GrossPolluter

iTrader: (1)

I'm gathering parts for a spare engine build and I am really interested in high compression turbo. My hp goals are only 300hp or more, but mainly I want response. I know 91 octane is a big limitation. I want to run water/meth injection also, but I was actually thinking about tuning without it and just adding it for extra insurance.

j9fd3s

iTrader: (3)

don't do it, its a trap! https://www.rx7club.com/2nd-generati...rs-use-401993/ it wasn't quite the post i was looking for, but iirc, he went back to the normal turbo rotors.

and you could get better spool with one of the newer compressor wheels, which are lighter, or a better exhaust, or suchlike

Cosmo_TT

iTrader: (11)

I tried this as well, after only being able to hit 12psi before the motor blowing up on 91 octane with one tuner and a very well known tuner telling me that he would only run 10psi on 91 octane with 9.4 rotors, I decided to go back to 8.5 so I can go with higher boost on pump gas, it did give me great results on low boost though, practically instant boost and 361-362whp at 12 psi but I know I can run 20psi on pump gas with 8.5cr, and that's over 450whp.
If I was keeping it strictly C16 I probably would have stayed with the 9.4cr

GrossPolluter

iTrader: (1)

Darn, did you ever run it at low boost before deciding 12psi? Which turbo are you running?

Cosmo_TT

iTrader: (11)

At that time it was just a BNR stage 4.5, now I am running an S467.
And no I never ran low boost, 12psi was gonna be my low boost, now 17psi is gonna be my low boost :-)

Maxthe7man

Excuse the crude picture and that I didnt draw out rotors instead of cylinders.
If you have a fuel air mixture with a maximum cylinder pressure limit before auto ignition occurs and have 2 engines that are turbocharged, which one will make more power, the low compression one or the high compression one?
The low compression engine will make more power because its effectively larger than the high compression engine, even though actual displacement stays the same. There is more volume at TDC which holds more air and fuel.
Also compression done by the cylinder alone does not have the benefit of an intercooler to remove heat due to the compression, while the turbo does have the benefit of intercooling after doing compression of the incoming charge..
There is a bottom limit to compression though, since you still need a certain amount to allow the engine to run off-boost.
In Japan a few shops have 7.8:1 rotors for turbo applications.

RotaryEvolution

iTrader: (30)

yes, a lower compression engine will sacrifice low end response for top end power and is safer for big numbers.

finding the happy median is the issue you run into with changing compression ratios. IMO the 9.0:1 engines are the perfect ideal compromise between both worlds for a street driven car, engines that never see less than 5k RPMs can get by with even less compression and have more potential power limit(8.5:1CR and lower).

the 9.4:1 engines just can't handle pump fuel and high boost, 15psi even with 500cc's of water/methanol still resulted in a blown engine. 13psi and just distilled water alone was the highest i have managed on pump fuel with the high compression engine on pump gas with a hybrid turbo(6 port n/a block with BNR stage 3, 332WHP @12.7psi). a more refined turbo could probably have gone further due to less back pressure retaining heat in the chambers. so a higher compression engine needs a bigger turbo and compromises spool to do a similar job as a lower compression ratio engine and has a less safe power limit.

i do have another customer running a T6 footprint BW S475(low backpressure) with E85 and 9.4:1 rotors without water injection(solely E85) so we will see how far that can go. according to my numbers it should be able to go slightly past 20psi on the ethanol alone which should push it upwards of 600whp still without sacrificing too much spool.

it's always a balancing act, a standard factory rotating assembly in a turbo engine is really only good for 8,000-8300 RPMs. balancing it will push the limit higher but then rev limiters will cause issues again as does the chewing of the bendable seals which are still the best compromise for turbo engines. in an ideal world an engine might be torn down after a season, but that rarely happens unless the owner is also the builder. ceramic seals eliminate chatter but they also are the most fragile when it comes to detonation abuse, causing catastrophic damage in the event of failure.


plenty of people have turbocharged higher compression engines, but you don't see too many putting up big numbers or for very long. E85, race fuel or lower boost are just the limiting factors.

i also have a customer with a 10:1 PP exhaust renesis i'm building for track/street use on pump gas with water injection but i'm going to limit it to 10psi. you can see the trending boost limits in relation to the fuel used..

