after searching for an hour or so trying to find a good thread about using 9.7:1 rotors in an s5 TII engine, i found literally nothing. a few people running 9.4:1 rotors, but very little about the s5 na rotors... everyone who seems to know what they're talking about just says "oh just use as low of compression as possible" but thats not the point. i dont want to big boost. i want to run under 10psi and make the same power that i was at 14-15psi on my 9.0 rotors, with quicker spooling and lowend response. someone please just tell me what the big deal is... why not just run less boost, run a little less timing under boost, and have better response/lowend making the same power?

i'm sure this thread will fill up with "you'll just blow your engine if you boost high compression rotors" but thats not the point. i want to hear from people who have actually done it...

 

haha, you probabaly haven't searched enough!

Here you go:

https://www.rx7club.com/showthread.p...ght=9.7+rotors

 

well first of all 9.7:1 compression is high compression period, even with 5-7 PSI youre have to run 93 + octane on your car so it doesnt detonate and 2 me personally i had a gslse engine with turbo and i didnt felt any quicker spool of my turbo. about power yes you get more power but i dont think the same as with 14 psi with 9.0 compression. less timing on your car means that youre not gonna have power until that turbo kick in. trust me i did that and i prefer my turbo II engine more that my gslse with turbo

 

this is exactly the type of stuff i was expecting to see when i made this thread.

 

You can run high comp turbo, n/a guys prove it time and time again. Plus it's sexy as hell. Standalone makes it easier too.

 

I've got a question about this:

since the r-tek 1.7 chip retards timing, would it be good to use this chip with the 9.4:1 rotors?

 

Standalone would be better. Stock computer retards timing under boost too, but I'm guessing the rtek does it more?

 

My experience is a nice low and and top end all around, the only downside I experienced was that the car was more willing to boost at any given time, or that more boost was generated with less throttle application. As a result it made the car feel "surgy" on the highway and even at a 75mph cruise the car wanted to go into boost or sit just below atmospheric. This made gas mileage suffer, IMO. I eventually modded the engine to the point that I broke the rear iron's dowelpin landing, and had to do a rebuild because of it (didnt break any internals). This was in my vert, with GSLSE rotors and a healthy streetport.

 

I run 9.7.1 rotors with a turbo. Started with 6lbs now I am up to 10lbs. Yes, I run 93 octane all the time with standalone. No pinging and plenty of "quick" power after 4500 rpm. Fastest I have had it was 130 mph and the car still had plenty of power left.

 

that is probably what i will do....
9.7:1 at stock psi...
not too crazy but a nice daily driver, and the turbo should last a while

 

i thought about it but i didn't want to sacrifice the reliability of the engine, i may build one just to swap in and see the differences.

 

why would that sacrifice reliability of the engine?

 

Because bumping compression up on any engine always brings it a bit closer to the threshold of pinging, which tears stuff up. Boosted or not. This is why we don't normally see street cars with 11:1 compression ratios even if theyre not boosted.

I was once told by some piston/GN racer that (and this may be bs, but it sounds pretty close) for about every 7psi boost you run through an engine, it's like you increase it's effective compression ratio by a full point. For example, if you have an engine with 8.5:1 compression ratio, and you run 14psi through that, it is roughly equivalent to a 10.5:1 CR engine in NA form.

Taking that same 14psi boost and applying it to an engine with 9.7:1 CR would yeild almost 12:1 CR...now you can see why it's so hard to keep a hicomp boosted engine in one piece. Your first tank of bad gas or boost spike and it's all over with. And don't say that you are going to "run low boost" because it doesnt always work that way.

1) rx7 turbos usually creep on the top end

2) high compression drives the turbo a bit more

3) boost tends to come in a bit earlier with hi compresion, making a midrange ping more likely

 

I run series 4 na (9.4:1) in my TII engine. A bit better low end as expected and better turbo response on an engine that saw no other changes. I have a big single so I do not quite have the same issues that RR (above) has with it wanting to boost all the time and mileage suffering as a result. My mileage sucks because I have a lead foot.

Being limited to run 7psi less does not make up the difference in the top end. My dyno's show that. But if your not worried about high boost (you say that now...), no big deal. Right now I make about the same amount of power at 12 psi as I made with 17 psi with the 8.4:1 rotors.
I opted for the na rotors originally to help with spool up which comes on about 300-400 rpm sooner and better 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 as I have been told by Steve Kan who tuned my car.

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.

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He was wrong. Calculating the effective CR of a boosted engine is quite easy. It's just the static CR multiplied by the manifold pressure ratio, where the manifold pressure ratio is the absolute manifold pressure divided by atmospheric pressure. So for the example above:

ECR = 8.5 x ((14 + 14.7) / 14.7) = 8.5 x 1.95 = 16.6:1

You can see that running 1 atmoshere of boost (14.7psi) doubles the absolute manifold pressure and hence doubles the ECR.

Using the above formula, the correct ECR is 18.9:1. So if you dropped boost from 14psi to 10.5psi you'd get the same 16.6:1 ECR as above.

 

btw, didn't you just start a thread on this a few days ago?

 

You obviously havent had a decent intercooler before. Its all about the tunning and cooling of the intake charge.

 

thank you jason, this is some info that everyone can benefit from.