post 86' rotors in 85' and older housings
 

I have a GSLSE engine to be rebuilt, the 11.5 and lower compression seems to be a bunch of compromise. I would like to grab some S5 NA rotors or perhaps rennisis ones, (i am familiar that there is apex seal depth concerns, and that there is no real power difference between S5 and RX8 rotors). Racing Beat suggests against using the older housing and the newer housings due to a slight change in the trochoid. Is this truly an issue for a non track driven car, or is Racing Beat's sage advice to be taken for gospel. Come to think of it, is the 2 pounds saved off each rotor worth the cost and effort? I would assume I will loose some redline, but being a street car not sure if that really important. Then there is the 3mm apex seals, which popular consensus is the 2mm is a better design, but once again... perhaps for a street car there is little benefit to the 2mm seals.

opinions?

I don't have opinions, I have facts.

I have put GSL-SE and FC rotor housings up to each other on dowels, and found no place anywhere around the circumference where the chrome surface is in a different place. I know RB says they are different, and I wanted to see where, but I was surprised to find there was no difference. I also did it with a 12A rotor housing just to be sure.

I have about 40-50k on a S4 rotating assembly in a more or less GSL-SE engine and I rev the snot out of it, no issues extant. It's a little bit tired now, but it's had a very hard life.The 2mm seal rotors are superior in every way. The apex seals seal better, are easier on the chrome surface, and can handle RPM better. The 12 pin rotor gears reduce loading on the stationary gears considerably compared to the 9 pin gears that 3mm seal rotors have.

Basically 3mm seals are only good for salvaging worn out 2mm seal rotors. And even then, they limit your RPM and will be harder on the oh so precious rotor housing chrome.

For a street car, the 2mm seals are better, for a race car, the 2mm seals are better. For all high high boost drag engines, it probably doesn't matter since you will be breaking this regardless :)

IF I were to do another 3mm seal 13B, I'd hunt down some carbon seals to save the rotor housings...

Peejay, you pretty well described what I understood until I read RB tech article. seemed strange that only the folks in Anaheim raise this concern.

Hmm.. i have a digital camera and access to GSL-SE, FC, and 12A rotor housings right now. I may take some pics tonight.

My thought was , maybe the 13Bs USED to be different, then they made replacement rotorhousings on '86 up tooling? But then i stuck the known-vintage 12A unit up and it was the same too...

I would be interested of course, but you say you have S4 rotors in a SE keg... works thus far.

The interesting thing about trying to prove something you'd previously known is when you get conflicting data. Some people throw out the data and stick to their beliefs, honest people examine the data further.

I grabbed one (1) GSL-SE rotor housing and one (1) FC N/A rotor housing, laid them dowel to dowel and ran a straightedge across the chrome. Taking photos was hard to juggle light balance but here is what I saw last night: GSL-SE is always on the bottom or in the background, FC is on top or in the foreground.

http://www.photoberkit.com/copper/al...213_193235.jpg

Hmm, not so similar after all! The FC pinch is a lot less pinched, maybe .002-005".

http://www.photoberkit.com/copper/al...213_193403.jpg
http://www.photoberkit.com/copper/al...213_193422.jpg

Different lighting, no straightedge. You can clearly see the very slight discrepancy.

http://www.photoberkit.com/copper/al...213_193441.jpg
http://www.photoberkit.com/copper/al...213_193501.jpg

What is interesting is that the rotor housing is also a little larger at the largest radius of the trochoid surface.

In conclusion, I dunno? Clearly THESE two rotor housings ARE different. Enough to make a clearance issue? Probably not... if you're going to be whacking the pinch with the face of the rotor, you're probably going to be doing it with either type of rotor housing.

One thing that a builder should do, regardless of which rotors and rotor housings are used, is to verify the clearance to the pinch. A slightly misclocked stationary gear can cause you grief here.

Trust but verify!

When I get the engine apart in my '84, I will also compare the shape of its S4 rotors to the shape of GSL-SE rotors. If the ROTORS are the same dimensionally, then who cares?

are you planning a rebuild soon?

Yes. Actually I have been planning to rebuild since 2013.

I've done this rotor housing comparison myself and didn't see a noticeable difference. But it was in the past and I thought it was a dead subject until now.

I do have some housings here to compare with each other. I wonder if I should go as far as comparing FD and FC to GSL-SE and 12A.

I'd compare the FD to FC first just to make sure there is no difference. The housings also have different amounts of wear. Some are high mileage and others are about medium. Some have a little bit of missing edge chrome. And I'm sure I could dig up a chattered housing or two.

Speaking of chatter marks and wear, what are your thoughts on the thickness of remaining chrome on your results, peejay?

Something to add while I'm thinking about it. I've successfully ran an S4 NA assembly in GSL-SE housings. Likewise I ran an R5 assembly (basically GSL-SE) in S4 NA housings. No issues with either.

Do you think maybe one of your housings could be warped? When I did a similar test with an overheated housing, the dowel pins didn't line up perfectly. Thus the overall shape was just a little bit off, but still within the factory crush limit, so I used it with no issues. It only had a tight dowel pin during assembly.

I did the same check but skipped some generations. The front is an S4 13B housing and the rear is a from a pre-74(?) 12A. (It has 2 tension bolt holes that don't align and doesn't have the steel lining.)
My straight edge is a 1/2" piece of steel that has one end milled flat enough that a .0015 feeler blade can't get through on a granite block.

With a .002" feeler blade I found they were pretty even all the way around, especially at the pinch.
However, wear is probably throwing it off in places. On the 13B a .002" blade can pass through parts of the center but not near the edges.
So, I don't think I can be definite unless we raid Mazda's museum and measure pristine, unused housings.

https://farm5.staticflickr.com/4612/...ab31ee03_o.jpg

i've run newer rotors in the older engines too, a couple of times. no downside as far as i've experienced.