Ok. I think I have the front counterweights figured out. The wider (1.02")is for the S4 rotors and other (0.92") is for the S5/S6 rotors.

My question is what rear counterweight (0.55" wide)do I have in this pic? Is it for the heavier S4 rotors or the lighter S5/S6 rotors?

Thanks
Justin

 

Someone mentioned all rear CW's for FC's are *unbalanced* if you buy them brand new from the Mazda dealer?
This means you need to drill them yourself.

The FC Zenki versus Kouki CW's are identical in size and shape.
Hell, even the weight is only off by a few hundreths of a pound.


-Ted

 

So is there any way difference between the rear counterweights from the different years? They have different part numbers.

Justin

 

Front counterweights are pretty easy to tell apart. s4 is the thicker one. s5 is the thinner one. s6 has the larger diameter "pad" for the larger needle bearings and spacer. s3 GSLSE weight is even thicker than the s4.

IT's the rear auto weights are are impossible to tell apart visually. They are visually identical from 84-95 and weight only varies by .02lb according to my observations. The only way to make a guess is by observing the drill holes. 1 or 2 holes usually means it's an s4, and 2.5-4 holes usually means it's an s5/6.

Keep in mind rotor weights are not absolute either, so unless you're getting the assembly balanced as a whole it's a moot point most of the time. You could run 2 E weight s4 rotors with s5 counterweights and be okay...because A weight s5/6 rotors weigh right about the same. E is the lightest weight range in a series, and A the heaviest. So even though an s4 rotor is supposed to weigh 10.5lb and an s5/6 is supposed to weigh 9.5lb, in reality an E weight s4 might weigh closer to 10.1lb and an A weight s5/6 rotor might weigh closer to 9.9. So it is not absolute.

 

Thanks! Very informative.

I am planning on cutting about an 1.5" long bevel on the trailing edges of the rotors to increase intake duration. I was both concerned with selecting the right counterweights to modify and also deciding how much material to remove from the counterweights. From the sound of it the engine doesnt seem to respond very badly to a slight unbalance in this rotor/counterweight system. It looks like I will be removing approx. 0.6lb from each rotor. Since I am using FD rotors, I will shoot to remove half of the mass removed from the S5 to S4 counterweights. I also need to see what letter rotors I have.

Justin

 

That's just about what I did.
After 2500 miles - so far so good..

 

Yea, I saw your post back in December. I like to see people trying new things.

I have been thinking of this for a while, just wanted to do some more research as to how sensitive the engine is to balancing etc. From my initial calcs we are probably fine. It looks like the weight removed is within Mazda's original casting variance.

https://www.rx7club.com/single-turbo-rx-7s-23/my-beveled-s5-turbo-rotors-%2Apics%2A-505525/


Another innovation I will be trying is posted here:

https://www.rx7club.com/single-turbo-rx-7s-23/my-twin-scroll-ssac-manifold-505519/


Justin

 

In regards to:

I did this. I removed approx. 30g from each rotor. Pics are here: https://www.rx7club.com/showthread.php?t=505525

However I decided NOT to remove any material from the counterweights. This is why:

Justin