RPMs and Turbos
 

I once heard from someone that if you turbo a RX-8 you can't go over a certain rpm or you'll blow up the engine, I doubt this is correct but it may have something to do with that person's tune. Do you have to get a multistage boost controller or some such thing?

I believe that it had something to do with when the ports open after that number the engine would suck in more air and cause the turbo to increase boost PSI and cause the engine to run extremely lean/rich (i forget).

Can someone explain this (or this logic) to me?

You can still rev to 9000 but the renesis makes best power when turboed in the 7000-7500 range . Going much over that does not gain a lot and it does stress the motor and cooling system capacity .

peak power at 7-8k rpm and as Brettus will still cause stress in other parts of the engine/cooling system

Where it makes peak power is largely a function of mass flow rate.

...no reason why you can't make peak power at 9,000 rpms if your turbo can shove enough air of sufficient density into the engine & you've got the fuel/spark to keep up with it.

So, in a word, no. This urban legend doubtless comes from running turbos that can't flow enough up top.

Sounds like an old wives tale. When a turbo kicks in, you'll have more air flowing, but your MAF/MAP sensor will see it and inject the proper amount of fuel for the measured air. Only way that doesn't happen is if your fuel management doesn't work or you don't have a proper tune.

^ha play nice guys!!! in a sense you both are right stock 8 will start to lose power at around 8500 or so give or take a few hundred however it you have a turbo that can continue to flow more in at 9k (hence a bigger turbo than say a stock greddy) you can easily make sufficient power at 9k and even higher

come on now, i already unintentionally filled the weekly drama requirement for the board in other places.

lol

thanks for clearing that up.

you will need a turbo with a big hotside to flow up to 9000 rpm.

Dude, MySQL is intelligent. Therefore, he is not agreeing with you.

Have a look at this: http://en.wikipedia.org/wiki/Mass_flow_rate

& this: http://www.mustangsandmore.com/ubb/D...orqueVsHP.html

Horsepower is the first derivative of torque. The more oxygen molecules and gasoline molecules you can combust *per unit time* yields more power. Therefore, you will get more power at higher RPMs if you have a salubrious A/F ratio, the spark to burn it, and a means to get rid of the combusted gas quickly enough.

With a correctly sized turbo, the Renesis will make peak horsepower at redline.

Also, the downside of short shifting (7500rpm) is that you drop below the fattest part of the Renesis power curve since the transmission is geared to put you back in the sweet spot with every shift.

So, that's not one but *two* downsides to a too-small turbo.

Given that a gt3071 turbo (think Satan's Hairdryer) will spool every bit as fast as the POS 18g GReddy turbo, will make power up top, and last much longer there is no reason to choose the 18g over the 3071r except up-front cost. Since you'll be replacing the 18g after 20k miles or less, you're still $$$ ahead to buy the gt3071 up front if you plan to keep the car for awhile.

we have nice big exhaust ports, but they are horribly shaped...

Firstly - show me this dyno with peak hp at 9000 . There have been about 6-7 kits come out that all peak well below 8000 . The only way you might do it is with a massive turbo that spools at 6-7000 rpm and it would be totally useless for the street . So your "correctly sized turbo" is possibly suitable for a drag car and totally irelevant to this conversation .

secondly - what is all this irrelevant stuff about Greddy vs 3071 ? That was not even part of the conversation either .

No. Significant modifications other than the "correct" turbo would be required to do this. I guess you could intentionally reduce output below 9000 RPM to achieve this goal if you really wanted to, but what would be the point in that?

You're assuming you know what the torque curve looks like without knowing what FI system we're talking about. It's quite possible to increase output substantially below 4000 RPM, in which case shifting early is more than acceptable. Also, only the 1-2 shift is even close to putting you from redline to the beginning of the "sweet spot" on an NA 8... the rest put you at much higher RPM for drivability reasons. It's geared quite horribly IMO, but that's another topic all together.


To sum up: in theory nothing is stopping one from running a turbo'd renesis all the way to 10,000 RPM. In practice, things like intake design, port shape, exhaust port restrictions, fuel delivery, and a whole host of other things make it impractical.

Dream20B, I am doing this for your benefit, not Brettus, since he does not seem particularly susceptible to reasoned logical/mathematical proofs.

Since there are people pushing 400WHP with the Renesis, it obviously *can* flow enough in/out. The rotary gurus agree there's a problem with the exhaust ports that might cause premature failure of the engine due to (asymmetric) thermal stress, but that problem will exist whether you shift at 7500 rpm or 9000 rpm.

...don't need to show you a dyno. This assertion can be proven mathematically (see links). A gt3071 will do the trick and spool lower/faster than the 18g because of its superior design (ball bearing vs journal bearing for starters).

(GReddy vs 3071 *is* salient since you need a properly sized turbo to not have power drop off above 7500 rpms).

http://www.turbobygarrett.com/turbob...o_tech103.html

http://www.turbobygarrett.com/turbob...30/GT3071R.htm

Really, this stuff is dead simple for anyone who got through high school physics.

It has already been proven in practice , no maths is necessary - end of story.

You may have gone to high school but they don't run classes in common sense and logical thinking which i'm sure you would have benefitted from ....

My brother ran into this with his 2.2 honda revving to 10k. had a t3 t4 on there, fell off past 7500, put a t66 on there was great up top, no bottom end though, grabbed a VNT (variable nozzel turbine) for sh*ts and giggles off a volvo turbo diesel truck ~55mm, and it made power all through the whole RPM range past 3500....until the turbine fried (diesil turbines are made for lower EGT's)......I was worried about this with our cars, you guys have reaffirmed it.....and so far the only company i know with VNT aftermarket is turbonetics and they want an ARM and a testical.......

Dear Brettus.

Are you sure you're not really danobre posting under a second screen name?

Regards,

mysql and mysql101

dear mysql ,
WTF ?
sincerely
Brettus

+1

:lol2:

^haha it may get nasty again!!!....for the above our exhaust ports are ok but I PERSONALLY feel they could use a good porting including the intake ports and will make is sound meaner also

on the exhaust side the problem is there is very little that can be done in comparison to past traditional porting. RG ported Charles R Hill's motor a while back and talked about it a bit. The ports are large, but poorly shaped. Its sort of an inherant property of side exhaust ports as they are designed, and with the water jackets in the way....

Forgive me im new to the renesis (not new to engineering), but from my understanding (I could have this backwards), with a lack of cams here, our actual, physical timing can be modified by porting the exhaust right? or am i wrong, correct me here. so if you put more material on the fore side of the port and shave off on the aft side, it should retard timing slightly enough to lower EGT.

so if you can retard the timing it should make it managable to not fry ports, or there is the alternative of tossing a little ethanol in there to lower combustion temps.

Maybe the OP is referring about our SDAIS and how opening the auxiliary ports changed the airflow.

That would indeed change the timing, but it's pretty labor-intensive.

Normally tuners override the stock ECU's timing either with a 'piggyback' ECU (not so good) or hack the ECU (very good, since you give it new targets to chase a tune to & thus gain the benefits of the stock ECU's 'continuous tuning' features).

Why are people snapping at each other over this? No point in getting bent out of shape over what is essentially a purely academic argument.