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Chris R
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Joined: March 09, 2006
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Location: Minnesota
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 Posted: Thu Aug 19, 2010 8:20 pm Post subject: Where to have 912 Crank Turned Down? |
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I'm looking for suggestions on an experienced shop for turning down a 912 crank to fit the 36hp case?
Thanks-
Chris R |
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Mr. Motorhead
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Location: Practitioner of 36hp alchemy
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| Posted: Fri Aug 20, 2010 10:10 am Post subject: |
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Talk to Jose at DPR, he's done several of mine. Make sure you know what you want done as there are a couple ways to go. For a plug and play, 50mm on the main journals with what ever rod journal/stroke you plan on useing.
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GTV
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| Posted: Fri Aug 20, 2010 7:22 pm Post subject: |
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What all is involved when "dropping in" a Porsche crankshaft? Sorry to hijack, but I did a search and couldn't find much...
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Chris R
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| Posted: Fri Aug 20, 2010 8:03 pm Post subject: Way more info than you asked for... But interesting! |
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| MR COSMIC wrote: |
| What all is involved when "dropping in" a Porsche crankshaft? Sorry to hijack, but I did a search and couldn't find much... |
There are several posts about this but you really have to dig to get to the facts on it. Someone will correct me if I'm wrong but... I believe 356A and pre-A will drop right in without machining. Later 356 and 912 Cranks had 2 increased main bearing's that were 55mm. So, when using the later cranks you either:
1- Take down the Crank to fit the case
or
2- Bore out the case to fit the crank
Here's a lengthy but very interesting email from a long-time Porsche and VW engine builder regarding the 912 crank in a 36hp engine. After his suggestion which makes total sense, I believe I am still going to go with the 912 crank, turned down in my 36hp case.
"If you are going strictly for an all-out record holder, then going the exotic route of modifying lots of late-model Porsche 616 parts and fitting them into a 36 hp VW case is one way to go, but will be a LOT of work. But in my opinion, as I stated earlier, you will end up with a Porsche engine of the second rank. Porsche was aware of all the limitations of the original 25 hp/36 hp VW case layout back in the early 1950's........why else do you think they essentially redesigned the entire engine, keeping only a few of the basic dimensions such as overall crank length, camshaft length and gear size and changed more than they kept?
Yes, you can stuff a 912 crank into a stock 36 hp case by grinding down the main journals (number 2 and 3) to VW diameter, but then you kinda' eliminate the reasons why Porsche made it with the dimensions that they did. First off, ever consider why Porsche increased the diameter for those two journals from 50 mm to 55 mm? Yes, the obvious answer was to "make the crank stronger", but in what way? I once thought it was because of the higher output, but by consulting several engineering texts, it became obvious that higher rpm's and hp-outputs were the cause for the cranks were flexing (and breaking!), but not for the reasons I first thought. I thought it was due to the higher torque resulting from bigger pistons and higher compression ratios. While these "external loads" did contribute to flexing and breaking of the cranks, by far the biggest cause was the "internal loads" caused by the mass distribution of the crank throws, rods, and pistons themselves. All this stuff flying around a high rpm's is the dominant cause of failure. And remember, the stresses on the rotating stuff goes up by the mathematical square of rpm and linearly with stroke. As an example, one text pointed out that for most automotive engines the load imposed by the rotating mass of the big end of the rod on crankshaft bearings will exceed that of the external load caused by combustion somewhere in the neighborhood of 2,200 rpm.........which means that load goes WAY up really fast as you rev up the engine.
