So I've been reading up on some threads and see a lot more questions relating to popping S5 rotors into an S4 TII. I don't know how common it is to do the reverse, IE S4 TII rotors into an REW but I know it's talked about a little. Forgive me if my thoughts are all over the place. I'm kinda winging it here.

I have some questions about the potential output using lower compression rotors. It is worth it and what are some experiences of those who've actually done it, specifically low RPM (city) drivability and for lack of a better term, the fun factor? And what's the typical use of your vehicle?

So at a given PSI, the higher compression rotor should yield more power just because of the nature of having higher compression, but that = more heat = more chance of detonation after a certain point. Take that same PSI number with a lower compression rotor and you're a little safer as you have a little less heat, etc but at that PSI you make a little less power but have the potential of running more boost overall. For the sake of arguement, let's say this is all done with the same grade of fuel.

In addition, they're a little heavier, but let's say it's a moot point as they'd be lightened.

So, at what point does it not matter if that point is even reachable at all?

It is better to run 30 psi with a 9:1 or 35 with 8.5:1...as an example?

 

On pump gas the 8.5 can handle more boost safely before detonation/pre-ignition sets in.
Power difference is not noticible but torque especially in the off boost area is a little lower. At higher boost due to some type of race fuel being used 99% of the times compression does not matter that much at least in my experience.

 

what about having 9.7:1 rotors instead of the 9:1 rotors?? I know this is a stupid question considering I just built a 13B-RE with these, but I guess I'm always curious about that "little bit" of info that I might have forgotten about. Well, then again, I guess it's pretty self explanatory.......that thin red line between a running engine and a blown engine is much thinner.

 

Let's not veer off to higher compression. 99.999% of the time, you're not going to increase compression with forced induction applications from stock.

Well, right. I said that. And again, we're sticking with the same fuel, whether it's C16 or 93...doesn't matter. I'm basically looking for an answer of what can be gained (or lost) by using one over another and peak power potential between the two.

 

words I've read

 

I think crispeed said it pretty good. You will lose some off boost reponse. (might not feel as crisp)
But overall feel will be about the same. More boost is always fun Keep in mind high comp. and boost can yield nice numbers but it gives very little room for error. On a 11/1 honda we did 340whp@4.5-5lbs not too bad. The better thing with the lower comp rotors are they are much more thicker where it counts. That is the main reason I would look at them. All else being equal if your only going to run around 20lbs or so and mainly pump gas, the FD rotors wil be fine.

-S-

 

Does any one know were i can get a couple 8.5:1 rotors in good condition or perferably new?

 

I'd rather keep my 8.5 rotors.
You can always turn up the boost.
If you want better response, just drop the turbine A / R or drop a whole turbo size.
At least the 8.5 rotors will not dent under detonation...


-Ted

 

Alright, my friend and I were just discussing this the other day and I have a few questions for you guys. We had a strong discussion for like an hour, and I'm just going to shorten this, due to my lack of actual knowledge on the subject.

reguarding this- would a motor w/high compression rotors tuned to the maximum possible PSI be just as powerful (top end wise) as a low compression motor tuned to its maximum PSI? I understand that it would leave little room for error, but if you tuned the car w/lower compression rotors, could you not max out your PSI on that as well?

I hope this does not sound stupid, but from what I have read and heard it seems like some people just think about the amount of boost, or the PSI I guess, and is your power always directly proportional to your boost? Why is more boost better? I understand that you can run more boost w/lower compression rotors, but what does it matter if you can make the same power w/less PSI and higher comp. rotors (basically this is me asking-is it possible to make the same power? w/higher comp +lower PSI) I thought I read somewhere that a guy w/a S4 TII was running somewhere around 20PSI on the stock turbo (ported of course IIRC) and it was making less power than, I can't remember, but like maybe 10 PSI? My point being, the psi of the turbo was not the deciding factor of his power, however, the maximum flow of his engine. I may have this ALL WRONG, and I'm going to guess I probably do, so don't be too harsh on me. I would just like to learn once and for all

What makes since to me (but may or may not be correct) is this:
You can only squeeze so much air into an engine - correct? Can you accomplish the same atmospheric density with a lower boost number (psi) and higher compression rotors that you can accomplish with a higher PSI and lower comp. rotors? To me, it seems like you could only achieve one maximum density. And that density, could be achieved via (a bad illustration im sure, but here goes) 9.7/1 rotors w/10psi or 9.0/1 rotors w/15psi? something along those lines. I hope I have made my questions clear, and maybe I will get an answer...i should probably proof read this, but meh. im sure you guys can edit this for me :X lol

 

I believe you are correct. There has to be a threshold at which more compression is detrimental. Wether it be from boost or compression ratio. Lets call the threshhold xpsi.

