What pistons do I have here ??
11-27-2005, 11:17 AM
#17
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I was gonna say, you know what they are, lol. For one, the releifs are not in the normal flame path, even after very short run time, they will hold carbon, atleast a trace.
I've been doing some homework on the subject. I don't think that you added much CC volume to the chamber. I've read that many don't even bother to recheck the chamber volume after releif cutting due to the minor change.
Impressive work, especially for DIY, is there, or will there be a write-up?
I've been doing some homework on the subject. I don't think that you added much CC volume to the chamber. I've read that many don't even bother to recheck the chamber volume after releif cutting due to the minor change.
Impressive work, especially for DIY, is there, or will there be a write-up?
11-28-2005, 12:35 AM
#19
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Thanks for the compliments and I am glad you also find it useful.
Ibkevin, as to the amount removed see post #28 on the previous thread (=1.5cc/piston).
The only edge removal I did was de-burring with a knife. I have looked at many pistons and valve pocketing appears to be the norm rather than the exception. The .075” wall at the periphery is only at medium height (.190”) compared to what I have seen. Since aluminum is a very good heat conductor, I don’t think that this wall (or any other edge introduced) will present any significant heat rise. As a matter of fact I am willing to bet that the face of the exhaust valve is significantly hotter than any area of the piston. In lieu of actual measurements the type of deposits will be a good indicator.
Porsche-O Phile, that corresponds to a compression ratio of 7.9:1 … on an unshaved head (see post #1 on the previous thread ). Assuming that the cross-sectional area of the head’s combustion chamber is 80% of the cylinder area the amount of head-shaving needed to return to the original 8:1 CR is .024cm or .009”, found from (10/2)^2*Pi*h*80% = 1.5cc
Zero 10, yeah I thought the clan pockets were a sure giveaway.
Epic2112, I have been a little conservative with the boost on this car since the headgasket was unknown, but no longer. My occasional 22 psi boost finally gently pushed, what apparently was the original HG, over the limit, by “ovalizing” 3 cylinder seals. The gasket was weakened because the corrugated metal base was rusted away behind the deformed rings. Here is a picture of my old HG and “new” MLS gasket, which I (as usual) have done something unusual to.
Nine-44 & theedge, the tool used took a lot of careful planning, design and machining and it was also done with the intent of leasing it. As to the cost, I have not yet set a price, but a significant (returnable) deposit will be needed to cover the replacement and potential partial damage cost of the tool. I will also write up a procedure for setting the cutting depth and carrying out the cutting. The actual process is surprisingly simple and quick. It was the planning and tool design that took time.
Laust
Ibkevin, as to the amount removed see post #28 on the previous thread (=1.5cc/piston).
The only edge removal I did was de-burring with a knife. I have looked at many pistons and valve pocketing appears to be the norm rather than the exception. The .075” wall at the periphery is only at medium height (.190”) compared to what I have seen. Since aluminum is a very good heat conductor, I don’t think that this wall (or any other edge introduced) will present any significant heat rise. As a matter of fact I am willing to bet that the face of the exhaust valve is significantly hotter than any area of the piston. In lieu of actual measurements the type of deposits will be a good indicator.
Porsche-O Phile, that corresponds to a compression ratio of 7.9:1 … on an unshaved head (see post #1 on the previous thread ). Assuming that the cross-sectional area of the head’s combustion chamber is 80% of the cylinder area the amount of head-shaving needed to return to the original 8:1 CR is .024cm or .009”, found from (10/2)^2*Pi*h*80% = 1.5cc
Zero 10, yeah I thought the clan pockets were a sure giveaway.
Epic2112, I have been a little conservative with the boost on this car since the headgasket was unknown, but no longer. My occasional 22 psi boost finally gently pushed, what apparently was the original HG, over the limit, by “ovalizing” 3 cylinder seals. The gasket was weakened because the corrugated metal base was rusted away behind the deformed rings. Here is a picture of my old HG and “new” MLS gasket, which I (as usual) have done something unusual to.
Nine-44 & theedge, the tool used took a lot of careful planning, design and machining and it was also done with the intent of leasing it. As to the cost, I have not yet set a price, but a significant (returnable) deposit will be needed to cover the replacement and potential partial damage cost of the tool. I will also write up a procedure for setting the cutting depth and carrying out the cutting. The actual process is surprisingly simple and quick. It was the planning and tool design that took time.
Laust
Last edited by Laust Pedersen; 01-14-2013 at 03:33 PM.
11-28-2005, 03:51 PM
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Seems like a brilliant idea. I just wonder why the porsche boys did or didn't think of it ? Exactly how much piston to valve clearance do you have now ? Are you going to post pictures of the tool you designed, is this something that most when changing a headgasket would want to perform to their engines ?
. No don't do that.
11-28-2005, 09:43 PM
#24
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He did turn it over by hand, that's a good enough test I suppose.
Laust, did you ensure that at least one valve was fully open when you tested this, or did you set the crank at TDC, and turn the cam?...
Just want to be sure, lol.
Other than that, now you get the chance to stretch out your t-belt changing intervals, see how long they really go
Laust, did you ensure that at least one valve was fully open when you tested this, or did you set the crank at TDC, and turn the cam?...
Just want to be sure, lol.
