Converting to non-interference engine
02-01-2005, 12:48 PM
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Cutting valve pockets is common practice and this doesn't look very deep either. IMHO this would most likely work out well. CR change from 8 to 7.9 is not a noticable difference either.
However, with proper sevice, this mod should not be required.
Bengt
However, with proper sevice, this mod should not be required.
Bengt
02-08-2005, 02:06 PM
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Thanks for all your responses and concerns.
Weakening the piston:
Again the real question is if the piston still will have adequate strength to handle the forces it is exposed to. The deepest points of the pockets are buried in the solid section that provides connection to the gudgeon pin. The combustion pressure counteracts the inertia of the piston around TDC and as such HP is not challenging the strength as much as low end torque and high rpm would, which is good since our engines do not produce much low end torque not are high revving. So in my judgment it will be able to handle the forces in our engines (incl. modified engines).
Hot spot at the “thin wall section”:
I have seen many pistons on various occasions (SEMA shows in particular) from JE, Ross, Arias, Weiseco, KB, CP, Ford, Chrysler, GM pistons etc. and some have thinner outer pocket walls that our proposed modification (see for example this pdf file ). Remember also that aluminum is a very good thermal conductor and that the heat flows out of the pistons through the outer edges into the cylinder walls. That means that the hottest part of the piston will be at the center and the edges will be coolest. A thin ridge at the edge will not be hotter than the center.
The strength of the ridge should not be of concern either, since the pressure is the same on the outside as on the inside, hence no resulting bending force.
Mixing properties:
It almost appeared to be a fad in the mid 80’s to have flat top of dished piston, but such simple geometries seem to have been abandoned to the extent that some high compression pistons almost appear to have “cancerous growth” on the top to fill out the cylinder head cavity. That valve pockets are of little concern to the manufacturers can be illustrated by the fact that the next generation Corvette Z06 has 4 valve pockets, but only two valves per cylinder (I am not sure why, symmetric installation? or preparation for a 4-valve head?).
“A non-interference engine interferes at high rpm”:
Huh, If all components are standard, the rpm is kept below red-line, the carbon deposits are within reason, then a non-interference engine is not interfering, otherwise it is just mislabeled. Any non-modified or properly modified engine in a reasonably good condition will not have valve float below red-line. Should a collision happen with valve float above redline, then there is no geometric lock to bend the valves and a small but noisy collision will likely not bent the valves.
What is the valve-piston clearance for the modified piston?:
The more cases we want to incorporate in one design the larger the valve pockets. Without any special needs* expressed we intend to dimension the valve pockets to have .020” sideways clearance and .010” depth clearance with the following worst case scenarios added: 1) .005” combined con rod stretch, journal oil-film eccentricities, crank and piston pin bend (essentially high rpm dynamic vs. static piston height), 2) cam lift tolerance .002” (standard cam assumed), 3) hard carbon build up on valves and piston exhaust .020” and intake .010”, 4) minimum head gasket thickness .040” and 5) maximum head shave .018” (8.2 CR with valve pockets). All of this gives a static (idle) depth clearance of .055” for the exhaust and .045” on the intake on an engine with a .040” compressed HG and no carbon deposits.
We intend to include special needs if they are clearly expressed and quantified.
* Special needs are non standard valves, cams, headgaskets and extra room for thermal coating on piston crowns.
Just change the T-belt often:
While the belt itself is only ~$25 the cost if done professionally is ~$500 and somewhere in between for a DIY job, if you value your own time. If the water pump is changed much before its service life, then there is some added cost there as well. Alternatively the WP and T-belt change intervals could come out of sync adding to the risk of T-belt breakage. Unfortunately changing the T-belt apparently does not give you 100% certainty for no breakage within for example 30k miles, see this thread .
