krazyjl

sup all, i'm new to the rx7 world.. been lounging mostly at supraforums.com, so i decided to come and take a look at rx-7's, since they seem to be the other good high-end jap sports car. (3000gt's are a little too fat for my tastes, 300zx too crude?)

anyway, so my question is why do rx7 engines run significantly lower boost? I've seen talk about mostly 12-15psi or so, whereas supras run up to 20 on pump gas, 30 on race gas... for street cars

does it have something to do with the design of the rotary engine?

Kurgan

That's a pretty broad generalization. Most people will run lower boost to preserve their engine to some extent. The RX7 doesn't need to run wild boost numbers to be a fast street car... but they can.

For the most part, an rx7 with 450 rwhp is gonna be just as fast (straight line here fellas) as a MKIV with 600+ rwhp. The power to weight ratio is better.

There is a myriad of possible responses to this...

HWO

as a general answer i think piston owners cant destroy there motor with a single ping (detonation) and therefore are prepared to run much closer to the wind than rotary owners are.

krazyjl

a single detonation will kill a rotary???

peejay

A turbo rotary, yes. If you're lucky, you get three or four pings, and if you're really lucky, you let off after the first couple and before the fatal one.

My N/A detonated dozens of times and it still lives strong. It sounds absolutely horrible, the car kicks back and it makes a noise that's a cross between a belt chirp and a bone snapping.

HWO

one single ping is enough to either
a) brake an apex seal, or
b)crack an end plate

krazyjl

ahhh i see

so it seems the apex seal is the achilles heel of the engine

HWO

Nope, BAD tuning and incorrect modifications is.

the stock apex seals will handle 1200 odd HP if you dont ping.

Maxthe7man

Wierd, someone posted before that it sounds like popcorn popping, which is pretty close to what I heard one day, almost like a loose piece of plastic tapping the firewall, I let off, the engine survived, funny thing is it happened at only 9 psi of boost, and it was about 5 degrees ambient air when it did it, my guess is that with the cold air, charge cooling wasn't the problem, air density and not enough fuel for it was, I always run premium, I think the turbo moves alot more lb's of air per minute with a cold charge than a hot one.. I find that wiht colder air outside I have to turn the s-afc mixtures up to compensate, or I start to run lean, but as the outside air gets warmer, I can turn it down again, so much for intercooling eh? I think a J&S knock sensor is in my future for sure
Anyone try a MSD knock-alert on a rotary??
Max

HWO

time for a quick update - Rice Racing's 13BPPT is currently making 830HP, not bad for a 2 rotor

I am pretty sure you'll find the puerto rican guys are making similar HP, the difference is, they know how to make that kinda HP, and the rest of people DONT.

Node

iTrader: (1)

puerto ricans are crazy bastards when it comes to import dragracing.

peejay

Why haven't we heard of a 2-rotor runner over 1000hp period?

Answer... they're freakin' tiny engines and they don't rev as high as a full-out race boinger. Unless *you* figure out how to make a 13B live at 17k. Torque or RPM, you need to make more of either or both to make more HP... you make more torque with more displacement or higher combustion pressures, and you can't get more displacement out of a 13B.

nyt

iTrader: (1)

wow you seem to know a lot about an engine you didnt design and havent seen.

crispeed

Is your opinion based on real hard facts or just rumors!
I happen to know a few people that are making well over 700RWHP and running with stock seals that are not rebuilding their motors after 3 runs. It just so happens that a very good friend of mine have over 60 runs on his motor and he's running at 40+psi. One of the top rotary runners in the US is making well over 790RWHP and he's not rebuilding his motor after 3 runs!
As mentioned before by HWO(Shane) correct tuning is the key to motor longevity may it be piston or rotary!

