Glyphon

well, what about twin setup on the 8...one turbo for each rotor? more of a hypothetical question than anything else.

zoom44

how would you split the exhaust? i was thinking that but the siamese center is annoying. much easier on a V piston engine when they are already completly seperate.

tuj

Wrong. It most certainly matters where the compression occurs at. Go look at any turbo drag car and tell me it doesn't have low-compression pistons/rotors. The whole point of low-compression rotors/pistons is that they allow you to run more boost without detonating. More air means more power. More boost and less compression will always make more power than more compression and less boost.

Remember, the FI compression is shoving more air IN the engine, while the static compression of the engine determines how efficiently the air the engine has is used.

zoom44

yes perhaps i mis stated that. it was not my intention to say they were the same thiong. only that in terms of knock it doesnt matter where they are and in terms of making power highere static pressure does not mean you cant make the same amount of power as a lower static pressure

this statement
is what i disagree with. i believe we could make the same power with the normal compression on this car and a little less boost that we could by lowering the compression and getting higher boost. its just a matter of tuning.

tuj

Sorry dude, but that's not going to happen. Let's pretend that the max boost on the Renesis is 14.7 psi or 1 bar. So that means the dynamic compression is roughly 2 bar (the total pressure of the air) * 10 (the 10:1 static CR) = 20 bar. And if I have a 13B-rew (9:1 static CR) with the same amount of boost, I get 18 bar (pretending the rew has zero overlap). Now if Renesis is maxed out when it hits a 20 bar effective compression, the 13B-rew can still take more boost. Meaning we can now boost to 2.2 bar.

Now following that logic, what you've said makes sense; the 13B-rew has more air in our scenario, but the Renesis is more efficient. However, the faulty assumption is that there is one effective compression ratio at which detonation occurs. This isn't true.

Why? Because the combustion process isn't as perfect as we'd like to pretend it is. Hot-spots on the rotor face or housing or the spark plug electrodes will cause pre-ignition before the charge auto-ignites because of pressure.

So why does static CR matter? Because if you increase static CR, you are decreasing the surface area of the combustion chamber, therefore more of the charge is concentrated along the surfaces. These surfaces are where the hotspots are, and the mixture will pre-ignite if the concentration of the charge in the vicinity is high enough.

What this effectively means is that if our Renesis engine was at the limit with an effective CR of 20 bar, our 13B-rew has a limit higher than that since it has less static compression (of course we are ignoring that the rew has overlap, but that's ok).

Basically what this means is that the relationship between the static compression ratio and the maximum effective compression ratio is nonlinear.

zoom44

i get what your saying.

but cant that be offset with higher octane and cooling?

secondly what if we stay in the same engine. rene vs rene but lower compression rotors in one?

tuj

Ways to prevent detonation are:

• decrease timing advance (loses power)
• lower compression ratio
• cool the surfaces of the housings and rotors more (loses power due to less themal efficiency)
• higher octane (burns slower, more detonation resistant)
• water injection (cools charge, internals, stratifies charge)
• fuel additives (tolune, increase detonation resistance)
• alternative fuel (methanol, you'd be nuts)
• run less boost

Water injection could work well, but its kind of a pain. Higher octane (still pump) probably won't net much. Lower compression rotors are a great idea in the Renesis because the zero overlap design is actually beneficial to FI.

The scary thing about FI on a rotary is that one big ping can mean the end of your engine in a hurry. The same can be true for a piston engine, but its less likely.

Oh yeah, the other thing I forgot to mention is that the relationship between static CR and hp is basically diminishing, meaning that as you step up in CR, you get less and less gains. But for boost, the relationship to hp is roughly linear. 1 bar of boost basically means twice the hp.

The good news is that going to say a 9:1 rotor would probably only loose about 7 hp, which you can more than make back in the increased boost.

Japan8

Temporarily back from a very long hospital stay (got a weekend pass)...

I seem to remember Charles Hill saying that the Renesis is actually quite resilent to abuse (including detonation). He put his through quite a bit as he worked on nitro on his 8...

RG has talked the topic of CR and FI to death. Run a search. The higher static CR will give you a more drivable car as you'll have better off-boost response. Ultimate power limits will probably got to the low static CR car, but aren't most of us driving our cars daily on the street?

mikeschaefer

That's what I meant by equation. If that's the true relationship then I can understand why low cr + high boost is the better option. Thanks!

neit_jnf

pressure and temperature inside the combustion chamber is what affects power output and the probability of knock

The same levels of in-chamber pressure and temperature can be obtained with lower static compression and higher boost as with higher static compression and lower boost.

That said, the lower compression motor will take a higher "safe" maximum boost level than the high compression one. This safe level is determined by the in-chamber temperature and pressure levels that will create maximum power without detonating. This level will be reached at a lower boost with the higher compression motor.

The most important thing here is that if the pressure and temperature inside the chamber is the same it will produce the same power regardless of how it got to that level!! The advange of the higher compression motor is that, given the same airflow capabilities of the turbo and the motor, it will spool faster and have a better power/tq curve.

Most people mistake higher boost with higher power. Something like "If I can boost 30 psi at 8:1 CR but only 20 psi at 10:1, therefore lower compression is best yo!"

Just as an example, at stock 10 psi my FD (9:1 CR) dynoes 215~220whp. A Greddy turbo Renesis at 7 psi dynoes what? 240whp? More power at lower boost? Hmm...

Another example, my FD dynoed 315whp at 13.5 psi all the way to redline. The Interceptor EMS Greddy RX-8 got nearly 300 whp at 12.5 psi but choking down to 5.5 psi at redline!! Similar power at lower boost again! Of course this also has to do with the different intake and exhaust porting but you get the idea.



On a separate note about the sequential twin turbo system on the FD. Yes it is very complicated and hard to troubleshoot, but it's the best of both worlds! What single turbo (or non-sequential conversion) can give you positive boost pressure at 1600 rpm and full boost at 2600~2800 rpm without choking in the high end? Using the greddy RX-8 as an example again, it does spool really quickly but can't produce more than 6psi of boost at the high end. I really love the way the sequential system works.

DreamWarrior

I'd love to see the flaws of a sequential system worked out and perfected for this engine. I think it'd be a beast of a system if it could both be made to fit and "debugged" until its reliable. I wonder if new technology would allow that?

neit_jnf

I believe Mazda already worked it out. For the 99 refresh the sequential system got revamped with a much better vaccuum (sp?) line "black box" instead of the dreaded rats nest we have and more efficient, faster spooling turbos.

Cost is probably the ultimate culprit as a set of twins and related components cost the same or even more than the engine itself...

rotarygod

neit is correct about pressure and temperature.

tuj

You have ignored the fact that the 13B-rew has overlap, therefore it has a lower effective compression. If the 13b-rew had no overlap, in theory it would produce higher cylinder pressures, resulting in more power than the Renesis (assuming the other new design factors in the Renesis, like seals, etc are negligible power contributors).

The math:
((10/14.7)+1)*9 = 15.12
((7/14.7)+1)*10 = 14.76

I'm sorry, but even if you ignore overlap, this statement isn't true IMHO:

Um, I don't think those people are mistaken. The relationship between boost and hp increases FASTER than the relationship between cr and hp. If the numbers really were as you say, the one with more boost will make more power. And there isn't an absolute effective compression ratio that will always cause boost. Surface area in the combustion chamber is crucial to detonation resistance.

To summarize: you're right that effective compression ratio (aka internal pressure @precombustion) determines power output, but you're wrong that effective compression ratio determines detonation (try and see if you can achieve the same ECR on an 11:1 as on a 8:1).