Read a couple articles about knife edging long long ago and was always interested, especially considering how slow-revving the 4.9L I6 is.
Figure with the group here we'd have people that got heavily into extreme mods like that and other stuff. Anyone done anything way out of the normal like that and want to go into what kind of differences you saw and whether it was ultimately necessary for that build?
We've knife-edged cranks in the past. Never did a proper A:B test, though. I'd probably install a crank scraper instead if I were doing it today, it has much the same effect but other benefits.
Knife edge is old news. If you order a high end crank these days it will usually have a bull nose with a taper edge on the rear. Something like a teardrop shape. It's meant to reduce windage. There is power there, especially if you combine it with a dry sump. That is one of those last couple % kind of things. There are much lower hanging fruit than that.
Keith Tanner said:We've knife-edged cranks in the past. Never did a proper A:B test, though. I'd probably install a crank scraper instead if I were doing it today, it has much the same effect but other benefits.
I thought that knife edging was designed to help reduce mass of the primary rotating assembly. Oil shedding was an added bonus. Do I have that wrong?
Patientzero said:Knife edge is old news.
I mean, the last factory improvement on my truck's engine was 1996, and mine doesn't even have that. Considering how many oddball engines we have on this site, I figured that ordering a high end crank wasn't really an option for many. Also, I can do most of the knife edging at home if I'm patient and have a good scale.
Mr_Asa said:Keith Tanner said:We've knife-edged cranks in the past. Never did a proper A:B test, though. I'd probably install a crank scraper instead if I were doing it today, it has much the same effect but other benefits.
I thought that knife edging was designed to help reduce mass of the primary rotating assembly. Oil shedding was an added bonus. Do I have that wrong?
If you want to reduce rotating mass, the place to do that is the flywheel. The primary reason for knife-edging (in my understanding) is to address the significant aerodynamic drag of the crank moving at high speed through a surprisingly thick atmosphere of oil mist. The scraper addresses this by cutting down on the oil in the air.
In reply to Mr_Asa :
I don't think bespoke engine work has a good ROI for the sort of performance applications on the forum. Generally the money is better spent on power adders (usually turbo) and suspension work.
On my challenge car I built a scale to balance my rods/pistons to .5g, but I doubt it's going to help for performance and reliability. Fun to do though.
Knife-edge cranks are one way to minimize windage and drag in the oil/mist at the expense of being the most labor-intensive and expensive way of doing it. Take a perfectly balanced rotating assembly, completely disassemble it, spend days cutting and hacking while making sure you take the same amount of material off each counterweight, re-balance the whole thing which may require rods and pistons or drilling and Mallory... or a lightweight flywheel and crank scraper does the same thing for $200 instead of a complete teardown.
Plus, you can brag that you made a crank scraper with $1 of leftover sheet steel and a pair of tin snips. :)
Plus, you can brag that you made a crank scraper with $1 of leftover sheet steel and a pair of tin snips. :)
Stop saying that or I'll waste another week making a crank scraper out of washing machine scrap. This engine needs to get put back together ASAP.
In reply to CrustyRedXpress :
You mean you haven't done that yet? I just assumed you had.
In reply to CrustyRedXpress :
You can call it the "Maytag mod" or the "Whirlpool Windage mod"
Get moving. I expect a build thread tomorrow 
Mr_Asa said:Patientzero said:Knife edge is old news.
Also, I can do most of the knife edging at home if I'm patient and have a good scale.
I doesn't work that way. That is going to affect the balance of your whole rotating assembly. If you reduce the weight of the crank you also need to reduce the weight of rods/pistons. But it will need to be rebalanced by a machine shop. Alternatively you could add heavy metal back to the crank but again, it would still need to be rebalanced and the weight added in the correct location.
In reply to Curtis73 (Forum Supporter) :
Haha! You guys are killing me. Check out the CRX thread in the challenge forum for a DIY rod balancing scale and some pretty serious dremel work on an engine block.
Here is what I mean about this kind of stuff not being worth it for most people here. There actually is a crank scraper for my engine-it's about $300 and improves NA performance by 1-3%, depending on RPM. If my understanding of the physics are right, the majority of that improvement will certainly come in the highest RPM range.
https://www.crank-scrapers.com/Honda-Acura_D-SERIES.html
$300 could be new tires for the season, a set of shocks, two more nights at the test and tune, etc. There's just a lot of things that are lower hanging fruit than engine mods that give 3% more power at 7000 RPM.
on my 2.2 ecotec engine
Keith Tanner said:Mr_Asa said:Keith Tanner said:We've knife-edged cranks in the past. Never did a proper A:B test, though. I'd probably install a crank scraper instead if I were doing it today, it has much the same effect but other benefits.
I thought that knife edging was designed to help reduce mass of the primary rotating assembly. Oil shedding was an added bonus. Do I have that wrong?
If you want to reduce rotating mass, the place to do that is the flywheel. The primary reason for knife-edging (in my understanding) is to address the significant aerodynamic drag of the crank moving at high speed through a surprisingly thick atmosphere of oil mist. The scraper addresses this by cutting down on the oil in the air.
The better designs I have seen looked more like wings, and were biased towards the main journals, which is where the air isn't.
It's kind of amazing to consider that for all of the displacement of an engine, there is TWICE as much air moving around in the crankcase, or rather the same amount of air moving twice as often.
(unless you have a rotary, then crankcase volume stays constant with no major motion involved 
)
CrustyRedXpress said:$300 could be new tires for the season...
CrustyRedXpress said:In reply to Curtis73 (Forum Supporter) :
Here is what I mean about this kind of stuff not being worth it for most people here. There actually is a crank scraper for my engine-it's about $300 and improves NA performance by 1-3%, depending on RPM. If my understanding of the physics are right, the majority of that improvement will certainly come in the highest RPM range.
https://www.crank-scrapers.com/Honda-Acura_D-SERIES.html
$300 could be new tires for the season, a set of shocks, two more nights at the test and tune, etc. There's just a lot of things that are lower hanging fruit than engine mods that give 3% more power at 7000 RPM.
It's a piece of sheetmetal. You can make one yourself with a little work.
That said, I have witnessed someone painfully go through many Honda D15s/D16s (whatever can be found in time for the next event) because the oil turns to merengue at sustained RPM and the bearing clearance becomes measurable with a tape measure. I'd think that a crank scraper would reduce the amount of oil clinging to the crank, so maybe the oil pickup has a chance of delivering it to the pump.
Even if doing it all store-bought, $300 is cheaper than replacing a few engines per year.
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