1950split
Samba Member
Joined: December 08, 2005
Posts: 485
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 Posted: Fri Feb 17, 2012 4:05 am Post subject: DPR cranks - internal or external groove main bearings ? |
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Hi,
I found this extremely interesting post in which SRP1 explains the reason for which one type of bearings should be used and not the other :
http://www.thesamba.com/vw/forum/viewtopic.php?t=4...p;start=20
My question is DPR stroker cranks are, I guess, made from 1600cc cranks ? According to SRP1's explanation, crossdrilled cranks, and if my assumption is correct, DPR's crank would therefore require external grooved main bearings.
However, in the case of a 180hp+, street and strip set up would it be of advantage to use internal groove bearings over external ?
Thanks,
Phil |
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AlteWagen
Troll
Joined: February 23, 2007
Posts: 8697
Location: PNW
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| Posted: Fri Feb 17, 2012 2:55 pm Post subject: |
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| SRP1 wrote: |
Ok here's the deal.
In the early 36hp, 40hp, and 1500cc engines the crankshafts where straight drilled for oiling. The straight drill design made the crank very strong, and those engines used the early KS internal oil groove center main bearing, the center main and groove feeds oil to #1 and #4 rod bearings at all 360* of crankshaft roatation.
Later on VW changed the crank design to a crossdrilled oiling set up in the crank, when they did so they also changed the center main to an external oiling groove. Now here's the key; when VW went to the crossdrilled crank they also incorporated that huge tear drop in the crank to increase oil pick up from the now small slot in the bearing (no longer 360*)
With the tear drop slot the #1 and #4 bearings are feed oil about 280* of crankshaft rotation, that's a pretty good amount of oil, at least enough IMO to keep the rods well oiled even in a big stroker engine, little lone a stock engine.
Now lets back up to the straight drilled crank, you know that little oiling hole in the main journal, but stronger by design (most all stroker cranks are drilled with straight shot oiling to increase crank strenght) When used with an external grooved bearing the rods are oiled about 90* of crankshaft rotation, before the hole in the crank passes past the slot in the bearing, the remaining 270* degree's of rotation the rod bearings are left out in the breeze so to speak for oil. We all know that as RPM increases, and centrifical forces increase, the oil is pushed (slung) forced outward faster, that increases the demand for oil at the rods. Add a short 90* fill time from the oiling galley to the rod, so at as low as even 4000rpm, and the next thing you know, BOOM.
At the very least your rod bearings are starving for oil.
I should touch just a little on what is called hydro dynamic wedge, that is the locked layer of oil trapped between the bearing and journal. FWIW I am also sure that VW was paying attention to this with the new bearing design and crossdrilled cranks. With the new design the oil galleys are closed so to speak when the piston is at TDC and BDC. With the galley blocked off it does increase the strenght of the wedge as the oil can not creap back into the galley as the piston is loaded and forces the rod into the crank journal. With the 360* slot that effect is there, just not as strong so to speak. However I have never seen any adverse effect from it in the VW engines. Just something to include in this.
You have to keep oil running through the center main, bottom line. That journal feeds not 1, but 2 rod journals, clearence is also key too, but I won't go into that here.
Now someone mentioned the lack of internal groove bearings, that's true it's is a problem now. You can get KS bearings, but your up against using the aluminum backed bearing vs the steel back.
I noticed this oiling issue many many years ago, and I did not always have a set of KS bearings to use. So what I have done many times in the past is I cut an internal groove into the external groove bearing, but only a steel back bearing, never aluminum back. I don't think the aluminum back has the strenght to handle this mod, I know for a fact that the steel back does, I have done to many of them to be concered about failure with this mod, it works.
The other mod out there for those who have one (yamaducci is one of them) is to cut a 360* groove in the center main of the crankshaft. That mod
works, but worries me in high HP applications, because it gives the crank a place to flex. I have seen those cranks beat out the center main pretty bad, like I said, I believe that this is a high HP based problem. Yamaducci's crank I believe is a full circle unit, that one should hold up fine??? We'll see I guess as time is the mother of all tests for that one.
Last FWIW I have also grooved the bearings in all out effort engines for the #2 and #3 rods also, even though they will live a good life without it. This is due to the fact that the oiling galley and main bearing for that particular journal is only feeding one rod.
If you groove the main bearings at all the journal positions (skip the little bearing at the pulley) the bearing life increases 10 fold, and you will also see a drop in oil temp due to the ability of the oiling system to increase flow.
I hope this sheds some light on this subject, I'm sure there will be the naysayers that don't understand or have an engine that "run's just fine"
That's ok, no need to reply, I know more than I need to about this subject already. Everything I post on this site comes from years of experience, countless engine builds, and miles of R & D. I hope someone can learn from all this.
SRP1 |
Just to keep the info in this thread too |
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modok
Samba Member
Joined: October 30, 2009
Posts: 27660
Location: Colorado Springs
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| Posted: Fri Feb 17, 2012 6:14 pm Post subject: |
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if it's a 1600 crank use either bearing type
if it is a 1500 single drilled crank then use the inside grooved center main
Usually, the 1500 cranks are used for strokes longer than 78, and the 1600 cranks are used for 78 and shorter |
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