I have sent my engine to MarvelSpeed to be rebuilt. It's an S4 with the 8.5 rotors. I really thought I wanted the 9:1 's but I can't find any used and the price for brand new rotors is outrageous.

My plans are for a GT35/40 and around 400+ at the rear wheels on an almost street only vehicle. I think 400 is conservative so I was going to use the higher compression rotors.

My questions are 1. Is it worth spending the extra dough on the 9:1 rotors? and 2. How much difference is there between the two in mid-range torque, off boost response, etc?

 

if you're spending the extra dough why not just use the 9.4:1 and you won't need to run as much boost.

 

I suppose I thought that the 9:1's were the best compromise. Does anyone have any experience running 9.4:1 on a turbocharged engine?

 

I'm pretty sure there is a reason people don't generally run the 9.4's in a turbo. I *think* and hopefully someone will chime in if this isn't correct, that power is more a function of the air mass than the amount it is compressed. You could get more air initially into the engine a lower compression rotor. So more volume (with lower comp rotors) with slightly higher boost will do better than less vol, higher comp with less boost.

I dunno, does that make sense?

 

No, you would not want to run 9.4:1. It would limit boost to ~10Psi to be safe, and the turbo you are looking at is not real efficient at 10psi.


Go with 9:1 or 8.5:1



-Zach

 

Limit would be more like 14-15 psi on pump gas assuming all other variables are within spec. Although I would run water injection on this setup.

If you have a pair of used 8.5:1 rotors in good condition, I would use them. I personally would rather replace the rotor housings than the rotors during a rebuild. Just run a 17lbs or lighter flywheel, and those 8.5:1s will feel like a beast off boost .

http://www.rpmperformancecentre.com.au/ has cheap lightened flywheels.

 

Yeah 14-15 would be the limit.. i said ~10psi to be safe.. So i think we agree


I have driven cars with 8.5:1 and they are not bad at all. Although 9:1 is nicer on the street.


-Zach

 

9.4:1 is the way to go! even with the stock flywheel this thing was a beast off boost ^_^

 

I have the SR Motorsports 9.5lb flywheel. I just wanted the ultimate street/road race set up. My plans include the One Lap of America and hopefully some amatuer autocross. I know that I would at least need the 9:1's for autocross.

Importboi, are you using the 9.4:1 on a turbocharged engine? How much boost are you running?

 

Hi,

I'm new to the forum, but not new to rotaries and rx7's. I'm running (sort of) an S5 turbo on an engine with 9.4:1 compression ratio. The reason I say "sort of" is that I don't have the intercooler plumbed in yet and I'm - obviously - still in the tuning phase, but I'll say, "so far, so good."

The reason I hadn't already posted on this is that my car/engine is a total mutant, and I'm not fully tuned yet, so any input at this point is preliminary. In fact, I've probably not run over 5 - 6 psi of boost.

The car is a '73 Porsche 914. The engine is an S4 NA. I've built a turbo setup around an S5 turbo/manifold and a diamond star 1st gen intercooler. (Intercooling in a midengine is awarkward.) The exhaust is three inch through about 6 feet and one flowmaster, then tapers down to 2.5 inch and a Walker Super Turbo. I don't want a loud car. This thing is a total sleeper. Oh yeah, it's all controlled by a Haltech E6K.

As far as spool up time for boost and off-boost response, all I can compare it to is a stock '87 TII that I bought new long ago. The spool up is definitely quicker, but that could be in part due to the exhaust mods. Also, the flywheel arrangement is much lighter than a stock 7. The off-boost torque of the car feels as good or better than it did as an NA, which is to say very crisp - even below 2000 rpm. I'm only planning to boost in the 8 - 10 psi range, but I'll repost when I know more.

BTW, great forum and great discussions.

Danny

 

I was very confused when I built my first turbo motor so I went with the highest compression ratio 9.7:1 Rotors. My reasoning for using the high compression rotors was I wanted to make a lot of power with very little boost. So far so good I used the 9:7:1 Rotors from a 1989-1991 GTU type motor. The most boost I have ever run in my motor was 25psi out a of custom 60-1 I had built by Turbonetics. The car made roughly 480RWHP. So my theory and reasoning have done me well so far. Most Rotary Engine builders do not recommend doing what I did but it works.
If your looking to use low boost and make a approximately 400 HP I would use the high compression stuff. The more boost you introduce into this little motors the more modifications are needed.

 

i havent got to boost it yet... only takin it t 4k rpms with very little throttle

 

rotaryengineering,

Your reasoning is consistent with some fatory piston engine designs of the last few years. Though they are not high horsepower engines, the 1.8 liter turbo from VW/Audi and a small displacement five cylinder from Volvo both use high compression with turbos and have gotten great reviews for their flexibility and good behavior. As much as it may not be a popular view, we can often take a hint from the factory cars. (At least as far as what will be tractable for the street.)

 

The big difference tuning wise between using higher or lower compression rotors is going to be in the mount of timing advance used for each compression ratio. The lower the compression ratio the more timing you can run in relation to the higher compression engine at the same amount of boost. You will come to a certain point in the horsepower game where the timing retard will be so much on the high compression engine that the low compression engine is making more power at the same amount of boost. This point is still somewhere above 400 hp though. There is also more room for tuning error on the lower compression engine. You can't necessarily say that you are limited to only so much psi for any given ratio. It all depends on the tuning, how efficient your intercooler setup is, octane gas used, etc. Many variables. If you are just looking for 400 or so at the wheels then I say use whatever rotors you like. Just tune it properly. Nothing wrong with the 8.5:1 though but lowend drivability suffers a little.

 

Thanks for all of the input so far. Is anyone out there making 400+ hp on the high compression rotors? Either the 9.4 or 9.7:1's.

 

I believe that John Duarte was making this on his 10th a few years back. Well actually maybe he was more like 500. Seems like he ran a best time of 10.8 in the 1/4 and I think he was using the 9.4's.

 

I searched the life out of the internet to try to find some 9:1 rotors. No luck, so I am bumping an old thread. Would any heavyweights like to chime in and give me some feedback on using the 9.4:1 rotors while trying to achieve 400bhp?

 

Dont mark my words but i believe the GSL SE rotors are higher compression than 9.7:1 arent they around 10.5???

 

gsl-se is 9.4:1 with factory 3mm seals

 

which i do have some by the way :-D

 

which arer the highest comp.? the 9.7 gtus?

 

right

 

any series5 N/a rotors....there all 9.7:1

 

cool thanks guys i just bought a pair of GSL rotors last night. We'll see how they do with a t51R water injection and nos in a 3rd gen BP

 

I was running 19-20 psi on 9.4 rotors for 9 months. Turbo was T3 60-1. I dynoed at 400rwhp at 6500rpm's, maxed the injectors out. Car ran high 11 secound's at 122mph in the 1/4 mile. 1.8# 60 foot times. Car ran like a N/A car, no turbo lag. Currently running a T66 on 8.5 rotors at 22psi and mid 11's at 128mph in the 1/4 mile with 1.9# 60 foot times. I feel my first set up was better for the street. When the peddle was pushed to the floor the car responded. Who drives around under boost anyway. Tried to race a Mustang with the old set up once. We were going about 60mph. Right after we made eye contact I down shifted to 3rd and took off. The Mustang did not race. Meet the guy a week later and he said he did not race because my car was gone. By the time he started to race he said the race was over. If I were to rebuild another motor it will have 9.4 rotors with big street ports. The 9.4 T3 60-1 motor was quick, this 8.5 T66 motor is fast. What do you want, big HP numbers ( Earl Cambell ) or a quick street car( Berry Sanders )?
chuck