jd to rescue

iTrader: (3)

Mmmmm. Not to disagree, as I always thought there was a difference between pre-ignition and detonation, but can you give an example of where excessive temps would occur in the intake mixture? Are we talking, for example, about temperature increases caused by extreme compression?

Ehron

iTrader: (1)

Compressing a substance causes it to heat. As the mixture is compressed in the combustion chamber it heats up. If the temperature of the compressed mixture reaches it's flash point it can ignite with no outside source...

RotaryEvolution

iTrader: (30)

nope, detonation is when combustion acts prematurely before the power stroke and basically tries to force the engine backwards which effectively sounds like a bag of popcorn in the microwave in rotaries to a sledge hammer on a block in piston engines.

pre-ignition and detonation are almost always in direct correlation with one another in a rotary engine but not often are they used in the correct context.

Kento

And when do you think pre-ignition occurs? They are both premature ignition events...

maxcooper

My understanding is:
- pre-ignition: burning starts before the spark
- detonation: an "explosion" in the combustion chamber, unlike the normal, slower burn that starts at the spark plug(s) and propagates out in a controlled fashion

Detonation can happen after the ignition spark event.

-Max

Kento

Correct. Because pre-ignition is caused by an outside source other than the plug, it usually still has a flame front as the mixture burns. Detonation is just that; a uncontrolled spontaneous ignition of the mixture due to excessive combustion chamber temps caused by high intake temps/high compression of the mixture.
True, although it generally occurs before the spark event because of the compression stroke causing the spontaneous ignition.

RotaryEvolution

iTrader: (30)

you're contradicting yourself, reread what you posted last.

Kento

Contradiction where?

Gorilla RE

iTrader: (1)

Pre-ignition as the name states: occurs from A POINT in the combustion chamber during compression stroke that is so hot it creates its own ignition and causes the air fuel mixture to ignite BEFORE the plug fires.

Detonation on the other hand can occur any time before, after and during the compression cycle. It is caused when excessive pressure and or heat are present in the combustion chamber and the air fuel mixture ignites. When detonation occurs they have multiple "mini explosions" if you will that collide with one another in a very violent and destructive manner.
~Jonathan

adictd2b00st

methanol plus pump gas for the win :P its done me well!

Kento

"Before" the compression cycle? Highly unlikely, as the intake cycle more often than not cools the combustion chamber; if your intake temps are higher than the combustion chamber temps, you've got other more important problems to worry about.
"During" the compression cycle is what we're talking about here, by far the most common area of occurrence.
"After" the compression cycle is another highly unlikely occurrence, as the degree of crank/eccentric shaft rotation before the ignition event is pretty darn small-- but it can happen. "Contradiction"? Hardly.

Wargasm

Just thinking out loud here... because I honestly don't know the answer to the OP's question...

I wonder... What was the commonly-accepted "max safe on pump" before the 1993 RX-7 started getting modded? Back in say, 1991?

The reason that I ask myself this is that the FD MAP sensor stops being accurate at about 17 PSI.

Step 1:
Bunch of tuners start modding the newly-released FD by cranking up boost and stuff...

Step 2:
Around 16-17 PSI, the MAP maxes out and it goes BOOM.

Step 3:
Post on internet (the old email 'big list' probably) says, "I was running 15+ PSI and I blew up!"

Step 4:
Repeat x 20 people (and don't forget your friend's brother's mom's boyfriend that you heard about running 20 PSI who blew up)

Keep in mind that anyone running over 15 PSI successfully back when the FD was new was probably competitively racing (like for money!) it and was most likely dealing with a standalone ECU, race fuel, custom everything. You think they're going to chime in and say, "oh here's how to run X PSI" on some internet mailing list?

Result:
15-17 PSI becomes the commonly used "Max" value.

I guess what I'm trying to say is that

Everyone knows you can run 10 PSI because that's stock.

Most people agree 12 is safe because a LOT of people run that, and the risk is low because you're only +2 over stock (hard to screw that up!).