These modified pistons are primarily aimed at those who’ve had an unfortunate and unplanned failure. Most of these unexpected failures have been under normal everyday driving, not racing or revving the engine to high rpm. Since it’s going to cost you an unexpected $1200+ repair bill adding these pistons could give you a large amount of peace of mind, in that if you have an unexpected timing-belt failure again, you can simply replace the broken timing-belt and not have to yank the head and replace the (expensive exhaust) valves as well.
I once had a 1985 Toyota MR2 from new (as a frequent driver) with a 7500 rpm redline, which was approached often. I deliberately pushed the typical 60k timing-belt change limit until I got nervous at 130k miles at which point it was finally changed. Looking at the removed belt I thought that there was at least another 30k in it. The car died of natural causes (old age) with an intact 2nd belt at 230k miles.
The decision to change a timing-belt after a certain mileage could be approached statistically for example by requiring a 99.5% confidence level that the belt will not break. That could be lowered (interval lengthened) if the consequences were not that severe. However I have no idea of the distribution function of the belt life-span (to breakage).
Back to the specifics: Danno (GURU Racing) and I have decided to go ahead with the project of offering a piston modification service that converts our 951 interference engine to a non-interference engine and believe that the conversion can be done for $195 per piston set.
So please contact either Danno or me if you want to have this done.
Laust
Weakening the piston:
Again the real question is if the piston still will have adequate strength to handle the forces it is exposed to. The deepest points of the pockets are buried in the solid section that provides connection to the gudgeon pin. The combustion pressure counteracts the inertia of the piston around TDC and as such HP is not challenging the strength as much as low end torque and high rpm would, which is good since our engines do not produce much low end torque not are high revving. So in my judgment it will be able to handle the forces in our engines (incl. modified engines).
Hot spot at the “thin wall section”:
I have seen many pistons on various occasions (SEMA shows in particular) from JE, Ross, Arias, Weiseco, KB, CP, Ford, Chrysler, GM pistons etc. and some have thinner outer pocket walls that our proposed modification (see for example this pdf file ). Remember also that aluminum is a very good thermal conductor and that the heat flows out of the pistons through the outer edges into the cylinder walls. That means that the hottest part of the piston will be at the center and the edges will be coolest. A thin ridge at the edge will not be hotter than the center.
The strength of the ridge should not be of concern either, since the pressure is the same on the outside as on the inside, hence no resulting bending force.
Mixing properties:
It almost appeared to be a fad in the mid 80’s to have flat top of dished piston, but such simple geometries seem to have been abandoned to the extent that some high compression pistons almost appear to have “cancerous growth” on the top to fill out the cylinder head cavity. That valve pockets are of little concern to the manufacturers can be illustrated by the fact that the next generation Corvette Z06 has 4 valve pockets, but only two valves per cylinder (I am not sure why, symmetric installation? or preparation for a 4-valve head?).
“A non-interference engine interferes at high rpm”:
Huh, If all components are standard, the rpm is kept below red-line, the carbon deposits are within reason, then a non-interference engine is not interfering, otherwise it is just mislabeled. Any non-modified or properly modified engine in a reasonably good condition will not have valve float below red-line. Should a collision happen with valve float above redline, then there is no geometric lock to bend the valves and a small but noisy collision will likely not bent the valves.
What is the valve-piston clearance for the modified piston?:
The more cases we want to incorporate in one design the larger the valve pockets. Without any special needs* expressed we intend to dimension the valve pockets to have .020” sideways clearance and .010” depth clearance with the following worst case scenarios added: 1) .005” combined con rod stretch, journal oil-film eccentricities, crank and piston pin bend (essentially high rpm dynamic vs. static piston height), 2) cam lift tolerance .002” (standard cam assumed), 3) hard carbon build up on valves and piston exhaust .020” and intake .010”, 4) minimum head gasket thickness .040” and 5) maximum head shave .018” (8.2 CR with valve pockets). All of this gives a static (idle) depth clearance of .055” for the exhaust and .045” on the intake on an engine with a .040” compressed HG and no carbon deposits.
