How to build a 8500rpm safe 968 engine?
09-03-2020, 03:23 PM
#91
Rennlist Member
Hello Michael. I would like to hear more about the custom JE pistons that you used for Henk's S2 engine. They look completely different from all the skirted pistons I see for 944/951/968. What material are they made from? What piston-to-cylinder wall clearance did you use? I think you covered the type of rings you chose but what about coatings were used for the piston sides and the top? That thread also mentioned a custom piston pin and higher compression height.
Did you ever go back and revisit the cams in this build? You mentioned maybe re-camming to lower the rpms range a little bit.
There were two pistons in this image, though I think it's clear that focus is on the piston to the left. I don't see any coatings in these pics, but maybe coatings were applied after the mock-ups and clearance checks. I swiped this image from this build thread: https://rennlist.com/forums/924-931-...r-400hp-4.html
Now that JE has made those pistons once, is that design stored in JE's database to be ordered again.
-Bill
Did you ever go back and revisit the cams in this build? You mentioned maybe re-camming to lower the rpms range a little bit.
There were two pistons in this image, though I think it's clear that focus is on the piston to the left. I don't see any coatings in these pics, but maybe coatings were applied after the mock-ups and clearance checks. I swiped this image from this build thread: https://rennlist.com/forums/924-931-...r-400hp-4.html
Now that JE has made those pistons once, is that design stored in JE's database to be ordered again.
-Bill
09-04-2020, 11:31 AM
#92
Rennlist Member
Responses
Thanks.
Questions:
”Use a properly designed dry sump oiling system (this is a subject within itself)”
If we take out the extreme g forces with slicks on track and limit ourselves to street cars on street tires, is the dry sump system necessary for a reliable 8500rpm with the 968 engine?
I don't know how efficient the Porsche oil pump will be at 8500. The risk, of course, is cavitation and air is a very poor lubricant. If you decide to try it, you risk losing a rod bearing at a minimum, and kicking a rod out of the block at worst. You may want to consider a dry sump system as insurance.
“Use a billet rod to maintain big end concentricity and limit stretch. Use a lightweight piston with strutted skirt support (engineered for the application)“
For the 928 crank and deck height, we found similar enough modern raw forgings for both pistons and rods that we could have the suppliers start from those and get the strength of forged parts. I wonder if the same is possible for the 968.
I don't see any reason to adapt other rods or pistons to use in a 968 since both can be commercially sourced to fit and be proper for the application. I'd reconsider my earlier suggestion for these items.
What ring pack runs reliably at 8500rpm with 968 stroke? I think that the stock 968 1.75mm ring doesn’t model well even for the stock 968 stroke and redline.
I'd use a bespoke ring pack with a bespoke piston; both designed for the application.
“Convert to mechanical lifters to maintain valve control”
Is this solid lifter conversion necessary because of the time spent on the base circle getting short or because the camshaft profile and 8500 rpm engine speed combine to large forces when opening the valve? Suppose that one would keep the camshaft lift low and duration long, would the hydraulic lifters work at 8500 rpm?
I don't know if the Porsche hydraulic followers would operate at 8500. I've taken them to 7600 and they work fine, but 8500 is another universe. Regarding the cam profile, if it's not designed to make power at 8500, what's the point of going there?
“Limit oil to top end by blocking oil to followers“
Is this only the case with solid lifter conversion or would you do that even with hydraulic lifters?
Questions:
”Use a properly designed dry sump oiling system (this is a subject within itself)”
If we take out the extreme g forces with slicks on track and limit ourselves to street cars on street tires, is the dry sump system necessary for a reliable 8500rpm with the 968 engine?
I don't know how efficient the Porsche oil pump will be at 8500. The risk, of course, is cavitation and air is a very poor lubricant. If you decide to try it, you risk losing a rod bearing at a minimum, and kicking a rod out of the block at worst. You may want to consider a dry sump system as insurance.
“Use a billet rod to maintain big end concentricity and limit stretch. Use a lightweight piston with strutted skirt support (engineered for the application)“
For the 928 crank and deck height, we found similar enough modern raw forgings for both pistons and rods that we could have the suppliers start from those and get the strength of forged parts. I wonder if the same is possible for the 968.
