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Crankshaft knife edging!!!

4K views 25 replies 4 participants last post by  garryt 
#1 ·
iv'e been doing a little research on this, and it would seem that a lot of companies remove material from the counterweights only:confused: now I can see that it would balance ok at a couple of hundred rpms, but you have lost the individual big end journal to counterweight balance (per cylinder). opinions please???
 
#3 ·
I understand that part of things. but it would seem to me, that reducing the counterweights would upset the weight distribution from front of crank to rear. for example= if you were to cut a standard crank in half, both halves should still be perfectly balanced. but if you were to cut a knife edged crank in half???
 
#4 ·
I'm lost on what you are asking.

Knife edging a crank is a total waste of money for any street engine. If you got money to waste, I would go for it. I have with a PG engine that I have but that was never assembled.

Knife edging has been around for decades and there have not been any issues that I've read about.
 
#7 ·
If I was going to design a race engine from the ground up, then it's something to think about.

I for one think people think too much. I fit in that category sometimes so I believe I'm experienced enough to comment about it.

We play with VW engines. Most of these are daily drivers. None are really that efficient [valve/port size, etc] or ever designed to run at a really high RPM. If someone wants to express that knife edging a crank is foolish, have at it. I would suggest that they do not have it done on their engine. If the other person understands the advantages of it, then they should have it done on their engine.

In that link, someone mentioned that knife edge cranks spit out rods. Many people make stupid comments like that. They have no proof of that statement. There are so many variables and to grab on one of them is insane. Just like all conspiracies, some people will grab on to that idea and never question it. Then the rumor comes out saying all knife edged cranks are junk.

I like power. The power delivery that works for me is a turbo. I like smaller turbos that run out of breath at redline [6.5-7k]. Most of those turbos will provide a good low end boost too. I totally understand that for max power, I should have a turbo that is at max efficiency at redline. The problem is boost may not develop until 4-5k. That would not work for me but I understand that it works for someone else. Does that make what I want wrong? I'll take 250HP street engine vs a 400HP race engine for my daily driver.

Same way with a knife edge crank. If you have the money, have at it. Will it add more power? Probably not enough that you can feel but the bragging rights is worth something. I doubt if it was done right that it would cause a reliability issue. To get the max out of that crank, you'll have to run it north of 8k and that is twice what I do as a daily driver.

The whole 5 pages was an interesting read but none of that applies to a street car that most of us drive. If I was going to build a street engine with maximum everything, I would have it done. If I was going to build an engine to tow my 5th wheeler, I would not. That's my opinion.
 
#8 ·
well the way I see it is, if you take a factory crank and have it "just" balanced to 0 grams, you should be able to spin it up on a balancer to very high rpms (10k maybe) and see no crank deflection whatsoever. if you were to take that same crank and carve weight off the counterweights only, then spin it up to 10k. i'm pretty sure you would see some deflection then.
 
#9 ·
IF you could rev it to 10k, you would have other issues.

With all those main bearings, I suspect that you would not see any deflection. Many V8's have two rods between the main bearings and maybe, just maybe, you may have an issue.

I'm always down with a good hypothetical question, but opinions are just that. I do not have the knowledge or tools to prove what is right or not. The common sense is, in a blind test, you would not know if the crank is knife edged or not. In the street, race track or dyno.
 
#10 ·
no I don't mean revving the engine to 10k, but spinning the crank itself on a lathe. I guess when it comes to buying a lightened crank from a company (other than vw) it's a bit hit or miss. some cranks are likely going to be better made/balanced than others:rolleyes:
 
#11 ·
So installing a crank on a lathe and seeing it distort at a high RPM would tell me nothing. The crank would never see that condition once it's installed. So what good is that information? I'm not a machinist but I believe they have supports that prevent the object from whipping around. Are cranks ever turned at that high RPM?

Again, I enjoy reading peoples reasons why something is good/bad but I believe knife edging a crank looks good on paper and done properly will not cause future issues. Normally it is done on engines that will be running max load. Loads possibly double/triple what it was ever designed to run. When those engines fail [and they often do], it's hard for me to accept that it was because of the knife edging. I believe most have failed because they have pushed the envelope so far the envelope torn open.
 
#22 ·
the crank isn't really supported by the bearings. the bearings are supporting the thin film of oil that the crank is running in. so any significant distortion is going to compromise that oil layer. I guess higher oil pressure can help up to a point though.
 
#23 ·
Yes, it's common knowledge that without oil, the engine will not last long. In discussions, it's assumed that we all know this and not really brought up. Yes, the oil is supporting the crank, but the bearing is supporting the oil, and the bearing is supported by the main cap/mounting bolts. So somewhere in the support process, the bearings are part of that team.
 
#25 ·
These engines were never suppose to be race engines. 7k is enough for me. They were never designed to go past that. Every engine will fail past their design parameters, no matter how good they are. If someone wants to knife edge a crank, spin the crank to 9k, and then blame the knife edged crank on the failure, that's fine with me. I believe most of the blame is based on opinion vs facts. Lots of thinking going on but very little engineered facts.
 
#26 ·
I couldn't agree more. push the limits and pay the price:laugh: I do recall some guy over this way with a mk3 2.0 agg/ crossflow head and bike carbs. standard bottom end and it would spin to 8k. blew up after a while, but seemed to last for a while too. my rev limiter is about 6300k which will do me just fine for a 2.0 block.
 
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