Many people say 15 is safe because a pretty fair number of us are around 15 with good results, but the risks at 15 are definitely starting to increase, particularly with single turbo installs starting to become more common at 15+ PSI. (who gets a single to run sub-15??).

20 PSI? Hmm how many people here run that on pump with/without WI or AI? Scared huh?

Bottom line is that the farther you go from stock, the more failures you get. I don't think it's linear either.

10->12 is pretty darn foolproof.
10->15 requires tuning, planning, research, etc. to pull it off
10->15 on a single turbo requires MORE planning, more research... starting to be a lotta variables now!
10->20 - you better be a pretty good tuner and know how to build it RIGHT the first time.. 0 room for mistakes.


Sorry for ramblin.

I guess I'm NOT trying to give anyone a hard answer here. I go with the flow like most other people here. We have a lot of results that worked out well at 15-17 PSI with some careful planning, and above that point, we start having a lot of examples of bad results. Therefore, I stick with the rest of the people and say "15" or "17" or whatever.

To more directly answer the original poster's question... I think that heat and "pushing the envelope" factor into the 15 PSI pump limit heavily. The heat part of the equation can be partially solved by the right tuning, enough fuel, water or alc injection. The pushing the envelope factor is something that takes time. The bleeding edge folks find the edge for us by constantly going near it (or going over the edge and learning an expensive lesson).

Gorilla RE

iTrader: (1)

COMMON OR NOT IT IS WHAT IT IS.... I GAVE A DEFFINTION OF DETONATION NOT WHAT IS MOST COMMON. ALL THE THINGS I STATED CAN AND DO HAPPEN. NOW I DEFFENATLY HAVE TO AGREE THAT SOME ARE MORE LIKELY THAN OTHERS.
~J

BLitzed33

or the trailing plug acts like a glow plug because the heat range is too hot with too much boost, haha

BDC

If that were the case, the engine would begin to randomly and intermittently misfire as it heated up over time while being loaded up over and over. Those results would be repeatable.

B

BDC

I've been reading this thread with some enthusiasm. I'm glad to see this subject coming up. It's I feel a very important subject as "the standard" on what's "tuned" on pump fuel is set way too high. It's being done too aggressively, in my opinion. I've been saying this over and over and over again for I don't know how long now -- It's all in the fuel. Every bit of this problem, when it comes to the sheer number of blown motors that create stacks of scarred rotor housings and obliterated rotors not only in my workspace but also in other RE shops abroad, is caused by an overall imbalance in how we're "engineering" our engines to run. By this I mean how we're looking at the specifics; the categorical aspects of modifying our RE's (intercooler, fuel system components, modifications done to the engine, turbocharging, etc.). One important component we're not looking at rightly is the fuel we're using. Basically, we're expecting to create race-like horsepower out of a fuel that's designed for only street-like performance, at best. We're pushing pump fuels way too hard. It's no wonder these things blow up all the time. While it's certainly possible to hit 400rwhp with pump fuel, since when has it ever been properly established that it's reliable? This problem evidences itself during the compression stroke -- an inferior fuel like pump fuel, with its low auto-ignition temperature, will suddenly auto-ignite due to the heat produced in this stroke and create pre-ignition, which is largely the cause of our engines blowing up. Taking spark advance out does not solve or sidestep this issue since it's not a problem with when the spark plug fires; it's a problem with the inferiority of the fuel being used in the first place. Ever notice how race gas guys making 500+hp are able to run tons of timing (18*+) with barely any split at all and do it reliably? Not only is there an issue of pre-ignition during compression, there's also a potential problem of detonation post ignition-event that could occur due to the over-rich condition we're purposefully creating -- running in the 10's:1 on a pump fuel, that's highly volatile, that we're using to hack (yes, hack) our way into making more power by washing the chambers to death in it.

Btw, "boost" is an ambigious term. It means nothing apart from arguably the somewhat similarity in IAT's produced at the compressor outlet. Otherwise, "boost" is only meaningful when it's followed by the type of turbo that's producing it. Boost does not equal standardized power output of the engine. Load would probably be a better way of characterizing this, in my opinion.