Getting back to your issue, the long explanation goes something like this: It turns out that because of the off-set nature of the cylinders on opposite sides of the flat-four engine, there are two "rocking couples" operating on the crankshaft resulting from the off-set crankpin throws...kinda like zig-zag dumbell weights.....one for the front pair of cylinders and one for the rear pair (don't worry, all reciprocating engines with off-set cylinders have these rocking couples, not just VW/Porsche. All in-line engines have them). In order to counter these rocking couples that exist when the crank is rotating (they don't exist when the crank is at rest), the main bearings, in addition to making the crank run true and to bear the "external load" of combustion, must also provide a countering force to the rocking-couples through the bearings. This is where the no. 2 main bearing comes to our attention........if you look at the crank throws when the crank is placed on the work bench with the flywheel end away from you and the pulley end toward you, you will notice that two of the throws are on the left and two are on the right. But also note that if the crank is placed so that no.3 (the one closest to the flywheel) is at TDC, the throws do not alternate left-right-left-right.....they alternate left-right-right-left! Why they do it this way I won't go into now, but suffice it to say it is for "over-all" engine balance.
The result is, if you can imagine the crank rotating at this instant on the bench, the front half of the crank (throws 3 and 1) will form a rocking couple that will want to rotate the crank in a counterclockwise direction (when viewed from above) about an axis perpendicular to the bench running through the web connecting the two throws. To counter this, main bearings no. 1 and 2 will provide the required force. So far, so good, since both main bearings are roughly the same size, they each carry an equal load. But now take a look at the throws forming the rear half of the crankshaft......they too are offset......so what? Well, if you take a look, they form the mirror-image of the front pair.....which means that THEIR rocking couple is a mirror image as well, that is, in a CLOCKWISE direction. So what?.......main bearings no. 2 and 3 will still take care of these loads as well. Except, because of the left-right-right-left layout of the crank pin throws, main bearing no.2 has to provide counter-support for BOTH rocking couples that are now additive in nature at the same time. In other words, bearing no. 2 has to carry TWICE the load as bearings 1 and 3!
This rocking couple load not only doubles the load on bearing no.2, it also make the crank to want to bend and flex in the middle, right at this point.....which is one of the famous places where Porsche 74 mm cranks break. Now we get to the evolution of ever increasing costly fixes Porsche tried to reduce this flexing. First, they went to an aluminum case to provide more rigidity than the magnesium case. Cranks still broke. Next they "shuffle pinned" the case web at no.2 bearing saddle in order to provide more rigidity at that point and reduce flexing. It helped, but as engines became faster, the cranks resumed the breakage habit. Next, they increased the diameter of the crank at no.2 main (the so-called "C" crank) to 55 mm to make the crank stiffer in cross-section at this point, which it did, but eventually even higher rpms lead to additional loads...and flexing....and breakage (as a side note, no 3 bearing was made bigger as well, but this was done more to take advantage of the larger bearing bore in the case needed to be able to make no. 2 larger during manufacture, rather than any load-bearing or stiffness issues at this point.....it still has no more load than main no.1, which is still at 50 mm). Lastly, Porsche finally decided to fully counter-weight the crank with built-in counter-weights in order to provide some semblance of self-countering to the original rocking-couples by providing ADDITIONAL rocking couples, but staggered in such as way as to work in opposition, thus reducing the loads on the main bearings (my guess is that they held off on this solution since it is much more expensive to make a forged crank with counter-weights than without). This really, kinda, sorta worked!!!....except for the fact that all these fixes do not reduce an additional rocking couple that is present that no amount of counter-balancing can eliminate. This additional rocking couple results from the reciprocating forces imposed by the changes in direction of the pistons at top-dead and bottom-dead center of crank rotation. This recirocating couple is periodic in both time and magnitude (unlike the rotating rocking couple of the throws, which is constant in time, but not magnitude) and will, if great enough, cause crank flex and breakage.
So, what is the upshot of all this, and what is my recommendation? Well, if you plan on driving this car more than at just the drag races, I would not go with a 74 mm stroke Porsche crank reduced to 50 mm at bearings 2 and 3. It won't be any stiffer at that point than a VW 36 hp crank and will have much greater flexural loads (the hardened bearing surface at tis point will be removed during the grinding as well) Also, going to Chevy rods may not be good in the long run since they may have a different mass than the Porsch rods, whose big-end weight was probably taken into account when Porsche calculated how large to make the counterweights. Either keep the original 64 mm VW stroke, or if you want to go to a bigger stroke, stop at the 69 mm being offered with the counter-weighted crank from Mr. Okrasa. (If you insist on the 74 mm stroke of the Porsche, modify the case if possible and keep the original 55 mm diameter.)
