what crank should I use?

[tig321]

I'm sure there have been in depth discussions about this but I have searched and i'm having trouble coming up with anything. more than just
6m= not as smooth, +250rmp redline
7m= smoother, lower redline
I have a 7m crank from my 90 turbo but it's quite scored. Def needs machining. I also have a 6m crank from a jdm engine I can't use cause of a cracked block. this crank looks mint. I'm having the 90 engine rebuilt with forged pistons(ross most likely) Then balanced. My eventual plans are a gt series turbo and to hopefully break 400 rwhp. Does it really matter what crank I use?
Thanks

 

[Justin727]

no it doesnt matter. Use the better of the 2. I'm using a 6m crank

 

[tig321]

O.K. good. thanks for the fast response

 

[NashMan]

i find that peopel use the 6m crank more then the 7m

cause it genaly lighter and stroung eghough

 

[IJ.]

The redline is lower because of the ECU it's nothing to do with the Crank itself I routinely ran mine to 7700+.

I put my 6M crank back in and it's noticably harsher and if I had a choice I'd go 7M everytime.

 

[jdub]

The redline is lower because of the ECU it's nothing to do with the Crank itself I routinely ran mine to 7700+.

I put my 6M crank back in and it's noticably harsher and if I had a choice I'd go 7M everytime.

Agreed! The 7M crank is better balanced from the factory...I used a 6M crank (had it fully balanced) and got the same results as IJ.

 

[Supra]

One more on IJ/JDUB's side - 7M all the way! It takes away so much more vibration. My 87T rides like a Cressida, moves like a 4th gen Supra, burns coolant like a 3rd gen. :icon_bigg

 

[DaSuprawolf]

lol

 

[tig321]

Thanks very much for chiming in. I'll give them my 7m crank back. that was close:biglaugh:

 

[IJ.]

The 6M crank smooths out over 6000 rpm

It's only harsh when cruising at moderate rpm compared to the 7M so I'm guessing it was all done as an NVH exercise.

 

[Nick M]

Which to use...the upgraded crankshaft. Toyota went to a crank with smaller but better distributed counter weights.

If you can.

 

[Justin727]

Although there are technical reasons why they changed crank shafts in the end it comes down to personal prefference.
I have never seen a 6m nor 7m crank fail just yet. So until one does agian its just what YOU want. I stuck with the 6m crank!

 

[Insidious Surmiser]

i've got a 6m, not by choice... just curious though, anyone know if the 7m cranks were ever sold in japan, seems to me, the 7m was not sold any longer when the 1jz came out, and this switch was the same time they started to produce 7m cranks for us (in the states)

edit: also, no harshness noted, in fact my car drive MUCH smoother than any other car i've driven (i've driven a LOT of cars)

 

[IJ.]

Trust me there's a difference !
(had both cranks in the same motor within months of each other)

 

[jkbmkiii]

7m all the way! I have owned a couple of 89+ cars and a couple of 87-88 cars, my new motor for my 88 has a 7m crank in it!

 

[Supra]

i've got a 6m, not by choice... just curious though, anyone know if the 7m cranks were ever sold in japan, seems to me, the 7m was not sold any longer when the 1jz came out, and this switch was the same time they started to produce 7m cranks for us (in the states)

edit: also, no harshness noted, in fact my car drive MUCH smoother than any other car i've driven (i've driven a LOT of cars)

It's not a valid comparision to drive an 1987 and then go drive a 1992 model. There's 5 years of wear and tear + the early model has hard rubber motor mounts verses the later model fluid motor mounts. There's alot of variables beyond the crank there that will transfer vibration to the driver.

As I said before, I've driven my 1987 with the 6M crank for 6+ years with various builds. This year I switched to a 7M crank and I can tell you that without ANY OTHER changes to the same car / motor, there is a significant differance between the 7M and 6M crank in terms of vibration. The 7M is much smoother over all.