B

BDC

Here's what I'm talking about -- here's what is possible when the fuel aspect of our engineered powerplant is addressed (from two separate runs):





B

Boostn7

Funny how many continue to set 15psi as the limit of pump gas (93oct). Yet, I hear of too many blown motors @ 15psi or less!!!!!!
With a bad tune you will break a seal @ 17psi or 10psi !!!!

Due to the rotary design of long narrow chambers with a high surface to volume ratio heat is the biggest issue.
With all the proper upgrades there's no reason why a properly tuned RX7 couldn't run higher then 15psi !!!

I have run 20psi on 9.0:1 rotors with 93 octane with a rich 10.8 A/F tune. The extra fuel actually loweres EGTs. We're talking 3-4-5th pull to 160+mph.
Let's see, 48xrwh on Sunoco 94 @ 19psi.
With 8.5:1 rotors (turboII) I ran 20- 22psi on Sunoco 94.

When I tune customers cars, I set max pump boost @ 18psi at the same 10.8A/F.

As long as you monitor your A/Fs and EGTs under load and lift whenever your EGTs start climbing from extended load you should not blow a seal !!!!!

BDC: do you need methanol to make a 400rwh motor reliable ?
Some of your statements make no sense.....
Removing spark advance does help but you will reach threshold before EGTs start climbing.
Over-rich conditions in the 10's will not break seals, but it will help maintain combustion pressure under control.
Sorry, but you're the one hacking your way to making more power and washing away any lubrication for all your seals.

Since you're relying on the AI for the added 10psi what failsafes do you have to avoid a blown motor?

Alcohol/Methanol Injection is nothing new...GN guys were big users and love it 'till systems started to fail.
Lots of Evo guys are running them and blowing motors because of failures.
Sounds like a time-bomb: blocked jets- complete or partial, system pressure drop, flow descrepancy, failed relays, electrical contact short, dying pump, stuck solenoid, clogged filter, etc.... you rely on the methanol/Alcohol for extra fueling of higher boost.

Running race gas is safest method.
Using Water Inj as an added margin of safety is also very safe.

JD

CMonakar

Well put.

jimlab

I don't know why I bother, but what the hell...

I didn't see anyone mention the stock fuel system or the factory ECU being a contributing factor to the "15 PSI max" rule of thumb. The facts are that A) the stock injectors reach maximum duty cycle at about 15 PSI and the factory ECU, even with a piggy-backed reprogrammed chip, does not have fuel maps for over 15 PSI. Past that point, fuel delivery flatlines and the stock injectors are maxed out at 100% duty cycle. That's predominantly where the 15 PSI limit originally came from.

You have to keep in mind that fully programmable aftermarket ECUs and a wide variety of injectors and fuel pumps to choose from are relatively recent developments. In the 90s, you were basically stuck with having your factory ECU reprogrammed by XS Engineering, Pettit, or Mostly Mazda (later M2 Performance) or trying to deliver more fuel for higher boost with a PFS "gold" or "purple" PMC, which had extremely limited (the gold more so than the purple) "tuning" capabilities. I don't know of anyone who had an engine live for very long using either of the Peter Farrell computers, and the same goes for the reprogrammed ECU options available back then.

People looking to run more boost strapped additional injectors on any surface that would support them to get more fuel into the engine, and some people (Kevin Wyum and Carlos Iglesias, especially) started tinkering with the first aftermarket engine management solutions from Motec, Wolf, and Haltech. However, at the point at which you can reliably deliver more fuel to the engine, the stock turbos (this was prior to the BNR upgrades being available) become a limiting factor and once again you're limited to about 15-17 PSI, at least with pump gas. Out of their efficiency range, the stock twins simply produce more heat than the extra boost is worth. This was another factor that helped solidify the 15 PSI "limit". However, with the turbo and ECU options available today, that's no longer the case.

