Is it worth using the 9:1 rotors if I have 8.5's???
 

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 :D


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

Thank You Chuck!!! That's exactly what I wanted to hear. I read/listen a lot, but I needed my suspicions confirmed. Me thinks I may now procure some 9.4:1's!!!!

does that make me crazy for wanting to use 9.7:1 rotors?

if i didnt basically get the 9.4s for next to natta i would have done the 9.7s. What do those 9.7s go for anyway, price wise?

I would go lower compression to prevent knocking with more boost. Higher compression if your gonna run race gas all the time.

I really don't know shit though, thats my guess.

"I really don't know shit though, thats my guess."
LOL , OK what are you talking about??? The reason to use high compression rotors is that you can get more power from the same boost levels, pump/race gas, either one. Look at Vosko's #s 459hp at 15psi, that is attainable by high comp. rotors.

i got some gsl-se rotors... 9.4:1

yea thats what i just got, decided to put them in the FD as well as a t51kai, meth inj, and maybe a 50 shot. I was wondering how much do those GTUs 9.7:1 run in price?

Be careful with the GSL-SE rotors. They are shaped ever so slightly different than the post '86 rotors. Many people have used them but the difference is enough the Mazdatrix, Racing Beat and others don't recommend interchanging them. They are also heavier than your old rotors. I hope you have the proper counterweights to match as well as an aftermarket flywheel. Your engine will shake violentely if you mess this area up. The 9.7:1's would be the best choice due to the same shape and weight. They are just a little hard to come by though.

i always thought lower compression is better for a turbo setup?? all this high comp talk has got me crosseyed @¿@

All you have to do is replace the side seals and counter weights(i believe, dont quote me on that). Also with the 9.7 rotors your looking at race gas almost all the time. Basically i was told by several different tuners to go with the 9.4s if i want the max power on pump gas.Im always still learning though so if i have been misinformed please tell me.
-JT

The GSL-SE rotors are 9.4:1.They use larger side seals and 3 mm apex seals. They are also the heaviest of the 13B rotors. As I said earlier their shape is ever so slightly different from the current rotors.

The '86-'88 2nd gen n/a rotors are also 9.4:1. These use the same size side and apex seals (2mm) as all the later rotaries up until the Renesis. They are heavier than the 3rd gen rotors by 1/2 lb but are 1 lb lighter than the GSL-SE rotors. Any time you change year model rotors you should also change counterweights unless you already know what you have. Remember that the rear counterweight is built into the flywheel from the factory.

You don't neccesarily need race gas on any setup. The key is in how well it is tuned and how much boost you plan to run. Low boost would not require a high octane. High boost would. You get the picture.

i have the new side seals and 3mm hurleys for them and kne i had to change the counter weight but didnt know that they weighed that much more! Thanks. Any idea who sells the 9.7s and what year and model that they come on.

o yea one more Q. Is there a point in the amount of boost where a higher compression rotor(9.7s) would not be benificial or even be not as efficiant as a lower comp one(8.5/9.4)? Thanks for the help.

no, never. a higher compression ratio is always more thermally efficient. the only problem with high compression ratios is that it "concentrates" the heat of the charge more, meaning that to run an equivalent amount of boost at a higher compression ratio with exactly the same detonation resistance in the charge, you'll have to have a cooler charge coming in... lots cooler.

as you reduce your compression ratio, you can run far more lbs of air into the chamber, which often means more heat with a compression system of the same efficiency, but your thermal efficiency drops, as does your off-boost performance.

great thank you! Also(kind of hard to explain but ill try) If i go from the 8.5s to the 9.4s, this is just an example, lets say i pick up 20hp at 15 psi. Now say i go to the 9.7s will there be a noticable difference? how much, roughly? Thanks
JT

Actually I need to correct that statement. Wakeech is a damn smart guy though! (No I'm not being sarcastic!) There is a point where lower compression would be more beneficial. One thing about having lower compression rotors is that you can run more timing. This of course assumes that you are at a power level high enough where the high compression rotors are more prone to detonation. To avoid detonation you would need to start to retard the timing moe and more as the power level rises (high compression). The lower compression may not need any timing retard at this level however. It is hard to say exactly where this power level is however. the higher the octane gas used the higher the limit. You will eventually get to a point where there is so much more advance on the low compression per boost level than on the high compression that the low compression engine is making more power. It would also be doing it with much more margin for error. If I were to guess I would say that this would start taking effect around the 450+ hp mark. Again it all depends on the type of fuel being used and just how good the state of tuning really is. The higher compression engines will spool the turbo faster which is good and be more drivable when not under boost. Gas mileage would be a little better when your foots not planted either.

Can someone sum up the pros and cons of each type in one post?