I don't see any reason to adapt other rods or pistons to use in a 968 since both can be commercially sourced to fit and be proper for the application. I'd reconsider my earlier suggestion for these items.
What ring pack runs reliably at 8500rpm with 968 stroke? I think that the stock 968 1.75mm ring doesn’t model well even for the stock 968 stroke and redline.
I'd use a bespoke ring pack with a bespoke piston; both designed for the application.
“Convert to mechanical lifters to maintain valve control”
Is this solid lifter conversion necessary because of the time spent on the base circle getting short or because the camshaft profile and 8500 rpm engine speed combine to large forces when opening the valve? Suppose that one would keep the camshaft lift low and duration long, would the hydraulic lifters work at 8500 rpm?
I don't know if the Porsche hydraulic followers would operate at 8500. I've taken them to 7600 and they work fine, but 8500 is another universe. Regarding the cam profile, if it's not designed to make power at 8500, what's the point of going there?
“Limit oil to top end by blocking oil to followers“
Is this only the case with solid lifter conversion or would you do that even with hydraulic lifters?
09-04-2020, 12:02 PM
#93
Nordschleife Master
On the dry sump: The main question on 968 dry sump systems that I have in mind is whether any 8500 rpm oiling problems come from the scavenge side or the pressure side. If the problems only come from the scavenge side, then "semi dry sump" sort of system would be one solution. A semi dry sump baffles the deep part of the wet sump system more and then uses a scavenge pump system to evacuate various problem points in the engine to the sump, while using the stock pump as the pressure stage. If the stock pump has problems say with cavitation (and I am not talking about the pickup sucking air here) at 8500 rpm, then a full external dry sump system is needed. Has anyone seen cavitation damage in the 968 oil pump gears?
On forged rods and pistons: I don't think that there is sufficient demand for 968 forged pistons and rods to get a company to make a completely new raw forging for them. Right? So in some sense all forged 968 custom rods and pistons are adapted from other engines. We got lucky in our 928 project and found very close match on the raw forging and got forged pistons and rods that are almost exactly the way we wanted them, while still being actually forged. It is my understanding that one could have a company cut any sort of piston or rod from a blank metal, but then it wouldn't have the strength of a forged part which comes from the part being forged relatively close to the final shape.
On the rings: For our 8000 rpm / 78.9mm stroke 928 project, we went with a 1.0/1.0/2.0 nikasil-compatible ring pack if my memory serves me well. I was wondering if the 8500 rpm 968 with its 88mm stroke would work with those or if even skinnier rings are needed.
On the hydraulic lifters: The reason for using hydraulic lifters is that shimming the twin cam lifter bucket head is very time consuming. We've used lighter VW/Audi lifters with good success, but our loads are not extreme. Mike Simard who runs aggressive camshafts modified the VW/Audi hydraulic lifters to minimize the travel range of the lifter such that it's almost solid, just doesn't require any adjustment. How quickly can solid lifters be adjusted in the 968 engine?
Relating to camshaft profiles: 8500 rpm duration isn't a problem for lifters, but it's true that if the accelerations are limited, one is leaving money on the table in terms of power with that compromise. Are there good performance camshaft options that retain the variocam feature?
On forged rods and pistons: I don't think that there is sufficient demand for 968 forged pistons and rods to get a company to make a completely new raw forging for them. Right? So in some sense all forged 968 custom rods and pistons are adapted from other engines. We got lucky in our 928 project and found very close match on the raw forging and got forged pistons and rods that are almost exactly the way we wanted them, while still being actually forged. It is my understanding that one could have a company cut any sort of piston or rod from a blank metal, but then it wouldn't have the strength of a forged part which comes from the part being forged relatively close to the final shape.
On the rings: For our 8000 rpm / 78.9mm stroke 928 project, we went with a 1.0/1.0/2.0 nikasil-compatible ring pack if my memory serves me well. I was wondering if the 8500 rpm 968 with its 88mm stroke would work with those or if even skinnier rings are needed.
