Eagle Crank Shaft... Knife-Edging or Lightening?
10-01-2005, 01:39 AM
#1
Eagle Crank Shaft... Knife-Edging or Lightening? I just opened up my Eagle 4.0" crank and it must weigh at least 55lbs. Is this typical for a stroker crank? I was wondering if it is possible or even worth it to lighten or knife-edge the crank to knock some weight off and hopefully get a more reliable high RPM range.
Thanks!
Thanks!
10-01-2005, 12:47 PM
#5
Cranks are heavy period. The Lunati I have is a pretty healthy chunk. 55 isn't to uncommon for the average crank. Some of the more expensive cranks are lighter, but that is why they cost more. The cost of the labor to get it lightened or knife edged would be my determining factor of whether I would do it or not.
10-01-2005, 10:07 PM
#6
However, I don't know of anyone 'knife-edging a crank. The rod and main journals can be gundrilled. This is very common. The counter weights get undercut, so the mass is on the end. This requires the least mass for balancing the pistons, pin, and rods.
However, the biggest thing is if you reduce your bobweight. If that cranks was balanced for a 600gm piston, 140gm pin, on a 700gm rod changing to a 400gm piston on a 100gm pin connected to a 500gm rod would reduce the bobweight, would reduce the bobweight by nearly a pound and the crank by a lot.
Was your crank balanced for your rotating assembly?
However, the biggest thing is if you reduce your bobweight. If that cranks was balanced for a 600gm piston, 140gm pin, on a 700gm rod changing to a 400gm piston on a 100gm pin connected to a 500gm rod would reduce the bobweight, would reduce the bobweight by nearly a pound and the crank by a lot.
Was your crank balanced for your rotating assembly?
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10-02-2005, 02:38 AM
#8
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Although the lightest possible rotating assembly is desireable, it is not that easy. As mentioned above the assembly needs to be balanced together so if you get a ridiculously light crank and use normal rods/pistons you then have to add weight ($$$) to counter the weight and more so the inertia of the the rod/piston combo to be balanced. There are shelf cranks out there that come knife edged and gun drilled right out of the box, the knife edging also cuts down on windage by aerodynamic standards. Although I wouldnt pay someone to knife edge my crank I would buy one that way already, they usually aren't a whole lot lighter but anything fancier has GOT to make more power, right?
Just as a point of reference, a standard small block Chevy 350 (3.480" stroke) from a factory motor typically weighs in at 53-55lbs.....
Just as a point of reference, a standard small block Chevy 350 (3.480" stroke) from a factory motor typically weighs in at 53-55lbs.....
10-02-2005, 01:44 PM
#9
Cranks come balanced for a designated bobweight. 1600-1800gms is common. My race engine with a 3.48 strong and 1430gm bobweight as a 36# crank. Crower ultralight...undercut but not windage adjusted. If it used a 3.25" stroke, Crower MaxLite4 (maybe 20gms lighter), JE SuperFlys (320gm) pistons, it may have gone down to 32-34#.
If you look at the Crower catalog online, they have pictures and descriptions of their different cranks. Again, you would aero shape a crank (tear drop shape), rather than knife edge it. However, before doing that, talk with some good pan manufacturers to make sure your have the right scrapers and screens. There is no point shaping the crank to manage oil if you don't have the pan to keep it away.
Another point, there is no point shaping the crank to manage oil if you haven't done a no-hole balance. Holes in the crank for balancing just trap oil. No something you want to do.
Getting the most from your motor is a matter of picking and preparing the right pieces to get the best results for your rules (or intended use) and budget.
Do you have a lightweight small diameter multiplate clutch to go with the lighted crank? For example, our race car has a 5.5" Quartermaster. And first gear removed from the transmission. Etc., etc., etc. (to quote Yul Brenner).
If you look at the Crower catalog online, they have pictures and descriptions of their different cranks. Again, you would aero shape a crank (tear drop shape), rather than knife edge it. However, before doing that, talk with some good pan manufacturers to make sure your have the right scrapers and screens. There is no point shaping the crank to manage oil if you don't have the pan to keep it away.
Another point, there is no point shaping the crank to manage oil if you haven't done a no-hole balance. Holes in the crank for balancing just trap oil. No something you want to do.
Getting the most from your motor is a matter of picking and preparing the right pieces to get the best results for your rules (or intended use) and budget.
Do you have a lightweight small diameter multiplate clutch to go with the lighted crank? For example, our race car has a 5.5" Quartermaster. And first gear removed from the transmission. Etc., etc., etc. (to quote Yul Brenner).
Last edited by DavidNJ; 10-02-2005 at 01:51 PM.
10-02-2005, 02:03 PM
#10
If you a building a 400+ cu. in. street/strip engine, crank weight shouldn't be your issue. If you are building an engine for a displacement restricted class, you want the biggest bore possible (for better breathing), then the lightest rotating assembly (which should be very light with the short stroke). If bore and stroke are fixed, the lightest allowed by the rules (many classes stipulate rotating assembly weight).
Then you can spend the extra $$$ on head, cams, intake, exhaust, and tuning.
Then you can spend the extra $$$ on head, cams, intake, exhaust, and tuning.