BTW, this is an excellent thread, very informative.

thats what i was wondering. Basically im going to be running in the ballpark of 500hp every day with a BP, t51 and meth inj at around 17-18 psi, with the 9.4s. At the track I was looking at maybe uping the boost to around25-30psi with c16. At the tracks 2 bar setting, are you saying that i would be making more power with the low compr. rotors and c16? but with 93 oct pump gas i would be making a significant gain using the high compressions?
Thanks
JT

HA, I guess I do know SHIT! Its only obvious, the most important thing is to prevent knocking. Lower compression, less tendency to knock. Notice how I said "if your gonna use race gas". I know I don't want to be race gas dependant because my car is for the street. Whats race gas $4 a gallon. If i do want max power, say for a track day, then I'll buy some race fuel, change my settings and bam, more power.


This thread asked for opinions, don't trash, especially without an explanation.

Wasnt trying to knock ya bro, my bad. You are right that there is less knock at higher boost levels with the low comp. I am just wondering that if you can run lower and make the same power why wouldnt you want to. For example if you can make significant power at lets say 15psi(Vosko-459/ErnieT10.7@20psi, many others) why would you want to have to run another 3,4 psi to have the same power? My car is going to be daily driven/shown/and raced(mybe once a month). Everything seems like it makes sense in my head, maybe im missing something.

The overlap is what im kinda wondering about, from what ive been told from the different people that are blueprinting the car i should be seeing AROUND 500hp at 17,18psi while on 93oct(while using the meth. inj), and this can be used pretty much everyday. So if you can make a great deal of power on pump gas with the high comp/inj when you want to go to the track and be tuned for C16 you can still run less boost and be making as much power as the low compr. at higher boost. I could be wrong but the way i see it is lets say we want to make 600hp. With the high comp. you may need to run 25 psi while on low comp. your looking at 30( these arent exact but a guestimate) i could be way off and if i am please let me know. Lets get RICE RACING/VOSKO/ERNIET/RX_Rotary etc in here for their input. Damn, WTF ever happened to AnthonyRodriguez, i would like to hear what he has to say.
-JT

What is our goal here, a fast daily driver or a weekend Vet/Mustage killer with going to the track once a month?. If you want a daily driver buy a tow vehicle for your weekend play vehicle. For 9 moths I was running 9.4 rotor and dynoed at 400rwhp at 6500rpm's. THis car was driven on the weekends and to the track. I used 93 pump gas for everday driving. I only bought 6 gallons and added 104 octane booster. The boost was 19-20 psi, water temp was under 160F and my air temp was under 100f. Turbo was T3 60-1. ( I think the T3 .82 a/r stage three wheel is a little to small though ) Throttle responce was instant. For street racing, all I can say it was quick. 122mph in the 1/4 mile. Now because of the pear pressure I am running 8.5 rotor with a T66. .96 a/r p-trim. Yes I am producing more power at 22-25 psi ( higher rpm range ) but the throttle responce is not a crisp as the 9.4 and smaller turbo. That lack of throttle responce will cause you to loose a street race more often then not. Who here has accully driven a 2700lb, 500 HP rotary car? I know rotarygod and I have and for the steet it is fast enough in my opinion. I always thought/believe that a 500 hp rotary motor equaled a 600 HP piston motor. ( not torque ) Please correct me on this If I am wrong, it's just my gut feeling. I know of a 360rwhp 3rd gen that runs 11.7 at 115mph. This car is fast. He has spent allot of money on the drive line, trans, diff, drive shaft and axles. this cost alone was allot he said. I could not imagine his car with 450rwhp. More upgraded drive line parts, more money.
Sorry for the long reply.
chuck

I don't make 500hp I'm guessing, but I can tell you that what I do have is plenty for the street. If you are not 100% focused on keeping the car in a straight line you will end up in the ditch. 500hp in a 2700# car is almost a death trap on the street in my opinion. But on the other hand putting a small turbine side on the rotary probably isn't the best idea either. It's hard not to make 350-400hp with just a decent turbo.
My vote, I like the off boost power I have for my car (9.0 rotors). I can climb small hills in 5th gear at 2700rpm so thats good enough for me. But my 13b turbo is in FB so it doesn't require as much low end power to get it moving as a FC or FD.

My goal.....well....i guess it would be to have a daily driven, show quality car that, because of the power capable, makes me nervous when i get in and gives me that stupid grin after I get out and look back at the ride that made me want to piss my pants. Thats MY goal , probably not everyone elses.I have driven alot of high power cars as well as raced shifter carts so know about extreme power to weight ratios. I dont understand fully though how you compare running a relatively smaller turbo at 6psi less than a bigger turbo and comparing that there is more power made or less lag.. If you only switched the rotors out, that would be a much better comparison, if it was the only variable. I dont know about 500 being too much for the street. I mean you do have a boost controller that hopefully has high and low settings, as well as a ECU that can be mapped for other applications as well. I guess some people are just more comfortable with power than others.
Thanks
JT

It's probably a different story when it's 500hp in a FB than a FD. I'm guessing the FD can handle the power a little better. Maybe 600hp would be a better "limit" for a street FD?

What happened to the topic of high comp rotors? LOL
Anyway is Rice,evil,AJ13b, any of the big guys able to give me a little more info. on the "overlap" of where high comp. rotors start to not be benificial.
Thanks all
JT