On the hydraulic lifters: The reason for using hydraulic lifters is that shimming the twin cam lifter bucket head is very time consuming. We've used lighter VW/Audi lifters with good success, but our loads are not extreme. Mike Simard who runs aggressive camshafts modified the VW/Audi hydraulic lifters to minimize the travel range of the lifter such that it's almost solid, just doesn't require any adjustment. How quickly can solid lifters be adjusted in the 968 engine?
Relating to camshaft profiles: 8500 rpm duration isn't a problem for lifters, but it's true that if the accelerations are limited, one is leaving money on the table in terms of power with that compromise. Are there good performance camshaft options that retain the variocam feature?
09-04-2020, 12:45 PM
#94
Rennlist Member
Pistons
Hello Michael. I would like to hear more about the custom JE pistons that you used for Henk's S2 engine. They look completely different from all the skirted pistons I see for 944/951/968. What material are they made from? What piston-to-cylinder wall clearance did you use? I think you covered the type of rings you chose but what about coatings were used for the piston sides and the top? That thread also mentioned a custom piston pin and higher compression height.
Did you ever go back and revisit the cams in this build? You mentioned maybe re-camming to lower the rpms range a little bit.
There were two pistons in this image, though I think it's clear that focus is on the piston to the left. I don't see any coatings in these pics, but maybe coatings were applied after the mock-ups and clearance checks. I swiped this image from this build thread: https://rennlist.com/forums/924-931-...r-400hp-4.html
Now that JE has made those pistons once, is that design stored in JE's database to be ordered again.
-Bill
Did you ever go back and revisit the cams in this build? You mentioned maybe re-camming to lower the rpms range a little bit.
There were two pistons in this image, though I think it's clear that focus is on the piston to the left. I don't see any coatings in these pics, but maybe coatings were applied after the mock-ups and clearance checks. I swiped this image from this build thread: https://rennlist.com/forums/924-931-...r-400hp-4.html
Now that JE has made those pistons once, is that design stored in JE's database to be ordered again.
-Bill
These are custom pistons made from 2618. I used .004 clearance in a ductile iron bore (4.155"). No coatings were used. Ring pack is .043/.043/2mm with an expander sized to adjust the oil ring tension. I used a 22mm tool steel pin 2.250 long for support, and with a .150 straight wall. The compression height was to accommodate the additional .100" stroke and to use as long a rod as I could for this application.
The piston on the left is prepped for installation, while the one on the right is right out of JE's box. Notice the additional radius on the valve pockets, the removal of sharp edges, and the bead blasting to eliminate stress risers and add a bit of strength.
All of my pistons are on file at JE. I've done enough over the years that I can usually call up a particular file and make a few changes to suit most 944 applications. I've also got access to the JE engineers. Although I usually know what works, I'm not shy about consulting them for any odd-ball stuff.
What are you building?
MM
09-05-2020, 12:03 PM
#96
Drifting
Watching this thread closely as I have a spare 968 engine I’d like to build.
What engine management system is ideal for this type of build we’re talking about? The car needs to be behave well on the streets (ie stopping and idling properly at stop lights and crawling along in traffic).
What engine management system is ideal for this type of build we’re talking about? The car needs to be behave well on the streets (ie stopping and idling properly at stop lights and crawling along in traffic).
09-05-2020, 01:27 PM
#97
Nordschleife Master
8500 rpm capable 16V 3.0L variocam 968 engine with a twin-scroll turbo would be tempting option for a cabrio street car. It could idle like stock and have a flat torque curve all the way to 8500 rpm, if built right.
09-06-2020, 12:12 AM
#98
Rennlist Member
Well, this is a little off topic because I’m not shooting for anything close to 8,500rpm so if you guys want me to flame me, I’ll understand.
I’m build a 50/50 street/track car for HPDE and autocross. Translation - I want to drive it to the track and hopefully, home again too. Unfortunately, I about two hours drive from everywhere - Summit, NCCar, and NJMP, but there are two local autocross venues at FedEx Field and the PCA event at BWI.