I believe Rynberg pointed out that most people claiming "X" boost on stock twins likely don't run that on the street, nor would they run that without water injection or high octane fuel as cushions to help prevent detonation. I'm far more interested in what someone actually runs on the street, on a daily basis, with pump gas, than what they run on the dyno for bragging rights. I ran 20-21 PSI on a daily basis for three years with my Supra on 91-92 octane without a problem. By comparison, I ran 14-15 PSI daily with my RX-7 with an M2 reprogrammed ECU, Crane HI-6, a Cosmo fuel pump, and other supporting mods and lost two engines in less than a year (only one to detonation, but that's another story).

The bottom line is that you cannot compare boost levels between two different cars or even necessarily two engines produced by the same manufacturer and draw any sort of conclusion on how much boost they're capable of running. The amount of boost you can safely run is ultimately determined by how efficiently you can produce the desired pressure level, and how much fuel you can deliver and still maintain a reasonable margin of safety with the air fuel ratio. Run too lean for too long and you can blow even a naturally aspirated engine.

Boostn7

Good point Jim. That should account for many blown motors back then.
Stock fuel system maxes @ 14-15psi
Run 4x 850cc + fuel pump and you're good for ~400rwh @ 9x % inj duty.
Run 2x850cc + 2x1200cc + fuel pump and you're good for ~475rwh @ 9x% inj duty.
At this point there was a slight fuel pressure drop at higher rpms, most likely the limit on the Walbro#341 and stock fuel lines.

Original poster(SidewaysR1) asks about the 15psi limit even with a complete fuel system and single turbo.

Rotaries are simply not as forgiving when not properly done/tuned thanks to their poor thermal efficiency and their weakest link......apex seals.

As far as the twins here's rwh #'s from my 1/2003 dyno:
.90-.92kg/cm2= 333.6 rwh 280.7 rwt - 3rd gear pull
.94-.95 = 342.5 rwh 290.2 rwt - " "
.98-1.00 = 353.8 rwh 302.8 rwt - " "
1.02-1.05 = 363.5 rwh 307.2 rwt - " "
1.07-1.05 = 360.1 rwh 312.0 rwt - " "
1.07-1.10 = 367.8 rwh 316.6 rwt - " "
1.10-1.12 = 376.7 rwh 331.8 rwt - " "
1.10- 1.13 = 381.3 rwh 331.1 rwt - " "
1.12-1.14 = 390.0 rwh 348.2 rwt - " "
1.13-1.14 = 392.8 rwh 348.9 rwt - " " -(after cool down)
1.14 = 394.5 rwh 346.9 rwt - " "
1.14-1.15 = 395.4 rwh 351.2 rwt- " " -(map sensor hit max, 18psi on gauge)
1.14-1.15 = 398.5 rwh 357.7 rwt- 4th gear, 17-18psi, 93% ID)
1.15 = 397.7 rwh 363.5 rwt - 4th gear, 19 psi dropping to 17-18psi
1.15 = 402.0 rwh 356.0 rwt - 4th gear, 17-18psi, 6 degree split, 92 % ID
*That last run was pull #46 and inlet temps started @ 39°C dropping to 36° and A/Fs were 10.6 @ 6.5krpm and went leaner to 11.3-11.4 by 7-7.5krpm.

Very obvious their eficiency does not stop @ 15psi?
FYI: I was running a Greddy 3 row FMI, 4x 850cc injs w/ Walbro #341 and full tank of Sunoco 94.
I have all the logs and dyno sheets and 2 forum members were present at the dyno.

Later,
JD

fzr100069

interesting...as in what EGT range?

GoodfellaFD3S

iTrader: (213)

1500 degrees F is the highest peak value I'm comfortable seeing on my setup.

kuroi FD

1500? I thought ideal was around 12-1300 degrees F?

GoodfellaFD3S

iTrader: (213)

Depends on who you talk to. My peak EGTs with the BNRs at 17 psi, pump gas/water inj: 2nd gear 8000rpm, 1200. 3rd gear 8000 rpm, 1300. 4th gear 8000 rpm, 1400.