i was just curious what kind of power differences people were seeing from running a few points leaner afr's? Or the effects on spooling. that type of thing. like say from 10.9 to 11.5. Or in whatever case you experienced. Open for discussion.

Brent

 

still curious what people have experienced. I do all street tuning so i can feel a difference but i cant tell how much it is

 

anything in the 10's afr it floods the engine a little. more backfire than ususal. but from what im told its supposed to be that way when running pump because of the detonation. the higher the afr the higher the horsepower! but i think you know that!

 

lol yeah i know it makes more power if its leaner. i was just curious how much of a difference in power people see when tuning.

I know years ago when i was still running the twins i picked up about 20 hp but i dont remember what afr it started at and what i tuned it to.

 

higher AFR only make more power to a certain point right? like 12.5:1, and about the detonation on pump gas, it would depend on how much boost, timing, ect.

 

What boost and AFR are you currently running?

Here's my experience on running lean... Going from 11 AFR to 12 AFR gave me an extra 30-35 HP at the wheels at 17 PSi but you can see the result. Tune only lasted 4 laps at Mosport and then engine went boom without any warning or sound, just drop in boost to 0 PSi at 230-240 km/h. In reality, I didn't mind blowing a motor, I wanted to find this mysterious limit all the well known tuners kept telling me about.

Also, I could never really consider myself a tuner until I popped a motor first hand



thewird

 

Tune for a consistant AFR of 10.8-11.2 under boost on pump gas......

-J

 

It makes a huge diffrence in my opinion and depending on the gas you are using. My particulat case i ran like 9-10psi on 90 octance with AFR about 11.8-12.0 and ran the same time running 16psi on C16 with AFR about 10.5 or so.

 

This depends on boost. You can run 11.5-12.0 AFR depending on the gas your using at 10 PSi no problem. And richen it up as you increase the boost. 11.0 AFR is a good target for 16-17 PSi with good gas.

This is from experience at flogging my car at the track almost every weekend this car. The generally accepted AFR's are safer of course.

thewird

 

I know what afrs are safe. I tune mine on pump gas 93 and run 12.1- 12.2 from 1- 6 psi then taper to about 11.2- 11.3 at 15 psi. A couple people have touched on the total power gained from running leaner. And obviously this is up to a point. I don't think anymore power can be gained past 12.5 or so in boost. And thats too lean for higher boost on pump fuel. I would guess anywhere from 20- 30 hp could be gained going from 10.5- 11.5 but its just speculation on my part. The gain is probably larger the richer you are. Anything lower then 9ish really bogs the car down. My highest p row is tuned exactly this way to prevent engine damage from over boosting. The car doesn't even want to rev higher if it boosts more then 16.5. It's Much safer then fuel a fuel cut limiter. I also have the timing pulled way back.

 

The best way to answer your question is to get some dyno time and find out first hand.

 

true. but i do all my tuning on the street, there are no dyno's near by so i would have to drive then spend money at the dyno too. Not really worth the trouble for me to find out. It's just a question i had out of curiosity that i thought would be an interesting discussion

 

I'm not running a single (stock turbos on 3rd gen), and I think that as far as wheel hp/tq goes, "your results may vary" depending on

1. dyno
2. AFR (duh)
3. timing (duh)
4. air temp (as read by IAT also ambient)
5. coolant temp
6. fuel temp
7. turbo/exhaust/porting/compression ratio/intake manifold/intercooler + piping + ducting/boost pressure/etc.
8. fan on car
9. gear used (I used 3rd, from what I read, it will show slightly lower numbers in general than using 4th)
EDIT: 10. gas (I used 91 octane)

etc... as everyone already knows...

BUT I will tell you that last time I was on the dyno, I took my AFRs under boost from about 10.2-10.5 starting, and leaned it out to 10.7-11 (about 12-13 psi stock turbos) and only picked up about 10 whp/10 wtq. My AFR curve has generally the same shape, it's mostly leaner up top. I decreased split by 2 degrees throughout the rev range and got another 10 whp/wtq (and better torque down low!)... something to chew on, hope this helps...