As far as going to Porsche heads and cylinders.......if you are going for the "period correct" look, why bother? Especially since the head-stud spacing is different. As I said before, even after you do all the conversion you will, in my opinion, have a second-rate Porsche. I say go for first-rate VW. Keep your modifications reasonable and do-able. And you don't want Dell'ortos? Hmmmm, who is going to know, other than experts? But they are going to see alot of other stuff "not period correct" anyway. I think you will find the Dell'ortos good, dependable carbs that are fairly easy to tune and re-jet. Good Luck and work slowly.
D" |
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gimmesomeshelter
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Joined: May 08, 2004
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Location: San Carlos, CA
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| Posted: Fri Aug 20, 2010 9:47 pm Post subject: |
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Hello-
Does anyone know how much it costs to prep a 36hp case for a 356 C/SC crank? From what I've read, it requres the following:
1) Align bore case (std Porsche bearing = 1st over VW bearing)
2) Machine oil groove into 2 & 3 bearing saddles
3) Machine notches (to hold bearings) into bearing saddles 2 & 3
4) Add Porsche premium because they're the only people who do this work 
Thanks,
Paul
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henry roberts
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Joined: February 24, 2003
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| Posted: Sat Aug 21, 2010 1:32 am Post subject: |
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thanks for posting that letter. it confirms the validity of some ideas that have been floating about in my head for a while. plus makes me question another that i had had.
now when is that money tree going to bear fruit so i can test them all?  |
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grueni
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| Posted: Sat Aug 21, 2010 4:17 am Post subject: |
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good reasons, but i think there is more to think about it.
ballancing (crank, pulley,flywheel and pressure plate) can do more than you think.
lighter pistons can also save alot...do not calculate it jey correct, but 50g less in piston makes about 15% less stress on the crank.
and at the end, how much hp will you make with that engine. it is really hard to make some real HP with the 36er. and nobody of the racer at boneville ever breake a crank...and bengtH's engine with over 80hp is in use on the street since a long time.
50mm mains are enough for all of the current hp done. maybe some bigger projekts will need bigger main journaly...we will see.
i would stay with 50mm main.
the moste crank breakes comes from other reasons maybe like ignition,rod bearing failure, dirty oil, oilpressure problems +++
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henry roberts
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| Posted: Sat Aug 21, 2010 6:52 pm Post subject: |
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wouldn't a larger center main reduce the flex in the crank, therefore reducing friction (and wear). less friction, less power lost (less heat generated as well). every fraction of a hp helps when you are starting with only 30bhp.
with my "dream crank" i was thinking at least 55mm center main but was hoping to find a suitable bearing that was even bigger. |
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grueni
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| Posted: Sun Aug 22, 2010 1:30 am Post subject: |
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for sure will it help alot!
the way is not easy and defintly not cheap.
here in germany i didn't find jet a machinist that woult machine just the middle to 55 bearing or 2. and 3. and the others to first over, new pins are not that hard, that can you do at your own with a mill( drill the old ones deeper befor open to bigger bearing) the bearings from porsche are nearly 10x more money than 36hp. and porsche did use 40 50 50 50 up to 75hp in serial cars. those engines was build for long live.
if you will build everything hi-end than do it+alu case 25hp.
maybe one day i will do it too.
i would put that money in the heads to reach the 75+hp.
just my 2 cent
wishes
chris
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TomSimon
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Posts: 751
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| Posted: Wed Aug 25, 2010 9:30 am Post subject: |
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| Mr. Motorhead wrote: |
| Talk to Jose at DPR, he's done several of mine. Make sure you know what you want done as there are a couple ways to go. For a plug and play, 50mm on the main journals with what ever rod journal/stroke you plan on useing. |
X2 Jose knows his stuff, has done work on several 36hp enthusiasts cranks. I dropped and picked up my cranks off in person, but he can help you over the phone and ship as well.
