Polishing Rotors - Pros and Cons

[vex]

Originally Posted by RETed

I doubt this is the case...
I bet the rotor faces are already black with soot and carbon deposits.
You'd be surprised how well deposits after combustion will stick to surfaces.
Since this is a rebuild, the engine would've been burning inefficiently due to inferior compression from the seals settling in.
This is more than enough time to leave a fine layer of soot on the rotor faces.
Too bad we can't tear the motor down just to confirm this...


-Ted

Solution: Teflon coated rotors! It's the next big thing!

[NoDOHC]

I don't know if Teflon will take the combustion temperature of a rotary.

ReTed, I will be putting a turbo on the engine soon, when I take the headers off, I will get some pictures in the exhaust ports. I am just as curious as you are.

Don't worry, there should be no issue seeing in the exhaust ports, I didn't want them to be the bottleneck when I ported the engine.

[NoDOHC]

Updates and further questions:

I will be removing the engine from the car (hopefully before winter) and installing 9.7:1 rotors, hardened planetary gears and S5 counterweights.

My question that pertains to this thread is:

Should I polish the 9.7:1 rotors? I will already be increasing the side flex clearance by 0.0005"

They are much smoother and more refined than the 8.5:1 rotors were when I started. I also do not want to have the negative effects that I had from the 8.2:1 rotors.

The negative effects (mostly just inability to run lean) could be partly from my terrible compression ratio.

Basically, I am looking to pick up another 30 Hp and 3 mpg (desktop dyno indicates these gains). I get about 26 mpg right now, I got 29 before I put this terrible 0.78:1 overdrive transmission in my car.

I will put the 8.2:1 rotors back in and install the GT37 after I have determined what power gains are to be had from higher compression rotors.

I read one article that indicates that all compression ratios from 8:1 to 11:1 make approximately the same power in a rotary, but that the high compression rotors give better low-end torque.

Everything else I have read says that 9.7:1 is good and 10:1 Renesis rotors are even better.

If Desktop dyno (designed mostly for piston engines) is right, I should have 300 Hp at 7500 rpm with 9.7:1 rotors and about 220 lb-ft of torque at 6100 rpm. (240 WHp - 176 Wlb-ft).

It is also possible that I will lose too much power to the additional flow work required to force the air through a smaller dish (as the rotor passes TDC) and I will make less horsepower and similar torque to what I do now.

This is why I want to make this change (that and I already have the rotors).

Summary:
Perceived Pros:
Better flow at TDC
Allow more timing advance without detonation
Better heat rejection
Carbon buildup resistance

Perceived Cons:
Poor chamber mixing (no bumps to generate turbulence)
More difficult to light charge (no localized hot spots to act as activation points)
Removal of special factory coating (which is actually visible on these rotors)

edit: Oh yeah, I will post pictures of the rotors when I take them out so that we can see if there is carbon buildup or not.

Bearing in mind that this will be an NA only engine (I will put low compression rotors back in before I turbo it), I am looking for input.

What do you guys think?

[NoDOHC]

Well, I think I have a polishing bug.

I couldn't resist and went ahead and polished the rotors.

They are now down to 9.5:1 CR (unfortunately).

According to Desktop dyno, this compression ratio increase should give a 12% increase in torque.

Has anyone tried dynoing an engine with low compression rotors and then dynoing the same engine with only a rotor change?

[alexyjay]

a nice cleaning and your ready to rock. that what i always do.

[NoDOHC]

Back from the dead!

I finally got around to replacing the engine. I didn't tear it down, as it is in great shape, makes good compression and purrs like a kitten with 13,000 miles on it - it seemed a shame to tear it down.

I built a new engine, but as I compared the two engines side by side, the new one is not as carefully ported as the old one (I wish I had the time, but I realized that winter was coming so I didn't spend as much time on the second build). I still think it should run well, just not as good as the first one.

Anyway, I took the old engine out and snapped a few photos of the condition of the rotors after 13,000 miles (OMP and Premix).



This is the only one that turned out. You can see the carbon deposit in the exhaust port reflecting in the rotor face.

All faces of both rotors were a nice golden-brown and there was no carbon to be found.

I wish that I had a better camera, I can see down in the chamber fine, but the camera can't. I will try messing with the lighting tomorrow and get some better pictures.


EDIT:
Better Pictures.... Well it seems that I got 2 3-piece seals and 4 2-piece seals the last time I bought some, I was building another engine and found this out, requiring me to tear the old 8.2:1 engine down for it's seals. I was sad to do it, but not too much, as I wanted better pictures anyway.

All three faces of the rear rotor looked the same:


One face of the front rotor didn't look so good:

[NoDOHC]

So I was wondering... Was it assembly lube that charred and stuck to the face of that rotor, was it an oil injection problem? It is also possible that this rotor face had a stuck side seal (it did not come up when pressed down).

I did not polish the rotors on the engine I have in the car now. Maybe I will look at them after 13,000 miles.

ReTed, You are completely right about the factory coating. I found that the 9.4:1 rotors that I put in the engine the second time were coated. The only place that carbon had stuck to them was in the combustion chamber 'bathtub' and where the coolant that was regularly floating around in the engine (bad coolant seals) had caused the surface to rust. I have had two other NA engines apart and many turbo engines and have never seen that coating until this time (although I may not have been expressly looking for it).