EDIT: Oh, and my car doesn't really backfire a lot... especially under boost.... even with the AFRs in the 10s. I get tiny pop here and there but that's usually under deceleration and it's not that loud. I'm sporting a high flow cat instead of a midpipe however. I got my tune fairly well sorted off boost.

 

Leaning it out from 10.5 to 11.5 picks up a lot of HP. I have noticed it many times while tuning on the dyno. I dont have an exact figure as I have never written it down but its probably in the 25-35 RWHP range.

 

damn it just jumpped outa there as if it wanted to fly



oh and dudeman if your looking for more horsepower gains look at upgraded throttlebodys. take a look
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/making-car-faster-but-still-reliable-799595/


oh and my tuner has some lightweight billet rotors for sale that will give you 70-100hp guarnteed. $4000 though

 

On all the cars I've tuned I always felt the most difference from going from anywhere in the 10's to about 11:1, then you start getting diminishing returns.

 

darkphantom - thats interesting, although i'm not really looking for more HP. I'm happy with my numbers... whatever they may be. I just like to know more details about the effects of certain things. Which this discussion has definitely had some good information i think. Thanks everyone that had input for my curiosity.

 

hahahahhaha i love this thread!

 

^^^ I'm no expert, but I never run any richer than 11.9:1.
I have been told and thoroughly believe that the detonation that everyone is so afraid of is caused by hot spots in the combustion chamber. One of these hot spots is the trailing spark plug. I have had my motor together (first build btw) since about late march of this year. I run it hard, though on the stock s5 turbo.
Things that I looked at were adding surface area to the coolant passages around the trailing plug. I also run evans npg+ which boils at almost 400*F which will virtually eliminate any and all localized boiling in the cooling system which can cause a lack of cooling in that area especially under WOT. I also have mazdacomp underdrive pulleys which slow the water pump to a sane rpm so that it doesn't cavitate the coolant.
The FD water pump is the worst for cavitation btw.
Also having a heat barrier of some type between the exhaust manifold/turbo and the lower intake to keep the intake air temps down helps a lot.
I have dyno'd at 258.1whp on a mustang dyno (stock s5 turbo) uncorrected at 4500ft altitude in the middle of june (~95*F ambient) with no fan on the intercooler/radiator and no changes from my street tune at 12 psi. 8.5:1 rotors in an s5 based motor. Average afr at wot in boost was 12.5:1.
I transition into boost in the 13's because it picks up a lot better. Anything richer than 12.0:1 and the thing feels squishy as hell. There is just more aggressive pull from the motor when you are not chocking it with fuel trying to cool the intake charge so it doesn't detonate.
Running that rich is a bandaid as far as I am concerned. Decent intercooling and proper prep work make all the difference.
I run br10eix ngk plugs .023 gap
BTW my motor runs 91 octane **** fuel here in utah where there isn't anything better unless you buy race fuel.

For anyone that is reading that hasn't ever tuned, there is something that a lot of people miss....... The necessary afr for a particular situation will vary greatly depending on ignition timing.
Detonation is caused by heat igniting the combustion mixture before the spark plug does.

I am fully prepared to be shot down by some of the more experienced people. I am merely stating my position based on 3 years that I have been tuning. 1 of which has been on my own custom built motor; the first two were with stock used motors.


Learn by doing. Take notes......

 

I'll be the first to shoot you down .

First lets take a look at the facts you stated.

1) 12.5 AFR under WOT
2) 12 PSi
3) 8.5:1 compression rotors

Now your understanding is that most people consider 12.5 AFR (for boosted rotary) unsafe. Now is is true but that is not the whole part of the story.

Let's quickly explain what detonation really is. Detonation is caused by the fuel pre-igniting uncontrollably before the spark. Now what causes this? At some point heat is greater then the fuel's resilience to heat, octane rating. The higher the octane, the more heat the fuel can take before self-combustion (detonation).