We intend to include special needs if they are clearly expressed and quantified.
* Special needs are non standard valves, cams, headgaskets and extra room for thermal coating on piston crowns.
Just change the T-belt often:
While the belt itself is only ~$25 the cost if done professionally is ~$500 and somewhere in between for a DIY job, if you value your own time. If the water pump is changed much before its service life, then there is some added cost there as well. Alternatively the WP and T-belt change intervals could come out of sync adding to the risk of T-belt breakage. Unfortunately changing the T-belt apparently does not give you 100% certainty for no breakage within for example 30k miles, see this thread .
These modified pistons are primarily aimed at those who’ve had an unfortunate and unplanned failure. Most of these unexpected failures have been under normal everyday driving, not racing or revving the engine to high rpm. Since it’s going to cost you an unexpected $1200+ repair bill adding these pistons could give you a large amount of peace of mind, in that if you have an unexpected timing-belt failure again, you can simply replace the broken timing-belt and not have to yank the head and replace the (expensive exhaust) valves as well.
I once had a 1985 Toyota MR2 from new (as a frequent driver) with a 7500 rpm redline, which was approached often. I deliberately pushed the typical 60k timing-belt change limit until I got nervous at 130k miles at which point it was finally changed. Looking at the removed belt I thought that there was at least another 30k in it. The car died of natural causes (old age) with an intact 2nd belt at 230k miles.
The decision to change a timing-belt after a certain mileage could be approached statistically for example by requiring a 99.5% confidence level that the belt will not break. That could be lowered (interval lengthened) if the consequences were not that severe. However I have no idea of the distribution function of the belt life-span (to breakage).
Back to the specifics: Danno (GURU Racing) and I have decided to go ahead with the project of offering a piston modification service that converts our 951 interference engine to a non-interference engine and believe that the conversion can be done for $195 per piston set.
So please contact either Danno or me if you want to have this done.
Laust
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Jay Wellwood (10-13-2023)
02-08-2005, 03:23 PM
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It sounds like a worthy product, but it still doesn't address my point about changing the shape and configuration of the combustion chamber. It will obviously work and run, and it looks like you have addressed the reliability concerns.
Yes, this is "common practice" and dished and domed pistons are commonplace. There is, however, a known or assumed tradeoff for these band-aid cures. This is beyond compression ratio. In this case, compression ratio reduction is minimal, and probably the smallest variable with regard to changing the combustion chamber configuration.
Does it affect plug selection, timing, or fuel requirements, flame propogation? We don't know. I am not trying to step on your efforts, because they are laudable. In fact, I wish I had some in my 928 as we speak so I don't have to flatbed it into the shop tomorrow for a timing belt since the light just came on. I would rather drive it in and spent the $100.00 at Hooters.
Maybe dyno and record a/f ratios with a wideband before and after with your first one to determine any effects the change to the combustion chamber will have, involving only that one single variable only. It may well be none, but we don't know. Remember, the combustion chamber is where it all happens.
Jeff
'86 951 2.8. Lots of extra stuff
'87 928 S4. Some extra stuff (SC in progress)
'92 968 Cab. Mostlly stock, and staying that way
Yes, this is "common practice" and dished and domed pistons are commonplace. There is, however, a known or assumed tradeoff for these band-aid cures. This is beyond compression ratio. In this case, compression ratio reduction is minimal, and probably the smallest variable with regard to changing the combustion chamber configuration.
Does it affect plug selection, timing, or fuel requirements, flame propogation? We don't know. I am not trying to step on your efforts, because they are laudable. In fact, I wish I had some in my 928 as we speak so I don't have to flatbed it into the shop tomorrow for a timing belt since the light just came on. I would rather drive it in and spent the $100.00 at Hooters.
Maybe dyno and record a/f ratios with a wideband before and after with your first one to determine any effects the change to the combustion chamber will have, involving only that one single variable only. It may well be none, but we don't know. Remember, the combustion chamber is where it all happens.