Other than that, now you get the chance to stretch out your t-belt changing intervals, see how long they really go
11-29-2005, 10:43 PM
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Epic2112 & Campeck,
As you can see on the original gasket has 4 large triangular holes for coolant passage behind cylinder # 4 as well as two very small holes on the exhaust side of cylinder # 1. This guides the main coolant flow through the front wall (from the water pump), around the cylinders, behind cylinder #4, up through the 4 holes, to the back of cylinder head and then to the front of the cylinder head where it exits. The Cometic (MLS) gasket has a number of smaller holes on the exhaust side on all cylinders and only one large triangular hole at the back. To me it looks as if the back of the engine gets inadequately cooled, especially since there is much less air flow at speed (compared to the front) and the back of the engine at slow speed and standstill is heated by the cross-over pipe and other exhaust parts. So I drilled a couple of holes in the back of the MLS gasket mimicking the OEM gasket. Afterwards I was very careful in de-burring and removing all the shavings especially between the layers.
I also increased the cylinder-head torque using a 30-60-90 ft-lb tightening sequence (in preparation for some serious boost
).
And lastly … hold on for this one … I actually reused the MLS gasket after it had been used only 100 miles, making sure that there was adequate copper spray at the few spots where the Viton coating was compromised. I wanted to buy a new one, but was unable to find one in time before the holidays, which I had scheduled for piston modification and other stuff. I actually talked to a technician at Cometic who said, that it was possible to reuse their gasket with the addition of some copper spray, but he reiterated that they recommend a new gasket.
Rip It,
I believe that the engine designers in the early days were oversold on the benefits of the “silent belt” and only later realized how often it needed to be changes. Also designs were made under the assumption of scheduled service by a qualified Porsche technician.
The valve piston clearance (in case of belt breakage) is equal to the HG thickness (=.040”) minus various valve and pocket deposits. The exhaust valve deposits should be taken seriously, since they are very hard and I measured a worst case deposit thickness of .017”.
The pictures of the tool will wait until I have written the procedure and am ready to lease the tool to whoever wants to make the modification with the pistons in their car. Since I did it on my car, I obviously think the benefits outweigh the drawbacks (excluding the effort of making the tool) and in my estimation the process is simple enough for anybody who can remove and install the cylinder head.
Robert D and Zero10,
I do intend to run the timing belt to its end or until it visibly appears to be close to breaking. However it may take “forever” driving it only 5k miles per year. Converting to a non-interference engine also gave me confidence in removing the very annoying belt covers for easy belt inspection, easy access to water pump and maybe even better cooling on the front of the engine.
As mentioned I did set the engine (cylinder #1 and #4) to TDC and turned the cam a number of times. This is not a full verification, since the hydraulic lifters will collapse without oil pressure, but should be good enough for verifying that the pocket and valve axis are coaxial. However the needed cut depth is very easy to measure and implement, so I am confident it is adequate.
Laust
As you can see on the original gasket has 4 large triangular holes for coolant passage behind cylinder # 4 as well as two very small holes on the exhaust side of cylinder # 1. This guides the main coolant flow through the front wall (from the water pump), around the cylinders, behind cylinder #4, up through the 4 holes, to the back of cylinder head and then to the front of the cylinder head where it exits. The Cometic (MLS) gasket has a number of smaller holes on the exhaust side on all cylinders and only one large triangular hole at the back. To me it looks as if the back of the engine gets inadequately cooled, especially since there is much less air flow at speed (compared to the front) and the back of the engine at slow speed and standstill is heated by the cross-over pipe and other exhaust parts. So I drilled a couple of holes in the back of the MLS gasket mimicking the OEM gasket. Afterwards I was very careful in de-burring and removing all the shavings especially between the layers.
I also increased the cylinder-head torque using a 30-60-90 ft-lb tightening sequence (in preparation for some serious boost
).And lastly … hold on for this one … I actually reused the MLS gasket after it had been used only 100 miles, making sure that there was adequate copper spray at the few spots where the Viton coating was compromised. I wanted to buy a new one, but was unable to find one in time before the holidays, which I had scheduled for piston modification and other stuff. I actually talked to a technician at Cometic who said, that it was possible to reuse their gasket with the addition of some copper spray, but he reiterated that they recommend a new gasket.
Rip It,
I believe that the engine designers in the early days were oversold on the benefits of the “silent belt” and only later realized how often it needed to be changes. Also designs were made under the assumption of scheduled service by a qualified Porsche technician.
The valve piston clearance (in case of belt breakage) is equal to the HG thickness (=.040”) minus various valve and pocket deposits. The exhaust valve deposits should be taken seriously, since they are very hard and I measured a worst case deposit thickness of .017”.
The pictures of the tool will wait until I have written the procedure and am ready to lease the tool to whoever wants to make the modification with the pistons in their car. Since I did it on my car, I obviously think the benefits outweigh the drawbacks (excluding the effort of making the tool) and in my estimation the process is simple enough for anybody who can remove and install the cylinder head.
Robert D and Zero10,
I do intend to run the timing belt to its end or until it visibly appears to be close to breaking. However it may take “forever” driving it only 5k miles per year. Converting to a non-interference engine also gave me confidence in removing the very annoying belt covers for easy belt inspection, easy access to water pump and maybe even better cooling on the front of the engine.
As mentioned I did set the engine (cylinder #1 and #4) to TDC and turned the cam a number of times. This is not a full verification, since the hydraulic lifters will collapse without oil pressure, but should be good enough for verifying that the pocket and valve axis are coaxial. However the needed cut depth is very easy to measure and implement, so I am confident it is adequate.
Laust