rx7london

ive ot a single turbo 93 rx7 just wondered what a safe limit to rev i to as it dont make full boost to 5000rpm so need to rev it high
thanks
steve

88fc3sw/HX83

I'd say 9k, but don't hold it there too long IMHO!

Smilodon

There are several parameters that determine how high you can rev an engine. Generally, the strictly rotating portions of the engine are not the limiting factors. It is the portions of the engine that engage in movements other than rotational that are the limiting factor

The major moving parts of a rotary:
Rotors
Seals
Rotor shaft

Since all three of these parts exhibit oscillation in addition to or instead of rotation, what can be done to better enable them to rotate at a higher speed?

For starters, the rotor shaft needs to be free from elastic or plastic deformation at whatever speed range you are looking at. In other words, it needs to be stiff and rigid, and dynamically balanced

The rotors, since they are the major mass being flung around inside the motor, need to be stiff, rigid, and light. They also need to be dynamically balanced so that they not only have a center of mass that is in their physical center, (static balance) but also exhibit no vibration when rotated about their physical center. (dynamic balance)

Thirdly, the only spring-controlled oscillating portion of the motor, the apex seals. Since the apex seals are pushed out to seal the interface between the rotor and the case by spring pressure and centrifugal force, and are pushed away from the case by the "ski jump" portions of the case that are between the exhaust/intake and compression/ignition spaces, it follows that the apex seals need to be as light (and long-wearing) and tough as possible.
The stock cast-iron seals are long-wearing, but not light. Graphite seals are very light, but not all that tough or long-lasting. There are some that make steel seals, ceramic seals, and one type in particular was tough, long-lasting and light, according to the maker, but it's been a while so I don't know which it was. I have heard that ceramics are light and long-lasting but fragile.

The ideal rotor shaft would be infinitely rigid and have no mass. The ideal rotor would have infinite dimensional stability and no mass. The ideal apex seal would have infinite dimensional stability and no mass.

So, rationally, the stiffest, strongest, best-balanced rotor shaft, with the lightest rotor you can find, dynamically balanced, and lightest apex seals all of exactly the same mass with stiffer than stock springs will give you high-revving pleasure.

To keep the peak stresses on the motor the same for any given RPM, you have to use the square of the RPM ratio increase to determine what you need.

If you double your RPM, you need all the oscillating pieces to be 1/4 their mass to leave the stresses on the motor at the same level.

If you increse the RPM from 8000 to 12,750 (as a 24-hour Mazda endurance racing motor in IMSA years ago) you will either have about 2.54 times the stress, or you will reduce the mass to 1/2.54 (39 percent) of what it was, or some compromise in between.

Supposing, for example, that you reduced the rotor mass from, say, 4500 grams to 3750 grams. That is a ratio of 1.2. So, if you take the square root of 1.2, then you have the ratio of engine speed increase that you can enjoy without putting higher rotor-relates stresses on the engine. √1.2= 1.09544 So, from 8000 RPM you could go to 8763 RPM.

Of course, these aren't the only stresses the engine undergoes, but the ones most dramatically affected by RPM increase.

Lightweight rotor design is the one area the rotary really needs improvement, and where it is the most sorely lagging.

chohakai

iTrader: (1)

very good break down above...

Here's my take based on experience:

running FD like oil pressure (100 psi+), an average 13B can rev to 10K without any problems, above 11K you could run into issues with sized bearings depending on lubrications, contact of rotors with rotor housings... etc...

having said that, I've known well built/lightened/race clearanced bridgeports running upto 11.5K all day.

Fred

Smilodon

Thanks. Hi revs rocque. And to know I can run a 13b at 10,000 stock is a very nice thing to know. Do the GSL-SEs have a rev limiter? Or just that beeeeeep?

diabolical1

iTrader: (3)

i'm not trying to take away anything from what Smilodon said, but it seems you also forgot to mention another factor that goes into being able to achieve RPM ... the type, size and quality of the PORTS.

also, it ceases to be simply just a good idea and becomes absolutely imperative that your rotating assembly be very well balanced.

now, the 10,000 RPM issue is not something that i'm going to touch because i wouldn't recommend it, for the simple fact that you're going to have conflicting issues between apex seal type and your turbo.

not trying to be a tool, but which stock 13B can run at 10,000 RPM? please tell.

the SE has an over-rev buzzer, not a limiter.

j9fd3s

iTrader: (3)

gsl-se's have a beep. but with the 12lbs rotors, 3mm steel seals, unhardened gears and not fd oil pressure, its not a good idea to rev it past 7500. not that the stock motor is even gonna make power that high anyways

Smilodon

Afirmnative. Er affirmative. What are the lightest rotors you have encountered? The record for my researching so far is about 3750 grams=about 8.25 lbs. Quite a dramatic difference.

So, for just the rotors, that would yield about 9650 rpm for strictly rotor-related stress equivalent to 12 lbs worth of rotor at 8000 rpm.

Of course, supposing you wanted to go to 12,000 rpm with an 8.25 lb rotor, then there would be about 1.55 times as much rotor-related stress on the motor than if you were to use the stock rotor at 8000 rpm.

Which, means, it isn't beyond the realm of reasonable possibility. Of course, by porting it carefully and putting on a turbo that will provide a nice boost rise at higher RPM, one might be able to have a powerful motor with a wide powerband AND an idle below 3000 rpm. Or below 2000. Or around 1000 woot!!!

chairchild

And just in case anyone's curious - theres a small potentiometer on the back of the rev counters, which deal with what RPM the overrev buzzers come into play

slpin

iTrader: (9)

as high as you dare to....

Smilodon

Why not dump that turbo and get one that is sized to provide boost from far lower RPM? Sounds to me like you have too much turbo for your motor.

R.P.M.

iTrader: (3)

My old 12A I had in my 1st gen would wrap the tach needle waaaay past 8000rpm. The needle would stop at 8 but it would just keep winding up. In second gear I would hit 120km/h.

Sooo many fun memeories

2rotorkid

http://images.realride.com/cgi-bin/i...mv&img=&tt=avi

dont know if this helps or if it is a repost but this seems like a good place to put it

j9fd3s

iTrader: (3)

dont forget that in the 80's they did run the heavy rotors up to 10k rpms.....

peejay

RPM = Ruins People's Motors.

Don't worry about extending your RPM range, worry about doing more with what you already have. Winding it out to something stupid like 10k is pointless, since you claim you don't make full boost until 5k, meaning you lose the race as soon as it starts....

...unless you are dyno racing, in which case Enzyte is a lot cheaper and works about as well.