Alright, you probably already had an idea on that but for the sake of being thorough, it has been included. Now what exactly causes heat? Well for one the constant combustion of the fuel and the friction of the parts. The actual compression the rotor creates also causes heat since when you compress air, heat is generated. Now since we are boosting our rotaries, the air entering the combustion chamber is already compressed, and then the rotor compresses it further generating even more heat. Last but not least, running lean also causes even more heat. The richer you run, the cooler its going to be due to the fuels cooling effect (which isn't that great btw).

One more thing that effects heat is timing, the more advanced you run the more power and heat your going to generate. Also, the less split you run the more power and heat your going to make since it makes the combustion more and more violent/powerful (might not be the best explanation but will give you an idea). Of course there's a point where timing stops making more power but keeps making the combustion hotter.

Anyway, to sum up what generates heat...

1) Combustion
2) Pressure of the air before it enters the combustion chamber (boost)
3) Compression of air from the compression stroke
4) A/F ratio
5) Ignition timing

Some stuff may have been generalized or left out but now that we got that out of the way lets look at the data you provided.

First you have 8.5:1 compression rotors which are s4 TII rotors if memory serves me right. Now these are very special rotors for boost. Why? Because they are lower compression then the other turbo rotaries. S5 turbo and S6 rotors are both 9:1 compression rotors. Well, this means that every compression stroke is generating less heat then the other rotors would. So this is a safer setup and can run more boost safely due to that.

Alright second, your running 12 PSi. This is still considered low boost and 12.5 AFR is close to what is considered safe (even moreso that you have the 8.5:1 compression rotors). Now when you hear people saying low 11's is safe or high 10's, they really mean it but it depends at what boost level your talking about. As you increase boost, combustion temperatures start going up. To avoid detonation, we must richen up the fuel to generate less heat. Your car may be safe at 12.5 AFR @ 12 PSi but increase that to 16 PSi, and it will most likely go boom. Might not on the first pull or gear but internal temperatures (or EGT's) will start going up until they start detonating the fuel.

Now I'm not sure if 12.5 AFR @ 12 PSi is truely safe on the 8.5:1 rotors vs 9.0:1 but like I mentioned, detonation isn't instant. EGT's have to build up and then it starts detonating.

Anyway, the point of the story is, AFR isn't what determines what is safe, EGT's do. At what point is your fuel going to detonate? Some people say EGT tuning is pointless but if you want to find limits, you need EGT's. Also, having a dyno handy is useful so you know what is making more power and at which point your getting diminishing returns for running a little more unsafe.

Well, there's me taking a shot at you. If you were running 9.0:1 compression rotors @ 12 PSi, the AFR I would recommend would be 11.5-11.7 for power however someone like Chuck would recommend 11.1 which is safe AND provides a margin for safety for inexperienced tuners (not that I'm experienced, I've just been willing to blow motors testing limits under track conditions). Again, this depends on timing and quality of fuel.

Also to note, how do you know your wideband is giving you the correct fuel ratio? I've seen multiple cases where different widebands have different readings.

Well, that was my first attempt at explaining why rotaries blow up

thewird

 

I know some people run 12:1 afr up to about 10 psi on 9:1 rotors but thats with 93 octane. I dont know how much difference the 91 octane and 8.5 rotors make, but to me it seems like a 12.5 afr would be pushing things a little too close @ 12 psi. and 91 octane I think those numbers are are a tiny bit in the red zone, so it might be okay for awhile, but i tend to like being more on the conservative side for peace of mind.
Also no fan on the dyno = no airflow through the intercooler which means little heat removed from intake charge, not a good combination for an iffy tune IMO. It sounds like the engine is still running though so thats good, just thought i would mention it.

 

Damn! I wrote this elaborate response and lost it!

Anyways, here goes again.....

I will now respond to the rebutle statements in the prior posts

Yes, I have s4TII rotors. The lower compression helps with detonation a bit.