It needs to run on pump gas and I want to stay NA - no turbos. I’m keeping the balance shafts, quest alternator mod, power steering, and A/C!
I have the some of the engine disassembled now. I purchased a 968 crankshaft. I have the head back already from rebuild and had 968 intake valves and beehive racing springs installed. I have an S2 intake and throttle body. The intake is back from bore hone and the throttle body have been bored to 64mm too. I just got some 4G63 con rods from Eagle and I’m planning to ship the crankshaft, rods, and ACL bearings to a place in Denver for the 95mm offset grind and other work.
I haven’t made definitive plans yet for the block, bore, pistons, or cams. Initially, I was planning to just reuse my 2.5L 944S block and have it sleeved and bored to 104mm with some block filler. My block appears to be in very good condition and there are a few (not many but a few) 104mm piston options available off the shelf that will allow me to increase compression a little bit to between 11:1 and 12:1.
In parallel, I’ve been searching for a cheap 3.0L block in good condition. I figure the cost of machining my block might be about the same as a nice 3.0: block if I can find one and it gives me more options later too.
I haven’t purchase the EM yet, but I’m planning to buy VEMS soon. I am not planning to convert to dry sump, so I’m limiting to about 7000 rpm.
The pistons and cams are the last two major pieces to this puzzle. I’m looking at CatCams Hot Street grind as it at allows me to keep hydraulic lifters, but I’m not sure how much more power I’ll get from this cam if in the 4-7k rpm range. I purchased the lightweight VW performance lifters too. I started a thread about looking for advice on the cam selection. I got some recommendation to contact John Milledge on cams, but I haven’t called him yet.
I also haven’t selected or purchased an exhaust header and have been hoping to get lucky on eBay but there are a few new options available from RSBarn, Lindsey, and RacersEdge.
I’d greatly appreciate any advice on any part of this plan.
-Bill
I’m build a 50/50 street/track car for HPDE and autocross. Translation - I want to drive it to the track and hopefully, home again too. Unfortunately, I about two hours drive from everywhere - Summit, NCCar, and NJMP, but there are two local autocross venues at FedEx Field and the PCA event at BWI.
It needs to run on pump gas and I want to stay NA - no turbos. I’m keeping the balance shafts, quest alternator mod, power steering, and A/C!
I have the some of the engine disassembled now. I purchased a 968 crankshaft. I have the head back already from rebuild and had 968 intake valves and beehive racing springs installed. I have an S2 intake and throttle body. The intake is back from bore hone and the throttle body have been bored to 64mm too. I just got some 4G63 con rods from Eagle and I’m planning to ship the crankshaft, rods, and ACL bearings to a place in Denver for the 95mm offset grind and other work.
I haven’t made definitive plans yet for the block, bore, pistons, or cams. Initially, I was planning to just reuse my 2.5L 944S block and have it sleeved and bored to 104mm with some block filler. My block appears to be in very good condition and there are a few (not many but a few) 104mm piston options available off the shelf that will allow me to increase compression a little bit to between 11:1 and 12:1.
In parallel, I’ve been searching for a cheap 3.0L block in good condition. I figure the cost of machining my block might be about the same as a nice 3.0: block if I can find one and it gives me more options later too.
I haven’t purchase the EM yet, but I’m planning to buy VEMS soon. I am not planning to convert to dry sump, so I’m limiting to about 7000 rpm.
The pistons and cams are the last two major pieces to this puzzle. I’m looking at CatCams Hot Street grind as it at allows me to keep hydraulic lifters, but I’m not sure how much more power I’ll get from this cam if in the 4-7k rpm range. I purchased the lightweight VW performance lifters too. I started a thread about looking for advice on the cam selection. I got some recommendation to contact John Milledge on cams, but I haven’t called him yet.
I also haven’t selected or purchased an exhaust header and have been hoping to get lucky on eBay but there are a few new options available from RSBarn, Lindsey, and RacersEdge.
I’d greatly appreciate any advice on any part of this plan.
-Bill
