Polishing Rotors - Pros and Cons
 

Right now I am in the act of freshening up an 88 4-port engine for in my '86.
I have seen many people polish the rotors before installing them, I decided to do the same.

The reasoning that I used is that it will decrease carbon buildup, improve heat rejection of the rotor and decrease exposed surface area which should minimize fuel precipitation and improve knock resistance.

My question here is whether the rotor requires some irregularities for improved turbulance as the air exchanges past the point between the leading and trailing plugs (right before it ignites the charge). Will polishing the rotor sacrifice startability and low-end toque by eliminating an important fuel mixing mechanism?

Maybe this question has yet to be researched. I only brought it up because I can't decide myself if it will improve flow or harm it.

I went ahead and polished the rotors, so I can only hope there are no negative side effects.

it theoretically should help keep carbon build up down for sure, which can keep hot spots from forming on the rotor.

They come coated from the factory.
Polishing them just just got rid of that special coating.
I don't bother, except to knock off all the crud.
No on see them once the motor is assembled anyways. :)


-Ted

I wouldn't have polished them if I knew that they were coated, they looked like rusted metal to me.

I hope I don't regret that.

Do you have pictures?

Here some pics i did for a 3 rotor custo couple off years back...

Regarding benefits by polishing the rotors, it will help with carbon build up control in theory . How much i really do not know. That 3 rotor i did for that custo is not do for a rebuild yet. I will also be interested to se how much it helped with the carbon control or if any at all. From looking at the first pic you can se how much carbon the rotor had on it:)...


http://img58.imageshack.us/img58/7260/picture122.jpg
http://img58.imageshack.us/img58/pic...jpg/1/w800.png
http://img58.imageshack.us/img58/5095/picture123.jpg
http://img58.imageshack.us/img58/pic...jpg/1/w800.png
http://img58.imageshack.us/img58/6624/picture161.jpg
http://img58.imageshack.us/img58/pic...jpg/1/w800.png
http://img58.imageshack.us/img58/4028/picture162.jpg
http://img58.imageshack.us/img58/pic...jpg/1/w800.png

isnt there some kind of coating you can get on the rotors too now a days?

Cermet.

Swaintech also provides a high temp coating for rotors..Ceramic based.

ah there we go! what company does the service?

This coating from the factory is just a anodize usually gold or green in color..
So depending on how harsh {or strong} the chemical you use to clean the rotors with, it will take it off anyway..

Dan

I've never seen anodizing appear like that.
I've never heard of any anodizing internal engine parts.
I've never heard of anyone anodizing steel / iron - only aluminum?
I don't think the rotors are aluminum.
Cadmiun perhaps?
SAE papers claims it's a special coating for oil adhesion.
It's on the rotors faces + on the outside edges of the rotor from the side seal groove to the edge of the rotor faces.


-Ted

Sorry Ted I ment cadmium plating on the rotor... On the other coating there is a small amount of teflon {I think} on the outside edge of the side seal on the rotor its self...

For a coating on tub side of the rotor it may be there but i have never seen it on brand new rotors {Rx7 or 8}

If you don't mind can you post a link or scan the SAE papers you are reading???

Dan

A few questions:

What would you call this? (Street port? Stock port?)
Attachment 4716
Attachment 4717

The intention was to avoid changing port timing while providing a nearly consistent cross-sectional area for the primary and secondary runners and ports. I am not sure if this is still a stock port due to stock port timing or if it is considered a street port.

Here is a picture of the finished rotors (I would post more, but it is too much work resizing them).
Attachment 4718

in my mind that is a street port..

as for the rotors good job and it looks good too..

Dan

I recently cleaned my rotors by hand and noticed they had a yellow coating on them. To top it off one rotor seemed to have a textured face while the other did not. Not sure if that plays a role or anything, but i thought it interesting.

Bling Bling... looking good :)

Some rotors have grooves and some don't
when removing material from the rotor to polish, I believe a balancing is in order before putting them into an engine.

For ceramic coating of rotors, housings, or irons check out JHB Performance in Canada. Here's my rotors that are going into my next setup. These have the cermet A applied to them.

Thanks Atkins Dan, I wasn't sure of the exact definition of each.

I intend to balance the engine JunpoweR, don't worry about that.

RX72NR: Why do you have that portion on the rotor machined off? Is this intended to dramatically lower compression? I don't think that you will get much combustion out at the edge of the rotor like that, too much quench area. Did you do that only to the one side of each face? What kind of power are you looking for?

I am only looking for 300 at the wheels. (Should be attainable with standalone ECU 550/720 injectors and a good FMIC). I think I will use the stock turbo for now.

NoDOHC

Your welcome and hope the info helped..