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TomSimon
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| Posted: Wed Aug 25, 2010 10:00 am Post subject: Re: Way more info than you asked for... But interesting! |
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| 1990coupequattro wrote: |
| MR COSMIC wrote: |
| What all is involved when "dropping in" a Porsche crankshaft? Sorry to hijack, but I did a search and couldn't find much... |
There are several posts about this but you really have to dig to get to the facts on it. Someone will correct me if I'm wrong but... I believe 356A and pre-A will drop right in without machining. Later 356 and 912 Cranks had 2 increased main bearing's that were 55mm. So, when using the later cranks you either:
1- Take down the Crank to fit the case
or
2- Bore out the case to fit the crank
Here's a lengthy but very interesting email from a long-time Porsche and VW engine builder regarding the 912 crank in a 36hp engine. After his suggestion which makes total sense, I believe I am still going to go with the 912 crank, turned down in my 36hp case. |
what an awesome explanation of rocking pairs and counterweighted cranks! A great argument for shuffle pinning the case's center main, couterweighting the crank, and using the largest diameter main bearing journal possible (on a race engine).
One thing not mentioned, above, but worth adding; larger 55mm main journals do a better job than the 50mm mains of resisting the destructive (bane of every Internal combustion engine (ICE), pulsed torsional loading, or 'crank twist with each ignition hit'. These 'hits', 2 hits for every 360 crankshaft degrees, set up this torsional shock wave in the crank that oscillates in this twisting fashion that beats the poor crank to death, looks for flaws and stress raisers in the crank from which cracks can and will initiate, well documented and known to be the precursors of crank failure.
Every ICE crankshaft experiences this, some cope better than others. The modern engineered solution is to add more main bearings, like Porsche did with the flat 6, Suburu followed suit. But we are stuck with a 3 main bearing crank. Not to worry, there are Type 1 race engines 3 main bearing cranks that put out over 300hp per cylinder, that live...
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TomSimon
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| Posted: Wed Aug 25, 2010 10:20 am Post subject: |
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one more thing... I can confirm that the 36hp VW case, with and uncut crank bore, is .25mm smaller than an uncut 356 Porsche crank bore. Porsche used a case bore that was almost the same size as VW, but a much thinner bearing shell.
If you have an uncut 36hp case, you can have it opened up by .25mm to factory 356 diameter, purchase standard OD 356 bearings (ID to fit your 356/912 crank). And yes, the Porsche main bearings cost anywhere from $250-$400usd
Is having 55mm crank journals worth the cost? For my race engine it is, but if I were just putting together a 25hp per cylinder hot rod engine, I wouldn't spend the extra money.
BTW There is a precedence for this thinking; VW drag racers (T1 based) have been doing this for years, using cranks specifically made with larger T4 main bearing journals. Most recently, fitting thin shell BMW main bearings to a case with a standard T1 case bore and a crank with T4 main journals has become common. I just had a T1 mag case machined for those BMW bearings, to accept my new Bugpack T1 (with T4 mains) crank.
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TomSimon
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| Posted: Mon Aug 30, 2010 4:20 pm Post subject: |
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A good explanation of crank twist, and how to minimize it by using a damper;
http://www.dragtimenews.com/dampers101.htm
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johanbbbe
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| Posted: Fri May 01, 2020 1:04 am Post subject: |
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Most recently, fitting thin shell BMW main bearings to a case with a standard T1 case bore and a crank with T4 main journals has become common. I just had a T1 mag case machined for those BMW bearings, to accept my new Bugpack T1 (with T4 mains) crank.[/quote]
Hello,i'm follow on this very interesting post.Can you please tell me which BMW bearings you can use? |
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