ganseg

iTrader: (5)

When I look at common HP curves, i assume i should shift at redline to straddle the peak, and not fall behind the dip just above 6,000 rpm. Of course i have been reading that rotors are not well enough balanced to be running that high. I also tried to see for myself how the car pulls on a Dynolicious graph.

Surprisingly it looks flat all thru the 2nd gear portion of my run. I don't remember what RPM it went down to when i shifted to second, but all the way up to the 8,000 rpm range, the G measurement stayed flat.

If true that means I will accellerate as well shifting at 8000. This is not what i would read from the dyno. Please help me to know how I am reading the dyno wrong. Thanks.

ganseg

iTrader: (5)

I should explain why it took 5 seconds to reach redline. I found the steepest long hill I could and did the pull there. I wanted it to make the pull long enough for the IPod app to distinguish changes in acceleration (or should I say Gs?).

Fencig

Ok, I'll take a stab at this...you have no pistons. Don't use the logic of a piston engine to plan shift points on a rotary. Running a piston engine to redline means you are entering a danger zone for the engine (and that it no longer produces power efficiently). The rotary in the RX-8 is actually very linear in power right up to the fuel cut off at about 9400RPM. This is because the cut off is there more to save your tranny than the engine itself.

Rotary engines are actually better "balanced" in the fact that they don't change motion direction like a piston engine does. If you think about it, it's easier to spin yourself around in a circle than to twist back and forth at high speed...it's same concept of motion. A rotary can spin very fast while a piston has to exert extra power to change directions, forcing a slow down.

If you'll look at some of the favorite performance products (access port mods) you will find that many actually increase the redline. They wouldn't do that if a rotary was not good at handling higher rpms.

elysium19

iTrader: (1)

Good 'stab' but this part isn't quite true. Above about 8500 rpm (at least in stock form) the renesis just makes more heat and not more power. For racing you have to think about total time spent at higher (power-producing) parts of the power band....but most people who want to raise the redline do it just because they think it's cool or probably a FEW who do it just so fuel doesnt cut off in the very rare racing scenario where it's better to just stay on the throttle than shift for a half second before a braking point, or something along those lines

ganseg

iTrader: (5)

This is the part i want to focus on. This year I plan to go to two tracks that allow for very high speeds. One, Brainerd, will allow me to get up to 120+ and go thru turn one without slowing and turn 2 at 115. The other track is Road America.

At Brainerd, that segment has a big impact on lap time. So, I would like to shift at points that maximize my acceleration and therefore the speed that i run that segment.

I have the same question on piston engines - their HP curves don't look that different. I drove a Spec Racer Ford at Brainerd. From the dyno run, there was no reason even to go to redline. The HP petered out where I could straddle the peak without hitting redline or hurting the engine - it didn't help to go any higher.

JinDesu

iTrader: (2)

You should send a PM to EricMeyer(s?), he talked a lot before in other threads about the exact thing you are asking. I believe he runs his 8 up to 8400rpms where the power peaks, and shifts at that point. He does a lot of track racing, so it seems like your right up his alley.

ganseg

iTrader: (5)

Yeah, i think that is the answer I will follow at the track for most laps. I need to adjust by 300 RPM because my tac is off. I'll have to shift at 8,500 indicated.

On the street, i think I will stop around 7,500 indicated. By that rpm, I think I am thru the opening of the last two ports, and the "dynamic effect" valve or whatever. If there is truely a balance issue, I assume it gets worse and worse as RPM rises.

Razz1

Max HP is at 8k sometimes less rev higher to compensate for the shift time to stay at peak power.

Kafka

Some highlights from Eric (Yes I stalk his posts all the time - he races rx8s and spend a professional level of time and money testing the stuff we argue about, even burn one of his...well that was an accident; Basically, its his opinion based on his experience, not his friend's aunt's relative's brother in law's friend's McTuner guy told him)

"Fast forward. The Mazda dry sump has a 12 gal/min flow rate. Modifications to the pump allow for 15 gal flow combined with opening up the galleys has solved this issue. He does this on our Renny race engines and those built for Pro Formula Mazda (spec engine with Motec engine mngt.). Note: This series is running the new 09 motors and currently a hard cut of 8,700 rpm I beleive. 04-08 motors had a 8,500 rpm limiter (so i tell everyone once again---don't drive your dumb engines at 9,000 thinking there is good power there----there is not)."

"Running these engines at 9,000 rpms makes the rotors wobble and upon tear down you can see witness marks on the irons (which I know many of you smart guys know on this forum). This leads to scored sealing surface(s) and reduced compression. There is no power up there."

