Mild Stoker Project Started, didn't get what I paid for!
12-21-2020, 08:54 PM
#16
Nordschleife Master
The crank stroke is measured over 180 degrees of swing. So if I move the rod center 2mm of the crank center, its 4mm of stroke. For example. A Chevy crank with a 3.75 inch stroke has it's crank to rod centers offset by 1.875".... I hope that's how it works and according to the SBC crank I just measured that appears to be the case. Another way to look at it is 2mm more arm, puts the piston 2mm higher at TDC and 2mm low BDC which add up to 4mm more of piston stroke. I think that's how it works.
The stroke length is the diameter.
D=2*R
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icsamerica (12-21-2020)
12-21-2020, 09:29 PM
#17
Burning Brakes
Thread Starter
Jerry.... Ya had me thinkin' there for a sec and I ran to the garage with a ruler to measure that SBC crank I have laying around becasue I know the stroke. LOL.
Reminds me of a house call I made two weeks ago. Old lady who bought 2 computers from me over 20 years... Shirley is her name...Sweet old lady, pleasant as one can be, with a nice laugh too. Her laptop needed some minor repairs. I dont do house calls anymore but I took a ride to her apartment early one Saturday morning for old time sake. I fixed the laptop, showed her how to use Zoom, did a test call made sure sound and video was OK so she can talk to her doctor. We got to talking...she loves travel shows but complained there is nothing left for here to see on cable but reruns so I showed her how to find "Bald and Bankrupt" on YouTube. I warned her the language can be a bit salty and she assured me that was OK. She informed me that as a retired high school science teacher in the 70's from Brooklyn it would be nothing she hadn't already heard. I showed her some picture of my kids. 2 hours later, parts an labor her bill was $170.00. She said I only have 20's do you have a 10. I gave her the 10 and she gave me eight 20's and my 10 back. She said there you go John, $170.00. I said oh you have to give me another 20 and she said... why and counted out 20, 40 60....170. I smiled, said oh right Shirley and got on my way thinking to myself good thing she taught science and not math.
Reminds me of a house call I made two weeks ago. Old lady who bought 2 computers from me over 20 years... Shirley is her name...Sweet old lady, pleasant as one can be, with a nice laugh too. Her laptop needed some minor repairs. I dont do house calls anymore but I took a ride to her apartment early one Saturday morning for old time sake. I fixed the laptop, showed her how to use Zoom, did a test call made sure sound and video was OK so she can talk to her doctor. We got to talking...she loves travel shows but complained there is nothing left for here to see on cable but reruns so I showed her how to find "Bald and Bankrupt" on YouTube. I warned her the language can be a bit salty and she assured me that was OK. She informed me that as a retired high school science teacher in the 70's from Brooklyn it would be nothing she hadn't already heard. I showed her some picture of my kids. 2 hours later, parts an labor her bill was $170.00. She said I only have 20's do you have a 10. I gave her the 10 and she gave me eight 20's and my 10 back. She said there you go John, $170.00. I said oh you have to give me another 20 and she said... why and counted out 20, 40 60....170. I smiled, said oh right Shirley and got on my way thinking to myself good thing she taught science and not math.
Last edited by icsamerica; 12-21-2020 at 09:39 PM.
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MFranke (12-29-2020)
12-23-2020, 01:01 PM
#18
Former Sponsor
Porsche Rod Journal is 52mm
1st possible Honda Journal size is 48mm (1.889in)
2nd possible alt Honda Journal Size is 45mm (1.77in)
1st possible leaves me with 4mm and but will have to use 1st honda over size bearging to get it round the journal during machining for a yeild of 5.63L with a 104mm piston.
2nd possible allows for 5mm with no oversize bearing and will yeild 5.7L with a 104mm piston.
All that said I've haven't gotten the crank out yet to have a look at the main to con rod overlap situation and see how far I can go with out welding. I dont think I'll need to weld or go billet to meet my HP goals of 360HP. I've got other big HP projects cars to scratch that itch and at my track skill level I dont need more than that in the 928 at this point and it may just get me in trouble. When I'm ready to do the go big or go home thing and I learn how to drive better I'll start over with a more modern platform.
My goal is Limerock in at 59.9. I'm 7 seconds away this year which is an eternity. I'm thinking there's 3 seconds left in me, 2 left in the suspension and 2 seconds with 60 more HP and better gearing. At present It's mostly 3rd gear loop and I'm at 6500 RPM for a very short time. Shorter gears will allow better use of 3rd and I then I can get to 4th on the straight making it a 2 gear track.
GB... At this point I know that I dont know what I dont know and I know that you know what I dont know so if you want to PM me some details and help me on a consulting basis I'm amenable to that and expect nothing for free.
1st possible Honda Journal size is 48mm (1.889in)
2nd possible alt Honda Journal Size is 45mm (1.77in)
1st possible leaves me with 4mm and but will have to use 1st honda over size bearging to get it round the journal during machining for a yeild of 5.63L with a 104mm piston.
2nd possible allows for 5mm with no oversize bearing and will yeild 5.7L with a 104mm piston.
All that said I've haven't gotten the crank out yet to have a look at the main to con rod overlap situation and see how far I can go with out welding. I dont think I'll need to weld or go billet to meet my HP goals of 360HP. I've got other big HP projects cars to scratch that itch and at my track skill level I dont need more than that in the 928 at this point and it may just get me in trouble. When I'm ready to do the go big or go home thing and I learn how to drive better I'll start over with a more modern platform.
My goal is Limerock in at 59.9. I'm 7 seconds away this year which is an eternity. I'm thinking there's 3 seconds left in me, 2 left in the suspension and 2 seconds with 60 more HP and better gearing. At present It's mostly 3rd gear loop and I'm at 6500 RPM for a very short time. Shorter gears will allow better use of 3rd and I then I can get to 4th on the straight making it a 2 gear track.
