Okay so the biggest problem I have with the PowerFC is that it cuts fuel instead of retarding ignition timing when you bounce off the rev limiter...my question is, how can I change the settings in my PowerFC to make it do the opposite? Is there a way I can play with the settings to make my PowerFC change ignition timing without cutting fuel at a certain RPM?

Car: 94 gt35r single turbo 550cc 2200cc injectors, water injection, PowerFC, AVC-R (can give more relevant information if needed)

 

Retard your timing in the map above the rpm where you want the timing retarded. Play with the break points to get it where you want it. What's wrong with cutting fuel?

 

the timing map is rpm and load....so yes. just change the ignition maps of leading and trailing for whatever boost level (PIM value) you want to have.

 

I was under the impression it's better for engine temps and the overall longevity of your internals (seals) to cut ignition instead of fuel at your rev limit. If I'm wrong, please correct me, that's just what I had come to understand.

Also, thank you lOOatme, I'll try to adjust those settings and see if I can get the desired effect

Edit: Okay after doing more research this is what I gathered:

Fuel cut: Injectors are set to false, completely cuts all fuel from the engine. Saves gas, less boom pop sizzle, and has a relatively low risk of a lean condition, as it is cutting ALL fuel so no combustion would take place...however; I'm going to go out on a limb and assume engine temps would climb VERY quickly in the few seconds you hit that rev limit (if you're autocrossing/drifting you might hit it a lot in a short span) because fuel cools the internals...which could result in warped seals, and in turn shitty compression (essentially, engine failure).

Ignition cut: Injectors are still spraying but ignition is retarded. Eats fuel, lots of boom pop sizzle, no real risk of a lean condition (is this true?), might destroy your exhaust if too much gas pools up.

What it comes down to, I guess, is what the car is being used for? I do want to autocross/drift it at some point and I'm not sure how often I'll be hitting that rev limit. I don't really care about my exhaust, I could go through 10+ exhausts before it even comes close to the cost of one rebuild.

 

1. how would engine temps climb if there is no combustion?
2. every OE engine uses fuel cut.

 

Friction creates heat, friction is happening when two surfaces make contact and rub, which is what the little 3mm thick apex seals do as the eccentric shaft spins the rotors. The OMP lubricates the apex seals to lower the coefficient of friction, which helps protect the seals. Combustion actually helps lower engine temps by carrying heat out the exhaust...lower engine temp, higher EGT- cut fuel and you just have a rotor spinning around, pushing hot air around the engine, and spark plugs firing...higher engine temps, lower EGT. Am I wrong? Please correct me if I am, I'm still learning all this stuff.

Also OEM being the preferable settings assumes the car is 100% OEM...which mine is not. Once you start modifying a car's physical parts, you have to consider changing how it runs by programming it differently. You can't expect to run a car that has been heavily modified with the same settings that it uses on an OEM ECU with the stock settings. That being said I don't know what is better, I'm just asking for opinions...but I don't think the manufacturers set the car's tune with single turbo on 18 PSI in mind.

 

Yes,
You are wrong. Engine temperature is directly related to engine combustion. Frictional heat is of no contribution engine temperature.

I honestly just disable the rev limiter and tune the engine to reduce power above my intended max rpm by adding fuel and pulling timing. I have seen more killed engines from bouncing off the rev limiter under load than I have ever seen any damage from the odd zing up to 10-11K.

 

I worded that very poorly- I meant to say assuming the engine was running with just spark for a minute and no fuel, frictional energy wouldn't cause a noticeable rise in engine temp/heat up the seals more than normal? Versus having fuel present to absorb the heat and shoot it out the exhaust- keeping in mind that I'm assuming no combustion is taking place in the combustion chamber because lack of spark.

But I guess I should add that I'm not concerned about the engine heat but the frictional heat and the thin 3mm seals. Over time the friction won't cause them to warp by hitting fuel cut? Anyways, you have a good point and I guess rotary engines were designed to rev high anyways, I'll just disable the rev limit or just set it super high and stay away from it

 

What about going down a long mountain using the engine to slow you down for 4-5 miles, we do this in CO all the time. There is no fuel being injected and the engine is just dragging down the mountain. What I see is the incoming air actually cools the engine from the inside out when we do this. The friction between the housing and seal should be fine since lube is all over the housings, and hopefully you are running an OMP.

 

friction is basically a constant, so the amount of frictional heat isn't going to change much in your example.

i didn't see combustion temps, but EGT on a tuned car will be around 950c, combustion temps are going to be somewhere around there.

Mazda has never published just a sliding friction number, but for it to be more than the heat of combustion, we probably wouldn't be able to turn the engine over by hand.

the sliding friction is, a, much less than the combustion temperatures, b, is dropping with rpm, and c the air the engine is ingesting (without fuel, or combustion) is going to cool everything much more than just fuel.

the D in the example is that cutting spark as a limiter has a couple of big drawbacks. the first one is that the mixture can still ignite via a hot spot, like the spark plug tip. the second is all of that fuel is washing away the oil film.

the reason the rotary doesn't like a rev limiter isn't because of the seals warping, its that the engine doesn't like the stop/start at high rpm and high loads. its ok to hit the limiter, but you don't want to stay there.

and E, if you're worried about sliding friction, 3mm seals are worse than 2mm seals, and then a piston engine is running ~1mm seals.

 

Thank you for the awesome information That's why I love this forum, I learn something new every day.

So the best approach is to just set my rev limit to something obnoxious like 11,000 RPMs and just never go near that? And yes I'm running an OMP and I premix when I plan on driving the car extra hard or when I change the tune to race gas.

 

depends, if you're just going to drive around at 8500rpm, limiter or not, then yes just raise it to something like 9500* and pull some timing between 8500-9500.

if you're just going to beat on the car and hit the limiter here and there, its totally fine.


*some things, like the clutch, are not happy with too many rpms.

 

Yea, I love the car but I didn't buy it to baby it. It's got a pretty tough ACT racing clutch, but my transmission is starting to feel the mileage and the abuse. Thanks for the input guys, I believe I got the answers I needed