I'm about to build a new motor form my third gen. I was using 3rdgen 9.0:1 compression rotors. I'm considering running Turbo II 8.5:1 compression rotors to be able to run more boost and take better advantage of my t-78.

What I'm worried about is how the turbo II rotors are half a pound heavier a piece. Anyone have any thoughts on the pluses or minus's of this?

Also, how much more boost will I be able to run reliably on pump and race gas with the lower compression rotors?

Thanks!
Bryan

 

I think that the extra weight of the rotors will be more than offset by the boost. Theoretically it will a rev slower but the heavier reciprocating mass will help when you are dumping the clutch.
You *might* think about a lightweight flywheel but I would just run it with what you have now.
Hopefully one of the people that have already been there/done that can give you more solid advice.
Theory only goes so far.

 

I already have a 9.5lb flywheel. I was also wondering if this would offset the weight increase from the heavier rotors.

 

I am not sure how much difference it will make. You are only adding 1 pound of reciprocating weight and that would not be the same as adding 1 pound to the flywheel because of the geometrics involved. The rotors are more contrained by their axis on the stationary gears and most of their weight is closer to the center of rotation since they are triangular.
Here in the South this could be called the "cheater bar" effect. The same amount of weight extended away from the center generates more leverage force.
This is where some physics kick in that alot of people do not think about. You could have two 15 lb flywheels. If one flywheel has more mass than the other further from center, one of the flywheels in effect would be lighter from a leverage standpoint. The farther out the weight is the more power it takes to turns it. By the same token it is harder to stop it from "x" rpm's.
That is why it takes longer to pick up the rpm's but easier to maintain them when launching.
If there was a way to design a flywheel with a centrifical weight system that could evenly force weight to the outer edge of the mass it would be incredible. The design would be a nightmare but the efficiency would be incredible.
I'm not very good at explaining **** and I am starting to babble

 

I believe I also someone with performance rotors that probably come designed for various applications...

Just checked - don't know if this is the only one - but Pineapple racing has them.

 

you have probably been getting little response because people are tired of answering the same questions every ywo weeks. there have been several threads on this topic where your questions have already been answered.

your answers are in here.....

https://www.rx7club.com/forum/showth...threadid=118575

https://www.rx7club.com/forum/showth...threadid=124620

PLEASE USE THE SEARCH FUNCTION IN THE FUTURE!

 

go with the higher compression rotors. Better spool and easier start when it's hot outside. Compresion is relative to the boost amount that you're running.

 

Heavier rotors will add rotational inertia, will stress the engine more at high RPM, and will require new front and rear counterweights. The lower compression will allow you to run more boost at the expense of some power off boost.

I asked that question once, and I got responses from people I trust that said that the increase in boost capacity would not be worth the decrease in "unboosted" power for most purposes. Higher compression is better if you can run the amount of boost you want. For most folks, the boost target is within reach with the 9:1 rotors and proper fuel and timing, so the higher compression rotors seem to be a better choice.

-Max

 

Heavier rotors will add rotational inertia, will stress the engine more at high RPM, and will require new front and rear counterweights. The lower compression will allow you to run more boost at the expense of some power at a given boost level (or on no boost).

I asked that question once, and I got responses from people I trust that said that the increase in boost capacity would not be worth the decrease in "unboosted" power for most purposes. Higher compression is better if you can run the amount of boost you want. For most folks, the boost target is within reach with the 9:1 rotors and proper fuel and timing, so the higher compression rotors seem to be a better choice.

-Max