intake valve is opening before TDC, so that the piston can immediately be "sucking" air into the cylinder on the downstroke. The closer the valve opens to TDC...the lower the volumetric efficiency.