Jeff
'86 951 2.8. Lots of extra stuff
'87 928 S4. Some extra stuff (SC in progress)
'92 968 Cab. Mostlly stock, and staying that way
02-08-2005, 09:57 PM
#21
This is VERY common place. Most are reading way too much into this. When changing Valve Timing or the Camshaft complete, the valve pockets often have to be relieved. This is done to ensure that the clearance is there at whatever Valve Timing the camshaft is set to. On most race engines the minimum clearance is 0.040". On street engines this is increased to 0.060".
I see no reason to do this if the Timing belt is replaced regulary. How many times, would you have to replace the belt to equal the cost of pulling the Pistons. If you cahnge the Valve Timing or change the Camshaft, everything changes again. I do not know what the clearances are now wityh the stock Valve Timing, as it changes with the Valve Timing. Depending upon the valve Timing or Camshaft lobe centerline, the Exhaust Valve maybe the closest at 5 degrees BTDC and the Intake will be at 5 ATDC. these will change with the centerlines, but the Exhaust and the Inatke will always be the same number of degrees BTDC and ATDC. So what does all this mean, it means that you could possibly gain clearance on the tightest Valve by advancing or reatrding either the Intake or Exhaust centerlines and maybe pick up some perfromance as well. It would be a good excerise to set the Camshaft centerlines at the stock position and measure the tightest point. There is a Camshaft under developemnt at present for Hydraulic Tappets, that just cleans up all of the bad harmonics etc, which so far as produced almost 12% increase in the upper revs. I will ask what the centerlines are and what the clearances are as well.
As for maching the Pockets, I have seen this done, and in some case the Flycuting Tool may be used, but I have seen where an End mill Cutter is used and with the correct Radius on the end, is plunged down into the Piston to the required depth. Then the corners of the Pockets are deburred and polished to remove any sharp edges. At no time would this be done without checking the Dome thickness.
"Does it affect plug selection, timing, or fuel requirements, flame propogation? We don't know."
This is not the case normally. I'm sure some things must change, but to obtain the correct Clearance and Comp Ratio, you have to do what is necessary. Not to be to critical, but the above comment is a little, "old lady with a new table" . You are good to go, no problem, as long as the thickness is not compromised.
I see no reason to do this if the Timing belt is replaced regulary. How many times, would you have to replace the belt to equal the cost of pulling the Pistons. If you cahnge the Valve Timing or change the Camshaft, everything changes again. I do not know what the clearances are now wityh the stock Valve Timing, as it changes with the Valve Timing. Depending upon the valve Timing or Camshaft lobe centerline, the Exhaust Valve maybe the closest at 5 degrees BTDC and the Intake will be at 5 ATDC. these will change with the centerlines, but the Exhaust and the Inatke will always be the same number of degrees BTDC and ATDC. So what does all this mean, it means that you could possibly gain clearance on the tightest Valve by advancing or reatrding either the Intake or Exhaust centerlines and maybe pick up some perfromance as well. It would be a good excerise to set the Camshaft centerlines at the stock position and measure the tightest point. There is a Camshaft under developemnt at present for Hydraulic Tappets, that just cleans up all of the bad harmonics etc, which so far as produced almost 12% increase in the upper revs. I will ask what the centerlines are and what the clearances are as well.
As for maching the Pockets, I have seen this done, and in some case the Flycuting Tool may be used, but I have seen where an End mill Cutter is used and with the correct Radius on the end, is plunged down into the Piston to the required depth. Then the corners of the Pockets are deburred and polished to remove any sharp edges. At no time would this be done without checking the Dome thickness.
"Does it affect plug selection, timing, or fuel requirements, flame propogation? We don't know."
This is not the case normally. I'm sure some things must change, but to obtain the correct Clearance and Comp Ratio, you have to do what is necessary. Not to be to critical, but the above comment is a little, "old lady with a new table" . You are good to go, no problem, as long as the thickness is not compromised.