"Personal note: Some people believe you need to run your car to 9,000 rpm or more. In reality the power drops off North of 8,700-8,800. Therefore, there is NO reason to ever rev your car in this range. In fact, you will find in some stock motors that the rotors will wobble and the tips near the corner seals will touch/ride/hit the irons. Stay away from 8,800+"

AFAIK,
Actual vs indicated tach reading differences

<5000RPM=100RPM (difference)
6000RPM=200RPM
7000RPM=300RPM
8000RPM=400RPM
9000RPM=almost 500RPM

So if stock RX8 redlines at "9000", and then rev limits at about 9300-9500 - really means 8500rpm actual...

ganseg

iTrader: (5)

Regarding the last line above, the engine knows the RPM more accurately than the tach displays. So, the cutoff is at 9,000 but the tach shows almost 500 over 9,000.

Dynos show the more accurate RPM. So if you see a HP peak of 8,200 then the tach would be indicating 8,600.

ganseg

iTrader: (5)

Regarding the flat dynolicious graph, can someone help me understand how to reconcile that to a dyno graph? It stays flat after the torque curve drops off and it stays flat before the HP peaks. Which would be telling me where to shift if the engine could rev with no problems (and if taking time to shift wasn't a factor - which it is in 0-60 times, 1/4 mile times but less so in my situation)?

9291150

Nobody seems to be getting that the only reason you'd want to run it to redline or cutoff so that the next shift would get you closer to peak power quicker. If you're shifting at only 8000rpm, then that's dropping you down into the 6000 range where where the renny is flat.

Kafka

AFAIK, if you shift at 9000rpm (shown at the tach), its actually 8500rpm.

The renny has a flat torque curve. If you shift at 9000rpm(tach) it will drop to 6500rpm+, and you won't feel the sudden drop of power.

As Eric said from his racing experience, repeated dynos, and Pro Formula Mazda cutoff line setting, you should not go over 9000rpm (which is 9500rpm in the tach) for the rensis only generate more heat and sound and no real power while it may damage the rotors if repeated for extended period of time...

9291150

^ Eric = one man's opinion. Again, I'm only saying if you're racing you want to stay as close to the 8200 peak as possble, hence the importance of your shift points. Maybe the changes he's made to his engines flattens the curve a bit, but in my experience the only way I avoid dropping behind (more than an 8 usually would) is to not let the engine fall under the 7's.

Kafka

Well I quote Eric cause its not only his opinion but the other professional he deal with in racing as well. I went to see a Pro Formula Mazda racing as well, and I met one of the drivers a few times and talked about it too. Eric is definitely not just someone's mom's aunt's coworker who is also a weekend street racer we heard on the forum.

Moreover, 8200rpm will be shown as 8600rpm in the tach of rx8....so yeah you're right in this perspective. =)

ganseg

iTrader: (5)

Thankfully the 4-5 shift only drops 1500, so it is easier to stay above the low 6000 (actual) where the two extra ports open. But I only make that shift once at Brainerd. (Three times at Road America.) The 3-4 shift is my most common and the rpm drops over 2000 rpm (8500 actual to where the extra two ports are opening).

Then I see racing beats software - when they port it it even moves the HP peak up.

YaXMaNGTO

Dude. Throw that dynoliscious graph in the trash UNLESS you can honestly say the "hill" you were on has a PERFECTLY consisent slope.

Any variation in the slope the hill will add tons of error in the readings. Unless you found a magical hill, the only time you're going to an accurate measurement is on a flat road.

Find as flat a road as you can and re-test and post the graph. I'm not against "G-techs" at all, but you sort of need to understand how accelerometer based instrumentation works before you use it analyze anything.

kersh4w

iTrader: (3)

welcome back yax, and again we putting the newbs where they should be.

wtf are you doing with an iphone ap? you honestly think that is accurate enough to give you usable data?


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YaXMaNGTO

LMAO What up kersh

threw your iphone... ON THE GREYOUND

04Green

iTrader: (1)

Followed this thread, got to thinking....

We have gear rations, final drive ratios and torque. What if you took the torque plot from an RPM dyno (some are mph), and calculated it through the gear ratios looking for torque at the rear drive shaft, as a function of RPM and gear? Then pick a shift point that gets you the best balance between shift points at the same dif output RPM?

The table looks like



It looks like redline has more torque in almost every case than anyplace in the next higher gear until you get to 3rd gear then the shift point is about 8,750 (at like 90 mph).

Now, the posts from smart people, say to stay below 9,000, or even 8,500. Given that, for performance, the shift point would be the max RPM you feel is safe. Since the torque curve is flat, you want to hold on to the gear multiplication as long as you can, and the higher revolutions make that possible.

Right?

Now, to get a decent tach, or calibrate the one I have to my AP for a real 8,500 RPM shift point.

DocBeech

iTrader: (1)

I am also interested in a tach recalibration. Not to mention I need to tune my speedo, it starts to show change at 50mph being off by 1mph and climbs substantially from there.

Beodude

iTrader: (3)

While that is true, the weight difference that has been noted in the rotors themselves is pretty big. There is about a 100 gram spread from what I've read. That is a pretty good difference. The front bearing wears down to the copper on cars that spent a lot of time close to, or over 9,000.

Engine design has nothing to do with the physics of acceleration though. Whether it's a piston or rotarty, no matter what RPM you run, it just depends on the power curve.