i got my engine all torn down and it appears that i need atleast 1 rotor and housing, my "good" rotor has some significant pitting from what is believed to be preignition so it may need replacing as well... my car is an 89 TII so the internals were all s5 TII, and i'm thinking about switching to lower compression or higher compression but i'm not really sure where to take it as far as the compression ratio is concerned...

 

hello everyone, my name is jacob

 

I am running s5 na rotors (9.4:1) in my turbo engine.
Better low end as expected but have to run water injection to get anything more than 10lbs. boost.
I make more power at the same boost level with all else being equal but there is not as much potential.
Overall I would rather have stuck with the s5-s6 turbo rotors so my suggestion is to just go with what it came with.

See engine specs here:
https://www.rx7club.com/vbgarage.php?do=view&id=2592

 

dp

 

Also, the boost response is much better with the same turbo and everything else the same.
Difference of about 300-400 rpm.
My old rotors were from an s4 TII (8.7:1 iirc).

 

Well I guess it depends on your goal...as everyone told me the lower compression S4 T2 rotors allow alot of room to play with boost and whereas the S5 T2 dont allow quite as much room but have better response and power. I am starting a rebuild to and still have yet to decide which rotors I am going with

 

S5 NA rotors are 9.7 compression, not 9.4. The S4 TII's are 8.7 and the S5 TII's are 9.0. If you don't have a stand alone ECU than stick with the compression that came stock with your motor. If you do have a stand alone ECU, than be all means, go with the higher compression rotors as you will have more useable power at lower rpms and make more power across the band but you will need to feed it more fuel at higher boost levels than a lower comp rotor at the same boost.

And, yes you can run more than 10 psi boost on S5 NA rotors, it's all in the tuning.

 

well aren't you fancy?
if you have a nice fuel system then the higher the compression, the better... better get some atkins apex seals thow...

 

i actually bought some new 2mm atkins seals last week

 

input

 

You are correct. I have series 4 na rotors currently. Sorry for the misinfo.
S6 rotors are also 9.0:1 btw. I agree on the ecu comment btw.
True at lower and equal boost pressures. However, being limited to run 7psi less does not make up the difference in the top end. My dyno's show that.
I opted for the na rotors originally to help with spool up and midrange power as I roadrace my car and these are definate benefits. Adding much more fuel in our case did not prevent detonation or enable us to run more psi. It would only start to cut out from being to rich. A higher compression ratio does justify a much richer fuel mixture. You still want the same afr.
Are you speaking from experience? Tell that to Steve Kan, he tuned my engine and I have a Haltech e6x fwiw.
We only have 91 octane readily available here in Cali. and that is what we tuned it with. Certainly with better gas you could run more boost.
We did run it up to 12 lbs. with no water injection but would get some noticable pinging at that point so backed it off to 10 lbs. for safety. Running two .8mm water nozzles moved the knocking up to 15 lbs. It could use a lot more water too but did not have any bigger nozzles then.
My fuel and ignition system are upgraded also to put it mildly.
Hopefully will get it up to 17+ lbs safely with more water injection on pump gas in April at the next tuning session. Should be well over 400rwhp.

It's your motor, run as much boost as you want. Hope you have better gas (as most of the Country does ).

 

I'm going with s4 N/A rotors in my s4 TII "re-rebuild". I have a friend who just rebuilt using the same setup, and his car is running GREAT. Granted he also did a decent street-port, he says that he's faster now using the high(er) compression rotors and running 6-7psi on stock turbo then he was before rebuilding, running 12psi (stock turbo).

He says the boost response is also better, as is the low-end.

So far at 6-7psi, he has only experienced positive results.

 

^he's running stock turbo, FD fuel pump, stock ignition, stock 500cc's etc.

He'll be upping the boost as soon as he installs his FMIC, Walbro fuel pump, 720cc's and the r-tek 1.7 chip.

 

Dave, sorry, I should have stated what octane that statement was regarding. We run 93 octane (pump gas) with either octane boost bottles or a 20% mix of toluene and are about 1 Bar boost. Higher compression gives us the ability to not need to run 18-20 psi and just run 15 and still have quite a bump in power from the increased comp benefit.

 

if its not a secret what was your timing advance at 12 psi of boost above 5000 rpm? or whenever you guys saw that it started pinging?

 

It started pinging right around 5500rpm iirc.
I'm sorry, but Steve wouldn't appreciate me giving away his tuning secrets on the forum. PM him and ask what his conservative timing settings are (username: pluto), he may just tell you. All I can say is that it was his standard conservative setting and retarding it did not prevent or postpone the pinging. Messing around with the trailing spark did nothing to prevent it either.
Simply just too much compression for the gas at that boost level.