hiroichi1515

iTrader: (17)

After reading this I may be changing my setup before I put it back together. I previously rebuilt my s4 n/a engine and threw on a hybrid T04B turbo that I built. Using the rtek 2.1, 820cc / 1000cc injectors and 600cc of pre turbo water injection I was able to run that turbo at 15 - 16 psi daily. I never had any issues whatsoever. During that time I still had the n/a exhaust diffusers in. I am not sure of the power it made because it was never taken to the dyno, but I was pleased with the results. I now have a sprint re, BNR 4.5 (60-1 wheel) turbo, turbo rotor housings, and a full 3in exhaust. My goal is to hit 400hp however, by reading this Im thinking that will be pushing it with these 9.4:1 rotors. I'm positive I won't try to pick up anymore power from this car after 400hp is reached, but I want to know if there are any other recommendations rather than lower compression rotors or ditching pump fuel.

Slides

You guys realize the difference in compression between even 8.5:1 and 9.4:1 is not that great, for matched peak internal compression 19psi on 8.5:1 gives 16psi on 9.4:1. You do however need less ignition timing and a little more fuel for cooling on the higher comp engine, there is less volume for the flame front to propagate through.

claiming they all fail below 1 bar with your 91 (ron+mon)/2 fuel is a bit silly.

There were Australian guys running turbo carbed rx4s with mild ported high comp 13bs (all mazda seals, nothing special) up to around 20PSI boost into 10s/9s over the quarter in the 90s on Australian 95RON fuel, with NO AI ie what you would see as 90-91 at the pump in US.

Less required ignition advance (people aim for this when designing heads/pistons on most motors to improve efficiency due to less negative pumping loss), a few % for fuel to damp minor increase in compression heat.

Slides

So far as fitting more charge mass in the chamber to make more power, yes at very high boost IF you can maintain decent IMP:EMP you will, but in vacuum/low/medium boost you just end up with more exhaust carry over with a weaker overlap effect due to less squish.

arghx

iTrader: (3)

With the higher compression rotors, you have to retard combustion to prevent knock. Funny thing about retarded combustion is that it heats up the exhaust, which can potentially spool the turbo better. It also reduces engine output, and you can only retard timing so far before you start getting misfiring or output just drops off a cliff.

Yukitama

Hm

Maxthe7man

Dave Vizards, How to build Horsepower volume 1 pages 35-38 covers it.
Although his books are based on piston motors, pretty much all of his findings in some way are universal, especially his intake manifold writings.
When I switched to 8.5 from 9.0 to one in the rew in my FC, there was a peak gain of about 28 rwhp , even with the 8.5 rotors being heavier.
My old dyno files are on a laptop that I borrowed to run the dyno, and I am not sure of its whereabouts after I got a new desktop to run the dyno, but my 8.5:1 engine made more power across the board on every wastegate spring than the 9:1 motor with the HKS T51s turbo with sweeps starting at 3000 rpm.
I eventually found my pump gas limit of 468 rwhp with 91 octane on 8.5:1 compared to 440 rwhp with the 9:1 version of the same motor. A turbo change to a HKS T51rkai then took me to 502 rwhp on 91 octane, with 8.5's. I would love to try that combo again with 7.8:1's, but I have moved onto a 20b with 9:1's and a HKS T62r. If the 20b was not fully assembled when I got it, it to would be stuffed with the lowest compression rotors I could get my hands on..

thewird

iTrader: (8)

I agree low compression is safer to push and make more power. But as devil's advocate, the RX-8 guys are pushing their 10:1's over 400rwhp. I have also personally tuned 2 turbo RX-8's in the mid to high 300 rwhp range running ~13 psi without issue. One has been running for 3-4 years and another was tuned last spring, both still running without issue. Assuming proper tuning, the only real issue with the RX-8's is the side seal failure from the ports in the wrong place. And yes, high compression spools turbo's quicker.

On another note though, I've been considering putting 8.5 compression rotors on my 20b so I could run more boost without AI. Maxthe7man, did the power come from being able to run more boost or the fact that you can run more timing?

thewird

GrossPolluter

iTrader: (1)

I want to do more research on e85 now. But I know it Would be a pain to fill up so often