Firstly, there are spots in the motor which can affect detonation as well. As I stated before, the trailing spark plug holes are a hot spot. Under full load, that is a very volatile place for the air/fuel mixture to be in.
Second, EGT doesn't necessarily AFFECT detonation. It can however, be an indicator of detonation. High air/fuel/combustion temps lead to high egt. That however, is not the only thing that affects egt and they can be misleading as well. Generally, a lean afr burns hot and a rich afr burns cool. That being said, ignition timing has the ultimate say in how hot the exhaust will be. I have seen a lean afr burn cool (relative to the situation) and a rich mixture burn very hot.
Furthermore, different afr's burn at different speeds. A slow burning afr like 10.5:1 and a retarded ignition timing would still end up with high egt's. Why? Because the mixture is burning while exiting the combustion chamber and into the manifold. Builds boost, but sloppy for power.

Also, one thing to remember is that the closer to stoich (gasoline 14.7:1) the mixture, the easier it is to ignite. Cool air is a very good thing to have. Carlos Lopez once told me that if he could put a refrigerator as an intercooler, he would. Cool air makes both power and safer combustion by cooling the combustion mixture.

And moreover, 12psi from one turbo isn't exactly the same as 12psi from another.
The stock FC turbo (which is what I have) running 12 psi is significantly hotter air than even the stock FD twins. Simply it is way too small to make good power on a 13b rotary engine. The pre-intercooler temps can reach almost 150*F on a hot summer day ~97*F. A good intercooler setup helps significantly. I rarely see more than 15*F above ambient (manifold temp) unless I just got back into the car on a hot day after running it and let it sit and soak.
The higher the manifold inlet temp, the more likely something is gonna blow chunks.

If anything affects egt's the most, its the ignition timing. It has the final say in what comes out the *** end of that combustion cycle. Most engines I have seen blow are in the area of max torque. This is usually where the highest average combustion chamber pressures; or brake mean effective pressures (bmep) are seen. Obviously this is because pressure builds heat quickly.
So, to keep from detonating, leveling off the ignition timing, or even taking a few degrees out helps with this. As for split, I usually have the highest split around max torque area and slowly taper down the split towards redline and/or max hp (which isn't always at redline lol)

The statement about no fan on my dyno run was more to the point that it survived, not that it was a good idea. I needed to see if it could handle the abuse as I do track days as well as auto-x. I had the shop 5gas analyzer mirroring my wideband, so I don't think that mine reads weird.

Basically when detonation is concerned, EVERYTHING has to be taken into consideration. I run BR10EIX plugs and I even deburred the edges of the bloody ground electrode.
My main point is that not only ignition timing and afr affect detonation. Ultimately it's heat management. How well you can keep heat out of the combustion mixture.
My motor has been together for 8 months so far and its great. I am a bit adventurous I will admit; however, my point is that everyone keeps putting most of the blame on a couple of things when they need to be looking at the big picture.
Sure, better fuel will make power safer. However, it doesn't mean you solved the problem either. Ask anyone making reliable power with an aggressive tune and they will tell you that heat management is a top priority.


I hope I am making sense. It's very late and I lost my initial responses.
Anyways, keep shooting. These discussions are what make people think. The more information there is on the table, the more people understand.

 

I did mention ignition timing and the other side of the coin is definitely heat. I thought my whole post was about heat but maybe I didn't make that clear enough :P. If you start with hotter air, of course its going to be even hotter during the compression cycle.

As for intake temps, this can all be controlled by tuning. I ran 90-100*C (194-212*F) intake temps after the intercooler before I upgraded my oil coolers and some better ducting all summer at the track running 16 PSi and had no issues. Was it safe, of course not, was it controlled with tuning, yes.

Anyway, its too early in the morning to type something long. And yes I agree the more people talk about it the better.

thewird

 

^^ As was my first post, but its all good.

 

i think another overlooked point was he was also running the car at 4500 feet. thinner air should also give a margin of safety.