GB... At this point I know that I dont know what I dont know and I know that you know what I dont know so if you want to PM me some details and help me on a consulting basis I'm amenable to that and expect nothing for free.
I didn't consider using a 45mm rod journal, as a method to get 5mm more stroke.
I still need to think about your 6.5" connecting rod.
Last edited by GregBBRD; 12-23-2020 at 01:15 PM.
12-23-2020, 04:39 PM
#19
Former Sponsor
It's like every single time I even think about it, my head hits a giant speed bump.
Really quickly, in my head: (Total disregard for deck height, pin location, big end bore or small end bore, or anything else, just thinking about something I build.)
For a 95.25mm stroke with a stock 968 piston, I use a 5.850" rod.
If you add 5mm to a 78,9mm stock crank, the new stroke is 83.9mm.
95.25mm - 83.9mm = 11.35mm (.447") So the rod needs to be .447" longer than the rod for the 95.25mm stroker, to put the 968 piston at the same place in the bore, with a 83.9mm stroke.
5.850" + .447" = 6.297" long rod.
What am I missing/doing wrong in my math?
Last edited by GregBBRD; 12-23-2020 at 04:50 PM.
12-23-2020, 05:18 PM
#20
Rennlist Member
For me the math is a bit simpler than that. If you add 5mm to the stroke the replacement rod simply needs to be 2.5mm shorter than the stock rod; center to center. I don't know how long the stock rod is, so I can't do the math.
Last edited by Jerry Feather; 12-23-2020 at 05:19 PM.
12-23-2020, 05:20 PM
#21
Former Sponsor
The wrist pin position on a 968 piston is different than stock, which is why it's not that simple.
12-23-2020, 05:26 PM
#22
Rennlist Member
For that small amount of difference I would be looking at some modification of the 968 piston tops, which they probably need anyway. Taking a bit off the top of the pistons will help by increasing the squish area and it will also help by putting the valve reliefs in proper orientation in relation to the offset of the wrist pins, so that they offset in respect to the crank rotation the same way on both sides of the block. It can also raise the compression ratio some.
Last edited by Jerry Feather; 12-23-2020 at 05:28 PM.
12-23-2020, 05:37 PM
#23
Rennlist Member
12-23-2020, 06:45 PM
#24
Former Sponsor
The distance from the center of the wrist pin to the top of the piston (compression height) is different on a 968 than a 928.
In a 968, Porsche increased the stroke, but used the same length rod. The only way to keep the piston from popping way out of cylinder (from the increase in stroke) was to move the wrist pin higher.
Last edited by GregBBRD; 12-23-2020 at 06:50 PM.
12-23-2020, 09:44 PM
#25
Burning Brakes
Thread Starter
Not using a "stock" 968 piston. I order some Wossner 104mm pistons that I recall are for a "stoker" 968 so the compression height is less so they can be used with a stock 968 rod. I'm not gonna get too hung up on it until the pistons arrive and I can measure them. They were the only new pistons I could find in stock and they have a coating that is useable with Alusil.
Last edited by icsamerica; 12-23-2020 at 09:54 PM.
12-23-2020, 10:25 PM
#26
Former Sponsor
Not using a "stock" 968 piston. I order some Wossner 104mm pistons that I recall are for a "stoker" 968 so the compression height is less so they can be used with a stock 968 rod. I'm not gonna get too hung up on it until the pistons arrive and I can measure them. They were the only new pistons I could find in stock and they have a coating that is useable with Alusil.
Yes, if they moved the pin up .200" (~5mm) from a stock 968 configuration, that would explain your rod length.
Be sure and get a written guarantee, from Wossner, on that coating, for use in a 928 engine.
(The loads on a 4 cylinder piston with a perfectly centered rod are way, way different than those of an 8 cylinder, with offset rods.)
12-29-2020, 12:10 AM
#27
Burning Brakes
Thread Starter
Inside the short block, See what's Broken ?
12-29-2020, 03:03 AM
#28
You did good with that engine !
Meanwhile, here in OZ, the parasites want 5 grand for an engine that was pulled from a car and left to rot on a pallet 8 years ago.
They're now all bitching that no-one wants to buy their overpriced junk.
Seriously, would YOU pay 5 grand for a condition unknown motor that's been sitting in a pile of fluids for a decade ?
Just suck it up and pay the shipping to get one from overseas I say.
I buy all my stuff from Roger (and Pelican...*shhhhhh*).
Meanwhile, here in OZ, the parasites want 5 grand for an engine that was pulled from a car and left to rot on a pallet 8 years ago.
They're now all bitching that no-one wants to buy their overpriced junk.
Seriously, would YOU pay 5 grand for a condition unknown motor that's been sitting in a pile of fluids for a decade ?
Just suck it up and pay the shipping to get one from overseas I say.
I buy all my stuff from Roger (and Pelican...*shhhhhh*).
12-29-2020, 05:51 AM
#29
Rennlist Member
Not using a "stock" 968 piston. I order some Wossner 104mm pistons that I recall are for a "stoker" 968 so the compression height is less so they can be used with a stock 968 rod. I'm not gonna get too hung up on it until the pistons arrive and I can measure them. They were the only new pistons I could find in stock and they have a coating that is useable with Alusil.
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Strosek Ultra (12-29-2020)
12-29-2020, 11:08 AM
#30
Rennlist Member
Join Date: Feb 2011
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Your only option is 968 iron-coated 104mm pistons. Aftermarket pistons can only be used in Nikasil-coated cylinders or cylinders having cast iron sleeves installed. Aftermarket pistons in alusil cylinders, whatever coating you have on the pistons, will end in a disaster.
Åke
Åke