6m v.s. 7m crank

[SilverSupraT]

Yeah I agree. Don't worry about the machines though. The CNC equipment I work with is accurate to within .0003" on the high end. Really it's a matter of whether I want to attempt it and risk doing something wrong.

What about dynamically balancing the whole rotating assembly afterward....what RPM could I expect to have it balance to? Before lightening and after lightening?

 

[IJ.]

Blake: As fast as the shop are comfortable spinning their machine?

 

[SilverSupraT]

Blake: As fast as the shop are comfortable spinning their machine?

lol that's what I was hoping to hear actually. I'm hoping to build it as perfectly as I can but honestly I'll probably still push it to the limit (hopefully not past that tho haha).

Thanks a lot IJ for all the info! I will definitely be using it.

 

[JustAnotherVictim]

The shop I used balanced everything at 10k rpm.
Came out nicely I think. :dunno:

 

[suprarich]

Taking weight off the crank is approx 400 usd. Where it is taken from has a lot to do with the natural balance. Spin balancing is much slower than the final RPM the balancing wishes the crank to acheive. To spin balance correctly, you need the weight of the pistons and rods, and the damper and flexplate/flywheel attached.

 

[SilverSupraT]

Taking weight off the crank is approx 400 usd. Where it is taken from has a lot to do with the natural balance. Spin balancing is much slower than the final RPM the balancing wishes the crank to acheive. To spin balance correctly, you need the weight of the pistons and rods, and the damper and flexplate/flywheel attached.

Thanks for the price. I might just consider that so that I don't have to think about it lol. I realize I need everything attached, that's why I won't be getting this done anytime soon. Gonna have to buy a could $K worth of parts first hehe.

 

[IJ.]

Blake: You missed the ? at the end of my sentence

I hand all my parts to the shop and tell them "balance it" it's their job they know what they're doing so I have NFI what speeds they spin it up at.

 

[suprarich]

Blake - IJ is correct - The spin balance is not all that fast in relation to a running motor.1k rpm on the balance machine will send you running with your head ducked down for fear of a flying crank. I see a post about 10,000 rpm, but to my understanding, that would be a finished "goal" rpm of a running motor, not the speed of the machine that balances the crank. I use a buddys "nascar certified" spin balance machine, and it, at most will turn 1300 rpm and will balance the crank to within .08 or so grams.

 

[SilverSupraT]

lol I saw the "?" at the end of the sentence. I think I got what you were getting at...I hope. I understand that they will spin it at X speed, whatever their machine works at. But am I wrong in thinking that a faster rpm (on the machine)+ more accuracy in the balance would indicate a higher potential running rpm in the motor?

If they spin it at 1 rpm and say it's balanced clearly that's different than it spinning at 100 rpm and say it's balanced. Obviously I don't know what I'm talking about, just assuming with what I do actually understand about physics.

I appreciate your guys' patience with a newb on the subject

 

[IJ.]

LMAO Blake!

It's a low speed process much like wheel balancing, it doesn't take high speeds to get the accuracy required.

I watched the rig at my shop in operation and no way known would it be safe at high speeds.

An I-6 has really good inherent balance to start with and as long as an even amount of weight is removed from each counterweight it usually only takes a touch up on the machine to get back to 0, then it's a matter of end balancing the rods and checking pistons pin and rings.

0 balance the Damper and Flywheel and you're set.

 

[bmoss85]

any cranks thats that ive ever balanced were done at 480rpm. it doesnt sound fast, but stand in front of crank with bob weights, flexplate and harmonic balancer attached to it and you will not want to spin it any faster.

 

[Shytheed Dumas]

I ran both cranks in the same motor within 2 weeks with the crank being the only change, what do ya want to know?

Side by side dyno sheets? :naughty:

 

[IJ.]

Side by side dyno sheets? :naughty:

Sorry didn't get around to dynoing them back to back.

I ran the 7M for awhile and had a "noise" that was a little like RK but only at high RPM, I pulled the engine down and found chatter marks on 2 of the rod journals so installed the "spare" 6M crank I had here.

The Chatter turned out to be crank flex above 8000 was letting the Pauter rods just brush the Oil Pump shaft, I turned the shaft down and while the 7M crank was out I had it lightened.

With the stock weight 7M crank it made 960 Nm of torque, I dyno'd again with the light 7M crank and it made 1154 Nm so I suspect a 6M crank being the same weight would react the same.

This doesn't translate into more Hp as such all that happens is on the dyno the lighter weight accelerates quicker ramping up the dyno and showing a higher torque reading.

 

[MRSUPRA]

Ive' run both the 6M and 7M cranks in my supra too. Just like IJ said, the 7M revs a little smoother, but the 6M feels like it spins up a slightly quicker.

Too me it's not even worth worrying about. Whatever crank is available is the one I would use. BTW, the most difference in engine smoothness to me was when I switched to a quieter exhaust.