02-09-2005, 02:57 PM
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Originally Posted by mroberts
My mechanic swears blind the turbo heads are non-interference. I noticed in one of he spec sheets that the US engines are 9.5:1, vs 10.5:1 in Europe - is this enough difference to turn a European "interference" engine into a non-interference one ?
02-09-2005, 03:37 PM
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You make a good point about the non-interference motor not making valve to piston contact at any RPM. I have only heard of such things happening and maybe carbon buildup was the cause.
Nevertheless I see little point in bothering to pull the pistons out and have them modified. Almost anyone who is getting serious with a Turbo is going to be in the engine more often than every 40K miles (head gaskets, other mods, etc.) and will change the timing belt at that time since it'll be off anyway. Anyone who is not serious is not going to go to the trouble.
Maybe you could sell the service to someone who has a higher mileage turbo and is getting rings and bearings done and is just going to keep driving the car more or less stock for another 100K miles?
-Joel.
Nevertheless I see little point in bothering to pull the pistons out and have them modified. Almost anyone who is getting serious with a Turbo is going to be in the engine more often than every 40K miles (head gaskets, other mods, etc.) and will change the timing belt at that time since it'll be off anyway. Anyone who is not serious is not going to go to the trouble.
Maybe you could sell the service to someone who has a higher mileage turbo and is getting rings and bearings done and is just going to keep driving the car more or less stock for another 100K miles?
-Joel.
02-09-2005, 03:41 PM
#24
Once this valve pocket cutting process is commonplace for the 951, a DIY kit for grinding the pockets with the pistons still in the motor(i.e. when the head is off after a t-belt breakage and a new one is going in)would be cool. I suppose a mount using the head bolt threads and a go-nogo(Navy term) angled piece to mount a drill for grinding. Might want a big o-ring around the piston to collect the shavings. I'd be very concerned if I had a t-belt break before the 30K mark(after an incompetent dealership mechanic using a Porsche tension tool still overtightens it) and was about to put on a new head or valves with their fate unknown.
02-17-2005, 11:20 AM
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Reading too much into this? Go back and look at the drawings. That is a significant amount of material that needs to be removed. It could even affect balance.
Enough with the "little old lady with a new table" business. This isn't an old American V8. We are talking about 300-400 horspower from only 2.5L. Small changes can mean a lot, especially in the combustion chamber. The point is, we don't know. In the time spent debating this, one could have done two dyno pulls, measured a/f ratios, with all else equal. Then we would know.
I have been studying engines longer than most on this board have been driving. I'm sorry for the curt comments, but when they fly, they will be responded to.
Jeff
'86 951 2.8. Lots of extra stuff
'87 928 S4. Some extra stuff (SC in progress)
'92 968. Mostlly stock and staying that way
Enough with the "little old lady with a new table" business. This isn't an old American V8. We are talking about 300-400 horspower from only 2.5L. Small changes can mean a lot, especially in the combustion chamber. The point is, we don't know. In the time spent debating this, one could have done two dyno pulls, measured a/f ratios, with all else equal. Then we would know.
I have been studying engines longer than most on this board have been driving. I'm sorry for the curt comments, but when they fly, they will be responded to.
Jeff
'86 951 2.8. Lots of extra stuff
'87 928 S4. Some extra stuff (SC in progress)
'92 968. Mostlly stock and staying that way
02-17-2005, 01:56 PM
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I don't think you'd want to try this mod without pulling the pistons. That sounds like you'd be using a jig to cut the pistons with a die grinder while vacuuming up the shavings or something. Yikes. I'd rather buy a timing belt every 40K miles and check the flippin' tension every now and then.