crispeed
87TII
9.20@150mph 1/4
5.90@120mph 1/8
1.31 60ft
2600lbs
651RWHP/520RWTQ

Maxthe7man

I think the eventual limiting factors would be the stationary gears, and the ability keep the housings clamped tightly, to stop them from shifting and what not, the eccentric shaft is pretty beefy and being that it does need see the entire load from from the rotors mostly just the thrust load, some of the load is carryied by the gears, it can handle alot of power. Eventually though the rotor faces do collapse from excessive boost as illustrated by racing beat, but psi alone is not what makes power, so as rotary port technology gets better along with better turbocharging tech, we should see rotary power increase with reduced boost levels as we have seen thanks to people brave enough to slap t04e's and what not on the sides of the their motors..
I really don't think people are rebuilding their motors every 3 races, there simply are not enough core motors and parts in North America to support such numbers, such numbers would have driven the cost of parts through the roof..Max

crispeed

Actually Max!
The E-shaft is much weaker than you think and is presently a problem for high HP rotaries. I've seen one break and many bent in the middle. Some people have figured out how to make a center bearing work and that should help. As for the rotor faces collapsing, that only happens when detonation/pre-ignition have occured.
I've seen more stock motors with dented rotors than I've seen in high hp motors. It's all about the tuning!

crispeed
87TII
9.20@150mph

nyt

iTrader: (1)

a t88 running 17psi is innefficient. if thats all youre aiming for you shoulda got a much smaller turbo

Maxthe7man

Interesting, it looks like a substanial piece considering it doesn not reall hold any of the static load of the rotors, or deal with the directional change of the rotors 100%, I should throw my RB book out the window I guess, they say that e-shaft failure is rare, except in the FC3s motors when pushed to the limit, apprently that oil cooling passage with the thermowax valve/stat is the weak link, out of interest what e-shaft are you using?
I can't really argue with you on this one, your sig says it all, but some of the pics I have seen of collapsed rotors had the apex seals intact, some unfortunate souls found the combustion pressure limit of a rotor face, and they simply collapsed, this is not intrinsic though to rotaries, this also happens in piston engine when the dome is simply not thick enough to with stand the force applied to out, even with perfect combustion, both rotors showed the same destruction in one motor, apparently at 30 psi of boost, but this could also depend on exactly what motor it was, and what generation, the FD stuff looks beefier than the FC stuff...Max

HWO

yes that is exactly the point, the apex seals didn't break, the end plates didn't crack, so the next weakest link - the rotor's face gave in.

Where is Peter in a time like this, he'd come up with more data than you can poke a stick at about how much HP each part of the motor can handle.

His 13BPPT making 830HP is only at mild boost and not running on super juice yet. he seems very confident it'll make over 1000HP when he gets it sorted.

crispeed

The Racing Beat book is good source for begginers and NA stuff.The best book actually written about rotaries was "How To Modify And Race Your Rx-7". That book features info from the gurus like Jim Downing and the Rotary God himself Rick Engman!
When it comes to high hp turbos you're on your own because no one has written a book mainly becasue most of the stuff is still new and everyday leads to a new discovery.
All the shafts are about the same with the early ones being the strongest. The same goes for the rotors. The 89 + rotors are the weakest mainly because of casting techniques for lightenning purposes from the factory.High boost is not responsible for dented rotors! DETONATION and PRE-IGNITION are. Like I said before I've seen more stock factory motors running stock boost with dented rotors.
The same is true for cracked end housings. There are many instances where people have cracked housings but not apex seals.

crispeed

crispeed

That's your problem!

To get those high hp numbers first you need an ECU that can read above 17.5psi of boost!

crispeed
87TII
9.20@150mph

Maxthe7man

Yah, I have that book to, not much on turbocharging as mentioned, the rotor section seems to be mainly concerned about lightening them for n/a hp applications than picking the best one for stomping 20+psi of boost through the engine..
Even the Turbo books, mention very little about rotary's and turbocharging, although I did find some very good SAE articles on it at the Library, the retail bookstore stuff, as thick and popular as they may be, are often not much more than a base primer on the aspects of turbocharging and basic combustion theory, its a good starting point, but one often finds themselves in literary pergatory when seeking hardcore info..
ON another note, do you see any increase in rotor blowby with mondo high boost levels, or do the side seal seem to be able to control it well enough, I had heard about some corner seal problems, but the side seals never seem to get mentioned...Max