Dan

My goal is around 500 h.p. on pump gas. The rotors are beveled/ flame relieved. I'm not a guru, maybe someone can chime in and explain the theory and benefits. I've got some NRS 3mm grey ceramic seals and springs going into the build. I'm thinking of having the housings and irons cermet A coated as well. Here's a pic of my twins...

The rotor bevels change the intake port timing, I believe earlier opening actually helping the engine breathe like a bridgeported one but without the drawbacks.

RX-8 rotors have that from the factory but smaller.

Ahh, I see now. That is a really cool Idea (I even complained about the distance between the side seal and face of the rotor while analyzing my port timing).

How do you balance the engine like that? I can see that only half of each rotor face was removed, but still the counterweights will have to get a lot lighter. I guess you can drill them a lot.

Thanks for the info (maybe I can do that next time).

500 Hp, wow!

I wonder if polishing the rotors has much effect on atomization? Alot of piston guys will sand blast their piston tops for atomization. Due to the path air and fuel take going into a rotary I doubt the effect would be the same, but I'm curious.

Brent

Update:

Sorry to resurrect this thread, but I have 4400 miles of driving to discuss.

Short story, Brent is 100% correct. The engine does not like high vacuums at lean mixtures at all. I have to run about 12:1 at idle to make it run. My fuel economy is down too (38 mpg instantaneous at 70 to 34 mpg instantaneous at 70, about 29 mpg on a tank). Most of the loss of economy is the fact that at cruising vacuums (~50kPA) I can't go lean of stoic (I was running 16:1 AFR before).

Long story: Too many other factors involved to discuss power (4-port vs. 6-port, bored-out intake runners, heavily modded UIM, etc.) but the detonation resistance is insane.
The engine will not pre-ignite at all. I have never heard it knock (I am still running NA). I advanced my timing until flooring the gas made the engine deccelerate the car and I still heard no knocking noise (70 degrees BTDC).

After those miles (very few actually, in the grand scheme of things) it pulls 70 kPA of Vacuum at idle, makes 100+ psi on all faces and otherwise appears to be in good condition.

Interrestingly enough, the engine makes more torque at 2,000 rpm than the 6-port before it. (It makes a whole lot more torque at 7500.) All in all, I am very pleased with the 4-port NA (my first). Unfortunately, Idle torque is not very good at all, the torque curve doesn't even start until 1500 rpm.

The car is very hard to drive with a stage 2 clutch, 8lb flywheel and no low end torque.

Are you saying the polished rotor faces are helping reduce detonation? And that they are causing you to need to run richer in vaccum?

I don't know for sure (too many factors). I did polish all the dry areas on the intake manifold to 400 grit and polished the wet (after injectors) surfaces to 150 grit. I also am running lower compression (I think 8.2:1 if I measured the water right). However at 40 degrees BTDC my 9.4:1 6-port sounded like a gnome was beating on my hood at 1000 rpm and this one will advance considerably past that (until the engine generates negative torque). I don't know if it is only due to the polished rotors, but that is what I am blaming it on.

Most detonation is actually the collision of two flame fronts. One is caused by the spark plug, the other is caused by a localized hot spot. By polishing the rotor I not only increased the heat rejection (think shiny handles vs. black handles in the sunlight) but I also minimized the possibility of localized hotspots (due to uniform surface finish, no protruding surfaces to absorb heat).

Actually I am very happy with the results on this engine, I recently discovered that the Borch handbook's advice to advance the timing until it detonates and then back it off 5 degrees is not very good advice on a rotary. I retarded my timing 30 degrees to 8 degrees BTDC at 1000 rpm and the car has surprisingly good low end torque (lugs to 300 rpm, no problem and will actually accelerate from there).

I will be interesting to see this engine with a turbo...

Reason I asked is typically, knock in a rotary isn't heard for long. USUSALLY its not even audible before its blown. I've only ever once heard the death rattle for about 0.5 seconds in all the engines I've blown.

My other point was the need for richer mixtures in vaccum would indicate you've disturbed the efficiency of the combustion chamber. If you're having to run closer to stoich, or under it, to achieve proper power/good running then it means the combustion chamber isn't "mixing" the air and fuel as effciently as it was which would cause the need for more fuel to be present for an even burn (good running) when the spark lights off.

Whether it is the polishing, runner smoothing or otherwise I couldn't say for sure.

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!

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.

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?

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?

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

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).

http://i594.photobucket.com/albums/t...C/P9120533.jpg

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:
http://i594.photobucket.com/albums/t...C/PA090549.jpg

One face of the front rotor didn't look so good:
http://i594.photobucket.com/albums/t...C/PA090548.jpg

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).