I think this mod (assuming no issues witht he deeper pockets) is only sensible when doing the whole bottom end, and then only sensible if the owner is not the sort of person who is going to need a head gasket more often (or about as often) as a timing belt. If you sell it as a way to allow the 'mechanic' who installs the belt to overtighten it and not bother to retension, and/or to neglect the belt retensions that's bad news. I think a lot of people put the new belts on stupid tight, which is hard on the water pump, rollers, and crank bearings. I suspect many waterpump failures, rebuilt or new, are because of this sort of work. Modifying the pistons is not a good cure for shoddy work. How many sensible people who do their own quality work have had timing belt failures?
-Joel.
I think this mod (assuming no issues witht he deeper pockets) is only sensible when doing the whole bottom end, and then only sensible if the owner is not the sort of person who is going to need a head gasket more often (or about as often) as a timing belt. If you sell it as a way to allow the 'mechanic' who installs the belt to overtighten it and not bother to retension, and/or to neglect the belt retensions that's bad news. I think a lot of people put the new belts on stupid tight, which is hard on the water pump, rollers, and crank bearings. I suspect many waterpump failures, rebuilt or new, are because of this sort of work. Modifying the pistons is not a good cure for shoddy work. How many sensible people who do their own quality work have had timing belt failures?
-Joel.
02-17-2005, 05:11 PM
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Seems like it would be more technically and economically feasible to create a timing chain system to replace the belt. Does anyone make one of these?
Another thought would be to start with 944 N/A pistons and machine them for non-interference and CR reduction.
Another thought would be to start with 944 N/A pistons and machine them for non-interference and CR reduction.
02-19-2005, 06:52 PM
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Jeff F, I weighed a piston (with rings) before and after the change to be 560g and 556g respectively. That is 4 grams removed (per piston) and with an estimated aluminum density of 2.7 that is about 1.5 cc, not a lot of material in “my book”.
The machining accuracy would also assure that the difference in weight change between the pistons is far less than ¼ gram.
Compare this to the general eagerness of knife-edging and lightening of the crankshaft and it should be obvious, that the weight change should be of no serious concern with respect to balancing.
Jns, I like your idea of a DIY kit a lot. Designing it to be fool proof would be a challenge though, but let’s see.
Laust
The machining accuracy would also assure that the difference in weight change between the pistons is far less than ¼ gram.
Compare this to the general eagerness of knife-edging and lightening of the crankshaft and it should be obvious, that the weight change should be of no serious concern with respect to balancing.
Jns, I like your idea of a DIY kit a lot. Designing it to be fool proof would be a challenge though, but let’s see.
Laust
02-19-2005, 11:28 PM
#29
"Once this valve pocket cutting process is commonplace for the 951, a DIY kit for grinding the pockets with the pistons still in the motor"
Please tell me I did not read what I thought I read!! Oh.....my .... Gosh, the ideas have sunk to an all time low!
Just leave it parked in the Garage, and you will never have to do the unthinkable as mentioned above!
Please tell me I did not read what I thought I read!! Oh.....my .... Gosh, the ideas have sunk to an all time low!
Just leave it parked in the Garage, and you will never have to do the unthinkable as mentioned above!
02-20-2005, 04:12 AM
#30
Please tell me I did not read what I thought I read!! Oh.....my .... Gosh, the ideas have sunk to an all time low!
Wow, I won the award for the worst idea ever. I'm truly honored. I'm sorry you are so shocked by the idea. With all this experience you claim to have, you must be in your golden years and should try to avoid sudden bouts of excitement. You're obviously so full of yourself that you could never learn anything from us low-lifes. This is a place for sharing ideas and supporting each other, not criticizing others. Try to remember that and obey any other orders I give you, boy.
Wow, I won the award for the worst idea ever. I'm truly honored. I'm sorry you are so shocked by the idea. With all this experience you claim to have, you must be in your golden years and should try to avoid sudden bouts of excitement. You're obviously so full of yourself that you could never learn anything from us low-lifes. This is a place for sharing ideas and supporting each other, not criticizing others. Try to remember that and obey any other orders I give you, boy.
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Dwizle (07-18-2021)