Here's how I think of it. Do you want to ignite 90cc at xpsi or do you want to ignite 120cc at xpsi. The lower compression rotors will have a larger combustion chamber than a high compression rotor. which one do you want filled to the max?

The larger one will let you burn more air/fuel at maximum density.

 

Yes, to simplify the turbo is a more efficient compressor than the rotor, so the lower the rotor compression and higher boost the more efficient the set-up hence the more horsepower potential.

This is why the old factory race turbo engines even running on alchohol (very high effective octane from charge cooling) used special cu$tom cast 7:1 compression ratio rotors and high boost.

There is a point where the turbo itself runs out of efficiency and you have to run it through another turbo (compounding), but even so it is more efficient than the rotor to a point.

Running on race gas or alchohol modern rotary tuners are at the point where they are "safely" maxing out boost (30-40psi) on a single turbo so they up the compression ratio for more power/streetability. If they could fit two of the monster turbos under the hood they would make more power at higher boost and lower compression.

If you drop the CR too much though I believe you would have too much lag between shifts and the higher total HP output wouldn't outweigh the loss of HP available right after the shift.

A dogmission and flat shifting would help this lag, but there would still be some loss in turbine speed just from the timing retard unless you had real anti-lag. Then you would probably melt down before finishing a race with a rotary...

At some point you would just ditch the rotary engine and use a turbine engine because it would have less lag

Where higher CR turbo really shines is streetability and driving feel; nice, but not diretly related to ultimate performance

 

Yeah. The best description of how a jet engine works, that I've heard, is to imagine a turbocharged internal combustion engine without the engine! Inject the fuel and burn it right before the exaust wheel.

Seem to make more sense than the simple diagrams in textbooks.

 

The only problem with that description is that air doesn't flow through compressor>CHRA>turbine.

-S-

 

There seems to be a lot of knowledge here about turbos and compression. Unfortunately this is something I have very little knowledge of. That being the case, allow me to ask 2 questions. I purchased a n/a S5 that is in need of a rebuild. I plan was to have it street-ported but no turbo. Well, I came across a T04 turbo, brand new for $250 so my plans have changed. Q #1 - Should I have the S5 rebuilt for lower compression so I can use the turbo or find another turbo engine? Q #2 - What is the difference between a blow-thru and a draw-thru setup in regards to performance?

If anyone can recommend some good reading on this subject I'd appreciate it.

 

Thanks for the responses everyone. S4 rotors are definitely on my to do list.

 

i think it comes down to a matter of priority and perspective. personally, i'd love to get a S5 13B-T or 13B-RE motor together for a future project, but generally speaking, i don't care which 13B-T i start with. i see a lot of guys on this board stressing over rotor weights and compression ratios simply because of theoretical numbers, but most of them will never build cars to power levels where these differences matter. however, you can't talk sense into them. the way i see it is the only property that needs to be stressed is the strength issues that surround the S6+ rotors.

S4 rotors will do just as good a job as S5+, it's just a matter of paying attention to tuning details.

 

im putting s4 n/a rotors in my 13bt
just a hybrid turbo, ext wg on stock manifold w/ spring at 8.6psi, will maybe run up to 12psi or so. may get a bit ballsy when i slap on alky injection
i think itll be badass, instant boost and crazy off-boost. itll be interesting to say the least.

truth is i was happy w/ my dp and filter only t2, well not totally, but w/ a portjob and all the other things im doing plus exhaust, i should be very happy with a quite peppy daily driver.
putting the cash down for a good ecu and tuner, well ive been told hes good, especially at drivability and lowend. has done a high compression turbo setup before also.
-Ben Martin

 

it depends, i think if had my choice of all new parts for a street turbo setup, id use the 8.5's, like everyone else has said, it gives you a larger margin for error on pump gas.

node's experimant is interesting, and should actually work well, its like a factory setup with more ic, better turbo, etc...

 

Nobody has mentioned that installing a bigger intercooler or cooling the charge more before it gets ignited will allow you to have higher compression, or higher boost, or the same of each with more reliability.